sshd_config(4) (man pages section 4: File Formats)

man pages section 4: File Formats

sshd_config(4)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO

NAME

sshd_config– sshd configuration file

SYNOPSIS

/etc/ssh/sshd_config

DESCRIPTION

The sshd(1M) daemon reads configuration data from /etc/ssh/sshd_config (or the file specified with sshd -f on the command line). The file contains keyword-value pairs, one per line. A line starting with a hash mark (#) and empty lines are interpreted as comments.

The sshd_config file supports the following keywords:

AllowGroups

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. Asterisk (*) and question mark (?) 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 regardless of the primary group.

AllowTcpForwarding

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.

AllowUsers

This keyword can be followed by a number of user names, separated by spaces. If specified, login is allowed only for user names that match one of the patterns. Asterisk (*) and question mark (?) 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 regardless of the user name.

Ciphers

Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The default is 3des-cbc,blowfish-cbc,aes-128-cbc.

CheckMail

Specifies whether sshd should check for new mail for interactive logins. The default is no.

DenyGroups

Can be followed by a number of group names, separated by spaces. Users whose primary group matches one of the patterns are not allowed to log in. Asterisk (*) and question mark (?) 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 regardless of the primary group.

DenyUsers

Can be followed by a number of user names, separated by spaces. Login is disallowed for user names that match one of the patterns. Asterisk (*) and question mark (?) 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 regardless of the user name.

DSAAuthentication

Specifies whether DSA authentication is allowed. The default is yes. Note that this option applies only to protocol version 2.

GatewayPorts

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.

HostKey

Specifies the file containing the private RSA host key (default /etc/ssh_host_key) used by SSH protocols 1.3 and 1.5. Note that sshd disables protocols 1.3 and 1.5 if this file is group/world-accessible.

IgnoreRhosts

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.

IgnoreUserKnownHosts

Specifies whether sshd should ignore the user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The default is no.

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, which can be an annoyance. On the other hand, if keepalives are not sent, sessions can 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.

KeyRegenerationInterval

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).

ListenAddress

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.

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).

LogLevel

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.

MaxStartups

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 (for example, 10:30:60). Referring to this example, sshd will refuse connection attempts with a probability of rate/100 (30% in our example) if there are currently 10 (from the start field) unauthenticated connections. The probabillity increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches full (60 in our example).

PasswordAuthentication

Specifies whether password authentication is allowed. The default is yes. Note that this option applies to both protocol versions 1 and 2.

PermitEmptyPasswords

When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is no.

PermitRootLogin

Specifies whether the root can log in using ssh(). 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. This might be useful for taking remote backups even if root login is normally not allowed.

PidFile

Allows you to specify an alternative to /var/run/sshd.pid, the default file for storing the PID of the sshd listening for connections. See sshd(1M).

Port

Specifies the port number that sshd listens on. The default is 22. Multiple options of this type are permitted.

PrintMotd

Specifies whether sshd should display the contents of /etc/motd when a user logs in interactively. (On some systems it is also displayed by the shell or a shell startup file, such as /etc/profile.) The default is yes.

Protocol

Specifies the protocol versions sshd should support. The possible values are 1 and 2. Multiple versions must be comma-separated. The default is 1.

RhostsAuthentication

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.

RhostsRSAAuthentication

Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is no.

RSAAuthentication

Specifies whether pure RSA authentication is allowed. The default is yes. Note that this option applies to protocol version 1 only.

ServerKeyBits

Defines the number of bits in the server key. The minimum value is 512, and the default is 768.

StrictModes

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.

Subsystem

Configures an external subsystem (for example, a file transfer daemon). Arguments should be a subsystem name and a command to execute upon subsystem request. The command sftp-server(1M) implements the sftp file transfer subsystem. By default, no subsystems are defined. Note that this option applies to protocol version 2 only.

SyslogFacility

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, and LOCAL7. The default is AUTH.

X11DisplayOffset

Specifies the first display number available for sshd's X11 forwarding. This prevents sshd from interfering with real X11 servers. The default is 10.

X11Forwarding

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.

XAuthLocation

Specifies the location of the xauth(1) program. The default is /usr/X/bin/xauth.