| .\" |
| .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| .\" All rights reserved |
| .\" |
| .\" As far as I am concerned, the code I have written for this software |
| .\" can be used freely for any purpose. Any derived versions of this |
| .\" software must be clearly marked as such, and if the derived work is |
| .\" incompatible with the protocol description in the RFC file, it must be |
| .\" called by a name other than "ssh" or "Secure Shell". |
| .\" |
| .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
| .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
| .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
| .\" |
| .\" Redistribution and use in source and binary forms, with or without |
| .\" modification, are permitted provided that the following conditions |
| .\" are met: |
| .\" 1. Redistributions of source code must retain the above copyright |
| .\" notice, this list of conditions and the following disclaimer. |
| .\" 2. Redistributions in binary form must reproduce the above copyright |
| .\" notice, this list of conditions and the following disclaimer in the |
| .\" documentation and/or other materials provided with the distribution. |
| .\" |
| .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| .\" |
| .\" $OpenBSD: sshd.8,v 1.277 2014/08/30 15:33:50 sobrado Exp $ |
| .Dd $Mdocdate: August 30 2014 $ |
| .Dt SSHD 8 |
| .Os |
| .Sh NAME |
| .Nm sshd |
| .Nd OpenSSH SSH daemon |
| .Sh SYNOPSIS |
| .Nm sshd |
| .Bk -words |
| .Op Fl 46DdeiqTt |
| .Op Fl b Ar bits |
| .Op Fl C Ar connection_spec |
| .Op Fl c Ar host_certificate_file |
| .Op Fl E Ar log_file |
| .Op Fl f Ar config_file |
| .Op Fl g Ar login_grace_time |
| .Op Fl h Ar host_key_file |
| .Op Fl k Ar key_gen_time |
| .Op Fl o Ar option |
| .Op Fl p Ar port |
| .Op Fl u Ar len |
| .Ek |
| .Sh DESCRIPTION |
| .Nm |
| (OpenSSH Daemon) is the daemon program for |
| .Xr ssh 1 . |
| Together these programs replace rlogin and rsh, |
| and provide secure encrypted communications between two untrusted hosts |
| over an insecure network. |
| .Pp |
| .Nm |
| listens for connections from clients. |
| It is normally started at boot from |
| .Pa /etc/rc . |
| It forks a new |
| daemon for each incoming connection. |
| The forked daemons handle |
| key exchange, encryption, authentication, command execution, |
| and data exchange. |
| .Pp |
| .Nm |
| can be configured using command-line options or a configuration file |
| (by default |
| .Xr sshd_config 5 ) ; |
| command-line options override values specified in the |
| configuration file. |
| .Nm |
| rereads its configuration file when it receives a hangup signal, |
| .Dv SIGHUP , |
| by executing itself with the name and options it was started with, e.g.\& |
| .Pa /usr/sbin/sshd . |
| .Pp |
| The options are as follows: |
| .Bl -tag -width Ds |
| .It Fl 4 |
| Forces |
| .Nm |
| to use IPv4 addresses only. |
| .It Fl 6 |
| Forces |
| .Nm |
| to use IPv6 addresses only. |
| .It Fl b Ar bits |
| Specifies the number of bits in the ephemeral protocol version 1 |
| server key (default 1024). |
| .It Fl C Ar connection_spec |
| Specify the connection parameters to use for the |
| .Fl T |
| extended test mode. |
| If provided, any |
| .Cm Match |
| directives in the configuration file |
| that would apply to the specified user, host, and address will be set before |
| the configuration is written to standard output. |
| The connection parameters are supplied as keyword=value pairs. |
| The keywords are |
| .Dq user , |
| .Dq host , |
| .Dq laddr , |
| .Dq lport , |
| and |
| .Dq addr . |
| All are required and may be supplied in any order, either with multiple |
| .Fl C |
| options or as a comma-separated list. |
| .It Fl c Ar host_certificate_file |
| Specifies a path to a certificate file to identify |
| .Nm |
| during key exchange. |
| The certificate file must match a host key file specified using the |
| .Fl h |
| option or the |
| .Cm HostKey |
| configuration directive. |
| .It Fl D |
| When this option is specified, |
| .Nm |
| will not detach and does not become a daemon. |
| This allows easy monitoring of |
| .Nm sshd . |
| .It Fl d |
| Debug mode. |
| The server sends verbose debug output to standard error, |
| and does not put itself in the background. |
