| .\" -*- nroff -*- |
| .\" |
| .\" 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 1.144 2001/08/29 23:02:21 stevesk Exp $ |
| .Dd September 25, 1999 |
| .Dt SSHD 8 |
| .Os |
| .Sh NAME |
| .Nm sshd |
| .Nd OpenSSH SSH daemon |
| .Sh SYNOPSIS |
| .Nm sshd |
| .Op Fl deiqD46 |
| .Op Fl b Ar bits |
| .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 p Ar port |
| .Op Fl u Ar len |
| .Op Fl V Ar client_protocol_id |
| .Sh DESCRIPTION |
| .Nm |
| (SSH 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. |
| The programs are intended to be as easy to |
| install and use as possible. |
| .Pp |
| .Nm |
| is the daemon that 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. |
| This implementation of |
| .Nm |
| supports both SSH protocol version 1 and 2 simultaneously. |
| .Nm |
| works as follows. |
| .Pp |
| .Ss SSH protocol version 1 |
| .Pp |
| Each host has a host-specific RSA key |
| (normally 1024 bits) used to identify the host. |
| Additionally, when |
| the daemon starts, it generates a server RSA key (normally 768 bits). |
| This key is normally regenerated every hour if it has been used, and |
| is never stored on disk. |
| .Pp |
| 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 |
| Next, the server and the client enter an authentication dialog. |
| The client tries to authenticate itself using |
| .Pa .rhosts |
| authentication, |
| .Pa .rhosts |
| authentication combined with RSA host |
| authentication, RSA challenge-response authentication, or password |
| based authentication. |
| .Pp |
| Rhosts authentication is normally disabled |
| because it is fundamentally insecure, but can be enabled in the server |
| configuration file if desired. |
| System security is not improved unless |
| .Xr rshd 8 , |
| .Xr rlogind 8 , |
| and |
| .Xr rexecd 8 |
| are disabled (thus completely disabling |
| .Xr rlogin 1 |
| and |
| .Xr rsh 1 |
| into the machine). |
| .Pp |
| .Ss SSH protocol version 2 |
| .Pp |
| Version 2 works similarly: |
| Each host has a host-specific key (RSA or DSA) used to identify the host. |
| However, when the daemon starts, it does not generate a server key. |
| Forward security is provided through a Diffie-Hellman key agreement. |
| This key agreement results in a shared session key. |
| .Pp |
| 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-sha1 or hmac-md5). |
| .Pp |
| Protocol version 2 provides a public key based |
| user (PubkeyAuthentication) or |
| client host (HostbasedAuthentication) authentication method, |
| conventional password authentication and challenge response based methods. |
| .Pp |
| .Ss Command execution and data forwarding |
| .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/IP connections, or forwarding the authentication agent |
| connection over the secure channel. |
| .Pp |
| Finally, 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. |
| .Pp |
| .Nm |
| can be configured using command-line options or a configuration |
| file. |
| Command-line options override values specified in the |
| configuration file. |
| .Pp |
| .Nm |
| rereads its configuration file when it receives a hangup signal, |
| .Dv SIGHUP , |
| by executing itself with the name it was started as, i.e., |
| .Pa /usr/sbin/sshd . |
| .Pp |
| The options are as follows: |
| .Bl -tag -width Ds |
| .It Fl b Ar bits |
| Specifies the number of bits in the ephemeral protocol version 1 |
| server key (default 768). |
| .It Fl d |
| Debug mode. |
| The server sends verbose debug output to the system |
| log, 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 -d options increase the debugging level. |
| Maximum is 3. |
| .It Fl e |
| When this option is specified, |
| .Nm |
| will send the output to the standard error instead of the system log. |
| .It Fl f Ar configuration_file |
| Specifies the name of the configuration file. |
| The default is |
| .Pa /etc/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 |
| 600 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 the file from which the host key is read (default |
| .Pa /etc/ssh_host_key ) . |
| This option must be given if |
| .Nm |
| is not run as root (as the normal |
| host file is normally not readable by anyone but root). |
| 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 inetd. |
