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

man pages section 4: File Formats

sshd_config(4)

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. Unless otherwise noted, keywords and their arguments are case-insensitive.

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 or supplementary group list 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. 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.

If a specified pattern takes the form user@host then user and host are checked separately, 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 can 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 can 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.

ChrootDirectory

Specifies a path to chroot(2) to after authentication. This path, and all its components, must be root-owned directories that are not writable by any other user or group.

The server always tries to change to the user's home directory locally under the chrooted environment but a failure to do so is not considered an error. In addition, the path might contain the following tokens that are expanded at runtime once the connecting user has been authenticated: %% 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.

The ChrootDirectory must contain the necessary files and directories to support the user's session. For an interactive SSH session this requires at least a user's shell, shared libraries needed by the shell, dynamic linker, and possibly basic /dev nodes such as null, zero, stdin, stdout, stderr, random, and tty. Additionally, terminal databases are needed for screen oriented applications. For file transfer sessions using sftp with the SSH protocol version 2, no additional configuration of the environment is necessary if the in-process sftp server is used. See Subsystem for details.

The default is not to chroot(2).

Ciphers

Specifies the ciphers allowed for protocol version 2. Cipher ordering on the server side is not relevant. Multiple ciphers must be comma separated.

Valid ciphers are: aes128-ctr, aes192-ctr, aes256-ctr, aes128-cbc, aes192-cbc, aes256-cbc, arcfour, arcfour128, arcfour256, 3des-cbc, and blowfish-cbc.

The default cipher list is:

aes128-ctr,aes192-ctr,aes256-ctr,arcfour128,
arcfour256,arcfour

Using CBC modes on the server side is not recommended due to potential security issues in connection with the SSH protocol version 2.

ClientAliveCountMax

Sets the number of client alive messages, (see ClientAliveInterval), that can be sent without sshd receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd disconnects the client, terminating the session. The use of client alive messages is very different from KeepAlive. The client alive messages are sent through the encrypted channel and therefore are not spoofable. The TCP keepalive option enabled by KeepAlive is spoofable. The client alive mechanism is valuable when a client or server depend on knowing when a connection has become inactive.

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

ClientAliveInterval

Sets a timeout interval in seconds after which, if no data has been received from the client, sshd sends a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages are not sent to the client. This option applies only to protocol version 2.

Compression

Controls whether the server allows the client to negotiate the use of compression. The default is yes.

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.

If a specified pattern takes the form user@host then user and host are checked separately, disallowing 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 bind remote port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports.

The argument can 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. See also RemoteForward in ssh_config(4).

GSSAPIAuthentication

Enables/disables GSS-API user authentication. The default is yes.

Currently sshd authorizes client user principals to user accounts as follows: if the principal name matches the requested user account, then the principal is authorized. Otherwise, GSS-API authentication fails.

GSSAPIKeyExchange

Enables/disables GSS-API-authenticated key exchanges. The default is yes.

This option also enables the use of the GSS-API to authenticate the user to server after the key exchange. GSS-API key exchange can succeed but the subsequent authentication using the GSS-API fail if the server does not authorize the user's GSS principal name to the target user account.

GSSAPIStoreDelegatedCredentials

Enables/disables the use of delegated GSS-API credentials on the server-side. The default is yes.

Specifically, this option, when enabled, causes the server to store delegated GSS-API credentials in the user's default GSS-API credential store (which for the Kerberos V mechanism means /tmp/krb5cc_<uid>).

Note –

sshd does not take any steps to explicitly destroy stored delegated GSS-API credentials upon logout. It is the responsibility of PAM modules to destroy credentials associated with a session.

HostbasedAuthentication

Specifies whether to try rhosts-based authentication with public key authentication. The argument must be yes or no. The default is no. This option applies to protocol version 2 only and is similar to RhostsRSAAuthentication. See sshd(1M) for guidelines on setting up host-based authentication.

