NAME
sshd_config
—
OpenSSH SSH daemon configuration
file
SYNOPSIS
/etc/ssh/sshd_config |
DESCRIPTION
sshd(8) 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. Arguments may optionally be enclosed in double quotes (") in
order to represent arguments containing spaces.
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 multipleAcceptEnv
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(8). Valid arguments are “any”, “inet” (use IPv4 only), or “inet6” (use IPv6 only). The default is “any”.
AllowAgentForwarding
- Specifies whether ssh-agent(1) forwarding is permitted. The default is “yes”. Note that disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.
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. Only group
names are valid; a numerical group ID is not recognized. By default, login
is allowed for all groups. The allow/deny directives are processed in the
following order:
DenyUsers
,AllowUsers
,DenyGroups
, and finallyAllowGroups
.See PATTERNS in ssh_config(5) for more information on patterns.
AllowTcpForwarding
- Specifies whether TCP forwarding is permitted. The available options are “yes” or “all” to allow TCP forwarding, “no” to prevent all TCP forwarding, “local” to allow local (from the perspective of ssh(1)) forwarding only or “remote” to allow remote forwarding only. 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. 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. The
allow/deny directives are processed in the following order:
DenyUsers
,AllowUsers
,DenyGroups
, and finallyAllowGroups
.See PATTERNS in ssh_config(5) for more information on patterns.
AuthenticationMethods
- Specifies the authentication methods that must be successfully completed
for a user to be granted access. This option must be followed by one or
more comma-separated lists of authentication method names. Successful
authentication requires completion of every method in at least one of
these lists.
For example, an argument of “publickey,password publickey,keyboard-interactive” would require the user to complete public key authentication, followed by either password or keyboard interactive authentication. Only methods that are next in one or more lists are offered at each stage, so for this example, it would not be possible to attempt password or keyboard-interactive authentication before public key.
For keyboard interactive authentication it is also possible to restrict authentication to a specific device by appending a colon followed by the device identifier “bsdauth”, “pam”, or “skey”, depending on the server configuration. For example, “keyboard-interactive:bsdauth” would restrict keyboard interactive authentication to the “bsdauth” device.
This option is only available for SSH protocol 2 and will yield a fatal error if enabled if protocol 1 is also enabled. Note that each authentication method listed should also be explicitly enabled in the configuration. The default is not to require multiple authentication; successful completion of a single authentication method is sufficient.
AuthorizedKeysCommand
- Specifies a program to be used to look up the user's public keys. The
program must be owned by root and not writable by group or others. It will
be invoked with a single argument of the username being authenticated, and
should produce on standard output zero or more lines of authorized_keys
output (see AUTHORIZED_KEYS in
sshd(8)). If a key supplied by AuthorizedKeysCommand does not
successfully authenticate and authorize the user then public key
authentication continues using the usual
AuthorizedKeysFile
files. By default, no AuthorizedKeysCommand is run. AuthorizedKeysCommandUser
- Specifies the user under whose account the AuthorizedKeysCommand is run. It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands.
AuthorizedKeysFile
- Specifies the file that contains the public keys that can be used for user
authentication. The format is described in the AUTHORIZED_KEYS FILE FORMAT
section of sshd(8).
AuthorizedKeysFile
may contain tokens of the form %T which are substituted during connection setup. 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. Multiple files may be listed, separated by whitespace. The default is “.ssh/authorized_keys .ssh/authorized_keys2”. AuthorizedPrincipalsFile
- Specifies a file that lists principal names that are accepted for
certificate authentication. When using certificates signed by a key listed
in
TrustedUserCAKeys
, this file lists names, one of which must appear in the certificate for it to be accepted for authentication. Names are listed one per line preceded by key options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). Empty lines and comments starting with ‘#
’ are ignored.AuthorizedPrincipalsFile
may contain tokens of the form %T which are substituted during connection setup. 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,AuthorizedPrincipalsFile
is taken to be an absolute path or one relative to the user's home directory.The default is “none”, i.e. not to use a principals file – in this case, the username of the user must appear in a certificate's principals list for it to be accepted. Note that
AuthorizedPrincipalsFile
is only used when authentication proceeds using a CA listed inTrustedUserCAKeys
and is not consulted for certification authorities trusted via ~/.ssh/authorized_keys, though theprincipals=
key option offers a similar facility (see sshd(8) for details). Banner
- The contents of the specified file are sent to the remote user before authentication is allowed. If the argument is “none” then no banner is displayed. This option is only available for protocol version 2. By default, no banner is displayed.