| .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 p Ar port |
| Specifies the port on which the server listens for connections |
| (default 22). |
| .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 |
| 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 |
| is 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 RhostsAuthentication , |
| .Cm RhostsRSAAuthentication , |
| .Cm HostbasedAuthentication |
| and using a |
| .Cm from="pattern-list" |
| option in a key file. |
| .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 4 |
| Forces |
| .Nm |
| to use IPv4 addresses only. |
| .It Fl 6 |
| Forces |
| .Nm |
| to use IPv6 addresses only. |
| .El |
| .Sh CONFIGURATION FILE |
| .Nm |
| reads configuration data from |
| .Pa /etc/sshd_config |
| (or the file specified with |
| .Fl f |
| on the command line). |
| The file contains keyword-argument pairs, one per line. |
| Lines starting with |
| .Ql # |
| and empty lines are interpreted as comments. |
| .Pp |
| The possible |
| keywords and their meanings are as follows (note that |
| keywords are case-insensitive and arguments are case-sensitive): |
| .Bl -tag -width Ds |
| .It Cm AFSTokenPassing |
| Specifies whether an AFS token may be forwarded to the server. |
| Default is |
| .Dq yes . |
| .It Cm AllowGroups |
| This keyword can be followed by a list of group names, separated |
| by spaces. |
| If specified, login is allowed only for users whose primary |
| group or supplementary group list matches one of the patterns. |
| .Ql \&* |
| and |
| .Ql ? |
| can be used as |
| wildcards in the patterns. |
| Only group names are valid; a numerical group ID isn't recognized. |
| By default login is allowed regardless of the group list. |
| .Pp |
| .It Cm AllowTcpForwarding |
| Specifies whether TCP forwarding is permitted. |
| The default is |
| .Dq yes . |
| Note that disabling TCP forwarding does not improve security unless |
| users are also denied shell access, as they can always install their |
| own forwarders. |
| .Pp |
| .It Cm AllowUsers |
| This keyword can be followed by a list of user names, separated |
| by spaces. |
| If specified, login is allowed only for users names that |
| match one of the patterns. |
| .Ql \&* |
| and |
| .Ql ? |
| can be used as |
| wildcards in the patterns. |
| Only user names are valid; a numerical user ID isn't recognized. |
| By default login is allowed regardless of the user name. |
| If the pattern takes the form USER@HOST then USER and HOST |
| are separately checked, allowing you to restrict logins to particular |
| users from particular hosts. |
| .Pp |
| .It Cm AuthorizedKeysFile |
| Specifies the file that contains the public keys that can be used |
| for user authentication. |
| .Cm AuthorizedKeysFile |
| may contain tokens of the form %T which are substituted during connection |
| set-up. The following tokens are defined: %% is replaced by a literal '%', |
| %h is replaced by the home directory of the user being authenticated and |
| %u is replaced by the username of that user. |
| After expansion, |
| .Cm AuthorizedKeysFile |
| is taken to be an absolute path or one relative to the user's home |
| directory. |
| The default is |
| .Dq .ssh/authorized_keys |
| .It Cm Banner |
| In some jurisdictions, sending a warning message before authentication |
| may be relevant for getting legal protection. |
| The contents of the specified file are sent to the remote user before |
| authentication is allowed. |
| This option is only available for protocol version 2. |
| .Pp |
| .It Cm ChallengeResponseAuthentication |
| Specifies whether challenge response authentication is allowed. |
| All authentication styles from |
| .Xr login.conf 5 |
| are supported. |
| The default is |
| .Dq yes . |
| .It Cm Ciphers |
| Specifies the ciphers allowed for protocol version 2. |
| Multiple ciphers must be comma-separated. |
| The default is |
| .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. |
| .It Cm ClientAliveInterval |
| Sets a timeout interval in seconds after which if no data has been received |
| from the client, |
| .Nm |
| will send a message through the encrypted |
| channel to request a response from the client. |
| The default |