HostbasedUsesNameFromPacketOnly

Controls which hostname is searched for in the files ~/.shosts, /etc/shosts.equiv, and /etc/hosts.equiv. If this parameter is set to yes, the server uses the name the client claimed for itself and signed with that host's key. If set to no, the default, the server uses the name to which the client's IP address resolves.

Setting this parameter to no disables host-based authentication when using NAT or when the client gets to the server indirectly through a port-forwarding firewall.

HostKey

Specifies the file containing the 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. sshd refuses 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 are not used in authentication. /etc/hosts.equiv and /etc/shosts.equiv are still used. The default is yes. This parameter applies to both protocol versions 1 and 2.

IgnoreUserKnownHosts

Specifies whether sshd should ignore the user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The default is no. This parameter applies to both protocol versions 1 and 2.

KbdInteractiveAuthentication

Specifies whether authentication by means of the “keyboard-interactive” authentication method (and PAM) is allowed. Defaults to yes. (Deprecated: this parameter can only be set to yes.)

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 is properly noticed. However, this means that connections 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 notices 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

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 what local address sshd should listen on. The following forms can be used:

ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port

If port is not specified, sshd listens 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 default is to listen on 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 (in seconds) 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. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with level DEBUG violates the privacy of users and is not recommended.

LookupClientHostnames

Specifies whether or not to lookup the names of client's addresses. Defaults to yes.

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-sha1-96,hmac-md5-96.

Match

Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file. Match blocks must be located at the end of the file, after all the global settings.

The arguments to Match are one or more criteria-pattern pairs. The available criteria are User, Group, Host, and Address. The match patterns can consist of single entries or comma-separated lists and can use the wildcard (Asterisk * and question mark ?) and negation (!) operators.

The patterns in a Host criteria should be hostname. The patterns in an Address criteria should be an IP address, which can additionally contain addresses to match in CIDR address/masklen format, for example, 192.0.2.0/24 or 2001:DB8::/32. The mask length provided must be consistent with the address - it is an error to specify a mask length that is too long for the address or one with bits set in this host portion of the address. For example, 192.0.2.0/33 and 192.0.2.0/8 respectively.

Only a subset of keywords can be used on the lines following a Match keyword. Available keywords are AllowTcpForwarding, Banner, ChrootDirectory, GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication, KbdInteractiveAuthentication, MaxAuthTries, PasswordAuthentication, PermitEmptyPasswords, PermitRootLogin, PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset, X11Forwarding, and X11UseLocalhost.

The following are four examples of using Match:

Disallowing user testuser to use TCP forwarding:
```
Match User testuser
  AllowTcpForwarding no
```
Displaying a special banner for users not in the staff group:
```
Match Group *,!staff
  Banner /etc/banner.text
```

Allowing root login from host rootallowed.example.com:

Match Host rootallowed.example.com
  PermitRootLogin yes

Allowing anyone to use GatewayPorts from the local net:
```
Match Address 192.168.0.0/24
  GatewayPorts yes
```

MaxStartups

Specifies the maximum number of concurrent unauthenticated connections to the sshd daemon. Additional connections are 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 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 probability 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. This option applies to both protocol versions 1 and 2.

PermitEmptyPasswords

When password or keyboard-interactive authentication is allowed, it specifies whether the server allows login to accounts with empty password strings.

If not set then the /etc/default/login PASSREQ value is used instead.

PASSREQ=no is equivalent to PermitEmptyPasswords yes. PASSREQ=yes is equivalent to PermitEmptyPasswords no. If neither PermitEmptyPasswords or PASSREQ are set the default is no.

PermitRootLogin

Specifies whether the root can log in using ssh(1). The argument must be yes, without-password, forced-commands-only, or no. without-password means that root cannot be authenticated using the “password” or “keyboard-interactive” methods (see description of KbdInteractiveAuthentication). forced-commands-only means that authentication is allowed only for publickey (for SSHv2, or RSA, for SSHv1) and only if the matching authorized_keys entry for root has a command=<cmd> option.

