Damien Miller | d4a8b7e | 1999-10-27 13:42:43 +1000 | [diff] [blame] | 1 | .\" -*- nroff -*- |
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| 3 | .\" $Id: RFC.nroff,v 1.1 1999/10/27 03:42:43 damien Exp $ |
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| 5 | .pl 10.0i |
| 6 | .po 0 |
| 7 | .ll 7.2i |
| 8 | .lt 7.2i |
| 9 | .nr LL 7.2i |
| 10 | .nr LT 7.2i |
| 11 | .ds LF Ylonen |
| 12 | .ds RF FORMFEED[Page %] |
| 13 | .ds CF |
| 14 | .ds LH Internet-Draft |
| 15 | .ds RH 15 November 1995 |
| 16 | .ds CH SSH (Secure Shell) Remote Login Protocol |
| 17 | .na |
| 18 | .hy 0 |
| 19 | .in 0 |
| 20 | Network Working Group T. Ylonen |
| 21 | Internet-Draft Helsinki University of Technology |
| 22 | draft-ylonen-ssh-protocol-00.txt 15 November 1995 |
| 23 | Expires: 15 May 1996 |
| 24 | |
| 25 | .in 3 |
| 26 | |
| 27 | .ce |
| 28 | The SSH (Secure Shell) Remote Login Protocol |
| 29 | |
| 30 | .ti 0 |
| 31 | Status of This Memo |
| 32 | |
| 33 | This document is an Internet-Draft. Internet-Drafts are working |
| 34 | documents of the Internet Engineering Task Force (IETF), its areas, |
| 35 | and its working groups. Note that other groups may also distribute |
| 36 | working documents as Internet-Drafts. |
| 37 | |
| 38 | Internet-Drafts are draft documents valid for a maximum of six |
| 39 | months and may be updated, replaced, or obsoleted by other docu- |
| 40 | ments at any time. It is inappropriate to use Internet-Drafts as |
| 41 | reference material or to cite them other than as ``work in pro- |
| 42 | gress.'' |
| 43 | |
| 44 | To learn the current status of any Internet-Draft, please check the |
| 45 | ``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow |
| 46 | Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), |
| 47 | munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or |
| 48 | ftp.isi.edu (US West Coast). |
| 49 | |
| 50 | The distribution of this memo is unlimited. |
| 51 | |
| 52 | .ti 0 |
| 53 | Introduction |
| 54 | |
| 55 | SSH (Secure Shell) is a program to log into another computer over a |
| 56 | network, to execute commands in a remote machine, and to move files |
| 57 | from one machine to another. It provides strong authentication and |
| 58 | secure communications over insecure networks. Its features include |
| 59 | the following: |
| 60 | .IP o |
| 61 | Closes several security holes (e.g., IP, routing, and DNS spoofing). |
| 62 | New authentication methods: .rhosts together with RSA [RSA] based host |
| 63 | authentication, and pure RSA authentication. |
| 64 | .IP o |
| 65 | All communications are automatically and transparently encrypted. |
| 66 | Encryption is also used to protect integrity. |
| 67 | .IP o |
| 68 | X11 connection forwarding provides secure X11 sessions. |
| 69 | .IP o |
| 70 | Arbitrary TCP/IP ports can be redirected over the encrypted channel |
| 71 | in both directions. |
| 72 | .IP o |
| 73 | Client RSA-authenticates the server machine in the beginning of every |
| 74 | connection to prevent trojan horses (by routing or DNS spoofing) and |
| 75 | man-in-the-middle attacks, and the server RSA-authenticates the client |
| 76 | machine before accepting .rhosts or /etc/hosts.equiv authentication |
| 77 | (to prevent DNS, routing, or IP spoofing). |
| 78 | .IP o |
| 79 | An authentication agent, running in the user's local workstation or |
| 80 | laptop, can be used to hold the user's RSA authentication keys. |
| 81 | .RT |
| 82 | |
| 83 | The goal has been to make the software as easy to use as possible for |
| 84 | ordinary users. The protocol has been designed to be as secure as |
| 85 | possible while making it possible to create implementations that |
| 86 | are easy to use and install. The sample implementation has a number |
| 87 | of convenient features that are not described in this document as they |
| 88 | are not relevant for the protocol. |
| 89 | |
| 90 | |
| 91 | .ti 0 |
| 92 | Overview of the Protocol |
| 93 | |
| 94 | The software consists of a server program running on a server machine, |
| 95 | and a client program running on a client machine (plus a few auxiliary |
| 96 | programs). The machines are connected by an insecure IP [RFC0791] |
| 97 | network (that can be monitored, tampered with, and spoofed by hostile |
| 98 | parties). |
| 99 | |
| 100 | A connection is always initiated by the client side. The server |
| 101 | listens on a specific port waiting for connections. Many clients may |
| 102 | connect to the same server machine. |
| 103 | |
| 104 | The client and the server are connected via a TCP/IP [RFC0793] socket |
| 105 | that is used for bidirectional communication. Other types of |
| 106 | transport can be used but are currently not defined. |
| 107 | |
| 108 | When the client connects the server, the server accepts the connection |
| 109 | and responds by sending back its version identification string. The |
| 110 | client parses the server's identification, and sends its own |
| 111 | identification. The purpose of the identification strings is to |
| 112 | validate that the connection was to the correct port, declare the |
| 113 | protocol version number used, and to declare the software version used |
| 114 | on each side (for debugging purposes). The identification strings are |
| 115 | human-readable. If either side fails to understand or support the |
| 116 | other side's version, it closes the connection. |
| 117 | |
| 118 | After the protocol identification phase, both sides switch to a packet |
| 119 | based binary protocol. The server starts by sending its host key |
| 120 | (every host has an RSA key used to authenticate the host), server key |
| 121 | (an RSA key regenerated every hour), and other information to the |
| 122 | client. The client then generates a 256 bit session key, encrypts it |
| 123 | using both RSA keys (see below for details), and sends the encrypted |
| 124 | session key and selected cipher type to the server. Both sides then |
| 125 | turn on encryption using the selected algorithm and key. The server |
| 126 | sends an encrypted confirmation message to the client. |
| 127 | |
| 128 | The client then authenticates itself using any of a number of |
| 129 | authentication methods. The currently supported authentication |
| 130 | methods are .rhosts or /etc/hosts.equiv authentication (disabled by |
| 131 | default), the same with RSA-based host authentication, RSA |
| 132 | authentication, and password authentication. |
| 133 | |
| 134 | After successful authentication, the client makes a number of requests |
| 135 | to prepare for the session. Typical requests include allocating a |
| 136 | pseudo tty, starting X11 [X11] or TCP/IP port forwarding, starting |
| 137 | authentication agent forwarding, and executing the shell or a command. |
| 138 | |
| 139 | When a shell or command is executed, the connection enters interactive |
| 140 | session mode. In this mode, data is passed in both directions, |
| 141 | new forwarded connections may be opened, etc. The interactive session |
| 142 | normally terminates when the server sends the exit status of the |
| 143 | program to the client. |
| 144 | |
| 145 | |
| 146 | The protocol makes several reservations for future extensibility. |
| 147 | First of all, the initial protocol identification messages include the |
| 148 | protocol version number. Second, the first packet by both sides |
| 149 | includes a protocol flags field, which can be used to agree on |
| 150 | extensions in a compatible manner. Third, the authentication and |
| 151 | session preparation phases work so that the client sends requests to |
| 152 | the server, and the server responds with success or failure. If the |
| 153 | client sends a request that the server does not support, the server |
| 154 | simply returns failure for it. This permits compatible addition of |
| 155 | new authentication methods and preparation operations. The |
| 156 | interactive session phase, on the other hand, works asynchronously and |
| 157 | does not permit the use of any extensions (because there is no easy |
| 158 | and reliable way to signal rejection to the other side and problems |
| 159 | would be hard to debug). Any compatible extensions to this phase must |
| 160 | be agreed upon during any of the earlier phases. |
| 161 | |
| 162 | .ti 0 |
| 163 | The Binary Packet Protocol |
| 164 | |
| 165 | After the protocol identification strings, both sides only send |
| 166 | specially formatted packets. The packet layout is as follows: |
| 167 | .IP o |
| 168 | Packet length: 32 bit unsigned integer, coded as four 8-bit bytes, msb |
| 169 | first. Gives the length of the packet, not including the length field |
| 170 | and padding. The maximum length of a packet (not including the length |
| 171 | field and padding) is 262144 bytes. |
| 172 | .IP o |
| 173 | Padding: 1-8 bytes of random data (or zeroes if not encrypting). The |
| 174 | amount of padding is (8 - (length % 8)) bytes (where % stands for the |
| 175 | modulo operator). The rationale for always having some random padding |
| 176 | at the beginning of each packet is to make known plaintext attacks |
| 177 | more difficult. |
| 178 | .IP o |
| 179 | Packet type: 8-bit unsigned byte. The value 255 is reserved for |
| 180 | future extension. |
| 181 | .IP o |
| 182 | Data: binary data bytes, depending on the packet type. The number of |
| 183 | data bytes is the "length" field minus 5. |
| 184 | .IP o |
| 185 | Check bytes: 32-bit crc, four 8-bit bytes, msb first. The crc is the |
| 186 | Cyclic Redundancy Check, with the polynomial 0xedb88320, of the |
| 187 | Padding, Packet type, and Data fields. The crc is computed before |
| 188 | any encryption. |
| 189 | .RT |
| 190 | |
| 191 | The packet, except for the length field, may be encrypted using any of |
| 192 | a number of algorithms. The length of the encrypted part (Padding + |
| 193 | Type + Data + Check) is always a multiple of 8 bytes. Typically the |
| 194 | cipher is used in a chained mode, with all packets chained together as |
| 195 | if it was a single data stream (the length field is never included in |
| 196 | the encryption process). Details of encryption are described below. |
| 197 | |
| 198 | When the session starts, encryption is turned off. Encryption is |
| 199 | enabled after the client has sent the session key. The encryption |
| 200 | algorithm to use is selected by the client. |
| 201 | |
| 202 | |
| 203 | .ti 0 |
| 204 | Packet Compression |
| 205 | |
| 206 | If compression is supported (it is an optional feature, see |
| 207 | SSH_CMSG_REQUEST_COMPRESSION below), the packet type and data fields |
| 208 | of the packet are compressed using the gzip deflate algorithm [GZIP]. |
| 209 | If compression is in effect, the packet length field indicates the |
| 210 | length of the compressed data, plus 4 for the crc. The amount of |
| 211 | padding is computed from the compressed data, so that the amount of |
| 212 | data to be encrypted becomes a multiple of 8 bytes. |
| 213 | |
| 214 | When compressing, the packets (type + data portions) in each direction |
| 215 | are compressed as if they formed a continuous data stream, with only the |
| 216 | current compression block flushed between packets. This corresponds |
| 217 | to the GNU ZLIB library Z_PARTIAL_FLUSH option. The compression |