| is 0, indicating that these messages will not be sent to the client. |
| This option applies to protocol version 2 only. |
| .It Cm ClientAliveCountMax |
| Sets the number of client alive messages (see above) which may be |
| sent without |
| .Nm |
| receiving any messages back from the client. If this threshold is |
| reached while client alive messages are being sent, |
| .Nm |
| will disconnect the client, terminating the session. It is important |
| to note that the use of client alive messages is very different from |
| .Cm Keepalive |
| (below). The client alive messages are sent through the |
| encrypted channel and therefore will not be spoofable. The TCP keepalive |
| option enabled by |
| .Cm Keepalive |
| is spoofable. You want to use the client |
| alive mechanism when you are basing something important on |
| clients having an active connection to the server. |
| .Pp |
| The default value is 3. If you set |
| .Cm ClientAliveInterval |
| (above) to 15, and leave this value at the default, unresponsive ssh clients |
| will be disconnected after approximately 45 seconds. |
| .It Cm DenyGroups |
| This keyword can be followed by a number of group names, separated |
| by spaces. |
| Users whose primary group or supplementary group list matches |
| one of the patterns aren't allowed to log in. |
| .Ql \&* |
| and |
| .Ql ? |
| can be used as |
| wildcards in the patterns. |
| Only group names are valid; a numerical group ID isn't recognized. |
| By default login is allowed regardless of the group list. |
| .Pp |
| .It Cm DenyUsers |
| This keyword can be followed by a number of user names, separated |
| by spaces. |
| Login is disallowed for user names that match one of the patterns. |
| .Ql \&* |
| and |
| .Ql ? |
| can be used as wildcards in the patterns. |
| Only user names are valid; a numerical user ID isn't recognized. |
| By default login is allowed regardless of the user name. |
| .It Cm GatewayPorts |
| Specifies whether remote hosts are allowed to connect to ports |
| forwarded for the client. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm HostbasedAuthentication |
| Specifies whether rhosts or /etc/hosts.equiv authentication together |
| with successful public key client host authentication is allowed |
| (hostbased authentication). |
| This option is similar to |
| .Cm RhostsRSAAuthentication |
| and applies to protocol version 2 only. |
| The default is |
| .Dq no . |
| .It Cm HostKey |
| Specifies the file containing the private host keys (default |
| .Pa /etc/ssh_host_key ) |
| used by SSH protocol versions 1 and 2. |
| Note that |
| .Nm |
| will refuse to use a file if it is group/world-accessible. |
| It is possible to have multiple host key files. |
| .Dq rsa1 |
| keys are used for version 1 and |
| .Dq dsa |
| or |
| .Dq rsa |
| are used for version 2 of the SSH protocol. |
| .It Cm IgnoreRhosts |
| Specifies that |
| .Pa .rhosts |
| and |
| .Pa .shosts |
| files will not be used in |
| .Cm RhostsAuthentication , |
| .Cm RhostsRSAAuthentication |
| or |
| .Cm HostbasedAuthentication . |
| .Pp |
| .Pa /etc/hosts.equiv |
| and |
| .Pa /etc/shosts.equiv |
| are still used. |
| The default is |
| .Dq yes . |
| .It Cm IgnoreUserKnownHosts |
| Specifies whether |
| .Nm |
| should ignore the user's |
| .Pa $HOME/.ssh/known_hosts |
| during |
| .Cm RhostsRSAAuthentication |
| or |
| .Cm HostbasedAuthentication . |
| The default is |
| .Dq no . |
| .It Cm KeepAlive |
| Specifies whether the system should send keepalive messages to the |
| other side. |
| If they are sent, death of the connection or crash of one |
| of the machines will be properly noticed. |
| However, this means that |
| connections will die if the route is down temporarily, and some people |
| find it annoying. |
| On the other hand, if keepalives are not sent, |
| sessions may hang indefinitely on the server, leaving |
| .Dq ghost |
| users and consuming server resources. |
| .Pp |
| The default is |
| .Dq yes |
| (to send keepalives), and the server will notice |
| if the network goes down or the client host reboots. |
| This avoids infinitely hanging sessions. |
| .Pp |
| To disable keepalives, the value should be set to |
| .Dq no |
| in both the server and the client configuration files. |