In Solaris, the default /etc/ssh/sshd_config file is shipped with PermitRootLogin set to no. If unset by the administrator, then CONSOLE parameter from /etc/default/login supplies the default value as follows: if the CONSOLE parameter is not commented out (it can even be empty, that is, “CONSOLE=”), then without-password is used as default value. If CONSOLE is commented out, then the default for PermitRootLogin is yes.

The without-password and forced-commands-only settings are useful for, for example, performing remote administration and backups using trusted public keys for authentication of the remote client, without allowing access to the root account using passwords.

PermitUserEnvironment

Specifies whether a user's ~/.ssh/environment on the server side and environment options in the AuthorizedKeysFile file are processed by sshd. The default is no. Enabling environment processing can enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.

Environment setting from a relevant entry in AuthorizedKeysFile file is processed only if the user was authenticated using the public key authentication method. Of the two files used, values of variables set in ~/.ssh/environment are of higher priority.

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. See also ListenAddress.

PreUserauthHook

Specifies an executable which is run prior to any of the processed authentication methods. The executable can be used to synchronize user information with a remote user-management facility using an arbitrary communication protocol.

The executable is run before any user validation is conducted by SSHD so the user is not required to be existent before she tries to log in.

The executable is invoked with two arguments in the following order: the name of the current authentication method and the username. The environment variable SSH_CONNECTION is also passed to the executable. If the executable returns a zero exit status, the current authentication method is processed as normal. See sshd(1M).

If the exit status is 1, the current authentication method is ignored and can not be used to validate the user. The executable must be owned by root and have permissions of 0500, otherwise it is treated as if it has exited with status 1.

There is no default value for this property.

PrintLastLog

Specifies whether sshd should display the date and time when the user last logged in. The default is yes.

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 in order of preference. The possible values are 1 and 2. Multiple versions must be comma-separated. The default is 2,1. This means that ssh tries version 2 and falls back to version 1 if version 2 is not available.

PubkeyAuthentication

Specifies whether public key authentication is allowed. The default is yes. This option applies to protocol version 2 only.

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. This parameter applies only to protocol version 1.

RhostsRSAAuthentication

Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is no. This parameter applies only to protocol version 1.

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 (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.

Alternately, the name internal-sftp implements an in-process sftp server. This can simplify configurations using ChrootDirectory to force a different filesystem root on clients.

By default, no subsystems are defined. 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.

UseOpenSSLEngine

Specifies whether sshd should use the OpenSSL PKCS#11 engine for offloading cryptographic operations to the Cryptographic Framework. Cryptographic operations are accelerated according to the available installed plug-ins. When no suitable plug-ins are present this option does not have an effect. The default is yes.

VerifyReverseMapping

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. (A yes setting means “verify”.) Setting this parameter to no can be useful where DNS servers might be down and thus cause sshd to spend much time trying to resolve the client's IP address to a name. This feature is useful for Internet-facing servers. The default is no.

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 yes. Disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders.

When X11 forwarding is enabled, there can 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). 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 can be exposed to attack when the ssh client requests forwarding (see the warnings for ForwardX11 in ssh_config(4)). A system administrator who wants to protect clients that expose themselves to attack by unwittingly requesting X11 forwarding, should specify a no setting.

Disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders.

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 might not function with this configuration. X11UseLocalhost can 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 location of the xauth(1) program. The default is /usr/X11/bin/xauth and sshd attempts to open it when X11 forwarding is enabled.

Time Formats

sshd command-line arguments and configuration file options that specify time can be expressed using a sequence of the form: time[qualifier,] 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 |: weeks

Each element of the sequence is added together to calculate the total time value. For example:

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.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Availability	SUNWsshu
Interface Stability	Uncommitted

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 recent 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.

SunOS 5.11 Last Revised 5 Dec 2009