ChallengeResponseAuthentication
- Specifies whether challenge-response authentication is allowed. All authentication styles from login.conf(5) are supported. The default is “yes”.
ChrootDirectory
- Specifies the pathname of a directory to
chroot(2) to after authentication. All components of the pathname
must be root-owned directories that are not writable by any other user or
group. After the chroot,
sshd(8) changes the working directory to the user's home directory.
The pathname may 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 session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and tty(4) devices. For file transfer sessions using “sftp”, no additional configuration of the environment is necessary if the in-process sftp server is used, though sessions which use logging do require /dev/log inside the chroot directory (see sftp-server(8) for details).The default is not to chroot(2).
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”, “aes128-gcm@openssh.com”, “aes256-gcm@openssh.com”, “arcfour128”, “arcfour256”, “arcfour”, “blowfish-cbc”, “cast128-cbc”, and “chacha20-poly1305@openssh.com”.
The default is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, aes128-gcm@openssh.com,aes256-gcm@openssh.com, chacha20-poly1305@openssh.com, aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, aes256-cbc,arcfour
The list of available ciphers may also be obtained using the
-Q
option of ssh(1). ClientAliveCountMax
- Sets the number of client alive messages (see below) which may be sent
without sshd(8) 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 byTCPKeepAlive
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
(see below) is set to 15, andClientAliveCountMax
is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. This option applies to protocol version 2 only. ClientAliveInterval
- Sets a timeout interval in seconds after which if no data has been received from the client, sshd(8) 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. Only group names are
valid; a numerical group ID is not recognized. By default, login is
allowed for all groups. The allow/deny directives are processed in the
following order:
DenyUsers
,AllowUsers
,DenyGroups
, and finallyAllowGroups
.See PATTERNS in ssh_config(5) for more information on patterns.
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.
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. The allow/deny directives are
processed in the following order:
DenyUsers
,AllowUsers
,DenyGroups
, and finallyAllowGroups
.See PATTERNS in ssh_config(5) for more information on patterns.
ForceCommand
- Forces the execution of the command specified by
ForceCommand
, ignoring any command supplied by the client and ~/.ssh/rc if present. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside aMatch
block. The command originally supplied by the client is available in theSSH_ORIGINAL_COMMAND
environment variable. Specifying a command of “internal-sftp” will force the use of an in-process sftp server that requires no support files when used withChrootDirectory
. GatewayPorts
- Specifies whether remote hosts are allowed to connect to ports forwarded
for the client. By default,
sshd(8) 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 (host-based
authentication). This option is similar to
RhostsRSAAuthentication
and applies to protocol version 2 only. The default is “no”. HostbasedUsesNameFromPacketOnly
- Specifies whether or not the server will attempt to perform a reverse name
lookup when matching the name in the ~/.shosts,
~/.rhosts, and
/etc/hosts.equiv files during
HostbasedAuthentication
. A setting of “yes” means that sshd(8) uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is “no”. HostCertificate
- Specifies a file containing a public host certificate. The certificate's
public key must match a private host key already specified by
HostKey
. The default behaviour of sshd(8) is not to load any certificates. 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, /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for protocol version 2. Note that sshd(8) 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”, “ecdsa”, “ed25519” or “rsa” are used for version 2 of the SSH protocol. It is also possible to specify public host key files instead. In this case operations on the private key will be delegated to an ssh-agent(1).
HostKeyAgent
- Identifies the UNIX-domain socket used to communicate with an agent that
has access to the private host keys. If “SSH_AUTH_SOCK” is
specified, the location of the socket will be read from the
SSH_AUTH_SOCK
environment variable. IgnoreRhosts
- Specifies that .rhosts and
.shosts files will not be used in
RhostsRSAAuthentication
orHostbasedAuthentication
./etc/hosts.equiv and /etc/shosts.equiv are still used. The default is “yes”.