| .It Cm KerberosAuthentication |
| Specifies whether Kerberos authentication is allowed. |
| This can be in the form of a Kerberos ticket, or if |
| .Cm PasswordAuthentication |
| is yes, the password provided by the user will be validated through |
| the Kerberos KDC. |
| To use this option, the server needs a |
| Kerberos servtab which allows the verification of the KDC's identity. |
| Default is |
| .Dq yes . |
| .It Cm KerberosOrLocalPasswd |
| If set then if password authentication through Kerberos fails then |
| the password will be validated via any additional local mechanism |
| such as |
| .Pa /etc/passwd . |
| Default is |
| .Dq yes . |
| .It Cm KerberosTgtPassing |
| Specifies whether a Kerberos TGT may be forwarded to the server. |
| Default is |
| .Dq no , |
| as this only works when the Kerberos KDC is actually an AFS kaserver. |
| .It Cm KerberosTicketCleanup |
| Specifies whether to automatically destroy the user's ticket cache |
| file on logout. |
| Default is |
| .Dq yes . |
| .It Cm KeyRegenerationInterval |
| In protocol version 1, the ephemeral server key is automatically regenerated |
| after this many seconds (if it has been used). |
| The purpose of regeneration is to prevent |
| decrypting captured sessions by later breaking into the machine and |
| stealing the keys. |
| The key is never stored anywhere. |
| If the value is 0, the key is never regenerated. |
| The default is 3600 (seconds). |
| .It Cm ListenAddress |
| Specifies the local addresses |
| .Nm |
| should listen on. |
| The following forms may be used: |
| .Pp |
| .Bl -item -offset indent -compact |
| .It |
| .Cm ListenAddress |
| .Sm off |
| .Ar host No | Ar IPv4_addr No | Ar IPv6_addr |
| .Sm on |
| .It |
| .Cm ListenAddress |
| .Sm off |
| .Ar host No | Ar IPv4_addr No : Ar port |
| .Sm on |
| .It |
| .Cm ListenAddress |
| .Sm off |
| .Oo |
| .Ar host No | Ar IPv6_addr Oc : Ar port |
| .Sm on |
| .El |
| .Pp |
| If |
| .Ar port |
| is not specified, |
| .Nm |
| will listen on the address and all prior |
| .Cm Port |
| options specified. The default is to listen on all local |
| addresses. Multiple |
| .Cm ListenAddress |
| options are permitted. Additionally, any |
| .Cm Port |
| options must precede this option for non port qualified addresses. |
| .It Cm LoginGraceTime |
| The server disconnects after this time if the user has not |
| successfully logged in. |
| If the value is 0, there is no time limit. |
| The default is 600 (seconds). |
| .It Cm LogLevel |
| Gives the verbosity level that is used when logging messages from |
| .Nm sshd . |
| The possible values are: |
| QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
| The default is INFO. |
| Logging with level DEBUG violates the privacy of users |
| and is not recommended. |
| .It Cm MACs |
| Specifies the available MAC (message authentication code) algorithms. |
| The MAC algorithm is used in protocol version 2 |
| for data integrity protection. |
| Multiple algorithms must be comma-separated. |
| The default is |
| .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
| .It Cm MaxStartups |
| Specifies the maximum number of concurrent unauthenticated connections to the |
| .Nm |
| daemon. |
| Additional connections will be dropped until authentication succeeds or the |
| .Cm LoginGraceTime |
| expires for a connection. |
| The default is 10. |
| .Pp |
| Alternatively, random early drop can be enabled by specifying |
| the three colon separated values |
| .Dq start:rate:full |
| (e.g., "10:30:60"). |
| .Nm |
| will refuse connection attempts with a probability of |
| .Dq rate/100 |
| (30%) |
| if there are currently |
| .Dq start |
| (10) |
| unauthenticated connections. |
| The probability increases linearly and all connection attempts |
| are refused if the number of unauthenticated connections reaches |
| .Dq full |
| (60). |
| .It Cm PAMAuthenticationViaKbdInt |
| Specifies whether PAM challenge response authentication is allowed. This |
| allows the use of most PAM challenge response authentication modules, but |
| it will allow password authentication regardless of whether |
| .Cm PasswordAuthentication |
| is disabled. |
| The default is |
| .Dq no . |
| .It Cm PasswordAuthentication |