| The server also will not fork and will only process one connection. |
| This option is only intended for debugging for the server. |
| Multiple |
| .Fl d |
| options increase the debugging level. |
| Maximum is 3. |
| .It Fl E Ar log_file |
| Append debug logs to |
| .Ar log_file |
| instead of the system log. |
| .It Fl e |
| Write debug logs to standard error instead of the system log. |
| .It Fl f Ar config_file |
| Specifies the name of the configuration file. |
| The default is |
| .Pa /etc/ssh/sshd_config . |
| .Nm |
| refuses to start if there is no configuration file. |
| .It Fl g Ar login_grace_time |
| Gives the grace time for clients to authenticate themselves (default |
| 120 seconds). |
| If the client fails to authenticate the user within |
| this many seconds, the server disconnects and exits. |
| A value of zero indicates no limit. |
| .It Fl h Ar host_key_file |
| Specifies a file from which a host key is read. |
| This option must be given if |
| .Nm |
| is not run as root (as the normal |
| host key files are normally not readable by anyone but root). |
| The default is |
| .Pa /etc/ssh/ssh_host_key |
| for protocol version 1, and |
| .Pa /etc/ssh/ssh_host_dsa_key , |
| .Pa /etc/ssh/ssh_host_ecdsa_key . |
| .Pa /etc/ssh/ssh_host_ed25519_key |
| and |
| .Pa /etc/ssh/ssh_host_rsa_key |
| for protocol version 2. |
| It is possible to have multiple host key files for |
| the different protocol versions and host key algorithms. |
| .It Fl i |
| Specifies that |
| .Nm |
| is being run from |
| .Xr inetd 8 . |
| .Nm |
| is normally not run |
| from inetd because it needs to generate the server key before it can |
| respond to the client, and this may take tens of seconds. |
| Clients would have to wait too long if the key was regenerated every time. |
| However, with small key sizes (e.g. 512) using |
| .Nm |
| from inetd may |
| be feasible. |
| .It Fl k Ar key_gen_time |
| Specifies how often the ephemeral protocol version 1 server key is |
| regenerated (default 3600 seconds, or one hour). |
| The motivation for regenerating the key fairly |
| often is that the key is not stored anywhere, and after about an hour |
| it becomes impossible to recover the key for decrypting intercepted |
| communications even if the machine is cracked into or physically |
| seized. |
| A value of zero indicates that the key will never be regenerated. |
| .It Fl o Ar option |
| Can be used to give options in the format used in the configuration file. |
| This is useful for specifying options for which there is no separate |
| command-line flag. |
| For full details of the options, and their values, see |
| .Xr sshd_config 5 . |
| .It Fl p Ar port |
| Specifies the port on which the server listens for connections |
| (default 22). |
| Multiple port options are permitted. |
| Ports specified in the configuration file with the |
| .Cm Port |
| option are ignored when a command-line port is specified. |
| Ports specified using the |
| .Cm ListenAddress |
| option override command-line ports. |
| .It Fl q |
| Quiet mode. |
| Nothing is sent to the system log. |
| Normally the beginning, |
| authentication, and termination of each connection is logged. |
| .It Fl T |
| Extended test mode. |
| Check the validity of the configuration file, output the effective configuration |
| to stdout and then exit. |
| Optionally, |
| .Cm Match |
| rules may be applied by specifying the connection parameters using one or more |
| .Fl C |
| options. |
| .It Fl t |
| Test mode. |
| Only check the validity of the configuration file and sanity of the keys. |
| This is useful for updating |
| .Nm |
| reliably as configuration options may change. |
| .It Fl u Ar len |
| This option is used to specify the size of the field |
| in the |
| .Li utmp |
| structure that holds the remote host name. |
| If the resolved host name is longer than |
| .Ar len , |
| the dotted decimal value will be used instead. |
| This allows hosts with very long host names that |
| overflow this field to still be uniquely identified. |
| Specifying |
| .Fl u0 |
| indicates that only dotted decimal addresses |
| should be put into the |
| .Pa utmp |
| file. |
| .Fl u0 |
| may also be used to prevent |
| .Nm |
| from making DNS requests unless the authentication |