IgnoreUserKnownHosts
- Specifies whether
sshd(8) should ignore the user's
~/.ssh/known_hosts during
RhostsRSAAuthentication
orHostbasedAuthentication
. The default is “no”. IPQoS
- Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are “af11”, “af12”, “af13”, “af21”, “af22”, “af23”, “af31”, “af32”, “af33”, “af41”, “af42”, “af43”, “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”, “ef”, “lowdelay”, “throughput”, “reliability”, or a numeric value. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automatically selected for interactive sessions and the second for non-interactive sessions. The default is “lowdelay” for interactive sessions and “throughput” for non-interactive sessions.
KbdInteractiveAuthentication
- Specifies whether to allow keyboard-interactive authentication. The
argument to this keyword must be “yes” or
“no”. The default is to use whatever value
ChallengeResponseAuthentication
is set to (by default “yes”). 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. The default is “no”. KerberosGetAFSToken
- If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before accessing the user's home directory. The default is “no”.
KerberosOrLocalPasswd
- If password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as /etc/passwd. The default is “yes”.
KerberosTicketCleanup
- Specifies whether to automatically destroy the user's ticket cache file on logout. The default is “yes”.
KexAlgorithms
- Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms
must be comma-separated. The default is
curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group-exchange-sha1, diffie-hellman-group14-sha1, diffie-hellman-group1-sha1
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(8) should listen on. The following forms may be used:
ListenAddress
host|IPv4_addr|IPv6_addrListenAddress
host|IPv4_addr:portListenAddress
[host|IPv6_addr]:port
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. MultipleListenAddress
options are permitted. Additionally, anyPort
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(8). 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 algorithms that contain
“-etm” calculate the MAC after encryption
(encrypt-then-mac). These are considered safer and their use recommended.
The default is:
hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com, umac-64-etm@openssh.com,umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com, hmac-md5-96-etm@openssh.com, hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com, hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 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 anotherMatch
line or the end of the file. If a keyword appears in multipleMatch
blocks that are satisified, only the first instance of the keyword is applied.The arguments to
Match
are one or more criteria-pattern pairs or the single tokenAll
which matches all criteria. The available criteria areUser
,Group
,Host
,LocalAddress
,LocalPort
, andAddress
. The match patterns may consist of single entries or comma-separated lists and may use the wildcard and negation operators described in the PATTERNS section of ssh_config(5).The patterns in an
Address
criteria may additionally contain addresses to match in CIDR address/masklen format, e.g. “192.0.2.0/24” or “3ffe:ffff::/32”. Note that 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 may be used on the lines following a
Match
keyword. Available keywords areAcceptEnv
,AllowAgentForwarding
,AllowGroups
,AllowTcpForwarding
,AllowUsers
,AuthenticationMethods
,AuthorizedKeysCommand
,AuthorizedKeysCommandUser
,AuthorizedKeysFile
,AuthorizedPrincipalsFile
,Banner
,ChrootDirectory
,DenyGroups
,DenyUsers
,ForceCommand
,GatewayPorts
,GSSAPIAuthentication
,HostbasedAuthentication
,HostbasedUsesNameFromPacketOnly
,KbdInteractiveAuthentication
,KerberosAuthentication
,MaxAuthTries
,MaxSessions
,PasswordAuthentication
,PermitEmptyPasswords
,PermitOpen
,PermitRootLogin
,PermitTTY
,PermitTunnel
,PubkeyAuthentication
,RekeyLimit
,RhostsRSAAuthentication
,RSAAuthentication
,X11DisplayOffset
,X11Forwarding
andX11UseLocalHost
. 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.
MaxSessions
- Specifies the maximum number of open sessions permitted per network connection. The default is 10.
MaxStartups
- Specifies the maximum number of concurrent unauthenticated connections to
the SSH daemon. Additional connections will be dropped until
authentication succeeds or the
LoginGraceTime
expires for a connection. The default is 10:30:100.Alternatively, random early drop can be enabled by specifying the three colon separated values “start:rate:full” (e.g. "10:30:60"). sshd(8) 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 “yes”.