| 218 | dictionary is not flushed between packets. The two directions are |
| 219 | compressed independently of each other. |
| 220 | |
| 221 | |
| 222 | .ti 0 |
| 223 | Packet Encryption |
| 224 | |
| 225 | The protocol supports several encryption methods. During session |
| 226 | initialization, the server sends a bitmask of all encryption methods |
| 227 | that it supports, and the client selects one of these methods. The |
| 228 | client also generates a 256-bit random session key (32 8-bit bytes) and |
| 229 | sends it to the server. |
| 230 | |
| 231 | The encryption methods supported by the current implementation, and |
| 232 | their codes are: |
| 233 | .TS |
| 234 | center; |
| 235 | l r l. |
| 236 | SSH_CIPHER_NONE 0 No encryption |
| 237 | SSH_CIPHER_IDEA 1 IDEA in CFB mode |
| 238 | SSH_CIPHER_DES 2 DES in CBC mode |
| 239 | SSH_CIPHER_3DES 3 Triple-DES in CBC mode |
| 240 | SSH_CIPHER_TSS 4 An experimental stream cipher |
| 241 | SSH_CIPHER_RC4 5 RC4 |
| 242 | .TE |
| 243 | |
| 244 | All implementations are required to support SSH_CIPHER_DES and |
| 245 | SSH_CIPHER_3DES. Supporting SSH_CIPHER_IDEA, SSH_CIPHER_RC4, and |
| 246 | SSH_CIPHER_NONE is recommended. Support for SSH_CIPHER_TSS is |
| 247 | optional (and it is not described in this document). Other ciphers |
| 248 | may be added at a later time; support for them is optional. |
| 249 | |
| 250 | For encryption, the encrypted portion of the packet is considered a |
| 251 | linear byte stream. The length of the stream is always a multiple of |
| 252 | 8. The encrypted portions of consecutive packets (in the same |
| 253 | direction) are encrypted as if they were a continuous buffer (that is, |
| 254 | any initialization vectors are passed from the previous packet to the |
| 255 | next packet). Data in each direction is encrypted independently. |
| 256 | .IP SSH_CIPHER_DES |
| 257 | The key is taken from the first 8 bytes of the session key. The least |
| 258 | significant bit of each byte is ignored. This results in 56 bits of |
| 259 | key data. DES [DES] is used in CBC mode. The iv (initialization vector) is |
| 260 | initialized to all zeroes. |
| 261 | .IP SSH_CIPHER_3DES |
| 262 | The variant of triple-DES used here works as follows: there are three |
| 263 | independent DES-CBC ciphers, with independent initialization vectors. |
| 264 | The data (the whole encrypted data stream) is first encrypted with the |
| 265 | first cipher, then decrypted with the second cipher, and finally |
| 266 | encrypted with the third cipher. All these operations are performed |
| 267 | in CBC mode. |
| 268 | |
| 269 | The key for the first cipher is taken from the first 8 bytes of the |
| 270 | session key; the key for the next cipher from the next 8 bytes, and |
| 271 | the key for the third cipher from the following 8 bytes. All three |
| 272 | initialization vectors are initialized to zero. |
| 273 | |
| 274 | (Note: the variant of 3DES used here differs from some other |
| 275 | descriptions.) |
| 276 | .IP SSH_CIPHER_IDEA |
| 277 | The key is taken from the first 16 bytes of the session key. IDEA |
| 278 | [IDEA] is used in CFB mode. The initialization vector is initialized |
| 279 | to all zeroes. |
| 280 | .IP SSH_CIPHER_TSS |
| 281 | All 32 bytes of the session key are used as the key. |
| 282 | |
| 283 | There is no reference available for the TSS algorithm; it is currently |
| 284 | only documented in the sample implementation source code. The |
| 285 | security of this cipher is unknown (but it is quite fast). The cipher |
| 286 | is basically a stream cipher that uses MD5 as a random number |
| 287 | generator and takes feedback from the data. |
| 288 | .IP SSH_CIPHER_RC4 |
| 289 | The first 16 bytes of the session key are used as the key for the |
| 290 | server to client direction. The remaining 16 bytes are used as the |
| 291 | key for the client to server direction. This gives independent |
| 292 | 128-bit keys for each direction. |
| 293 | |
| 294 | This algorithm is the alleged RC4 cipher posted to the Usenet in 1995. |
| 295 | It is widely believed to be equivalent with the original RSADSI RC4 |
| 296 | cipher. This is a very fast algorithm. |
| 297 | .RT |
| 298 | |
| 299 | |
| 300 | .ti 0 |
| 301 | Data Type Encodings |
| 302 | |
| 303 | The Data field of each packet contains data encoded as described in |
| 304 | this section. There may be several data items; each item is coded as |
| 305 | described here, and their representations are concatenated together |
| 306 | (without any alignment or padding). |
| 307 | |
| 308 | Each data type is stored as follows: |
| 309 | .IP "8-bit byte" |
| 310 | The byte is stored directly as a single byte. |
| 311 | .IP "32-bit unsigned integer" |
| 312 | Stored in 4 bytes, msb first. |
| 313 | .IP "Arbitrary length binary string" |
| 314 | First 4 bytes are the length of the string, msb first (not including |
| 315 | the length itself). The following "length" bytes are the string |
| 316 | value. There are no terminating null characters. |
| 317 | .IP "Multiple-precision integer" |
| 318 | First 2 bytes are the number of bits in the integer, msb first (for |
| 319 | example, the value 0x00012345 would have 17 bits). The value zero has |
| 320 | zero bits. It is permissible that the number of bits be larger than the |
| 321 | real number of bits. |
| 322 | |
| 323 | The number of bits is followed by (bits + 7) / 8 bytes of binary data, |
| 324 | msb first, giving the value of the integer. |
| 325 | .RT |
| 326 | |
| 327 | |
| 328 | .ti 0 |
| 329 | TCP/IP Port Number and Other Options |
| 330 | |
| 331 | The server listens for connections on TCP/IP port 22. |
| 332 | |
| 333 | The client may connect the server from any port. However, if the |
| 334 | client wishes to use any form of .rhosts or /etc/hosts.equiv |
| 335 | authentication, it must connect from a privileged port (less than |
| 336 | 1024). |
| 337 | |
| 338 | For the IP Type of Service field [RFC0791], it is recommended that |
| 339 | interactive sessions (those having a user terminal or forwarding X11 |
| 340 | connections) use the IPTOS_LOWDELAY, and non-interactive connections |
| 341 | use IPTOS_THROUGHPUT. |
| 342 | |
| 343 | It is recommended that keepalives are used, because otherwise programs |
| 344 | on the server may never notice if the other end of the connection is |
| 345 | rebooted. |
| 346 | |
| 347 | |
| 348 | .ti 0 |
| 349 | Protocol Version Identification |
| 350 | |
| 351 | After the socket is opened, the server sends an identification string, |
| 352 | which is of the form |
| 353 | "SSH-<protocolmajor>.<protocolminor>-<version>\\n", where |
| 354 | <protocolmajor> and <protocolminor> are integers and specify the |
| 355 | protocol version number (not software distribution version). |
| 356 | <version> is server side software version string (max 40 characters); |
| 357 | it is not interpreted by the remote side but may be useful for |
| 358 | debugging. |
| 359 | |
| 360 | The client parses the server's string, and sends a corresponding |
| 361 | string with its own information in response. If the server has lower |
| 362 | version number, and the client contains special code to emulate it, |
| 363 | the client responds with the lower number; otherwise it responds with |
| 364 | its own number. The server then compares the version number the |
| 365 | client sent with its own, and determines whether they can work |
| 366 | together. The server either disconnects, or sends the first packet |
| 367 | using the binary packet protocol and both sides start working |
| 368 | according to the lower of the protocol versions. |
| 369 | |
| 370 | By convention, changes which keep the protocol compatible with |
| 371 | previous versions keep the same major protocol version; changes that |
| 372 | are not compatible increment the major version (which will hopefully |
| 373 | never happen). The version described in this document is 1.3. |
| 374 | |
| 375 | The client will |
| 376 | |
| 377 | .ti 0 |
| 378 | Key Exchange and Server Host Authentication |
| 379 | |
| 380 | The first message sent by the server using the packet protocol is |
| 381 | SSH_SMSG_PUBLIC_KEY. It declares the server's host key, server public |
| 382 | key, supported ciphers, supported authentication methods, and flags |
| 383 | for protocol extensions. It also contains a 64-bit random number |
| 384 | (cookie) that must be returned in the client's reply (to make IP |
| 385 | spoofing more difficult). No encryption is used for this message. |
| 386 | |
| 387 | Both sides compute a session id as follows. The modulus of the server |
| 388 | key is interpreted as a byte string (without explicit length field, |
| 389 | with minimum length able to hold the whole value), most significant |
| 390 | byte first. This string is concatenated with the server host key |
| 391 | interpreted the same way. Additionally, the cookie is concatenated |
| 392 | with this. Both sides compute MD5 of the resulting string. The |
| 393 | resulting 16 bytes (128 bits) are stored by both parties and are |
| 394 | called the session id. |
| 395 | |
| 396 | The client responds with a SSH_CMSG_SESSION_KEY message, which |
| 397 | contains the selected cipher type, a copy of the 64-bit cookie sent by |
| 398 | the server, client's protocol flags, and a session key encrypted |
| 399 | with both the server's host key and server key. No encryption is used |
| 400 | for this message. |
| 401 | |
| 402 | The session key is 32 8-bit bytes (a total of 256 random bits |
| 403 | generated by the client). The client first xors the 16 bytes of the |
| 404 | session id with the first 16 bytes of the session key. The resulting |
| 405 | string is then encrypted using the smaller key (one with smaller |
| 406 | modulus), and the result is then encrypted using the other key. The |
| 407 | number of bits in the public modulus of the two keys must differ by at |
| 408 | least 128 bits. |
| 409 | |
| 410 | At each encryption step, a multiple-precision integer is constructed |
| 411 | from the data to be encrypted as follows (the integer is here |
| 412 | interpreted as a sequence of bytes, msb first; the number of bytes is |
| 413 | the number of bytes needed to represent the modulus). |
| 414 | |
| 415 | The most significant byte (which is only partial as the value must be |
| 416 | less than the public modulus, which is never a power of two) is zero. |
| 417 | |
| 418 | The next byte contains the value 2 (which stands for public-key |
| 419 | encrypted data in the PKCS standard [PKCS#1]). Then, there are |
| 420 | non-zero random bytes to fill any unused space, a zero byte, and the |
| 421 | data to be encrypted in the least significant bytes, the last byte of |
| 422 | the data in the least significant byte. |
| 423 | |
| 424 | This algorithm is used twice. First, it is used to encrypt the 32 |
| 425 | random bytes generated by the client to be used as the session key |
| 426 | (xored by the session id). This value is converted to an integer as |
| 427 | described above, and encrypted with RSA using the key with the smaller |
| 428 | modulus. The resulting integer is converted to a byte stream, msb |
| 429 | first. This byte stream is padded and encrypted identically using the |
| 430 | key with the larger modulus. |