| mechanism or configuration requires it. |
| Authentication mechanisms that may require DNS include |
| .Cm RhostsRSAAuthentication , |
| .Cm HostbasedAuthentication , |
| and using a |
| .Cm from="pattern-list" |
| option in a key file. |
| Configuration options that require DNS include using a |
| USER@HOST pattern in |
| .Cm AllowUsers |
| or |
| .Cm DenyUsers . |
| .El |
| .Sh AUTHENTICATION |
| The OpenSSH SSH daemon supports SSH protocols 1 and 2. |
| The default is to use protocol 2 only, |
| though this can be changed via the |
| .Cm Protocol |
| option in |
| .Xr sshd_config 5 . |
| Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys; |
| protocol 1 only supports RSA keys. |
| For both protocols, |
| each host has a host-specific key, |
| normally 2048 bits, |
| used to identify the host. |
| .Pp |
| Forward security for protocol 1 is provided through |
| an additional server key, |
| normally 768 bits, |
| generated when the server starts. |
| This key is normally regenerated every hour if it has been used, and |
| is never stored on disk. |
| Whenever a client connects, the daemon responds with its public |
| host and server keys. |
| The client compares the |
| RSA host key against its own database to verify that it has not changed. |
| The client then generates a 256-bit random number. |
| It encrypts this |
| random number using both the host key and the server key, and sends |
| the encrypted number to the server. |
| Both sides then use this |
| random number as a session key which is used to encrypt all further |
| communications in the session. |
| The rest of the session is encrypted |
| using a conventional cipher, currently Blowfish or 3DES, with 3DES |
| being used by default. |
| The client selects the encryption algorithm |
| to use from those offered by the server. |
| .Pp |
| For protocol 2, |
| forward security is provided through a Diffie-Hellman key agreement. |
| This key agreement results in a shared session key. |
| The rest of the session is encrypted using a symmetric cipher, currently |
| 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. |
| The client selects the encryption algorithm |
| to use from those offered by the server. |
| Additionally, session integrity is provided |
| through a cryptographic message authentication code |
| (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160, |
| hmac-sha2-256 or hmac-sha2-512). |
| .Pp |
| Finally, the server and the client enter an authentication dialog. |
| The client tries to authenticate itself using |
| host-based authentication, |
| public key authentication, |
| challenge-response authentication, |
| or password authentication. |
| .Pp |
| Regardless of the authentication type, the account is checked to |
| ensure that it is accessible. An account is not accessible if it is |
| locked, listed in |
| .Cm DenyUsers |
| or its group is listed in |
| .Cm DenyGroups |
| \&. The definition of a locked account is system dependant. Some platforms |
| have their own account database (eg AIX) and some modify the passwd field ( |
| .Ql \&*LK\&* |
| on Solaris and UnixWare, |
| .Ql \&* |
| on HP-UX, containing |
| .Ql Nologin |
| on Tru64, |
| a leading |
| .Ql \&*LOCKED\&* |
| on FreeBSD and a leading |
| .Ql \&! |
| on most Linuxes). |
| If there is a requirement to disable password authentication |
| for the account while allowing still public-key, then the passwd field |
| should be set to something other than these values (eg |
| .Ql NP |
| or |
| .Ql \&*NP\&* |
| ). |
| .Pp |
| If the client successfully authenticates itself, a dialog for |
| preparing the session is entered. |
| At this time the client may request |
| things like allocating a pseudo-tty, forwarding X11 connections, |
| forwarding TCP connections, or forwarding the authentication agent |
| connection over the secure channel. |
| .Pp |
| After this, the client either requests a shell or execution of a command. |
| The sides then enter session mode. |
| In this mode, either side may send |
| data at any time, and such data is forwarded to/from the shell or |
| command on the server side, and the user terminal in the client side. |
| .Pp |
| When the user program terminates and all forwarded X11 and other |
| connections have been closed, the server sends command exit status to |
