SSHD(8) System Manager's Manual SSHD(8)

sshd —
secure shell daemon

sshd [-diqDQ46] [-b bits] [-f config_file] [-g login_grace_time] [-h host_key_file] [-k key_gen_time] [-p port] [-u len] [-V client_protocol_id]

sshd (Secure Shell Daemon) is the daemon program for 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.

sshd is the daemon that listens for connections from clients. It is normally started at boot from /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 sshd supports both SSH protocol version 1 and 2 simultaneously. sshd works as follows.

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.

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.

Next, the server and the client enter an authentication dialog. The client tries to authenticate itself using .rhosts authentication, .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, or password based authentication.

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 rshd(8), rlogind(8), rexecd(8), and rexd(8) are disabled (thus completely disabling rlogin(1) and rsh(1) into the machine).

Version 2 works similarly: Each host has a host-specific DSA key 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. The rest of the session is encrypted using a symmetric cipher, currently Blowfish, 3DES or CAST128 in CBC mode or Arcfour. 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).

Protocol version 2 provides a public key based user authentication method (PubkeyAuthentication) and conventional password authentication.

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.

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.

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.

sshd can be configured using command-line options or a configuration file. Command-line options override values specified in the configuration file.

sshd rereads its configuration file when it receives a hangup signal, SIGHUP.

The options are as follows:

bits
Specifies the number of bits in the server key (default 768).
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 increases the debugging level. Maximum is 3.
configuration_file
Specifies the name of the configuration file. The default is /etc/sshd_config. sshd refuses to start if there is no configuration file.
login_grace_time
Gives the grace time for clients to authenticate themselves (default 300 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.
host_key_file
Specifies the file from which the host key is read (default /etc/ssh_host_key). This option must be given if sshd 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.
Specifies that sshd is being run from inetd. sshd 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 sshd from inetd may be feasible.
key_gen_time
Specifies how often the 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.
port
Specifies the port on which the server listens for connections (default 22).
Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged.
len
This option is used to specify the size of the field in the utmp structure that holds the remote host name. If the resolved host name is longer than 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 -u0 indicates that only dotted decimal addresses should be put into the utmp file.
When this option is specified sshd will not detach and does not become a daemon. This allows easy monitoring of sshd.
Do not print an error message if RSA support is missing.
client_protocol_id
SSH-2 compatibility mode. When this option is specified sshd assumes the client has sent the supplied version string and skips the Protocol Version Identification Exchange. This option is not intended to be called directly.
Forces sshd to use IPv4 addresses only.
Forces sshd to use IPv6 addresses only.

sshd reads configuration data from /etc/sshd_config (or the file specified with -f on the command line). The file contains keyword-value pairs, one per line. Lines starting with ‘#’ and empty lines are interpreted as comments.

The following keywords are possible.

Specifies whether an AFS token may be forwarded to the server. Default is “yes”.
This keyword can be followed by a number of group names, separated by spaces. If specified, login is allowed only for users whose primary group matches one of the patterns. ‘*’ and ‘’? 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 primary group.
Specifies whether TCP forwarding is permitted. The default is “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.
This keyword can be followed by a number of user names, separated by spaces. If specified, login is allowed only for users names that match one of the patterns. ‘*’ and ‘’? 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.
Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The default is “3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc”.
Specifies whether sshd should check for new mail for interactive logins. The default is “no”.
This keyword can be followed by a number of group names, separated by spaces. Users whose primary group matches one of the patterns aren't allowed to log in. ‘*’ and ‘’? 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 primary group.
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. ‘*’ and ‘’? 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.
Specifies whether public key authentication is allowed. The default is “yes”. Note that this option applies to protocol version 2 only.
Specifies whether remote hosts are allowed to connect to ports forwarded for the client. The argument must be “yes” or “no”. The default is “no”.
Specifies the file containing the private host keys (default /etc/ssh_host_key) used by SSH protocol versions 1 and 2. Note that sshd if this file is group/world-accessible. It is possible to have multiple host key files. “rsa1” keys are used for version 1 and “dsa” or “rsa” are used for version 2 of the SSH protocol.
Specifies that .rhosts and .shosts files will not be used in authentication. /etc/hosts.equiv and /etc/shosts.equiv are still used. The default is “yes”.
Specifies whether sshd should ignore the user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The default is “no”.
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 “ghost” users and consuming server resources.

The default is “yes” (to send keepalives), and the server will notice if the network goes down or the client host reboots. This avoids infinitely hanging sessions.