PermitEmptyPasswords
- When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is “no”.
PermitOpen
- Specifies the destinations to which TCP port forwarding is permitted. The
forwarding specification must be one of the following forms:
PermitOpen
host:portPermitOpen
IPv4_addr:portPermitOpen
[IPv6_addr]:port
Multiple forwards may be specified by separating them with whitespace. An argument of “any” can be used to remove all restrictions and permit any forwarding requests. An argument of “none” can be used to prohibit all forwarding requests. By default all port forwarding requests are permitted.
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 “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 log in.
PermitTunnel
- Specifies whether tun(4) device forwarding is allowed. The argument must be “yes”, “point-to-point” (layer 3), “ethernet” (layer 2), or “no”. Specifying “yes” permits both “point-to-point” and “ethernet”. The default is “no”.
PermitTTY
- Specifies whether pty(4) allocation is permitted. The default is “yes”.
PermitUserEnvironment
- Specifies whether ~/.ssh/environment and
environment=
options in ~/.ssh/authorized_keys are processed by sshd(8). The default is “no”. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such asLD_PRELOAD
. PidFile
- Specifies the file that contains the process ID of the SSH daemon. The default is /var/run/sshd.pid.
Port
- Specifies the port number that
sshd(8) listens on. The default is 22. Multiple options of this
type are permitted. See also
ListenAddress
. PrintLastLog
- Specifies whether sshd(8) 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(8) 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(8) 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.
RekeyLimit
- Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated, optionally followed a maximum amount of time
that may pass before the session key is renegotiated. The first argument
is specified in bytes and may have a suffix of ‘K’,
‘M’, or ‘G’ to indicate Kilobytes, Megabytes,
or Gigabytes, respectively. The default is between ‘1G’ and
‘4G’, depending on the cipher. The optional second value is
specified in seconds and may use any of the units documented in the
TIME FORMATS section. The default
value for
RekeyLimit
is “default none”, which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done. This option applies to protocol version 2 only. RevokedKeys
- Specifies revoked public keys. Keys listed in this file will be refused for public key authentication. Note that if this file is not readable, then public key authentication will be refused for all users. Keys may be specified as a text file, listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1). For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
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 1024.
StrictModes
- Specifies whether
sshd(8) 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”. Note that this does
not apply to
ChrootDirectory
, whose permissions and ownership are checked unconditionally. Subsystem
- Configures an external subsystem (e.g. file transfer daemon). Arguments
should be a subsystem name and a command (with optional arguments) to
execute upon subsystem request.
The command sftp-server(8) implements the “sftp” file transfer subsystem.
Alternately the name “internal-sftp” implements an in-process “sftp” server. This may simplify configurations using
ChrootDirectory
to force a different filesystem root on clients.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(8). 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”.
TrustedUserCAKeys
- Specifies a file containing public keys of certificate authorities that
are trusted to sign user certificates for authentication. Keys are listed
one per line; empty lines and comments starting with
‘
#
’ are allowed. If a certificate is presented for authentication and has its signing CA key listed in this file, then it may be used for authentication for any user listed in the certificate's principals list. Note that certificates that lack a list of principals will not be permitted for authentication usingTrustedUserCAKeys
. For more details on certificates, see the CERTIFICATES section in ssh-keygen(1). UseDNS
- Specifies whether sshd(8) 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. IfUsePrivilegeSeparation
is specified, it will be disabled after authentication. UsePrivilegeSeparation
- Specifies whether
sshd(8) 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”. If
UsePrivilegeSeparation
is set to “sandbox” then the pre-authentication unprivileged process is subject to additional restrictions. VersionAddendum
- Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection. The default is “none”.
X11DisplayOffset
- Specifies the first display number available for sshd(8)'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
“no”.
When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd(8) proxy display is configured to listen on the wildcard address (see
X11UseLocalhost
below), though 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 forForwardX11
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(8) 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(8) 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:
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(8). This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable.
SEE ALSO
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 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.