| the client, and both sides exit. |
| .Sh LOGIN PROCESS |
| When a user successfully logs in, |
| .Nm |
| does the following: |
| .Bl -enum -offset indent |
| .It |
| If the login is on a tty, and no command has been specified, |
| prints last login time and |
| .Pa /etc/motd |
| (unless prevented in the configuration file or by |
| .Pa ~/.hushlogin ; |
| see the |
| .Sx FILES |
| section). |
| .It |
| If the login is on a tty, records login time. |
| .It |
| Checks |
| .Pa /etc/nologin ; |
| if it exists, prints contents and quits |
| (unless root). |
| .It |
| Changes to run with normal user privileges. |
| .It |
| Sets up basic environment. |
| .It |
| Reads the file |
| .Pa ~/.ssh/environment , |
| if it exists, and users are allowed to change their environment. |
| See the |
| .Cm PermitUserEnvironment |
| option in |
| .Xr sshd_config 5 . |
| .It |
| Changes to user's home directory. |
| .It |
| If |
| .Pa ~/.ssh/rc |
| exists and the |
| .Xr sshd_config 5 |
| .Cm PermitUserRC |
| option is set, runs it; else if |
| .Pa /etc/ssh/sshrc |
| exists, runs |
| it; otherwise runs xauth. |
| The |
| .Dq rc |
| files are given the X11 |
| authentication protocol and cookie in standard input. |
| See |
| .Sx SSHRC , |
| below. |
| .It |
| Runs user's shell or command. |
| .El |
| .Sh SSHRC |
| If the file |
| .Pa ~/.ssh/rc |
| exists, |
| .Xr sh 1 |
| runs it after reading the |
| environment files but before starting the user's shell or command. |
| It must not produce any output on stdout; stderr must be used |
| instead. |
| If X11 forwarding is in use, it will receive the "proto cookie" pair in |
| its standard input (and |
| .Ev DISPLAY |
| in its environment). |
| The script must call |
| .Xr xauth 1 |
| because |
| .Nm |
| will not run xauth automatically to add X11 cookies. |
| .Pp |
| The primary purpose of this file is to run any initialization routines |
| which may be needed before the user's home directory becomes |
| accessible; AFS is a particular example of such an environment. |
| .Pp |
| This file will probably contain some initialization code followed by |
| something similar to: |
| .Bd -literal -offset 3n |
| if read proto cookie && [ -n "$DISPLAY" ]; then |
| if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then |
| # X11UseLocalhost=yes |
| echo add unix:`echo $DISPLAY | |
| cut -c11-` $proto $cookie |
| else |
| # X11UseLocalhost=no |
| echo add $DISPLAY $proto $cookie |
| fi | xauth -q - |
| fi |
| .Ed |
| .Pp |
| If this file does not exist, |
| .Pa /etc/ssh/sshrc |
| is run, and if that |
| does not exist either, xauth is used to add the cookie. |
| .Sh AUTHORIZED_KEYS FILE FORMAT |
| .Cm AuthorizedKeysFile |
| specifies the files containing public keys for |
| public key authentication; |
| if none is specified, the default is |
| .Pa ~/.ssh/authorized_keys |
| and |
| .Pa ~/.ssh/authorized_keys2 . |
| Each line of the file contains one |
| key (empty lines and lines starting with a |
| .Ql # |
| are ignored as |
| comments). |
| Protocol 1 public keys consist of the following space-separated fields: |
| options, bits, exponent, modulus, comment. |
| Protocol 2 public key consist of: |
| options, keytype, base64-encoded key, comment. |
| The options field is optional; |
| its presence is determined by whether the line starts |
| with a number or not (the options field never starts with a number). |
| The bits, exponent, modulus, and comment fields give the RSA key for |
| protocol version 1; the |
| comment field is not used for anything (but may be convenient for the |
| user to identify the key). |
| For protocol version 2 the keytype is |
| .Dq ecdsa-sha2-nistp256 , |
| .Dq ecdsa-sha2-nistp384 , |
| .Dq ecdsa-sha2-nistp521 , |
| .Dq ssh-ed25519 , |
| .Dq ssh-dss |
| or |
| .Dq ssh-rsa . |
| .Pp |
| Note that lines in this file are usually several hundred bytes long |
| (because of the size of the public key encoding) up to a limit of |
| 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA |
| keys up to 16 kilobits. |
| You don't want to type them in; instead, copy the |
| .Pa identity.pub , |
| .Pa id_dsa.pub , |
| .Pa id_ecdsa.pub , |
| .Pa id_ed25519.pub , |
| or the |
| .Pa id_rsa.pub |
| file and edit it. |
| .Pp |
| .Nm |