| 431 | |
| 432 | After the client has sent the session key, it starts to use the |
| 433 | selected algorithm and key for decrypting any received packets, and |
| 434 | for encrypting any sent packets. Separate ciphers are used for |
| 435 | different directions (that is, both directions have separate |
| 436 | initialization vectors or other state for the ciphers). |
| 437 | |
| 438 | When the server has received the session key message, and has turned |
| 439 | on encryption, it sends a SSH_SMSG_SUCCESS message to the client. |
| 440 | |
| 441 | The recommended size of the host key is 1024 bits, and 768 bits for |
| 442 | the server key. The minimum size is 512 bits for the smaller key. |
| 443 | |
| 444 | |
| 445 | .ti 0 |
| 446 | Declaring the User Name |
| 447 | |
| 448 | The client then sends a SSH_CMSG_USER message to the server. This |
| 449 | message specifies the user name to log in as. |
| 450 | |
| 451 | The server validates that such a user exists, checks whether |
| 452 | authentication is needed, and responds with either SSH_SMSG_SUCCESS or |
| 453 | SSH_SMSG_FAILURE. SSH_SMSG_SUCCESS indicates that no authentication |
| 454 | is needed for this user (no password), and authentication phase has |
| 455 | now been completed. SSH_SMSG_FAILURE indicates that authentication is |
| 456 | needed (or the user does not exist). |
| 457 | |
| 458 | If the user does not exist, it is recommended that this returns |
| 459 | failure, but the server keeps reading messages from the client, and |
| 460 | responds to any messages (except SSH_MSG_DISCONNECT, SSH_MSG_IGNORE, |
| 461 | and SSH_MSG_DEBUG) with SSH_SMSG_FAILURE. This way the client cannot |
| 462 | be certain whether the user exists. |
| 463 | |
| 464 | |
| 465 | .ti 0 |
| 466 | Authentication Phase |
| 467 | |
| 468 | Provided the server didn't immediately accept the login, an |
| 469 | authentication exchange begins. The client sends messages to the |
| 470 | server requesting different types of authentication in arbitrary order as |
| 471 | many times as desired (however, the server may close the connection |
| 472 | after a timeout). The server always responds with SSH_SMSG_SUCCESS if |
| 473 | it has accepted the authentication, and with SSH_SMSG_FAILURE if it has |
| 474 | denied authentication with the requested method or it does not |
| 475 | recognize the message. Some authentication methods cause an exchange |
| 476 | of further messages before the final result is sent. The |
| 477 | authentication phase ends when the server responds with success. |
| 478 | |
| 479 | The recommended value for the authentication timeout (timeout before |
| 480 | disconnecting if no successful authentication has been made) is 5 |
| 481 | minutes. |
| 482 | |
| 483 | The following authentication methods are currently supported: |
| 484 | .TS |
| 485 | center; |
| 486 | l r l. |
| 487 | SSH_AUTH_RHOSTS 1 .rhosts or /etc/hosts.equiv |
| 488 | SSH_AUTH_RSA 2 pure RSA authentication |
| 489 | SSH_AUTH_PASSWORD 3 password authentication |
| 490 | SSH_AUTH_RHOSTS_RSA 4 .rhosts with RSA host authentication |
| 491 | .TE |
| 492 | .IP SSH_AUTH_RHOSTS |
| 493 | |
| 494 | This is the authentication method used by rlogin and rsh [RFC1282]. |
| 495 | |
| 496 | The client sends SSH_CMSG_AUTH_RHOSTS with the client-side user name |
| 497 | as an argument. |
| 498 | |
| 499 | The server checks whether to permit authentication. On UNIX systems, |
| 500 | this is usually done by checking /etc/hosts.equiv, and .rhosts in the |
| 501 | user's home directory. The connection must come from a privileged |
| 502 | port. |
| 503 | |
| 504 | It is recommended that the server checks that there are no IP options |
| 505 | (such as source routing) specified for the socket before accepting |
| 506 | this type of authentication. The client host name should be |
| 507 | reverse-mapped and then forward mapped to ensure that it has the |
| 508 | proper IP-address. |
| 509 | |
| 510 | This authentication method trusts the remote host (root on the remote |
| 511 | host can pretend to be any other user on that host), the name |
| 512 | services, and partially the network: anyone who can see packets coming |
| 513 | out from the server machine can do IP-spoofing and pretend to be any |
| 514 | machine; however, the protocol prevents blind IP-spoofing (which used |
| 515 | to be possible with rlogin). |
| 516 | |
| 517 | Many sites probably want to disable this authentication method because |
| 518 | of the fundamental insecurity of conventional .rhosts or |
| 519 | /etc/hosts.equiv authentication when faced with spoofing. It is |
| 520 | recommended that this method not be supported by the server by |
| 521 | default. |
| 522 | .IP SSH_AUTH_RHOSTS_RSA |
| 523 | |
| 524 | In addition to conventional .rhosts and hosts.equiv authentication, |
| 525 | this method additionally requires that the client host be |
| 526 | authenticated using RSA. |
| 527 | |
| 528 | The client sends SSH_CMSG_AUTH_RHOSTS_RSA specifying the client-side |
| 529 | user name, and the public host key of the client host. |
| 530 | |
| 531 | The server first checks if normal .rhosts or /etc/hosts.equiv |
| 532 | authentication would be accepted, and if not, responds with |
| 533 | SSH_SMSG_FAILURE. Otherwise, it checks whether it knows the host key |
| 534 | for the client machine (using the same name for the host that was used |
| 535 | for checking the .rhosts and /etc/hosts.equiv files). If it does not |
| 536 | know the RSA key for the client, access is denied and SSH_SMSG_FAILURE |
| 537 | is sent. |
| 538 | |
| 539 | If the server knows the host key of the client machine, it verifies |
| 540 | that the given host key matches that known for the client. If not, |
| 541 | access is denied and SSH_SMSG_FAILURE is sent. |
| 542 | |
| 543 | The server then sends a SSH_SMSG_AUTH_RSA_CHALLENGE message containing |
| 544 | an encrypted challenge for the client. The challenge is 32 8-bit |
| 545 | random bytes (256 bits). When encrypted, the highest (partial) byte |
| 546 | is left as zero, the next byte contains the value 2, the following are |
| 547 | non-zero random bytes, followed by a zero byte, and the challenge put |
| 548 | in the remaining bytes. This is then encrypted using RSA with the |
| 549 | client host's public key. (The padding and encryption algorithm is |
| 550 | the same as that used for the session key.) |
| 551 | |
| 552 | The client decrypts the challenge using its private host key, |
| 553 | concatenates this with the session id, and computes an MD5 checksum |
| 554 | of the resulting 48 bytes. The MD5 output is returned as 16 bytes in |
| 555 | a SSH_CMSG_AUTH_RSA_RESPONSE message. (MD5 is used to deter chosen |
| 556 | plaintext attacks against RSA; the session id binds it to a specific |
| 557 | session). |
| 558 | |
| 559 | The server verifies that the MD5 of the decrypted challenge returned by |
| 560 | the client matches that of the original value, and sends SSH_SMSG_SUCCESS if |
| 561 | so. Otherwise it sends SSH_SMSG_FAILURE and refuses the |
| 562 | authentication attempt. |
| 563 | |
| 564 | This authentication method trusts the client side machine in that root |
| 565 | on that machine can pretend to be any user on that machine. |
| 566 | Additionally, it trusts the client host key. The name and/or IP |
| 567 | address of the client host is only used to select the public host key. |
| 568 | The same host name is used when scanning .rhosts or /etc/hosts.equiv |
| 569 | and when selecting the host key. It would in principle be possible to |
| 570 | eliminate the host name entirely and substitute it directly by the |
| 571 | host key. IP and/or DNS [RFC1034] spoofing can only be used |
| 572 | to pretend to be a host for which the attacker has the private host |
| 573 | key. |
| 574 | .IP SSH_AUTH_RSA |
| 575 | |
| 576 | The idea behind RSA authentication is that the server recognizes the |
| 577 | public key offered by the client, generates a random challenge, and |
| 578 | encrypts the challenge with the public key. The client must then |
| 579 | prove that it has the corresponding private key by decrypting the |
| 580 | challenge. |
| 581 | |
| 582 | The client sends SSH_CMSG_AUTH_RSA with public key modulus (n) as an |
| 583 | argument. |
| 584 | |
| 585 | The server may respond immediately with SSH_SMSG_FAILURE if it does |
| 586 | not permit authentication with this key. Otherwise it generates a |
| 587 | challenge, encrypts it using the user's public key (stored on the |
| 588 | server and identified using the modulus), and sends |
| 589 | SSH_SMSG_AUTH_RSA_CHALLENGE with the challenge (mp-int) as an |
| 590 | argument. |
| 591 | |
| 592 | The challenge is 32 8-bit random bytes (256 bits). When encrypted, |
| 593 | the highest (partial) byte is left as zero, the next byte contains the |
| 594 | value 2, the following are non-zero random bytes, followed by a zero |
| 595 | byte, and the challenge put in the remaining bytes. This is then |
| 596 | encrypted with the public key. (The padding and encryption algorithm |
| 597 | is the same as that used for the session key.) |
| 598 | |
| 599 | The client decrypts the challenge using its private key, concatenates |
| 600 | it with the session id, and computes an MD5 checksum of the resulting |
| 601 | 48 bytes. The MD5 output is returned as 16 bytes in a |
| 602 | SSH_CMSG_AUTH_RSA_RESPONSE message. (Note that the MD5 is necessary |
| 603 | to avoid chosen plaintext attacks against RSA; the session id binds it |
| 604 | to a specific session.) |
| 605 | |
| 606 | The server verifies that the MD5 of the decrypted challenge returned |
| 607 | by the client matches that of the original value, and sends |
| 608 | SSH_SMSG_SUCCESS if so. Otherwise it sends SSH_SMSG_FAILURE and |
| 609 | refuses the authentication attempt. |
| 610 | |
| 611 | This authentication method does not trust the remote host, the |
| 612 | network, name services, or anything else. Authentication is based |
| 613 | solely on the possession of the private identification keys. Anyone |
| 614 | in possession of the private keys can log in, but nobody else. |
| 615 | |
| 616 | The server may have additional requirements for a successful |
| 617 | authentiation. For example, to limit damage due to a compromised RSA |
| 618 | key, a server might restrict access to a limited set of hosts. |
| 619 | .IP SSH_AUTH_PASSWORD |
| 620 | |
| 621 | The client sends a SSH_CMSG_AUTH_PASSWORD message with the plain text |
| 622 | password. (Note that even though the password is plain text inside |
| 623 | the message, it is normally encrypted by the packet mechanism.) |
| 624 | |
| 625 | The server verifies the password, and sends SSH_SMSG_SUCCESS if |
| 626 | authentication was accepted and SSH_SMSG_FAILURE otherwise. |
| 627 | |
| 628 | Note that the password is read from the user by the client; the user |
| 629 | never interacts with a login program. |
| 630 | |
| 631 | This authentication method does not trust the remote host, the |
| 632 | network, name services or anything else. Authentication is based |
| 633 | solely on the possession of the password. Anyone in possession of the |
| 634 | password can log in, but nobody else. |
| 635 | .RT |
| 636 | |
| 637 | .ti 0 |
| 638 | Preparatory Operations |
