If you want to set the keep alive for the server, add this to /etc/ssh/sshdconfig: ClientAliveInterval 60 ClientAliveCountMax 2 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.
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. For each keyword, the first obtained value will be used. 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
SendEnv
and SetEnv
in ssh_config(5) for how to configure the client. The TERM
environment variable is always accepted whenever the client requests a pseudo-terminal as it is required by the protocol. 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
any
(the default), inet
(use IPv4 only), or inet6
(use IPv6 only).AllowAgentForwarding
- Is there a SSH config file (/.ssh/config) information page with all attributes you can have in that file? Who is managing the SSH protocol and its implementation in Mac (dont know what ssh client.
- If the key file in question is specified in the ssh config file, please include the relevant line(s) from the config file as well. – Kenster Apr 29 '15 at 10:10 To me was 'bad permissions' – insign May 25 '17 at 20:23.
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
DenyGroups
, AllowGroups
. See PATTERNS in ssh_config(5) for more information on patterns.
AllowStreamLocalForwarding
yes
(the default) or all
to allow StreamLocal forwarding, no
to prevent all StreamLocal forwarding, local
to allow local (from the perspective of ssh(1)) forwarding only or remote
to allow remote forwarding only. Note that disabling StreamLocal forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.AllowTcpForwarding
yes
(the default) 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. 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
DenyUsers
, AllowUsers
. See PATTERNS in ssh_config(5) for more information on patterns.
AuthenticationMethods
any
to indicate the default behaviour of accepting any single authentication method. If the default is overridden, then successful authentication requires completion of every method in at least one of these lists. For example, '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.
If the publickey method is listed more than once, sshd(8) verifies that keys that have been used successfully are not reused for subsequent authentications. For example, 'publickey,publickey' requires successful authentication using two different public keys.
Note that each authentication method listed should also be explicitly enabled in the configuration.
The available authentication methods are: 'gssapi-with-mic', 'hostbased', 'keyboard-interactive', 'none' (used for access to password-less accounts when PermitEmptyPasswords
is enabled), 'password' and 'publickey'.
AuthorizedKeysCommand
AuthorizedKeysCommand
accept the tokens described in the TOKENS section. If no arguments are specified then the username of the target user is used. The program should produce on standard output zero or more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). AuthorizedKeysCommand
is tried after the usual AuthorizedKeysFile
files and will not be executed if a matching key is found there. By default, no AuthorizedKeysCommand
is run.
AuthorizedKeysCommandUser
AuthorizedKeysCommand
is run. It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands. If AuthorizedKeysCommand
is specified but AuthorizedKeysCommandUser
is not, then sshd(8) will refuse to start.AuthorizedKeysFile
AuthorizedKeysFile
accept the tokens described in the TOKENS section. 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. Alternately this option may be set to none
to skip checking for user keys in files. The default is '.ssh/authorized_keys .ssh/authorized_keys2'.AuthorizedPrincipalsCommand
AuthorizedPrincipalsFile
. The program must be owned by root, not writable by group or others and specified by an absolute path. Arguments to AuthorizedPrincipalsCommand
accept the tokens described in the TOKENS section. If no arguments are specified then the username of the target user is used. The program should produce on standard output zero or more lines of AuthorizedPrincipalsFile
output. If either AuthorizedPrincipalsCommand
or AuthorizedPrincipalsFile
is specified, then certificates offered by the client for authentication must contain a principal that is listed. By default, no AuthorizedPrincipalsCommand
is run.
AuthorizedPrincipalsCommandUser
AuthorizedPrincipalsCommand
is run. It is recommended to use a dedicated user that has no other role on the host than running authorized principals commands. If AuthorizedPrincipalsCommand
is specified but AuthorizedPrincipalsCommandUser
is not, then sshd(8) will refuse to start.AuthorizedPrincipalsFile
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. Arguments to AuthorizedPrincipalsFile
accept the tokens described in the TOKENS section. 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 in TrustedUserCAKeys
and is not consulted for certification authorities trusted via ~/.ssh/authorized_keys, though the principals=
key option offers a similar facility (see sshd(8) for details).
Banner
none
then no banner is displayed. By default, no banner is displayed.CASignatureAlgorithms
Certificates signed using other algorithms will not be accepted for public key or host-based authentication.
ChallengeResponseAuthentication
yes
.ChrootDirectory
ChrootDirectory
accept the tokens described in the TOKENS section. 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), 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 may require /dev/log inside the chroot directory on some operating systems (see sftp-server(8) for details).
For safety, it is very important that the directory hierarchy be prevented from modification by other processes on the system (especially those outside the jail). Misconfiguration can lead to unsafe environments which sshd(8) cannot detect.
The default is none
, indicating not to chroot(2).
Ciphers
Ssh Config File Mac Location
Specifies the ciphers allowed. Multiple ciphers must be comma-separated. If the specified list begins with a ‘+’ character, then the specified ciphers will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified ciphers (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified ciphers will be placed at the head of the default set.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
- chacha20-poly1305@openssh.com
The default is:
The list of available ciphers may also be obtained using 'ssh -Q cipher'.