To disable keepalives, the value should be set to “no” in both the server and the client configuration files.

Specifies whether Kerberos authentication is allowed. This can be in the form of a Kerberos ticket, or if 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 “yes”.
If set then if password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as /etc/passwd. Default is “yes”.
Specifies whether a Kerberos TGT may be forwarded to the server. Default is “no”, as this only works when the Kerberos KDC is actually an AFS kaserver.
Specifies whether to automatically destroy the user's ticket cache file on logout. Default is “yes”.
The 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).
Specifies what local address sshd should listen on. The default is to listen to all local addresses. Multiple options of this type are permitted. Additionally, the Ports options must precede this option.
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).
Gives the verbosity level that is used when logging messages from 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.
Specifies the maximum number of concurrent unauthenticated connections to the sshd daemon. Additional connections will be dropped until authentication succeeds or the LoginGraceTime expires for a connection. The default is 10.

Alternatively, random early drop can be enabled by specifying the three colon separated values “start:rate:full” (e.g., "10:30:60"). sshd will refuse connection attempts with a probabillity of “rate/100” (30%) if there are currently “start” (10) unauthenticated connections. The probabillity increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches “full” (60).

Specifies whether password authentication is allowed. The default is “yes”. Note that this option applies to both protocol versions 1 and 2.
When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is “no”.
Specifies whether the root can log in using ssh(1). The argument must be “yes”, “without-password” or “no”. The default is “yes”. If this options is set to “without-password” only password authentication is disabled for root.

Root login with RSA authentication when the command option has been specified will be allowed regardless of the value of this setting (which may be useful for taking remote backups even if root login is normally not allowed).

Specifies the file that contains the process identifier of the sshd daemon. The default is /var/run/sshd.pid.
Specifies the port number that sshd listens on. The default is 22. Multiple options of this type are permitted.
Specifies whether sshd should print /etc/motd when a user logs in interactively. (On some systems it is also printed by the shell, /etc/profile, or equivalent.) The default is “yes”.
Specifies the protocol versions sshd should support. The possible values are “1” and “2”. Multiple versions must be comma-separated. The default is “1”.
Obsolete. Random number generation uses other techniques.
Specifies whether authentication using rhosts or /etc/hosts.equiv files is sufficient. Normally, this method should not be permitted because it is insecure. 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 “no”.
Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is “no”.
Specifies whether pure RSA authentication is allowed. The default is “yes”. Note that this option applies to protocol version 1 only.
Defines the number of bits in the server key. The minimum value is 512, and the default is 768.
Specifies whether skey(1) authentication is allowed. The default is “yes”. Note that s/key authentication is enabled only if PasswordAuthentication is allowed, too.
Specifies whether sshd 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 “yes”.
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 sftp-server(8) implements the “sftp” file transfer subsystem. By default no subsystems are defined. Note that this option applies to protocol version 2 only.
Gives the facility code that is used when logging messages from sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
Specifies whether login(1) is used for interactive login sessions. Note that login(1) is never used for remote command execution. The default is “no”.
Specifies the first display number available for sshd's X11 forwarding. This prevents sshd from interfering with real X11 servers. The default is 10.
Specifies whether X11 forwarding is permitted. The default is “no”. Note that disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders.
Specifies the location of the xauth(1) program. The default is /usr/X11R6/bin/xauth.

When a user successfully logs in, sshd does the following:
  1. If the login is on a tty, and no command has been specified, prints last login time and /etc/motd (unless prevented in the configuration file or by $HOME/.hushlogin; see the FILES section).
  2. If the login is on a tty, records login time.
  3. Checks /etc/nologin; if it exists, prints contents and quits (unless root).
  4. Changes to run with normal user privileges.
  5. Sets up basic environment.
  6. Reads $HOME/.ssh/environment if it exists.
  7. Changes to user's home directory.
  8. If $HOME/.ssh/rc exists, runs it; else if /etc/sshrc exists, runs it; otherwise runs xauth. The “rc” files are given the X11 authentication protocol and cookie in standard input.
  9. Runs user's shell or command.

The $HOME/.ssh/authorized_keys file lists the RSA keys that are permitted for RSA authentication in SSH protocols 1.3 and 1.5 Similarly, the $HOME/.ssh/authorized_keys2 file lists the DSA and RSA keys that are permitted for public key authentication (PubkeyAuthentication) in SSH protocol 2.0.

Each line of the file contains one key (empty lines and lines starting with a ‘#’ 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 “ssh-dss” or “ssh-rsa”.