| Specifies whether password authentication is allowed. |
| The default is |
| .Dq yes . |
| .It Cm PermitEmptyPasswords |
| When password authentication is allowed, it specifies whether the |
| server allows login to accounts with empty password strings. |
| The default is |
| .Dq no . |
| .It Cm PermitRootLogin |
| Specifies whether root can login using |
| .Xr ssh 1 . |
| The argument must be |
| .Dq yes , |
| .Dq without-password , |
| .Dq forced-commands-only |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .Pp |
| If this option is set to |
| .Dq without-password |
| password authentication is disabled for root. |
| .Pp |
| If this option is set to |
| .Dq forced-commands-only |
| root login with public key authentication will be allowed, |
| but only if the |
| .Ar command |
| option has been specified |
| (which may be useful for taking remote backups even if root login is |
| normally not allowed). All other authentication methods are disabled |
| for root. |
| .Pp |
| If this option is set to |
| .Dq no |
| root is not allowed to login. |
| .It Cm PidFile |
| Specifies the file that contains the process identifier of the |
| .Nm |
| daemon. |
| The default is |
| .Pa /var/run/sshd.pid . |
| .It Cm Port |
| Specifies the port number that |
| .Nm |
| listens on. |
| The default is 22. |
| Multiple options of this type are permitted. |
| See also |
| .Cm ListenAddress . |
| .It Cm PrintLastLog |
| Specifies whether |
| .Nm |
| should print the date and time when the user last logged in. |
| The default is |
| .Dq yes . |
| .It Cm PrintMotd |
| Specifies whether |
| .Nm |
| should print |
| .Pa /etc/motd |
| when a user logs in interactively. |
| (On some systems it is also printed by the shell, |
| .Pa /etc/profile , |
| or equivalent.) |
| The default is |
| .Dq yes . |
| .It Cm Protocol |
| Specifies the protocol versions |
| .Nm |
| should support. |
| The possible values are |
| .Dq 1 |
| and |
| .Dq 2 . |
| Multiple versions must be comma-separated. |
| The default is |
| .Dq 2,1 . |
| .It Cm PubkeyAuthentication |
| Specifies whether public key authentication is allowed. |
| The default is |
| .Dq yes . |
| Note that this option applies to protocol version 2 only. |
| .It Cm ReverseMappingCheck |
| Specifies whether |
| .Nm |
| should try to verify the remote host name and check that |
| the resolved host name for the remote IP address maps back to the |
| very same IP address. |
| The default is |
| .Dq no . |
| .It Cm RhostsAuthentication |
| Specifies whether authentication using rhosts or /etc/hosts.equiv |
| files is sufficient. |
| Normally, this method should not be permitted because it is insecure. |
| .Cm RhostsRSAAuthentication |
| should be used |
| instead, because it performs RSA-based host authentication in addition |
| to normal rhosts or /etc/hosts.equiv authentication. |
| The default is |
| .Dq no . |
| This option applies to protocol version 1 only. |
| .It Cm RhostsRSAAuthentication |
| Specifies whether rhosts or /etc/hosts.equiv authentication together |
| with successful RSA host authentication is allowed. |
| The default is |
| .Dq no . |
| This option applies to protocol version 1 only. |
| .It Cm RSAAuthentication |
| Specifies whether pure RSA authentication is allowed. |
| The default is |
| .Dq yes . |
| This option applies to protocol version 1 only. |
| .It Cm ServerKeyBits |
| Defines the number of bits in the ephemeral protocol version 1 server key. |
| The minimum value is 512, and the default is 768. |
| .It Cm StrictModes |
| Specifies whether |
| .Nm |
| should check file modes and ownership of the |
| user's files and home directory before accepting login. |
| This is normally desirable because novices sometimes accidentally leave their |
| directory or files world-writable. |
| The default is |
| .Dq yes . |
| .It Cm Subsystem |
| Configures an external subsystem (e.g., file transfer daemon). |
| Arguments should be a subsystem name and a command to execute upon subsystem |
| request. |
| The command |
| .Xr sftp-server 8 |
| implements the |
| .Dq sftp |
| file transfer subsystem. |
| By default no subsystems are defined. |
| Note that this option applies to protocol version 2 only. |
| .It Cm SyslogFacility |