| 639 | |
| 640 | After successful authentication, the server waits for a request from |
| 641 | the client, processes the request, and responds with SSH_SMSG_SUCCESS |
| 642 | whenever a request has been successfully processed. If it receives a |
| 643 | message that it does not recognize or it fails to honor a request, it |
| 644 | returns SSH_SMSG_FAILURE. It is expected that new message types might |
| 645 | be added to this phase in future. |
| 646 | |
| 647 | The following messages are currently defined for this phase. |
| 648 | .IP SSH_CMSG_REQUEST_COMPRESSION |
| 649 | Requests that compression be enabled for this session. A |
| 650 | gzip-compatible compression level (1-9) is passed as an argument. |
| 651 | .IP SSH_CMSG_REQUEST_PTY |
| 652 | Requests that a pseudo terminal device be allocated for this session. |
| 653 | The user terminal type and terminal modes are supplied as arguments. |
| 654 | .IP SSH_CMSG_X11_REQUEST_FORWARDING |
| 655 | Requests forwarding of X11 connections from the remote machine to the |
| 656 | local machine over the secure channel. Causes an internet-domain |
| 657 | socket to be allocated and the DISPLAY variable to be set on the server. |
| 658 | X11 authentication data is automatically passed to the server, and the |
| 659 | client may implement spoofing of authentication data for added |
| 660 | security. The authentication data is passed as arguments. |
| 661 | .IP SSH_CMSG_PORT_FORWARD_REQUEST |
| 662 | Requests forwarding of a TCP/IP port on the server host over the |
| 663 | secure channel. What happens is that whenever a connection is made to |
| 664 | the port on the server, a connection will be made from the client end |
| 665 | to the specified host/port. Any user can forward unprivileged ports; |
| 666 | only the root can forward privileged ports (as determined by |
| 667 | authentication done earlier). |
| 668 | .IP SSH_CMSG_AGENT_REQUEST_FORWARDING |
| 669 | Requests forwarding of the connection to the authentication agent. |
| 670 | .IP SSH_CMSG_EXEC_SHELL |
| 671 | Starts a shell (command interpreter) for the user, and moves into |
| 672 | interactive session mode. |
| 673 | .IP SSH_CMSG_EXEC_CMD |
| 674 | Executes the given command (actually "<shell> -c <command>" or |
| 675 | equivalent) for the user, and moves into interactive session mode. |
| 676 | .RT |
| 677 | |
| 678 | |
| 679 | .ti 0 |
| 680 | Interactive Session and Exchange of Data |
| 681 | |
| 682 | During the interactive session, any data written by the shell or |
| 683 | command running on the server machine is forwarded to stdin or |
| 684 | stderr on the client machine, and any input available from stdin on |
| 685 | the client machine is forwarded to the program on the server machine. |
| 686 | |
| 687 | All exchange is asynchronous; either side can send at any time, and |
| 688 | there are no acknowledgements (TCP/IP already provides reliable |
| 689 | transport, and the packet protocol protects against tampering or IP |
| 690 | spoofing). |
| 691 | |
| 692 | When the client receives EOF from its standard input, it will send |
| 693 | SSH_CMSG_EOF; however, this in no way terminates the exchange. The |
| 694 | exchange terminates and interactive mode is left when the server sends |
| 695 | SSH_SMSG_EXITSTATUS to indicate that the client program has |
| 696 | terminated. Alternatively, either side may disconnect at any time by |
| 697 | sending SSH_MSG_DISCONNECT or closing the connection. |
| 698 | |
| 699 | The server may send any of the following messages: |
| 700 | .IP SSH_SMSG_STDOUT_DATA |
| 701 | Data written to stdout by the program running on the server. The data |
| 702 | is passed as a string argument. The client writes this data to |
| 703 | stdout. |
| 704 | .IP SSH_SMSG_STDERR_DATA |
| 705 | Data written to stderr by the program running on the server. The data |
| 706 | is passed as a string argument. The client writes this data to |
| 707 | stderr. (Note that if the program is running on a tty, it is not |
| 708 | possible to separate stdout and stderr data, and all data will be sent |
| 709 | as stdout data.) |
| 710 | .IP SSH_SMSG_EXITSTATUS |
| 711 | Indicates that the shell or command has exited. Exit status is passed |
| 712 | as an integer argument. This message causes termination of the |
| 713 | interactive session. |
| 714 | .IP SSH_SMSG_AGENT_OPEN |
| 715 | Indicates that someone on the server side is requesting a connection |
| 716 | to the authentication agent. The server-side channel number is passed |
| 717 | as an argument. The client must respond with either |
| 718 | SSH_CHANNEL_OPEN_CONFIRMATION or SSH_CHANNEL_OPEN_FAILURE. |
| 719 | .IP SSH_SMSG_X11_OPEN |
| 720 | Indicates that a connection has been made to the X11 socket on the |
| 721 | server side and should be forwarded to the real X server. An integer |
| 722 | argument indicates the channel number allocated for this connection on |
| 723 | the server side. The client should send back either |
| 724 | SSH_MSG_CHANNEL_OPEN_CONFIRMATION or SSH_MSG_CHANNEL_OPEN_FAILURE with |
| 725 | the same server side channel number. |
| 726 | .IP SSH_MSG_PORT_OPEN |
| 727 | Indicates that a connection has been made to a port on the server side |
| 728 | for which forwarding has been requested. Arguments are server side |
| 729 | channel number, host name to connect to, and port to connect to. The |
| 730 | client should send back either |
| 731 | SSH_MSG_CHANNEL_OPEN_CONFIRMATION or SSH_MSG_CHANNEL_OPEN_FAILURE with |
| 732 | the same server side channel number. |
| 733 | .IP SSH_MSG_CHANNEL_OPEN_CONFIRMATION |
| 734 | This is sent by the server to indicate that it has opened a connection |
| 735 | as requested in a previous message. The first argument indicates the |
| 736 | client side channel number, and the second argument is the channel number |
| 737 | that the server has allocated for this connection. |
| 738 | .IP SSH_MSG_CHANNEL_OPEN_FAILURE |
| 739 | This is sent by the server to indicate that it failed to open a |
| 740 | connection as requested in a previous message. The client-side |
| 741 | channel number is passed as an argument. The client will close the |
| 742 | descriptor associated with the channel and free the channel. |
| 743 | .IP SSH_MSG_CHANNEL_DATA |
| 744 | This packet contains data for a channel from the server. The first |
| 745 | argument is the client-side channel number, and the second argument (a |
| 746 | string) is the data. |
| 747 | .IP SSH_MSG_CHANNEL_CLOSE |
| 748 | This is sent by the server to indicate that whoever was in the other |
| 749 | end of the channel has closed it. The argument is the client side channel |
| 750 | number. The client will let all buffered data in the channel to |
| 751 | drain, and when ready, will close the socket, free the channel, and |
| 752 | send the server a SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the |
| 753 | channel. |
| 754 | .IP SSH_MSG_CHANNEL_CLOSE_CONFIRMATION |
| 755 | This is send by the server to indicate that a channel previously |
| 756 | closed by the client has now been closed on the server side as well. |
| 757 | The argument indicates the client channel number. The client frees |
| 758 | the channel. |
| 759 | .RT |
| 760 | |
| 761 | The client may send any of the following messages: |
| 762 | .IP SSH_CMSG_STDIN_DATA |
| 763 | This is data to be sent as input to the program running on the server. |
| 764 | The data is passed as a string. |
| 765 | .IP SSH_CMSG_EOF |
| 766 | Indicates that the client has encountered EOF while reading standard |
| 767 | input. The server will allow any buffered input data to drain, and |
| 768 | will then close the input to the program. |
| 769 | .IP SSH_CMSG_WINDOW_SIZE |
| 770 | Indicates that window size on the client has been changed. The server |
| 771 | updates the window size of the tty and causes SIGWINCH to be sent to |
| 772 | the program. The new window size is passed as four integer arguments: |
| 773 | row, col, xpixel, ypixel. |
| 774 | .IP SSH_MSG_PORT_OPEN |
| 775 | Indicates that a connection has been made to a port on the client side |
| 776 | for which forwarding has been requested. Arguments are client side |
| 777 | channel number, host name to connect to, and port to connect to. The |
| 778 | server should send back either SSH_MSG_CHANNEL_OPEN_CONFIRMATION or |
| 779 | SSH_MSG_CHANNEL_OPEN_FAILURE with the same client side channel number. |
| 780 | .IP SSH_MSG_CHANNEL_OPEN_CONFIRMATION |
| 781 | This is sent by the client to indicate that it has opened a connection |
| 782 | as requested in a previous message. The first argument indicates the |
| 783 | server side channel number, and the second argument is the channel |
| 784 | number that the client has allocated for this connection. |
| 785 | .IP SSH_MSG_CHANNEL_OPEN_FAILURE |
| 786 | This is sent by the client to indicate that it failed to open a |
| 787 | connection as requested in a previous message. The server side |
| 788 | channel number is passed as an argument. The server will close the |
| 789 | descriptor associated with the channel and free the channel. |
| 790 | .IP SSH_MSG_CHANNEL_DATA |
| 791 | This packet contains data for a channel from the client. The first |
| 792 | argument is the server side channel number, and the second argument (a |
| 793 | string) is the data. |
| 794 | .IP SSH_MSG_CHANNEL_CLOSE |
| 795 | This is sent by the client to indicate that whoever was in the other |
| 796 | end of the channel has closed it. The argument is the server channel |
| 797 | number. The server will allow buffered data to drain, and when ready, |
| 798 | will close the socket, free the channel, and send the client a |
| 799 | SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the channel. |
| 800 | .IP SSH_MSG_CHANNEL_CLOSE_CONFIRMATION |
| 801 | This is send by the client to indicate that a channel previously |
| 802 | closed by the server has now been closed on the client side as well. |
| 803 | The argument indicates the server channel number. The server frees |
| 804 | the channel. |
| 805 | .RT |
| 806 | |
| 807 | Any unsupported messages during interactive mode cause the connection |
| 808 | to be terminated with SSH_MSG_DISCONNECT and an error message. |
| 809 | Compatible protocol upgrades should agree about any extensions during |
| 810 | the preparation phase or earlier. |
| 811 | |
| 812 | |
| 813 | .ti 0 |
| 814 | Termination of the Connection |
| 815 | |
| 816 | Normal termination of the connection is always initiated by the server |
| 817 | by sending SSH_SMSG_EXITSTATUS after the program has exited. The |
| 818 | client responds to this message by sending SSH_CMSG_EXIT_CONFIRMATION |
| 819 | and closes the socket; the server then closes the socket. There are |
| 820 | two purposes for the confirmation: some systems may lose previously |
| 821 | sent data when the socket is closed, and closing the client side first |
| 822 | causes any TCP/IP TIME_WAIT [RFC0793] waits to occur on the client side, not |
| 823 | consuming server resources. |
| 824 | |
| 825 | If the program terminates due to a signal, the server will send |
| 826 | SSH_MSG_DISCONNECT with an appropriate message. If the connection is |
| 827 | closed, all file descriptors to the program will be closed and the |