| enforces a minimum RSA key modulus size for protocol 1 |
| and protocol 2 keys of 768 bits. |
| .Pp |
| The options (if present) consist of comma-separated option |
| specifications. |
| No spaces are permitted, except within double quotes. |
| The following option specifications are supported (note |
| that option keywords are case-insensitive): |
| .Bl -tag -width Ds |
| .It Cm cert-authority |
| Specifies that the listed key is a certification authority (CA) that is |
| trusted to validate signed certificates for user authentication. |
| .Pp |
| Certificates may encode access restrictions similar to these key options. |
| If both certificate restrictions and key options are present, the most |
| restrictive union of the two is applied. |
| .It Cm command="command" |
| Specifies that the command is executed whenever this key is used for |
| authentication. |
| The command supplied by the user (if any) is ignored. |
| The command is run on a pty if the client requests a pty; |
| otherwise it is run without a tty. |
| If an 8-bit clean channel is required, |
| one must not request a pty or should specify |
| .Cm no-pty . |
| A quote may be included in the command by quoting it with a backslash. |
| This option might be useful |
| to restrict certain public keys to perform just a specific operation. |
| An example might be a key that permits remote backups but nothing else. |
| Note that the client may specify TCP and/or X11 |
| forwarding unless they are explicitly prohibited. |
| The command originally supplied by the client is available in the |
| .Ev SSH_ORIGINAL_COMMAND |
| environment variable. |
| Note that this option applies to shell, command or subsystem execution. |
| Also note that this command may be superseded by either a |
| .Xr sshd_config 5 |
| .Cm ForceCommand |
| directive or a command embedded in a certificate. |
| .It Cm environment="NAME=value" |
| Specifies that the string is to be added to the environment when |
| logging in using this key. |
| Environment variables set this way |
| override other default environment values. |
| Multiple options of this type are permitted. |
| Environment processing is disabled by default and is |
| controlled via the |
| .Cm PermitUserEnvironment |
| option. |
| This option is automatically disabled if |
| .Cm UseLogin |
| is enabled. |
| .It Cm from="pattern-list" |
| Specifies that in addition to public key authentication, either the canonical |
| name of the remote host or its IP address must be present in the |
| comma-separated list of patterns. |
| See PATTERNS in |
| .Xr ssh_config 5 |
| for more information on patterns. |
| .Pp |
| In addition to the wildcard matching that may be applied to hostnames or |
| addresses, a |
| .Cm from |
| stanza may match IP addresses using CIDR address/masklen notation. |
| .Pp |
| The purpose of this option is to optionally increase security: public key |
| authentication by itself does not trust the network or name servers or |
| anything (but the key); however, if somebody somehow steals the key, the key |
| permits an intruder to log in from anywhere in the world. |
| This additional option makes using a stolen key more difficult (name |
| servers and/or routers would have to be compromised in addition to |
| just the key). |
| .It Cm no-agent-forwarding |
| Forbids authentication agent forwarding when this key is used for |
| authentication. |
| .It Cm no-port-forwarding |
| Forbids TCP forwarding when this key is used for authentication. |
| Any port forward requests by the client will return an error. |
| This might be used, e.g. in connection with the |
| .Cm command |
| option. |
| .It Cm no-pty |
| Prevents tty allocation (a request to allocate a pty will fail). |
| .It Cm no-user-rc |
| Disables execution of |
| .Pa ~/.ssh/rc . |
| .It Cm no-X11-forwarding |
| Forbids X11 forwarding when this key is used for authentication. |
| Any X11 forward requests by the client will return an error. |
| .It Cm permitopen="host:port" |
| Limit local |
| .Li ``ssh -L'' |
| port forwarding such that it may only connect to the specified host and |
| port. |
| IPv6 addresses can be specified by enclosing the address in square brackets. |
| Multiple |
| .Cm permitopen |
| options may be applied separated by commas. |