| Gives the facility code that is used when logging messages from |
| .Nm sshd . |
| The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
| LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
| The default is AUTH. |
| .It Cm UseLogin |
| Specifies whether |
| .Xr login 1 |
| is used for interactive login sessions. |
| The default is |
| .Dq no . |
| Note that |
| .Xr login 1 |
| is never used for remote command execution. |
| Note also, that if this is enabled, |
| .Cm X11Forwarding |
| will be disabled because |
| .Xr login 1 |
| does not know how to handle |
| .Xr xauth 1 |
| cookies. |
| .It Cm X11DisplayOffset |
| Specifies the first display number available for |
| .Nm sshd Ns 's |
| X11 forwarding. |
| This prevents |
| .Nm |
| from interfering with real X11 servers. |
| The default is 10. |
| .It Cm X11Forwarding |
| Specifies whether X11 forwarding is permitted. |
| The default is |
| .Dq no . |
| Note that disabling X11 forwarding does not improve security in any |
| way, as users can always install their own forwarders. |
| X11 forwarding is automatically disabled if |
| .Cm UseLogin |
| is enabled. |
| .It Cm XAuthLocation |
| Specifies the location of the |
| .Xr xauth 1 |
| program. |
| The default is |
| .Pa /usr/X11R6/bin/xauth . |
| .El |
| .Ss Time Formats |
| .Pp |
| .Nm |
| command-line arguments and configuration file options that specify time |
| may be expressed using a sequence of the form: |
| .Sm off |
| .Ar time Oo Ar qualifier Oc , |
| .Sm on |
| where |
| .Ar time |
| is a positive integer value and |
| .Ar qualifier |
| is one of the following: |
| .Pp |
| .Bl -tag -width Ds -compact -offset indent |
| .It Cm <none> |
| seconds |
| .It Cm s | Cm S |
| seconds |
| .It Cm m | Cm M |
| minutes |
| .It Cm h | Cm H |
| hours |
| .It Cm d | Cm D |
| days |
| .It Cm w | Cm W |
| weeks |
| .El |
| .Pp |
| Each member of the sequence is added together to calculate |
| the total time value. |
| .Pp |
| Time format examples: |
| .Pp |
| .Bl -tag -width Ds -compact -offset indent |
| .It 600 |
| 600 seconds (10 minutes) |
| .It 10m |
| 10 minutes |
| .It 1h30m |
| 1 hour 30 minutes (90 minutes) |
| .El |
| .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 $HOME/.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 |
| .Pa $HOME/.ssh/environment |
| if it exists. |
| .It |
| Changes to user's home directory. |
| .It |
| If |
| .Pa $HOME/.ssh/rc |
| exists, runs it; else if |
| .Pa /etc/sshrc |
| exists, runs |
| it; otherwise runs xauth. |
| The |
| .Dq rc |
| files are given the X11 |
| authentication protocol and cookie in standard input. |
| .It |
| Runs user's shell or command. |
| .El |
| .Sh AUTHORIZED_KEYS FILE FORMAT |
| .Pa $HOME/.ssh/authorized_keys |
| is the default file that lists the public keys that are |
| permitted for RSA authentication in protocol version 1 |
| and for public key authentication (PubkeyAuthentication) |
| in protocol version 2. |
| .Cm AuthorizedKeysFile |
| may be used to specify an alternative file. |
| .Pp |
| Each line of the file contains one |
| key (empty lines and lines starting with a |
| .Ql # |
| are ignored as |
| comments). |
| Each RSA public key consists of the following fields, separated by |
| spaces: options, bits, exponent, modulus, comment. |
| Each protocol version 2 public key consists of: |
| options, keytype, base64 encoded key, comment. |
| The options fields |
| are optional; its presence is determined by whether the line starts |
| with a number or not (the option 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 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 RSA key modulus). |
| You don't want to type them in; instead, copy the |
| .Pa identity.pub , |
| .Pa id_dsa.pub |
| or the |
| .Pa id_rsa.pub |
| file and edit it. |
| .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 from="pattern-list" |
| Specifies that in addition to RSA authentication, the canonical name |
| of the remote host must be present in the comma-separated list of |
| patterns |
| .Pf ( Ql * |
| and |
| .Ql ? |
| serve as wildcards). |
| The list may also contain |
| patterns negated by prefixing them with |
| .Ql ! ; |
| if the canonical host name matches a negated pattern, the key is not accepted. |