| 828 | server will exit. If the program runs on a tty, the kernel sends it |
| 829 | the SIGHUP signal when the pty master side is closed. |
| 830 | |
| 831 | .ti 0 |
| 832 | Protocol Flags |
| 833 | |
| 834 | Both the server and the client pass 32 bits of protocol flags to the |
| 835 | other side. The flags are intended for compatible protocol extension; |
| 836 | the server first announces which added capabilities it supports, and |
| 837 | the client then sends the capabilities that it supports. |
| 838 | |
| 839 | The following flags are currently defined (the values are bit masks): |
| 840 | .IP "1 SSH_PROTOFLAG_SCREEN_NUMBER" |
| 841 | This flag can only be sent by the client. It indicates that the X11 |
| 842 | forwarding requests it sends will include the screen number. |
| 843 | .IP "2 SSH_PROTOFLAG_HOST_IN_FWD_OPEN" |
| 844 | If both sides specify this flag, SSH_SMSG_X11_OPEN and |
| 845 | SSH_MSG_PORT_OPEN messages will contain an additional field containing |
| 846 | a description of the host at the other end of the connection. |
| 847 | .RT |
| 848 | |
| 849 | .ti 0 |
| 850 | Detailed Description of Packet Types and Formats |
| 851 | |
| 852 | The supported packet types and the corresponding message numbers are |
| 853 | given in the following table. Messages with _MSG_ in their name may |
| 854 | be sent by either side. Messages with _CMSG_ are only sent by the |
| 855 | client, and messages with _SMSG_ only by the server. |
| 856 | |
| 857 | A packet may contain additional data after the arguments specified |
| 858 | below. Any such data should be ignored by the receiver. However, it |
| 859 | is recommended that no such data be stored without good reason. (This |
| 860 | helps build compatible extensions.) |
| 861 | .IP "0 SSH_MSG_NONE" |
| 862 | This code is reserved. This message type is never sent. |
| 863 | .IP "1 SSH_MSG_DISCONNECT" |
| 864 | .TS |
| 865 | ; |
| 866 | l l. |
| 867 | string Cause of disconnection |
| 868 | .TE |
| 869 | This message may be sent by either party at any time. It causes the |
| 870 | immediate disconnection of the connection. The message is intended to |
| 871 | be displayed to a human, and describes the reason for disconnection. |
| 872 | .IP "2 SSH_SMSG_PUBLIC_KEY" |
| 873 | .TS |
| 874 | ; |
| 875 | l l. |
| 876 | 8 bytes anti_spoofing_cookie |
| 877 | 32-bit int server_key_bits |
| 878 | mp-int server_key_public_exponent |
| 879 | mp-int server_key_public_modulus |
| 880 | 32-bit int host_key_bits |
| 881 | mp-int host_key_public_exponent |
| 882 | mp-int host_key_public_modulus |
| 883 | 32-bit int protocol_flags |
| 884 | 32-bit int supported_ciphers_mask |
| 885 | 32-bit int supported_authentications_mask |
| 886 | .TE |
| 887 | Sent as the first message by the server. This message gives the |
| 888 | server's host key, server key, protocol flags (intended for compatible |
| 889 | protocol extension), supported_ciphers_mask (which is the |
| 890 | bitwise or of (1 << cipher_number), where << is the left shift |
| 891 | operator, for all supported ciphers), and |
| 892 | supported_authentications_mask (which is the bitwise or of (1 << |
| 893 | authentication_type) for all supported authentication types). The |
| 894 | anti_spoofing_cookie is 64 random bytes, and must be sent back |
| 895 | verbatim by the client in its reply. It is used to make IP-spoofing |
| 896 | more difficult (encryption and host keys are the real defense against |
| 897 | spoofing). |
| 898 | .IP "3 SSH_CMSG_SESSION_KEY" |
| 899 | .TS |
| 900 | ; |
| 901 | l l. |
| 902 | 1 byte cipher_type (must be one of the supported values) |
| 903 | 8 bytes anti_spoofing_cookie (must match data sent by the server) |
| 904 | mp-int double-encrypted session key |
| 905 | 32-bit int protocol_flags |
| 906 | .TE |
| 907 | Sent by the client as the first message in the session. Selects the |
| 908 | cipher to use, and sends the encrypted session key to the server. The |
| 909 | anti_spoofing_cookie must be the same bytes that were sent by the |
| 910 | server. Protocol_flags is intended for negotiating compatible |
| 911 | protocol extensions. |
| 912 | .IP "4 SSH_CMSG_USER" |
| 913 | .TS |
| 914 | ; |
| 915 | l l. |
| 916 | string user login name on server |
| 917 | .TE |
| 918 | Sent by the client to begin authentication. Specifies the user name |
| 919 | on the server to log in as. The server responds with SSH_SMSG_SUCCESS |
| 920 | if no authentication is needed for this user, or SSH_SMSG_FAILURE if |
| 921 | authentication is needed (or the user does not exist). [Note to the |
| 922 | implementator: the user name is of arbitrary size. The implementation |
| 923 | must be careful not to overflow internal buffers.] |
| 924 | .IP "5 SSH_CMSG_AUTH_RHOSTS" |
| 925 | .TS |
| 926 | ; |
| 927 | l l. |
| 928 | string client-side user name |
| 929 | .TE |
| 930 | Requests authentication using /etc/hosts.equiv and .rhosts (or |
| 931 | equivalent mechanisms). This authentication method is normally |
| 932 | disabled in the server because it is not secure (but this is the |
| 933 | method used by rsh and rlogin). The server responds with |
| 934 | SSH_SMSG_SUCCESS if authentication was successful, and |
| 935 | SSH_SMSG_FAILURE if access was not granted. The server should check |
| 936 | that the client side port number is less than 1024 (a privileged |
| 937 | port), and immediately reject authentication if it is not. Supporting |
| 938 | this authentication method is optional. This method should normally |
| 939 | not be enabled in the server because it is not safe. (However, not |
| 940 | enabling this only helps if rlogind and rshd are disabled.) |
| 941 | .IP "6 SSH_CMSG_AUTH_RSA" |
| 942 | .TS |
| 943 | ; |
| 944 | l l. |
| 945 | mp-int identity_public_modulus |
| 946 | .TE |
| 947 | Requests authentication using pure RSA authentication. The server |
| 948 | checks if the given key is permitted to log in, and if so, responds |
| 949 | with SSH_SMSG_AUTH_RSA_CHALLENGE. Otherwise, it responds with |
| 950 | SSH_SMSG_FAILURE. The client often tries several different keys in |
| 951 | sequence until one supported by the server is found. Authentication |
| 952 | is accepted if the client gives the correct response to the challenge. |
| 953 | The server is free to add other criteria for authentication, such as a |
| 954 | requirement that the connection must come from a certain host. Such |
| 955 | additions are not visible at the protocol level. Supporting this |
| 956 | authentication method is optional but recommended. |
| 957 | .IP "7 SSH_SMSG_AUTH_RSA_CHALLENGE" |
| 958 | .TS |
| 959 | ; |
| 960 | l l. |
| 961 | mp-int encrypted challenge |
| 962 | .TE |
| 963 | Presents an RSA authentication challenge to the client. The challenge |
| 964 | is a 256-bit random value encrypted as described elsewhere in this |
| 965 | document. The client must decrypt the challenge using the RSA private |
| 966 | key, compute MD5 of the challenge plus session id, and send back the |
| 967 | resulting 16 bytes using SSH_CMSG_AUTH_RSA_RESPONSE. |
| 968 | .IP "8 SSH_CMSG_AUTH_RSA_RESPONSE" |
| 969 | .TS |
| 970 | ; |
| 971 | l l. |
| 972 | 16 bytes MD5 of decrypted challenge |
| 973 | .TE |
| 974 | This message is sent by the client in response to an RSA challenge. |
| 975 | The MD5 checksum is returned instead of the decrypted challenge to |
| 976 | deter known-plaintext attacks against the RSA key. The server |
| 977 | responds to this message with either SSH_SMSG_SUCCESS or |
| 978 | SSH_SMSG_FAILURE. |
| 979 | .IP "9 SSH_CMSG_AUTH_PASSWORD" |
| 980 | .TS |
| 981 | ; |
| 982 | l l. |
| 983 | string plain text password |
| 984 | .TE |
| 985 | Requests password authentication using the given password. Note that |
| 986 | even though the password is plain text inside the packet, the whole |
| 987 | packet is normally encrypted by the packet layer. It would not be |
| 988 | possible for the client to perform password encryption/hashing, |
| 989 | because it cannot know which kind of encryption/hashing, if any, the |
| 990 | server uses. The server responds to this message with |
| 991 | SSH_SMSG_SUCCESS or SSH_SMSG_FAILURE. |
| 992 | .IP "10 SSH_CMSG_REQUEST_PTY" |
| 993 | .TS |
| 994 | ; |
| 995 | l l. |
| 996 | string TERM environment variable value (e.g. vt100) |
| 997 | 32-bit int terminal height, rows (e.g., 24) |
| 998 | 32-bit int terminal width, columns (e.g., 80) |
| 999 | 32-bit int terminal width, pixels (0 if no graphics) (e.g., 480) |
| 1000 | 32-bit int terminal height, pixels (0 if no graphics) (e.g., 640) |
| 1001 | n bytes tty modes encoded in binary |
| 1002 | .TE |
| 1003 | Requests a pseudo-terminal to be allocated for this command. This |
| 1004 | message can be used regardless of whether the session will later |
| 1005 | execute the shell or a command. If a pty has been requested with this |
| 1006 | message, the shell or command will run on a pty. Otherwise it will |
| 1007 | communicate with the server using pipes, sockets or some other similar |
| 1008 | mechanism. |
| 1009 | |
| 1010 | The terminal type gives the type of the user's terminal. In the UNIX |
| 1011 | environment it is passed to the shell or command in the TERM |
| 1012 | environment variable. |
| 1013 | |
| 1014 | The width and height values give the initial size of the user's |
| 1015 | terminal or window. All values can be zero if not supported by the |
| 1016 | operating system. The server will pass these values to the kernel if |
| 1017 | supported. |
| 1018 | |
| 1019 | Terminal modes are encoded into a byte stream in a portable format. |
| 1020 | The exact format is described later in this document. |
| 1021 | |
| 1022 | The server responds to the request with either SSH_SMSG_SUCCESS or |
| 1023 | SSH_SMSG_FAILURE. If the server does not have the concept of pseudo |
| 1024 | terminals, it should return success if it is possible to execute a |
| 1025 | shell or a command so that it looks to the client as if it was running |
| 1026 | on a pseudo terminal. |
| 1027 | .IP "11 SSH_CMSG_WINDOW_SIZE" |
| 1028 | .TS |
| 1029 | ; |
| 1030 | l l. |
| 1031 | 32-bit int terminal height, rows |
| 1032 | 32-bit int terminal width, columns |
| 1033 | 32-bit int terminal width, pixels |
| 1034 | 32-bit int terminal height, pixels |
| 1035 | .TE |
| 1036 | This message can only be sent by the client during the interactive |
| 1037 | session. This indicates that the size of the user's window has |
| 1038 | changed, and provides the new size. The server will update the |
| 1039 | kernel's notion of the window size, and a SIGWINCH signal or |
| 1040 | equivalent will be sent to the shell or command (if supported by the |
| 1041 | operating system). |
| 1042 | .IP "12 SSH_CMSG_EXEC_SHELL" |
| 1043 | |
| 1044 | (no arguments) |
| 1045 | |
| 1046 | Starts a shell (command interpreter), and enters interactive session |
| 1047 | mode. |
| 1048 | .IP "13 SSH_CMSG_EXEC_CMD" |
| 1049 | .TS |
| 1050 | ; |
| 1051 | l l. |
| 1052 | string command to execute |
| 1053 | .TE |
| 1054 | Starts executing the given command, and enters interactive session |
| 1055 | mode. On UNIX, the command is run as "<shell> -c <command>", where |
| 1056 | <shell> is the user's login shell. |
| 1057 | .IP "14 SSH_SMSG_SUCCESS" |
| 1058 | |
| 1059 | (no arguments) |
| 1060 | |
| 1061 | This message is sent by the server in response to the session key, a |