| No pattern matching is performed on the specified hostnames, |
| they must be literal domains or addresses. |
| A port specification of |
| .Cm * |
| matches any port. |
| .It Cm principals="principals" |
| On a |
| .Cm cert-authority |
| line, specifies allowed principals for certificate authentication as a |
| comma-separated list. |
| At least one name from the list must appear in the certificate's |
| list of principals for the certificate to be accepted. |
| This option is ignored for keys that are not marked as trusted certificate |
| signers using the |
| .Cm cert-authority |
| option. |
| .It Cm tunnel="n" |
| Force a |
| .Xr tun 4 |
| device on the server. |
| Without this option, the next available device will be used if |
| the client requests a tunnel. |
| .El |
| .Pp |
| An example authorized_keys file: |
| .Bd -literal -offset 3n |
| # Comments allowed at start of line |
| ssh-rsa AAAAB3Nza...LiPk== user@example.net |
| from="*.sales.example.net,!pc.sales.example.net" ssh-rsa |
| AAAAB2...19Q== john@example.net |
| command="dump /home",no-pty,no-port-forwarding ssh-dss |
| AAAAC3...51R== example.net |
| permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss |
| AAAAB5...21S== |
| tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== |
| jane@example.net |
| .Ed |
| .Sh SSH_KNOWN_HOSTS FILE FORMAT |
| The |
| .Pa /etc/ssh/ssh_known_hosts |
| and |
| .Pa ~/.ssh/known_hosts |
| files contain host public keys for all known hosts. |
| The global file should |
| be prepared by the administrator (optional), and the per-user file is |
| maintained automatically: whenever the user connects from an unknown host, |
| its key is added to the per-user file. |
| .Pp |
| Each line in these files contains the following fields: markers (optional), |
| hostnames, bits, exponent, modulus, comment. |
| The fields are separated by spaces. |
| .Pp |
| The marker is optional, but if it is present then it must be one of |
| .Dq @cert-authority , |
| to indicate that the line contains a certification authority (CA) key, |
| or |
| .Dq @revoked , |
| to indicate that the key contained on the line is revoked and must not ever |
| be accepted. |
| Only one marker should be used on a key line. |
| .Pp |
| Hostnames is a comma-separated list of patterns |
| .Pf ( Ql * |
| and |
| .Ql \&? |
| act as |
| wildcards); each pattern in turn is matched against the canonical host |
| name (when authenticating a client) or against the user-supplied |
| name (when authenticating a server). |
| A pattern may also be preceded by |
| .Ql \&! |
| to indicate negation: if the host name matches a negated |
| pattern, it is not accepted (by that line) even if it matched another |
| pattern on the line. |
| A hostname or address may optionally be enclosed within |
| .Ql \&[ |
| and |
| .Ql \&] |
| brackets then followed by |
| .Ql \&: |
| and a non-standard port number. |
| .Pp |
| Alternately, hostnames may be stored in a hashed form which hides host names |
| and addresses should the file's contents be disclosed. |
| Hashed hostnames start with a |
| .Ql | |
| character. |
| Only one hashed hostname may appear on a single line and none of the above |
| negation or wildcard operators may be applied. |
| .Pp |
| Bits, exponent, and modulus are taken directly from the RSA host key; they |
| can be obtained, for example, from |
| .Pa /etc/ssh/ssh_host_key.pub . |
| The optional comment field continues to the end of the line, and is not used. |
| .Pp |
| Lines starting with |
| .Ql # |
| and empty lines are ignored as comments. |
| .Pp |
| When performing host authentication, authentication is accepted if any |
| matching line has the proper key; either one that matches exactly or, |
| if the server has presented a certificate for authentication, the key |
| of the certification authority that signed the certificate. |
| For a key to be trusted as a certification authority, it must use the |
| .Dq @cert-authority |
| marker described above. |
| .Pp |
| The known hosts file also provides a facility to mark keys as revoked, |
| for example when it is known that the associated private key has been |
| stolen. |
| Revoked keys are specified by including the |
| .Dq @revoked |
| marker at the beginning of the key line, and are never accepted for |