| The purpose |
| of this option is to optionally increase security: RSA 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 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 connection requests a pty; |
| otherwise it is run without a tty. |
| Note that if you want a 8-bit clean channel, |
| you 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 RSA 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/IP and/or X11 |
| forwarding unless they are explicitly prohibited. |
| .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. |
| .It Cm no-port-forwarding |
| Forbids TCP/IP 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-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 no-agent-forwarding |
| Forbids authentication agent forwarding when this key is used for |
| authentication. |
| .It Cm no-pty |
| Prevents tty allocation (a request to allocate a pty will fail). |
| .It Cm permitopen="host:port" |
| Limit local |
| .Li ``ssh -L'' |
| port forwarding such that it may only connect to the specified host and |
| port. 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. |
| .El |
| .Ss Examples |
| 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar |
| .Pp |
| from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula |
| .Pp |
| command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi |
| .Pp |
| permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 |
| .Sh SSH_KNOWN_HOSTS FILE FORMAT |
| The |
| .Pa /etc/ssh_known_hosts , |
| and |
| .Pa $HOME/.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: hostnames, |
| bits, exponent, modulus, comment. |
| The fields are separated by spaces. |
| .Pp |
| Hostnames is a comma-separated list of patterns ('*' and '?' 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. |
| .Pp |
| Bits, exponent, and modulus are taken directly from the RSA host key; they |
| can be obtained, e.g., from |
| .Pa /etc/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. |
| It is thus 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 |
| or by taking |
| .Pa /etc/ssh_host_key.pub |
| and adding the host names at the front. |
| .Ss Examples |
| .Bd -literal |
| closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi |
| cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= |
| .Ed |
| .Sh FILES |
| .Bl -tag -width Ds |
| .It Pa /etc/sshd_config |
| Contains configuration data for |
| .Nm sshd . |
| This file should be writable by root only, but it is recommended |
| (though not necessary) that it be world-readable. |
| .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key |
| These three 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 this file is group/world-accessible. |
| .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub |
| These three 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 . |
| .It Pa /etc/moduli |
| Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
| .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 pid of the one |
| started last). |
| The content of this file is not sensitive; it can be world-readable. |
| .It Pa $HOME/.ssh/authorized_keys |
| Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
| This file must be readable by root (which may on some machines imply |
| it being world-readable if the user's home directory resides on an NFS |
| volume). |
| It is recommended that it not be accessible by others. |
| The format of this file is described above. |
| Users will place the contents of their |
| .Pa identity.pub , |
| .Pa id_dsa.pub |
| and/or |
| .Pa id_rsa.pub |
| files into this file, as described in |
| .Xr ssh-keygen 1 . |
| .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
| These files are consulted when using rhosts with RSA host |
| authentication or protocol version 2 hostbased authentication |
| to check the public key of the host. |
| The key must be listed in one of these files to be accepted. |
| The client uses the same files |
| to verify that it is connecting to the correct remote host. |
| These files should be writable only by root/the owner. |
| .Pa /etc/ssh_known_hosts |
| should be world-readable, and |
| .Pa $HOME/.ssh/known_hosts |
| can but need not be world-readable. |
| .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. |