| 1062 | successful authentication request, and a successfully completed |
| 1063 | preparatory operation. |
| 1064 | .IP "15 SSH_SMSG_FAILURE" |
| 1065 | |
| 1066 | (no arguments) |
| 1067 | |
| 1068 | This message is sent by the server in response to a failed |
| 1069 | authentication operation to indicate that the user has not yet been |
| 1070 | successfully authenticated, and in response to a failed preparatory |
| 1071 | operation. This is also sent in response to an authentication or |
| 1072 | preparatory operation request that is not recognized or supported. |
| 1073 | .IP "16 SSH_CMSG_STDIN_DATA" |
| 1074 | .TS |
| 1075 | ; |
| 1076 | l l. |
| 1077 | string data |
| 1078 | .TE |
| 1079 | Delivers data from the client to be supplied as input to the shell or |
| 1080 | program running on the server side. This message can only be used in |
| 1081 | the interactive session mode. No acknowledgement is sent for this |
| 1082 | message. |
| 1083 | .IP "17 SSH_SMSG_STDOUT_DATA" |
| 1084 | .TS |
| 1085 | ; |
| 1086 | l l. |
| 1087 | string data |
| 1088 | .TE |
| 1089 | Delivers data from the server that was read from the standard output of |
| 1090 | the shell or program running on the server side. This message can |
| 1091 | only be used in the interactive session mode. No acknowledgement is |
| 1092 | sent for this message. |
| 1093 | .IP "18 SSH_SMSG_STDERR_DATA" |
| 1094 | .TS |
| 1095 | ; |
| 1096 | l l. |
| 1097 | string data |
| 1098 | .TE |
| 1099 | Delivers data from the server that was read from the standard error of |
| 1100 | the shell or program running on the server side. This message can |
| 1101 | only be used in the interactive session mode. No acknowledgement is |
| 1102 | sent for this message. |
| 1103 | .IP "19 SSH_CMSG_EOF" |
| 1104 | |
| 1105 | (no arguments) |
| 1106 | |
| 1107 | This message is sent by the client to indicate that EOF has been |
| 1108 | reached on the input. Upon receiving this message, and after all |
| 1109 | buffered input data has been sent to the shell or program, the server |
| 1110 | will close the input file descriptor to the program. This message can |
| 1111 | only be used in the interactive session mode. No acknowledgement is |
| 1112 | sent for this message. |
| 1113 | .IP "20 SSH_SMSG_EXITSTATUS" |
| 1114 | .TS |
| 1115 | ; |
| 1116 | l l. |
| 1117 | 32-bit int exit status of the command |
| 1118 | .TE |
| 1119 | Returns the exit status of the shell or program after it has exited. |
| 1120 | The client should respond with SSH_CMSG_EXIT_CONFIRMATION when it has |
| 1121 | received this message. This will be the last message sent by the |
| 1122 | server. If the program being executed dies with a signal instead of |
| 1123 | exiting normally, the server should terminate the session with |
| 1124 | SSH_MSG_DISCONNECT (which can be used to pass a human-readable string |
| 1125 | indicating that the program died due to a signal) instead of using |
| 1126 | this message. |
| 1127 | .IP "21 SSH_MSG_CHANNEL_OPEN_CONFIRMATION" |
| 1128 | .TS |
| 1129 | ; |
| 1130 | l l. |
| 1131 | 32-bit int remote_channel |
| 1132 | 32-bit int local_channel |
| 1133 | .TE |
| 1134 | This is sent in response to any channel open request if the channel |
| 1135 | has been successfully opened. Remote_channel is the channel number |
| 1136 | received in the initial open request; local_channel is the channel |
| 1137 | number the side sending this message has allocated for the channel. |
| 1138 | Data can be transmitted on the channel after this message. |
| 1139 | .IP "22 SSH_MSG_CHANNEL_OPEN_FAILURE" |
| 1140 | .TS |
| 1141 | ; |
| 1142 | l l. |
| 1143 | 32-bit int remote_channel |
| 1144 | .TE |
| 1145 | This message indicates that an earlier channel open request by the |
| 1146 | other side has failed or has been denied. Remote_channel is the |
| 1147 | channel number given in the original request. |
| 1148 | .IP "23 SSH_MSG_CHANNEL_DATA" |
| 1149 | .TS |
| 1150 | ; |
| 1151 | l l. |
| 1152 | 32-bit int remote_channel |
| 1153 | string data |
| 1154 | .TE |
| 1155 | Data is transmitted in a channel in these messages. A channel is |
| 1156 | bidirectional, and both sides can send these messages. There is no |
| 1157 | acknowledgement for these messages. It is possible that either side |
| 1158 | receives these messages after it has sent SSH_MSG_CHANNEL_CLOSE for |
| 1159 | the channel. These messages cannot be received after the party has |
| 1160 | sent or received SSH_MSG_CHANNEL_CLOSE_CONFIRMATION. |
| 1161 | .IP "24 SSH_MSG_CHANNEL_CLOSE" |
| 1162 | .TS |
| 1163 | ; |
| 1164 | l l. |
| 1165 | 32-bit int remote_channel |
| 1166 | .TE |
| 1167 | When a channel is closed at one end of the connection, that side sends |
| 1168 | this message. Upon receiving this message, the channel should be |
| 1169 | closed. When this message is received, if the channel is already |
| 1170 | closed (the receiving side has sent this message for the same channel |
| 1171 | earlier), the channel is freed and no further action is taken; |
| 1172 | otherwise the channel is freed and SSH_MSG_CHANNEL_CLOSE_CONFIRMATION |
| 1173 | is sent in response. (It is possible that the channel is closed |
| 1174 | simultaneously at both ends.) |
| 1175 | .IP "25 SSH_MSG_CHANNEL_CLOSE_CONFIRMATION" |
| 1176 | .TS |
| 1177 | ; |
| 1178 | l l. |
| 1179 | 32-bit int remote_channel |
| 1180 | .TE |
| 1181 | This message is sent in response to SSH_MSG_CHANNEL_CLOSE unless the |
| 1182 | channel was already closed. When this message is sent or received, |
| 1183 | the channel is freed. |
| 1184 | .IP "26 (OBSOLETED; was unix-domain X11 forwarding) |
| 1185 | .IP "27 SSH_SMSG_X11_OPEN" |
| 1186 | .TS |
| 1187 | ; |
| 1188 | l l. |
| 1189 | 32-bit int local_channel |
| 1190 | string originator_string (see below) |
| 1191 | .TE |
| 1192 | This message can be sent by the server during the interactive session |
| 1193 | mode to indicate that a client has connected the fake X server. |
| 1194 | Local_channel is the channel number that the server has allocated for |
| 1195 | the connection. The client should try to open a connection to the |
| 1196 | real X server, and respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION or |
| 1197 | SSH_MSG_CHANNEL_OPEN_FAILURE. |
| 1198 | |
| 1199 | The field originator_string is present if both sides |
| 1200 | specified SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It |
| 1201 | contains a description of the host originating the connection. |
| 1202 | .IP "28 SSH_CMSG_PORT_FORWARD_REQUEST" |
| 1203 | .TS |
| 1204 | ; |
| 1205 | l l. |
| 1206 | 32-bit int server_port |
| 1207 | string host_to_connect |
| 1208 | 32-bit int port_to_connect |
| 1209 | .TE |
| 1210 | Sent by the client in the preparatory phase, this message requests |
| 1211 | that server_port on the server machine be forwarded over the secure |
| 1212 | channel to the client machine, and from there to the specified host |
| 1213 | and port. The server should start listening on the port, and send |
| 1214 | SSH_MSG_PORT_OPEN whenever a connection is made to it. Supporting |
| 1215 | this message is optional, and the server is free to reject any forward |
| 1216 | request. For example, it is highly recommended that unless the user |
| 1217 | has been authenticated as root, forwarding any privileged port numbers |
| 1218 | (below 1024) is denied. |
| 1219 | .IP "29 SSH_MSG_PORT_OPEN" |
| 1220 | .TS |
| 1221 | ; |
| 1222 | l l. |
| 1223 | 32-bit int local_channel |
| 1224 | string host_name |
| 1225 | 32-bit int port |
| 1226 | string originator_string (see below) |
| 1227 | .TE |
| 1228 | Sent by either party in interactive session mode, this message |
| 1229 | indicates that a connection has been opened to a forwarded TCP/IP |
| 1230 | port. Local_channel is the channel number that the sending party has |
| 1231 | allocated for the connection. Host_name is the host the connection |
| 1232 | should be be forwarded to, and the port is the port on that host to |
| 1233 | connect. The receiving party should open the connection, and respond |
| 1234 | with SSH_MSG_CHANNEL_OPEN_CONFIRMATION or |
| 1235 | SSH_MSG_CHANNEL_OPEN_FAILURE. It is recommended that the receiving |
| 1236 | side check the host_name and port for validity to avoid compromising |
| 1237 | local security by compromised remote side software. Particularly, it |
| 1238 | is recommended that the client permit connections only to those ports |
| 1239 | for which it has requested forwarding with SSH_CMSG_PORT_FORWARD_REQUEST. |
| 1240 | |
| 1241 | The field originator_string is present if both sides |
| 1242 | specified SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It |
| 1243 | contains a description of the host originating the connection. |
| 1244 | .IP "30 SSH_CMSG_AGENT_REQUEST_FORWARDING" |
| 1245 | |
| 1246 | (no arguments) |
| 1247 | |
| 1248 | Requests that the connection to the authentication agent be forwarded |
| 1249 | over the secure channel. The method used by clients to contact the |
| 1250 | authentication agent within each machine is implementation and machine |
| 1251 | dependent. If the server accepts this request, it should arrange that |
| 1252 | any clients run from this session will actually contact the server |
| 1253 | program when they try to contact the authentication agent. The server |
| 1254 | should then send a SSH_SMSG_AGENT_OPEN to open a channel to the agent, |
| 1255 | and the client should forward the connection to the real |
| 1256 | authentication agent. Supporting this message is optional. |
| 1257 | .IP "31 SSH_SMSG_AGENT_OPEN" |
| 1258 | .TS |
| 1259 | ; |
| 1260 | l l. |
| 1261 | 32-bit int local_channel |
| 1262 | .TE |
| 1263 | Sent by the server in interactive session mode, this message requests |
| 1264 | opening a channel to the authentication agent. The client should open |
| 1265 | a channel, and respond with either SSH_MSG_CHANNEL_OPEN_CONFIRMATION |
| 1266 | or SSH_MSG_CHANNEL_OPEN_FAILURE. |
| 1267 | .IP "32 SSH_MSG_IGNORE" |
| 1268 | .TS |
| 1269 | ; |
| 1270 | l l. |
| 1271 | string data |
| 1272 | .TE |
| 1273 | Either party may send this message at any time. This message, and the |
| 1274 | argument string, is silently ignored. This message might be used in |
| 1275 | some implementations to make traffic analysis more difficult. This |
| 1276 | message is not currently sent by the implementation, but all |
| 1277 | implementations are required to recognize and ignore it. |
| 1278 | .IP "33 SSH_CMSG_EXIT_CONFIRMATION" |
| 1279 | |
| 1280 | (no arguments) |
| 1281 | |
| 1282 | Sent by the client in response to SSH_SMSG_EXITSTATUS. This is the |
| 1283 | last message sent by the client. |
| 1284 | .IP "34 SSH_CMSG_X11_REQUEST_FORWARDING" |
| 1285 | .TS |
| 1286 | ; |
| 1287 | l l. |
| 1288 | string x11_authentication_protocol |
| 1289 | string x11_authentication_data |
| 1290 | 32-bit int screen number (if SSH_PROTOFLAG_SCREEN_NUMBER) |
| 1291 | .TE |
| 1292 | Sent by the client during the preparatory phase, this message requests |
| 1293 | that the server create a fake X11 display and set the DISPLAY |
| 1294 | environment variable accordingly. An internet-domain display is |
| 1295 | preferable. The given authentication protocol and the associated data |