| authentication or as certification authorities, but instead will |
| produce a warning from |
| .Xr ssh 1 |
| when they are encountered. |
| .Pp |
| It is permissible (but not |
| recommended) to have several lines or different host keys for the same |
| names. |
| This will inevitably happen when short forms of host names |
| from different domains are put in the file. |
| It is possible |
| that the files contain conflicting information; authentication is |
| accepted if valid information can be found from either file. |
| .Pp |
| Note that the lines in these files are typically hundreds of characters |
| long, and you definitely don't want to type in the host keys by hand. |
| Rather, generate them by a script, |
| .Xr ssh-keyscan 1 |
| or by taking |
| .Pa /etc/ssh/ssh_host_key.pub |
| and adding the host names at the front. |
| .Xr ssh-keygen 1 |
| also offers some basic automated editing for |
| .Pa ~/.ssh/known_hosts |
| including removing hosts matching a host name and converting all host |
| names to their hashed representations. |
| .Pp |
| An example ssh_known_hosts file: |
| .Bd -literal -offset 3n |
| # Comments allowed at start of line |
| closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net |
| cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= |
| # A hashed hostname |
| |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa |
| AAAA1234.....= |
| # A revoked key |
| @revoked * ssh-rsa AAAAB5W... |
| # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org |
| @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... |
| .Ed |
| .Sh FILES |
| .Bl -tag -width Ds -compact |
| .It Pa ~/.hushlogin |
| This file is used to suppress printing the last login time and |
| .Pa /etc/motd , |
| if |
| .Cm PrintLastLog |
| and |
| .Cm PrintMotd , |
| respectively, |
| are enabled. |
| It does not suppress printing of the banner specified by |
| .Cm Banner . |
| .Pp |
| .It Pa ~/.rhosts |
| This file is used for host-based authentication (see |
| .Xr ssh 1 |
| for more information). |
| On some machines this file may need to be |
| world-readable if the user's home directory is on an NFS partition, |
| because |
| .Nm |
| reads it as root. |
| Additionally, this file must be owned by the user, |
| and must not have write permissions for anyone else. |
| The recommended |
| permission for most machines is read/write for the user, and not |
| accessible by others. |
| .Pp |
| .It Pa ~/.shosts |
| This file is used in exactly the same way as |
| .Pa .rhosts , |
| but allows host-based authentication without permitting login with |
| rlogin/rsh. |
| .Pp |
| .It Pa ~/.ssh/ |
| This directory is the default location for all user-specific configuration |
| and authentication information. |
| There is no general requirement to keep the entire contents of this directory |
| secret, but the recommended permissions are read/write/execute for the user, |
| and not accessible by others. |
| .Pp |
| .It Pa ~/.ssh/authorized_keys |
| Lists the public keys (DSA, ECDSA, Ed25519, RSA) |
| that can be used for logging in as this user. |
| The format of this file is described above. |
| The content of the file is not highly sensitive, but the recommended |
| permissions are read/write for the user, and not accessible by others. |
| .Pp |
| If this file, the |
| .Pa ~/.ssh |
| directory, or the user's home directory are writable |
| by other users, then the file could be modified or replaced by unauthorized |
| users. |
| In this case, |
| .Nm |
| will not allow it to be used unless the |
| .Cm StrictModes |
| option has been set to |
| .Dq no . |
| .Pp |
| .It Pa ~/.ssh/environment |
| This file is read into the environment at login (if it exists). |
| It can only contain empty lines, comment lines (that start with |
| .Ql # ) , |
| and assignment lines of the form name=value. |
| The file should be writable |
| only by the user; it need not be readable by anyone else. |
| Environment processing is disabled by default and is |
| controlled via the |
| .Cm PermitUserEnvironment |
| option. |
| .Pp |
| .It Pa ~/.ssh/known_hosts |
| Contains a list of host keys for all hosts the user has logged into |
| that are not already in the systemwide list of known host keys. |
| The format of this file is described above. |
