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SSHD_CONFIG (5) | File formats and conventions | Unix Manual Pages | :man

NAME

sshd_config - OpenSSH SSH daemon configuration file

CONTENTS

Synopsis
Description
Time Formats
Files
See Also
Authors

SYNOPSIS

/etc/ssh/sshd_config

DESCRIPTION

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):

AcceptEnv
Specifies what environment variables sent by the client will be copied into the session’s environ(7). See SendEnv in ssh_config(5) for how to configure the client. Note that environment passing is only supported for protocol 2. Variables are specified by name, which may contain the wildcard characters ‘*’ and ‘?’. Multiple environment variables may be separated by whitespace or spread across multiple AcceptEnv directives. Be warned that some environment variables could be used to bypass restricted user environments. For this reason, care should be taken in the use of this directive. The default is not to accept any environment variables.
AddressFamily
Specifies which address family should be used by sshd. Valid arguments are "any", "inet" (use IPv4 only) or "inet6" (use IPv6 only). The default is "any".
AllowGroups
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.
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 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.
AuthorizedKeysFile
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".
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. By default, no banner is displayed.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed. Specifically, in
.Fx , this controls the use of PAM (see pam(3)) for authentication. Note that this affects the effectiveness of the PasswordAuthentication and PermitRootLogin variables. The default is "yes".
Ciphers
Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The supported ciphers are "3des-cbc", "aes128-cbc", "aes192-cbc", "aes256-cbc", "aes128-ctr", "aes192-ctr", "aes256-ctr", "arcfour128", "arcfour256", "arcfour", "blowfish-cbc", and "cast128-cbc". The default is
‘‘aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr’’

ClientAliveCountMax
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 TCPKeepAlive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive 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.

ClientAliveInterval
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.
Compression
Specifies whether compression is allowed, or delayed until the user has authenticated successfully. The argument must be "yes", "delayed", or "no". The default is "delayed".
DenyGroups
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.
DenyUsers
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.
GatewayPorts
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 allow remote port forwardings to bind to non-loopback addresses, thus allowing other hosts to connect. The argument may be "no" to force remote port forwardings to be available to the local host only, "yes" to force remote port forwardings to bind to the wildcard address, or "clientspecified" to allow the client to select the address to which the forwarding is bound. The default is "no".
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed. The default is "no". Note that this option applies to protocol version 2 only.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user’s credentials cache on logout. The default is "yes". Note that this option applies to protocol version 2 only.
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 RhostsRSAAuthentication and applies to protocol version 2 only. The default is "no".
HostKey
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_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.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication or HostbasedAuthentication.

/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is "yes".

IgnoreUserKnownHosts
Specifies whether sshd should ignore the user’s ~/.ssh/known_hosts during RhostsRSAAuthentication or HostbasedAuthentication. The default is "no".
KerberosAuthentication
Specifies whether the password provided by the user for PasswordAuthentication 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".
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire an AFS token before accessing the user’s home directory. Default is "no".
KerberosOrLocalPasswd
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".
KerberosTicketCleanup
Specifies whether to automatically destroy the user’s ticket cache file on logout. Default is "yes".
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).
ListenAddress
Specifies the local addresses sshd should listen on. The following forms may be used:

ListenAddress
.Sm off host | IPv4_addr | IPv6_addr
.Sm on
ListenAddress
.Sm off host | IPv4_addr: port
.Sm on
ListenAddress
.Sm off [ host | IPv6_addr:port]
.Sm on

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.

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 120 seconds.
LogLevel 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.
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 "hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96".
MaxAuthTries Specifies the maximum number of authentication attempts permitted per connection. Once the number of failures reaches half this value, additional failures are logged. The default is 6.
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" (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).

PasswordAuthentication Specifies whether password authentication is allowed. The default is "no", unless sshd was built without PAM support, in which case the default is "yes". Note that if ChallengeResponseAuthentication is "yes", and the PAM authentication policy for sshd includes pam_unix(8), password authentication will be allowed through the challenge-response mechanism regardless of the value of PasswordAuthentication.
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 root can log in using ssh(1). The argument must be "yes", "without-password", "forced-commands-only" or "no". The default is "no". Note that if ChallengeResponseAuthentication is "yes", the root user may be allowed in with its password even if PermitRootLogin is set to "without-password".

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 log in.

PermitUserEnvironment 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.
PidFile Specifies the file that contains the process ID of the sshd daemon. The default is /var/run/sshd.pid.
Port Specifies the port number that sshd listens on. The default is 22. Multiple options of this type are permitted. See also ListenAddress.
PrintLastLog Specifies whether sshd should print the date and time of the last user login when a user logs in interactively. The default is "yes".
PrintMotd 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".
Protocol Specifies the protocol versions sshd supports. The possible values are "1" and "2". Multiple versions must be comma-separated. The default is "2". 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".
PubkeyAuthentication Specifies whether public key authentication is allowed. The default is "yes". Note that this option applies to protocol version 2 only.
RhostsRSAAuthentication 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.
RSAAuthentication Specifies whether pure RSA authentication is allowed. The default is "yes". This option applies to protocol version 1 only.
ServerKeyBits Defines the number of bits in the ephemeral protocol version 1 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 (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.
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, LOCAL7. The default is AUTH.
TCPKeepAlive 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 TCP keepalives are not sent, sessions may hang indefinitely on the server, leaving "ghost" users and consuming server resources.

The default is "yes" (to send TCP keepalive messages), and the server will notice if the network goes down or the client host crashes. This avoids infinitely hanging sessions.

To disable TCP keepalive messages, the value should be set to "no".

UseDNS Specifies whether sshd should look up 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 "yes".
UseLogin 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.
UsePAM Enables the Pluggable Authentication Module interface. If set to "yes" this will enable PAM authentication using ChallengeResponseAuthentication and PAM account and session module processing for all authentication types.

Because PAM challenge-response authentication usually serves an equivalent role to password authentication, you should disable either PasswordAuthentication or ChallengeResponseAuthentication.

If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user. The default is "yes".

UsePrivilegeSeparation 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".
VersionAddendum Specifies a string to append to the regular version string to identify OS- or site-specific modifications. The default is "FreeBSD-20050903".
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 argument must be "yes" or "no". The default is "yes".

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.

X11UseLocalhost 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".
XAuthLocation Specifies the full pathname of the xauth(1) program. The default is /usr/X11R6/bin/xauth.

Time Formats

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

<none>
seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W 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)

FILES

/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.

SEE ALSO

sshd(8)

AUTHORS


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