ClientAliveCountMax
TCPKeepAlive
. 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 unresponsive. The default value is 3. If ClientAliveInterval
is set to 15, and ClientAliveCountMax
is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. Setting a zero ClientAliveCountMax
disables connection termination.
ClientAliveInterval
Compression
yes
, delayed
(a legacy synonym for yes
) or no
. The default is yes
.DenyGroups
DenyGroups
, AllowGroups
. See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
DenyUsers
, AllowUsers
. See PATTERNS in ssh_config(5) for more information on patterns.
DisableForwarding
ExposeAuthInfo
SSH_USER_AUTH
environment variable. The default is no
.FingerprintHash
md5
and sha256
. The default is sha256
.ForceCommand
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 a Match
block. The command originally supplied by the client is available in the SSH_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 with ChrootDirectory
. The default is none
.GatewayPorts
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
no
.GSSAPICleanupCredentials
yes
.GSSAPIStrictAcceptorCheck
yes
then the client must authenticate against the host service on the current hostname. If set to no
then the client may authenticate against any service key stored in the machine's default store. This facility is provided to assist with operation on multi homed machines. The default is yes
.HostbasedAcceptedAlgorithms
The list of available signature algorithms may also be obtained using 'ssh -Q HostbasedAcceptedAlgorithms'. This was formerly named HostbasedAcceptedKeyTypes.
HostbasedAuthentication
no
.HostbasedUsesNameFromPacketOnly
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
HostKey
. The default behaviour of sshd(8) is not to load any certificates.HostKey
Note that sshd(8) will refuse to use a file if it is group/world-accessible and that the HostKeyAlgorithms
option restricts which of the keys are actually used by sshd(8).
It is possible to have multiple host key files. 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
SSH_AUTH_SOCK
environment variable.HostKeyAlgorithms
The list of available signature algorithms may also be obtained using 'ssh -Q HostKeyAlgorithms'.
IgnoreRhosts
HostbasedAuthentication
. The system-wide /etc/hosts.equiv and /etc/shosts.equivSsh Config File Mac Pro
are still used regardless of this setting.Accepted values are yes
(the default) to ignore all per-user files, shosts-only
to allow the use of .shosts but to ignore .rhosts or no
to allow both .shosts and rhosts.
IgnoreUserKnownHosts
HostbasedAuthentication
and use only the system-wide known hosts file /etc/ssh/known_hosts. The default is “no”.Include
Include
directive may appear inside a Match
block to perform conditional inclusion.IPQoS
af11
, af12
, af13
, af21
, af22
, af23
, af31
, af32
, af33
, af41
, af42
, af43
, cs0
, cs1
, cs2
, cs3
, cs4
, cs5
, cs6
, cs7
, ef
, le
, lowdelay
, throughput
, reliability
, a numeric value, or none
to use the operating system default. 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 af21
(Low-Latency Data) for interactive sessions and cs1
(Lower Effort) for non-interactive sessions.KbdInteractiveAuthentication
yes
or no
. The default is to use whatever value ChallengeResponseAuthentication
is set to (by default yes
).KerberosAuthentication
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
no
.KerberosOrLocalPasswd
yes
.KerberosTicketCleanup
yes
.KexAlgorithms
- curve25519-sha256
- curve25519-sha256@libssh.org
- diffie-hellman-group1-sha1
- diffie-hellman-group14-sha1
- diffie-hellman-group14-sha256
- diffie-hellman-group16-sha512
- diffie-hellman-group18-sha512
- diffie-hellman-group-exchange-sha1
- diffie-hellman-group-exchange-sha256
- ecdh-sha2-nistp256
- ecdh-sha2-nistp384
- ecdh-sha2-nistp521
- sntrup761x25519-sha512@openssh.com
The default is:
The list of available key exchange algorithms may also be obtained using 'ssh -Q KexAlgorithms'.
ListenAddress
ListenAddress
hostname|address [rdomain
domain]ListenAddress
hostname:port [rdomain
domain]ListenAddress
IPv4_address:port [rdomain
domain]ListenAddress
[hostname|address]:port [rdomain
domain]
The optional rdomain
qualifier requests sshd(8) listen in an explicit routing domain. If port is not specified, sshd will listen on the address and all Port
options specified. The default is to listen on all local addresses on the current default routing domain. Multiple ListenAddress
options are permitted. For more information on routing domains, see rdomain(4).
LoginGraceTime
LogLevel
LogVerbose
would enable detailed logging for line 1000 of kex.c, everything in the kex_exchange_identification
() function, and all code in the packet.c file. This option is intended for debugging and no overrides are enabled by default.