| 1296 | should be recorded by the server so that it is used as authentication |
| 1297 | on connections (e.g., in .Xauthority). The authentication protocol |
| 1298 | must be one of the supported X11 authentication protocols, e.g., |
| 1299 | "MIT-MAGIC-COOKIE-1". Authentication data must be a lowercase hex |
| 1300 | string of even length. Its interpretation is protocol dependent. |
| 1301 | The data is in a format that can be used with e.g. the xauth program. |
| 1302 | Supporting this message is optional. |
| 1303 | |
| 1304 | The client is permitted (and recommended) to generate fake |
| 1305 | authentication information and send fake information to the server. |
| 1306 | This way, a corrupt server will not have access to the user's terminal |
| 1307 | after the connection has terminated. The correct authorization codes |
| 1308 | will also not be left hanging around in files on the server (many |
| 1309 | users keep the same X session for months, thus protecting the |
| 1310 | authorization data becomes important). |
| 1311 | |
| 1312 | X11 authentication spoofing works by initially sending fake (random) |
| 1313 | authentication data to the server, and interpreting the first packet |
| 1314 | sent by the X11 client after the connection has been opened. The |
| 1315 | first packet contains the client's authentication. If the packet |
| 1316 | contains the correct fake data, it is replaced by the client by the |
| 1317 | correct authentication data, and then sent to the X server. |
| 1318 | .IP "35 SSH_CMSG_AUTH_RHOSTS_RSA" |
| 1319 | .TS |
| 1320 | ; |
| 1321 | l l. |
| 1322 | string clint-side user name |
| 1323 | 32-bit int client_host_key_bits |
| 1324 | mp-int client_host_key_public_exponent |
| 1325 | mp-int client_host_key_public_modulus |
| 1326 | .TE |
| 1327 | Requests authentication using /etc/hosts.equiv and .rhosts (or |
| 1328 | equivalent) together with RSA host authentication. The server should |
| 1329 | check that the client side port number is less than 1024 (a privileged |
| 1330 | port), and immediately reject authentication if it is not. The server |
| 1331 | responds with SSH_SMSG_FAILURE or SSH_SMSG_AUTH_RSA_CHALLENGE. The |
| 1332 | client must respond to the challenge with the proper |
| 1333 | SSH_CMSG_AUTH_RSA_RESPONSE. The server then responds with success if |
| 1334 | access was granted, or failure if the client gave a wrong response. |
| 1335 | Supporting this authentication method is optional but recommended in |
| 1336 | most environments. |
| 1337 | .IP "36 SSH_MSG_DEBUG" |
| 1338 | .TS |
| 1339 | ; |
| 1340 | l l. |
| 1341 | string debugging message sent to the other side |
| 1342 | .TE |
| 1343 | This message may be sent by either party at any time. It is used to |
| 1344 | send debugging messages that may be informative to the user in |
| 1345 | solving various problems. For example, if authentication fails |
| 1346 | because of some configuration error (e.g., incorrect permissions for |
| 1347 | some file), it can be very helpful for the user to make the cause of |
| 1348 | failure available. On the other hand, one should not make too much |
| 1349 | information available for security reasons. It is recommended that |
| 1350 | the client provides an option to display the debugging information |
| 1351 | sent by the sender (the user probably does not want to see it by default). |
| 1352 | The server can log debugging data sent by the client (if any). Either |
| 1353 | party is free to ignore any received debugging data. Every |
| 1354 | implementation must be able to receive this message, but no |
| 1355 | implementation is required to send these. |
| 1356 | .IP "37 SSH_CMSG_REQUEST_COMPRESSION" |
| 1357 | .TS |
| 1358 | ; |
| 1359 | l l. |
| 1360 | 32-bit int gzip compression level (1-9) |
| 1361 | .TE |
| 1362 | This message can be sent by the client in the preparatory operations |
| 1363 | phase. The server responds with SSH_SMSG_FAILURE if it does not |
| 1364 | support compression or does not want to compress; it responds with |
| 1365 | SSH_SMSG_SUCCESS if it accepted the compression request. In the |
| 1366 | latter case the response to this packet will still be uncompressed, |
| 1367 | but all further packets in either direction will be compressed by gzip. |
| 1368 | .RT |
| 1369 | |
| 1370 | |
| 1371 | .ti 0 |
| 1372 | Encoding of Terminal Modes |
| 1373 | |
| 1374 | Terminal modes (as passed in SSH_CMSG_REQUEST_PTY) are encoded into a |
| 1375 | byte stream. It is intended that the coding be portable across |
| 1376 | different environments. |
| 1377 | |
| 1378 | The tty mode description is a stream of bytes. The stream consists of |
| 1379 | opcode-argument pairs. It is terminated by opcode TTY_OP_END (0). |
| 1380 | Opcodes 1-127 have one-byte arguments. Opcodes 128-159 have 32-bit |
| 1381 | integer arguments (stored msb first). Opcodes 160-255 are not yet |
| 1382 | defined, and cause parsing to stop (they should only be used after any |
| 1383 | other data). |
| 1384 | |
| 1385 | The client puts in the stream any modes it knows about, and the server |
| 1386 | ignores any modes it does not know about. This allows some degree of |
| 1387 | machine-independence, at least between systems that use a POSIX-like |
| 1388 | [POSIX] tty interface. The protocol can support other systems as |
| 1389 | well, but the client may need to fill reasonable values for a number |
| 1390 | of parameters so the server pty gets set to a reasonable mode (the |
| 1391 | server leaves all unspecified mode bits in their default values, and |
| 1392 | only some combinations make sense). |
| 1393 | |
| 1394 | The following opcodes have been defined. The naming of opcodes mostly |
| 1395 | follows the POSIX terminal mode flags. |
| 1396 | .IP "0 TTY_OP_END" |
| 1397 | Indicates end of options. |
| 1398 | .IP "1 VINTR" |
| 1399 | Interrupt character; 255 if none. Similarly for the other characters. |
| 1400 | Not all of these characters are supported on all systems. |
| 1401 | .IP "2 VQUIT" |
| 1402 | The quit character (sends SIGQUIT signal on UNIX systems). |
| 1403 | .IP "3 VERASE" |
| 1404 | Erase the character to left of the cursor. |
| 1405 | .IP "4 VKILL" |
| 1406 | Kill the current input line. |
| 1407 | .IP "5 VEOF " |
| 1408 | End-of-file character (sends EOF from the terminal). |
| 1409 | .IP "6 VEOL " |
| 1410 | End-of-line character in addition to carriage return and/or linefeed. |
| 1411 | .IP "7 VEOL2" |
| 1412 | Additional end-of-line character. |
| 1413 | .IP "8 VSTART" |
| 1414 | Continues paused output (normally ^Q). |
| 1415 | .IP "9 VSTOP" |
| 1416 | Pauses output (^S). |
| 1417 | .IP "10 VSUSP" |
| 1418 | Suspends the current program. |
| 1419 | .IP "11 VDSUSP" |
| 1420 | Another suspend character. |
| 1421 | .IP "12 VREPRINT" |
| 1422 | Reprints the current input line. |
| 1423 | .IP "13 VWERASE" |
| 1424 | Erases a word left of cursor. |
| 1425 | .IP "14 VLNEXT" |
| 1426 | More special input characters; these are probably not supported on |
| 1427 | most systems. |
| 1428 | .IP "15 VFLUSH" |
| 1429 | .IP "16 VSWTCH" |
| 1430 | .IP "17 VSTATUS" |
| 1431 | .IP "18 VDISCARD" |
| 1432 | |
| 1433 | .IP "30 IGNPAR" |
| 1434 | The ignore parity flag. The next byte should be 0 if this flag is not |
| 1435 | set, and 1 if it is set. |
| 1436 | .IP "31 PARMRK" |
| 1437 | More flags. The exact definitions can be found in the POSIX standard. |
| 1438 | .IP "32 INPCK" |
| 1439 | .IP "33 ISTRIP" |
| 1440 | .IP "34 INLCR" |
| 1441 | .IP "35 IGNCR" |
| 1442 | .IP "36 ICRNL" |
| 1443 | .IP "37 IUCLC" |
| 1444 | .IP "38 IXON" |
| 1445 | .IP "39 IXANY" |
| 1446 | .IP "40 IXOFF" |
| 1447 | .IP "41 IMAXBEL" |
| 1448 | |
| 1449 | .IP "50 ISIG" |
| 1450 | .IP "51 ICANON" |
| 1451 | .IP "52 XCASE" |
| 1452 | .IP "53 ECHO" |
| 1453 | .IP "54 ECHOE" |
| 1454 | .IP "55 ECHOK" |
| 1455 | .IP "56 ECHONL" |
| 1456 | .IP "57 NOFLSH" |
| 1457 | .IP "58 TOSTOP" |
| 1458 | .IP "59 IEXTEN" |
| 1459 | .IP "60 ECHOCTL" |
| 1460 | .IP "61 ECHOKE" |
| 1461 | .IP "62 PENDIN" |
| 1462 | |
| 1463 | .IP "70 OPOST" |
| 1464 | .IP "71 OLCUC" |
| 1465 | .IP "72 ONLCR" |
| 1466 | .IP "73 OCRNL" |
| 1467 | .IP "74 ONOCR" |
| 1468 | .IP "75 ONLRET" |
| 1469 | |
| 1470 | .IP "90 CS7" |
| 1471 | .IP "91 CS8" |
| 1472 | .IP "92 PARENB" |
| 1473 | .IP "93 PARODD" |
| 1474 | |
| 1475 | .IP "192 TTY_OP_ISPEED" |
| 1476 | Specifies the input baud rate in bits per second. |
| 1477 | .IP "193 TTY_OP_OSPEED" |
| 1478 | Specifies the output baud rate in bits per second. |
| 1479 | .RT |
| 1480 | |
| 1481 | |
| 1482 | .ti 0 |
| 1483 | The Authentication Agent Protocol |
| 1484 | |
| 1485 | The authentication agent is a program that can be used to hold RSA |
| 1486 | authentication keys for the user (in future, it might hold data for |
| 1487 | other authentication types as well). An authorized program can send |
| 1488 | requests to the agent to generate a proper response to an RSA |
| 1489 | challenge. How the connection is made to the agent (or its |
| 1490 | representative) inside a host and how access control is done inside a |
| 1491 | host is implementation-dependent; however, how it is forwarded and how |
| 1492 | one interacts with it is specified in this protocol. The connection |
| 1493 | to the agent is normally automatically forwarded over the secure |
| 1494 | channel. |
| 1495 | |
| 1496 | A program that wishes to use the agent first opens a connection to its |
| 1497 | local representative (typically, the agent itself or an SSH server). |
| 1498 | It then writes a request to the connection, and waits for response. |
| 1499 | It is recommended that at least five minutes of timeout are provided |
| 1500 | waiting for the agent to respond to an authentication challenge (this |
| 1501 | gives sufficient time for the user to cut-and-paste the challenge to a |
| 1502 | separate machine, perform the computation there, and cut-and-paste the |
| 1503 | result back if so desired). |
| 1504 | |
| 1505 | Messages sent to and by the agent are in the following format: |
| 1506 | .TS |
| 1507 | ; |
| 1508 | l l. |
| 1509 | 4 bytes Length, msb first. Does not include length itself. |
| 1510 | 1 byte Packet type. The value 255 is reserved for future extensions. |
| 1511 | data Any data, depending on packet type. Encoding as in the ssh packet |
| 1512 | protocol. |
| 1513 | .TE |
| 1514 | |
| 1515 | The following message types are currently defined: |
| 1516 | .IP "1 SSH_AGENTC_REQUEST_RSA_IDENTITIES" |
| 1517 | |
| 1518 | (no arguments) |
| 1519 | |
| 1520 | Requests the agent to send a list of all RSA keys for which it can |
| 1521 | answer a challenge. |
| 1522 | .IP "2 SSH_AGENT_RSA_IDENTITIES_ANSWER" |
| 1523 | .TS |
| 1524 | ; |
| 1525 | l l. |
| 1526 | 32-bit int howmany |
| 1527 | howmany times: |
| 1528 | 32-bit int bits |
| 1529 | mp-int public exponent |
| 1530 | mp-int public modulus |
| 1531 | string comment |
| 1532 | .TE |
| 1533 | The agent sends this message in response to the to |
| 1534 | SSH_AGENTC_REQUEST_RSA_IDENTITIES. The answer lists all RSA keys for |
| 1535 | which the agent can answer a challenge. The comment field is intended |
| 1536 | to help identify each key; it may be printed by an application to |
| 1537 | indicate which key is being used. If the agent is not holding any |
| 1538 | keys, howmany will be zero. |