| .It Pa /etc/hosts.allow, /etc/hosts.deny |
| If compiled with |
| .Sy LIBWRAP |
| support, tcp-wrappers access controls may be defined here as described in |
| .Xr hosts_access 5 . |
| .It Pa $HOME/.rhosts |
| This file contains host-username pairs, separated by a space, one per |
| line. |
| The given user on the corresponding host is permitted to log in |
| without password. |
| The same file is used by rlogind and rshd. |
| The file must |
| be writable only by the user; it is recommended that it not be |
| accessible by others. |
| .Pp |
| If is also possible to use netgroups in the file. |
| Either host or user |
| name may be of the form +@groupname to specify all hosts or all users |
| in the group. |
| .It Pa $HOME/.shosts |
| For ssh, |
| this file is exactly the same as for |
| .Pa .rhosts . |
| However, this file is |
| not used by rlogin and rshd, so using this permits access using SSH only. |
| .It Pa /etc/hosts.equiv |
| This file is used during |
| .Pa .rhosts |
| authentication. |
| In the simplest form, this file contains host names, one per line. |
| Users on |
| those hosts are permitted to log in without a password, provided they |
| have the same user name on both machines. |
| The host name may also be |
| followed by a user name; such users are permitted to log in as |
| .Em any |
| user on this machine (except root). |
| Additionally, the syntax |
| .Dq +@group |
| can be used to specify netgroups. |
| Negated entries start with |
| .Ql \&- . |
| .Pp |
| If the client host/user is successfully matched in this file, login is |
| automatically permitted provided the client and server user names are the |
| same. |
| Additionally, successful RSA host authentication is normally required. |
| This file must be writable only by root; it is recommended |
| that it be world-readable. |
| .Pp |
| .Sy "Warning: It is almost never a good idea to use user names in" |
| .Pa hosts.equiv . |
| Beware that it really means that the named user(s) can log in as |
| .Em anybody , |
| which includes bin, daemon, adm, and other accounts that own critical |
| binaries and directories. |
| Using a user name practically grants the user root access. |
| The only valid use for user names that I can think |
| of is in negative entries. |
| .Pp |
| Note that this warning also applies to rsh/rlogin. |
| .It Pa /etc/shosts.equiv |
| This is processed exactly as |
| .Pa /etc/hosts.equiv . |
| However, this file may be useful in environments that want to run both |
| rsh/rlogin and ssh. |
| .It Pa $HOME/.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. |
| .It Pa $HOME/.ssh/rc |
| If this file exists, it is run with /bin/sh after reading the |
| environment files but before starting the user's shell or command. |
| If X11 spoofing is in use, this will receive the "proto cookie" pair in |
| standard input (and |
| .Ev DISPLAY |
| in environment). |
| This must call |
| .Xr xauth 1 |
| in that case. |
| .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 |
| if read proto cookie; then |
| echo add $DISPLAY $proto $cookie | xauth -q - |
| fi |
| .Ed |
| .Pp |
| If this file does not exist, |
| .Pa /etc/sshrc |
| is run, and if that |
| does not exist either, xauth is used to store the cookie. |
| .Pp |
| This file should be writable only by the user, and need not be |
| readable by anyone else. |
| .It Pa /etc/sshrc |
| Like |
| .Pa $HOME/.ssh/rc . |
| This can be used to specify |
| machine-specific login-time initializations globally. |
| This file should be writable only by root, and should be world-readable. |
| .El |
| .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. |
| .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 login.conf 5 , |
| .Xr moduli 5 , |
| .Xr sftp-server 8 |
| .Rs |
| .%A T. Ylonen |
| .%A T. Kivinen |
| .%A M. Saarinen |
| .%A T. Rinne |
| .%A S. Lehtinen |
| .%T "SSH Protocol Architecture" |
| .%N draft-ietf-secsh-architecture-09.txt |
| .%D July 2001 |
| .%O work in progress material |
| .Re |
| .Rs |
| .%A M. Friedl |
| .%A N. Provos |
| .%A W. A. Simpson |
| .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" |
| .%N draft-ietf-secsh-dh-group-exchange-01.txt |
| .%D April 2001 |
| .%O work in progress material |
| .Re |