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 identity.pub or the id_dsa.pub file and edit it.

The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported:

Specifies that in addition to RSA authentication, the canonical name of the remote host must be present in the comma-separated list of patterns (‘*’ and ‘’? serve as wildcards). The list may also contain patterns negated by prefixing them with ‘’!; 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).
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. 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.
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.
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 command option.
Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.
Forbids authentication agent forwarding when this key is used for authentication.
Prevents tty allocation (a request to allocate a pty will fail).

1024 33 12121...312314325 ylo@foo.bar

from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula

command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi

The /etc/ssh_known_hosts, /etc/ssh_known_hosts2, $HOME/.ssh/known_hosts, and $HOME/.ssh/known_hosts2 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.

Each line in these files contains the following fields: hostnames, bits, exponent, modulus, comment. The fields are separated by spaces.

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 ‘’! 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.

Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, e.g., from /etc/ssh_host_key.pub. The optional comment field continues to the end of the line, and is not used.

Lines starting with ‘#’ and empty lines are ignored as comments.

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.

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 /etc/ssh_host_key.pub and adding the host names at the front.

closenet,closenet.hut.fi,...,130.233.208.41 1024 37 159...93 closenet.hut.fi

/etc/sshd_config
Contains configuration data for sshd. This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable.
/etc/ssh_host_key
Contains the private part of the host key. This file should only be owned by root, readable only by root, and not accessible to others. Note that sshd does not start if this file is group/world-accessible.
/etc/ssh_host_key.pub
Contains the public part of the host key. This file should be world-readable but writable only by root. Its contents should match the private part. This file is not really used for anything; it is only provided for the convenience of the user so its contents can be copied to known hosts files. These two files are created using ssh-keygen(1).
/etc/primes
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
/var/run/sshd.pid
Contains the process ID of the sshd 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.
$HOME/.ssh/authorized_keys
Lists the RSA keys 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 identity.pub files into this file, as described in ssh-keygen(1).
$HOME/.ssh/authorized_keys2
Lists the DSA keys 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 id_dsa.pub files into this file, as described in ssh-keygen(1).
/etc/ssh_known_hosts and $HOME/.ssh/known_hosts
These files are consulted when using rhosts with RSA host 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 the remote host is the one it intended to connect. These files should be writable only by root/the owner. /etc/ssh_known_hosts should be world-readable, and $HOME/.ssh/known_hosts can but need not be world-readable.
/etc/nologin
If this file exists, sshd 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.
/etc/hosts.allow, /etc/hosts.deny
If compiled with LIBWRAP support, tcp-wrappers access controls may be defined here as described in hosts_access(5).
$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.

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.

$HOME/.shosts
For ssh, this file is exactly the same as for .rhosts. However, this file is not used by rlogin and rshd, so using this permits access using SSH only.
/etc/hosts.equiv
This file is used during .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 any user on this machine (except root). Additionally, the syntax “+@group” can be used to specify netgroups. Negated entries start with ‘-’.

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.

Warning: It is almost never a good idea to use user names in hosts.equiv. Beware that it really means that the named user(s) can log in as 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.

Note that this warning also applies to rsh/rlogin.

/etc/shosts.equiv
This is processed exactly as /etc/hosts.equiv. However, this file may be useful in environments that want to run both rsh/rlogin and ssh.
$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 ‘#’), 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.
$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 DISPLAY in environment). This must call xauth(1) in that case.

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.

This file will probably contain some initialization code followed by something similar to: "if read proto cookie; then echo add $DISPLAY $proto $cookie | xauth -q -; fi".

If this file does not exist, /etc/sshrc is run, and if that does not exist either, xauth is used to store the cookie.

This file should be writable only by the user, and need not be readable by anyone else.

/etc/sshrc
Like $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.

OpenSSH is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen, but with bugs removed and newer features re-added. Rapidly after the 1.2.12 release, newer versions of the original ssh bore successively more restrictive licenses, and thus demand for a free version was born.

This version of OpenSSH

  • has all components of a restrictive nature (i.e., patents, see ssl(8)) directly removed from the source code; any licensed or patented components are chosen from external libraries.
  • has been updated to support SSH protocol 1.5 and 2, making it compatible with all other SSH clients and servers.
  • contains added support for kerberos(8) authentication and ticket passing.
  • supports one-time password authentication with skey(1).

OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt, and Dug Song.

The support for SSH protocol 2 was written by Markus Friedl.

scp(1), sftp-server(8), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssl(8), rlogin(1), rsh(1)
September 25, 1999 BSD