| 1539 | .IP "3 SSH_AGENTC_RSA_CHALLENGE |
| 1540 | .TS |
| 1541 | ; |
| 1542 | l l. |
| 1543 | 32-bit int bits |
| 1544 | mp-int public exponent |
| 1545 | mp-int public modulus |
| 1546 | mp-int challenge |
| 1547 | 16 bytes session_id |
| 1548 | 32-bit int response_type |
| 1549 | .TE |
| 1550 | Requests RSA decryption of random challenge to authenticate the other |
| 1551 | side. The challenge will be decrypted with the RSA private key |
| 1552 | corresponding to the given public key. |
| 1553 | |
| 1554 | The decrypted challenge must contain a zero in the highest (partial) |
| 1555 | byte, 2 in the next byte, followed by non-zero random bytes, a zero |
| 1556 | byte, and then the real challenge value in the lowermost bytes. The |
| 1557 | real challenge must be 32 8-bit bytes (256 bits). |
| 1558 | |
| 1559 | Response_type indicates the format of the response to be returned. |
| 1560 | Currently the only supported value is 1, which means to compute MD5 of |
| 1561 | the real challenge plus session id, and return the resulting 16 bytes |
| 1562 | in a SSH_AGENT_RSA_RESPONSE message. |
| 1563 | .IP "4 SSH_AGENT_RSA_RESPONSE" |
| 1564 | .TS |
| 1565 | ; |
| 1566 | l l. |
| 1567 | 16 bytes MD5 of decrypted challenge |
| 1568 | .TE |
| 1569 | Answers an RSA authentication challenge. The response is 16 bytes: |
| 1570 | the MD5 checksum of the 32-byte challenge. |
| 1571 | .IP "5 SSH_AGENT_FAILURE" |
| 1572 | |
| 1573 | (no arguments) |
| 1574 | |
| 1575 | This message is sent whenever the agent fails to answer a request |
| 1576 | properly. For example, if the agent cannot answer a challenge (e.g., |
| 1577 | no longer has the proper key), it can respond with this. The agent |
| 1578 | also responds with this message if it receives a message it does not |
| 1579 | recognize. |
| 1580 | .IP "6 SSH_AGENT_SUCCESS" |
| 1581 | |
| 1582 | (no arguments) |
| 1583 | |
| 1584 | This message is sent by the agent as a response to certain requests |
| 1585 | that do not otherwise cause a message be sent. Currently, this is |
| 1586 | only sent in response to SSH_AGENTC_ADD_RSA_IDENTITY and |
| 1587 | SSH_AGENTC_REMOVE_RSA_IDENTITY. |
| 1588 | .IP "7 SSH_AGENTC_ADD_RSA_IDENTITY" |
| 1589 | .TS |
| 1590 | ; |
| 1591 | l l. |
| 1592 | 32-bit int bits |
| 1593 | mp-int public modulus |
| 1594 | mp-int public exponent |
| 1595 | mp-int private exponent |
| 1596 | mp-int multiplicative inverse of p mod q |
| 1597 | mp-int p |
| 1598 | mp-int q |
| 1599 | string comment |
| 1600 | .TE |
| 1601 | Registers an RSA key with the agent. After this request, the agent can |
| 1602 | use this RSA key to answer requests. The agent responds with |
| 1603 | SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE. |
| 1604 | .IP "8 SSH_AGENT_REMOVE_RSA_IDENTITY" |
| 1605 | .TS |
| 1606 | ; |
| 1607 | l l. |
| 1608 | 32-bit int bits |
| 1609 | mp-int public exponent |
| 1610 | mp-int public modulus |
| 1611 | .TE |
| 1612 | Removes an RSA key from the agent. The agent will no longer accept |
| 1613 | challenges for this key and will not list it as a supported identity. |
| 1614 | The agent responds with SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE. |
| 1615 | .RT |
| 1616 | |
| 1617 | If the agent receives a message that it does not understand, it |
| 1618 | responds with SSH_AGENT_FAILURE. This permits compatible future |
| 1619 | extensions. |
| 1620 | |
| 1621 | It is possible that several clients have a connection open to the |
| 1622 | authentication agent simultaneously. Each client will use a separate |
| 1623 | connection (thus, any SSH connection can have multiple agent |
| 1624 | connections active simultaneously). |
| 1625 | |
| 1626 | |
| 1627 | .ti 0 |
| 1628 | References |
| 1629 | |
| 1630 | .IP "[DES] " |
| 1631 | FIPS PUB 46-1: Data Encryption Standard. National Bureau of |
| 1632 | Standards, January 1988. FIPS PUB 81: DES Modes of Operation. |
| 1633 | National Bureau of Standards, December 1980. Bruce Schneier: Applied |
| 1634 | Cryptography. John Wiley & Sons, 1994. J. Seberry and J. Pieprzyk: |
| 1635 | Cryptography: An Introduction to Computer Security. Prentice-Hall, |
| 1636 | 1989. |
| 1637 | .IP "[GZIP] " |
| 1638 | The GNU GZIP program; available for anonymous ftp at prep.ai.mit.edu. |
| 1639 | Please let me know if you know a paper describing the algorithm. |
| 1640 | .IP "[IDEA] " |
| 1641 | Xuejia Lai: On the Design and Security of Block Ciphers, ETH Series in |
| 1642 | Information Processing, vol. 1, Hartung-Gorre Verlag, Konstanz, |
| 1643 | Switzerland, 1992. Bruce Schneier: Applied Cryptography, John Wiley & |
| 1644 | Sons, 1994. See also the following patents: PCT/CH91/00117, EP 0 482 |
| 1645 | 154 B1, US Pat. 5,214,703. |
| 1646 | .IP [PKCS#1] |
| 1647 | PKCS #1: RSA Encryption Standard. Version 1.5, RSA Laboratories, |
| 1648 | November 1993. Available for anonymous ftp at ftp.rsa.com. |
| 1649 | .IP [POSIX] |
| 1650 | Portable Operating System Interface (POSIX) - Part 1: Application |
| 1651 | Program Interface (API) [C language], ISO/IEC 9945-1, IEEE Std 1003.1, |
| 1652 | 1990. |
| 1653 | .IP [RFC0791] |
| 1654 | J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981. |
| 1655 | .IP [RFC0793] |
| 1656 | J. Postel: Transmission Control Protocol, RFC 793, USC/ISI, September |
| 1657 | 1981. |
| 1658 | .IP [RFC1034] |
| 1659 | P. Mockapetris: Domain Names - Concepts and Facilities, RFC 1034, |
| 1660 | USC/ISI, November 1987. |
| 1661 | .IP [RFC1282] |
| 1662 | B. Kantor: BSD Rlogin, RFC 1258, UCSD, December 1991. |
| 1663 | .IP "[RSA] " |
| 1664 | Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1994. See |
| 1665 | also R. Rivest, A. Shamir, and L. M. Adleman: Cryptographic |
| 1666 | Communications System and Method. US Patent 4,405,829, 1983. |
| 1667 | .IP "[X11] " |
| 1668 | R. Scheifler: X Window System Protocol, X Consortium Standard, Version |
| 1669 | 11, Release 6. Massachusetts Institute of Technology, Laboratory of |
| 1670 | Computer Science, 1994. |
| 1671 | .RT |
| 1672 | |
| 1673 | |
| 1674 | .ti 0 |
| 1675 | Security Considerations |
| 1676 | |
| 1677 | This protocol deals with the very issue of user authentication and |
| 1678 | security. |
| 1679 | |
| 1680 | First of all, as an implementation issue, the server program will have |
| 1681 | to run as root (or equivalent) on the server machine. This is because |
| 1682 | the server program will need be able to change to an arbitrary user |
| 1683 | id. The server must also be able to create a privileged TCP/IP port. |
| 1684 | |
| 1685 | The client program will need to run as root if any variant of .rhosts |
| 1686 | authentication is to be used. This is because the client program will |
| 1687 | need to create a privileged port. The client host key is also usually |
| 1688 | stored in a file which is readable by root only. The client needs the |
| 1689 | host key in .rhosts authentication only. Root privileges can be |
| 1690 | dropped as soon as the privileged port has been created and the host |
| 1691 | key has been read. |
| 1692 | |
| 1693 | The SSH protocol offers major security advantages over existing telnet |
| 1694 | and rlogin protocols. |
| 1695 | .IP o |
| 1696 | IP spoofing is restricted to closing a connection (by encryption, host |
| 1697 | keys, and the special random cookie). If encryption is not used, IP |
| 1698 | spoofing is possible for those who can hear packets going out from the |
| 1699 | server. |
| 1700 | .IP o |
| 1701 | DNS spoofing is made ineffective (by host keys). |
| 1702 | .IP o |
| 1703 | Routing spoofing is made ineffective (by host keys). |
| 1704 | .IP o |
| 1705 | All data is encrypted with strong algorithms to make eavesdropping as |
| 1706 | difficult as possible. This includes encrypting any authentication |
| 1707 | information such as passwords. The information for decrypting session |
| 1708 | keys is destroyed every hour. |
| 1709 | .IP o |
| 1710 | Strong authentication methods: .rhosts combined with RSA host |
| 1711 | authentication, and pure RSA authentication. |
| 1712 | .IP o |
| 1713 | X11 connections and arbitrary TCP/IP ports can be forwarded securely. |
| 1714 | .IP o |
| 1715 | Man-in-the-middle attacks are deterred by using the server host key to |
| 1716 | encrypt the session key. |
| 1717 | .IP o |
| 1718 | Trojan horses to catch a password by routing manipulation are deterred |
| 1719 | by checking that the host key of the server machine matches that |
| 1720 | stored on the client host. |
| 1721 | .RT |
| 1722 | |
| 1723 | The security of SSH against man-in-the-middle attacks and the security |
| 1724 | of the new form of .rhosts authentication, as well as server host |
| 1725 | validation, depends on the integrity of the host key and the files |
| 1726 | containing known host keys. |
| 1727 | |
| 1728 | The host key is normally stored in a root-readable file. If the host |
| 1729 | key is compromised, it permits attackers to use IP, DNS and routing |
| 1730 | spoofing as with current rlogin and rsh. It should never be any worse |
| 1731 | than the current situation. |
| 1732 | |
| 1733 | The files containing known host keys are not sensitive. However, if an |
| 1734 | attacker gets to modify the known host key files, it has the same |
| 1735 | consequences as a compromised host key, because the attacker can then |
| 1736 | change the recorded host key. |
| 1737 | |
| 1738 | The security improvements obtained by this protocol for X11 are of |
| 1739 | particular significance. Previously, there has been no way to protect |
| 1740 | data communicated between an X server and a client running on a remote |
| 1741 | machine. By creating a fake display on the server, and forwarding all |
| 1742 | X11 requests over the secure channel, SSH can be used to run any X11 |
| 1743 | applications securely without any cooperation with the vendors of the |
| 1744 | X server or the application. |
| 1745 | |
| 1746 | Finally, the security of this program relies on the strength of the |
| 1747 | underlying cryptographic algorithms. The RSA algorithm is used for |
| 1748 | authentication key exchange. It is widely believed to be secure. Of |
| 1749 | the algorithms used to encrypt the session, DES has a rather small key |
| 1750 | these days, probably permitting governments and organized criminals to |
| 1751 | break it in very short time with specialized hardware. 3DES is |
| 1752 | probably safe (but slower). IDEA is widely believed to be secure. |
| 1753 | People have varying degrees of confidence in the other algorithms. |
| 1754 | This program is not secure if used with no encryption at all. |
| 1755 | |
| 1756 | |
| 1757 | .ti 0 |
| 1758 | Additional Information |
| 1759 | |
| 1760 | Additional information (especially on the implementation and mailing |
| 1761 | lists) is available via WWW at http://www.cs.hut.fi/ssh. |
| 1762 | |
| 1763 | Comments should be sent to Tatu Ylonen <ylo@cs.hut.fi> or the SSH |
| 1764 | Mailing List <ssh@clinet.fi>. |
| 1765 | |
| 1766 | .ti 0 |
| 1767 | Author's Address |
| 1768 | |
| 1769 | .TS |
| 1770 | ; |
| 1771 | l. |
| 1772 | Tatu Ylonen |
| 1773 | Helsinki University of Technology |
| 1774 | Otakaari 1 |
| 1775 | FIN-02150 Espoo, Finland |
| 1776 | |
| 1777 | Phone: +358-0-451-3374 |
| 1778 | Fax: +358-0-451-3293 |
| 1779 | EMail: ylo@cs.hut.fi |
| 1780 | .TE |