| This file should be writable only by root/the owner and |
| can, but need not be, world-readable. |
| .Pp |
| .It Pa ~/.ssh/rc |
| Contains initialization routines to be run before |
| the user's home directory becomes accessible. |
| This file should be writable only by the user, and need not be |
| readable by anyone else. |
| .Pp |
| .It Pa /etc/hosts.equiv |
| This file is for host-based authentication (see |
| .Xr ssh 1 ) . |
| It should only be writable by root. |
| .Pp |
| .It Pa /etc/moduli |
| Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
| The file format is described in |
| .Xr moduli 5 . |
| .Pp |
| .It Pa /etc/motd |
| See |
| .Xr motd 5 . |
| .Pp |
| .It Pa /etc/nologin |
| If this file exists, |
| .Nm |
| refuses to let anyone except root log in. |
| The contents of the file |
| are displayed to anyone trying to log in, and non-root connections are |
| refused. |
| The file should be world-readable. |
| .Pp |
| .It Pa /etc/shosts.equiv |
| This file is used in exactly the same way as |
| .Pa hosts.equiv , |
| but allows host-based authentication without permitting login with |
| rlogin/rsh. |
| .Pp |
| .It Pa /etc/ssh/ssh_host_key |
| .It Pa /etc/ssh/ssh_host_dsa_key |
| .It Pa /etc/ssh/ssh_host_ecdsa_key |
| .It Pa /etc/ssh/ssh_host_ed25519_key |
| .It Pa /etc/ssh/ssh_host_rsa_key |
| These files contain the private parts of the host keys. |
| These files should only be owned by root, readable only by root, and not |
| accessible to others. |
| Note that |
| .Nm |
| does not start if these files are group/world-accessible. |
| .Pp |
| .It Pa /etc/ssh/ssh_host_key.pub |
| .It Pa /etc/ssh/ssh_host_dsa_key.pub |
| .It Pa /etc/ssh/ssh_host_ecdsa_key.pub |
| .It Pa /etc/ssh/ssh_host_ed25519_key.pub |
| .It Pa /etc/ssh/ssh_host_rsa_key.pub |
| These files contain the public parts of the host keys. |
| These files should be world-readable but writable only by |
| root. |
| Their contents should match the respective private parts. |
| These files are not |
| really used for anything; they are provided for the convenience of |
| the user so their contents can be copied to known hosts files. |
| These files are created using |
| .Xr ssh-keygen 1 . |
| .Pp |
| .It Pa /etc/ssh/ssh_known_hosts |
| Systemwide list of known host keys. |
| This file should be prepared by the |
| system administrator to contain the public host keys of all machines in the |
| organization. |
| The format of this file is described above. |
| This file should be writable only by root/the owner and |
| should be world-readable. |
| .Pp |
| .It Pa /etc/ssh/sshd_config |
| Contains configuration data for |
| .Nm sshd . |
| The file format and configuration options are described in |
| .Xr sshd_config 5 . |
| .Pp |
| .It Pa /etc/ssh/sshrc |
| Similar to |
| .Pa ~/.ssh/rc , |
| it can be used to specify |
| machine-specific login-time initializations globally. |
| This file should be writable only by root, and should be world-readable. |
| .Pp |
| .It Pa /var/empty |
| .Xr chroot 2 |
| directory used by |
| .Nm |
| during privilege separation in the pre-authentication phase. |
| The directory should not contain any files and must be owned by root |
| and not group or world-writable. |
| .Pp |
| .It Pa /var/run/sshd.pid |
| Contains the process ID of the |
| .Nm |
| listening for connections (if there are several daemons running |
| concurrently for different ports, this contains the process ID of the one |
| started last). |
| The content of this file is not sensitive; it can be world-readable. |
| .El |
| .Sh SEE ALSO |
| .Xr scp 1 , |
| .Xr sftp 1 , |
| .Xr ssh 1 , |
| .Xr ssh-add 1 , |
| .Xr ssh-agent 1 , |
| .Xr ssh-keygen 1 , |
| .Xr ssh-keyscan 1 , |
| .Xr chroot 2 , |
| .Xr login.conf 5 , |
| .Xr moduli 5 , |
| .Xr sshd_config 5 , |
| .Xr inetd 8 , |
| .Xr sftp-server 8 |
| .Sh AUTHORS |
| OpenSSH is a derivative of the original and free |
| ssh 1.2.12 release by Tatu Ylonen. |
| Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| Theo de Raadt and Dug Song |
| removed many bugs, re-added newer features and |
| created OpenSSH. |
| Markus Friedl contributed the support for SSH |
| protocol versions 1.5 and 2.0. |
| Niels Provos and Markus Friedl contributed support |
| for privilege separation. |