SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)

sshd_config —
OpenSSH SSH daemon configuration file

/etc/ssh/sshd_config
 

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

The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):

Specifies whether an AFS token may be forwarded to the server. Default is “no”.
This keyword can be followed by a list of group name patterns, separated by spaces. If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. ‘*’ and ‘?’ can be used as wildcards in the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups.
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 list of user name patterns, separated by spaces. If specified, login is allowed only 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 is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.
Specifies the file that contains the public keys that can be used for user authentication. 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, AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. The default is “.ssh/authorized_keys”.
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. By default, no banner is displayed.
Specifies whether challenge response authentication is allowed. All authentication styles from login.conf(5) are supported. The default is “yes”.
Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The default is 
  ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
    aes192-cbc,aes256-cbc''
Sets a timeout interval in seconds after which if no data has been received from the client, sshd 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.
Sets the number of client alive messages (see above) which may be sent without sshd receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from KeepAlive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by KeepAlive is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive.

The default value is 3. If ClientAliveInterval (above) is set to 15, and ClientAliveCountMax is left at the default, unresponsive ssh clients will be disconnected after approximately 45 seconds.

Specifies whether compression is allowed. The argument must be “yes” or “no”. The default is “yes”.
This keyword can be followed by a list of group name patterns, separated by spaces. Login is disallowed for users whose primary group or supplementary group list matches one of the patterns. ‘*’ and ‘?’ can be used as wildcards in the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups.
This keyword can be followed by a list of user name patterns, 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 is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.
Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify that sshd should bind remote port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be “yes” or “no”. The default is “no”.
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 RhostsRSAAuthentication and applies to protocol version 2 only. The default is “no”.
Specifies a file containing a private host key used by SSH. The default is /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for protocol version 2. Note that sshd will refuse to use a file if it 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 RhostsAuthentication, RhostsRSAAuthentication or HostbasedAuthentication.

/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 or HostbasedAuthentication. The default is “no”.
Specifies whether the system should send TCP 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 crashes. This avoids infinitely hanging sessions.

To disable keepalives, the value should be set to “no”.

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 “no”.
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”.
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).
Specifies the local addresses sshd should listen on. The following forms may be used:

If port is not specified, sshd will listen on the address and all prior Port options specified. The default is to listen on all local addresses. Multiple ListenAddress options are permitted. Additionally, any Port options must precede this option for non port qualified addresses.

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 120 seconds.
Gives the verbosity level that is used when logging messages from sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with a DEBUG level violates the privacy of users and is not recommended.
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 “hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96”.
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 probability of “rate/100” (30%) if there are currently “start” (10) unauthenticated connections. The probability 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”.
When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is “no”.
Specifies whether root can login using ssh(1). The argument must be “yes”, “without-password”, “forced-commands-only” or “no”. The default is “yes”.

If this option is set to “without-password” password authentication is disabled for root.

If this option is set to “forced-commands-only” root login with public key authentication will be allowed, but only if the 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.

If this option is set to “no” root is not allowed to login.

Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by sshd. The default is “no”. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.
Specifies the file that contains the process ID 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. See also ListenAddress.
Specifies whether sshd should print the date and time when the user last logged in. The default is “yes”.
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 supports. The possible values are “1” and “2”. Multiple versions must be comma-separated. The default is “2,1”. Note that the order of the protocol list does not indicate preference, because the client selects among multiple protocol versions offered by the server. Specifying “2,1” is identical to “1,2”.
Specifies whether public key authentication is allowed. The default is “yes”. Note that this option applies to protocol version 2 only.
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”. This option applies to protocol version 1 only.
Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is “no”. This option applies to protocol version 1 only.
Specifies whether pure RSA authentication is allowed. The default is “yes”. This option applies to protocol version 1 only.
Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 768.
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. The default is “no”. Note that login(1) is never used for remote command execution. Note also, that if this is enabled, X11Forwarding will be disabled because login(1) does not know how to handle xauth(1) cookies. If UsePrivilegeSeparation is specified, it will be disabled after authentication.
Enables PAM authentication (via challenge-response) and session set up. If you enable this, you should probably disable PasswordAuthentication. If you enable then you will not be able to run sshd as a non-root user.
Specifies whether sshd separates privileges by creating an unprivileged child process to deal with incoming network traffic. After successful authentication, another process will be created that has the privilege of the authenticated user. The goal of privilege separation is to prevent privilege escalation by containing any corruption within the unprivileged processes. The default is “yes”.
Specifies whether sshd 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 “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 argument must be “yes” or “no”. The default is “no”.

When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd proxy display is configured to listen on the wildcard address (see X11UseLocalhost below), however this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding is that the client's X11 display server may be exposed to attack when the ssh client requests forwarding (see the warnings for ForwardX11 in ssh_config(5) ). A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a “no” setting.

Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders. X11 forwarding is automatically disabled if UseLogin is enabled.

Specifies whether sshd should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, sshd binds the forwarding server to the loopback address and sets the hostname part of the DISPLAY environment variable to “localhost”. This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. X11UseLocalhost may be set to “no” to specify that the forwarding server should be bound to the wildcard address. The argument must be “yes” or “no”. The default is “yes”.
Specifies the full pathname of the xauth(1) program. The default is /usr/X11R6/bin/xauth.

sshd command-line arguments and configuration file options that specify time may be expressed using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier is one of the following:

seconds
|
seconds
|
minutes
|
hours
|
days
|
weeks

Each member of the sequence is added together to calculate the total time value.

Time format examples:

600
600 seconds (10 minutes)
10m
10 minutes
1h30m
1 hour 30 minutes (90 minutes)

/etc/ssh/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.

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.

sshd(8)
September 25, 1999 BSD