MACs
The algorithms that contain '-etm' calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended. The supported MACs are:
- hmac-md5
- hmac-md5-96
- hmac-sha1
- hmac-sha1-96
- hmac-sha2-256
- hmac-sha2-512
- umac-64@openssh.com
- umac-128@openssh.com
- hmac-md5-etm@openssh.com
- hmac-md5-96-etm@openssh.com
- hmac-sha1-etm@openssh.com
- hmac-sha1-96-etm@openssh.com
- hmac-sha2-256-etm@openssh.com
- hmac-sha2-512-etm@openssh.com
- umac-64-etm@openssh.com
- umac-128-etm@openssh.com
The default is:
The list of available MAC algorithms may also be obtained using 'ssh -Q mac'.
Match
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. If a keyword appears in multiple Match
blocks that are satisfied, only the first instance of the keyword is applied. The arguments to Match
are one or more criteria-pattern pairs or the single token All
which matches all criteria. The available criteria are User
, Group
, Host
, LocalAddress
, LocalPort
, RDomain
, and Address
(with RDomain
representing the rdomain(4) on which the connection was received).
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, such as 192.0.2.0/24 or 2001:db8::/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 are AcceptEnv
, AllowAgentForwarding
, AllowGroups
, AllowStreamLocalForwarding
, AllowTcpForwarding
, AllowUsers
, AuthenticationMethods
, AuthorizedKeysCommand
, AuthorizedKeysCommandUser
, AuthorizedKeysFile
, AuthorizedPrincipalsCommand
, AuthorizedPrincipalsCommandUser
, AuthorizedPrincipalsFile
, Banner
, ChrootDirectory
, ClientAliveCountMax
, ClientAliveInterval
, DenyGroups
, DenyUsers
, DisableForwarding
, ForceCommand
, GatewayPorts
, GSSAPIAuthentication
, HostbasedAcceptedAlgorithms
, HostbasedAuthentication
, HostbasedUsesNameFromPacketOnly
, IgnoreRhosts
, Include
, IPQoS
, KbdInteractiveAuthentication
, KerberosAuthentication
, LogLevel
, MaxAuthTries
, MaxSessions
, PasswordAuthentication
, PermitEmptyPasswords
, PermitListen
, PermitOpen
, PermitRootLogin
, PermitTTY
, PermitTunnel
, PermitUserRC
, PubkeyAcceptedAlgorithms
, PubkeyAuthentication
, RekeyLimit
, RevokedKeys
, RDomain
, SetEnv
, StreamLocalBindMask
, StreamLocalBindUnlink
, TrustedUserCAKeys
, X11DisplayOffset
, X11Forwarding
and X11UseLocalhost
.
MaxAuthTries
MaxSessions
MaxSessions
to 1 will effectively disable session multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while still permitting forwarding. The default is 10.MaxStartups
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).
ModuliFile
PasswordAuthentication
yes
.PermitEmptyPasswords
no
.PermitListen
PermitListen
portPermitListen
host:port
Multiple permissions may be specified by separating them with whitespace. An argument of any
can be used to remove all restrictions and permit any listen requests. An argument of none
can be used to prohibit all listen requests. The host name may contain wildcards as described in the PATTERNS section in ssh_config(5). The wildcard ‘*’ can also be used in place of a port number to allow all ports. By default all port forwarding listen requests are permitted. Note that the GatewayPorts
option may further restrict which addresses may be listened on. Note also that ssh(1) will request a listen host of “localhost” if no listen host was specifically requested, and this name is treated differently to explicit localhost addresses of “127.0.0.1” and “::1”.
PermitOpen
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. The wildcard ‘*’ can be used for host or port to allow all hosts or ports respectively. Otherwise, no pattern matching or address lookups are performed on supplied names. By default all port forwarding requests are permitted.
PermitRootLogin
yes
, prohibit-password
, forced-commands-only
, or no
. The default is prohibit-password
. If this option is set to prohibit-password
(or its deprecated alias, without-password
), password and keyboard-interactive authentication are 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.
PermitTTY
yes
.PermitTunnel
yes
, point-to-point
(layer 3), ethernet
(layer 2), or no
. Specifying yes
permits both point-to-point
and ethernet
. The default is no
. Independent of this setting, the permissions of the selected tun(4) device must allow access to the user.
PermitUserEnvironment
environment=
options in ~/.ssh/authorized_keys are processed by sshd(8). Valid options are yes
, no
or a pattern-list specifying which environment variable names to accept (for example 'LANG,LC_*'). The default is no
. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD
.PermitUserRC
yes
.PerSourceMaxStartups
MaxStartups
, whichever is lower. The default is none
.PerSourceNetBlockSize
32:128
, which means each address is considered individually.PidFile
none
to not write one. The default is /var/run/sshd.pid.Port
ListenAddress
.PrintLastLog
yes
.PrintMotd
yes
.PubkeyAcceptedAlgorithms
The list of available signature algorithms may also be obtained using 'ssh -Q PubkeyAcceptedAlgorithms'.
PubkeyAuthOptions
none
(the default; indicating no additional options are enabled), touch-required
and verify-required
. The touch-required
option causes public key authentication using a FIDO authenticator algorithm (i.e. ecdsa-sk
or ed25519-sk
) to always require the signature to attest that a physically present user explicitly confirmed the authentication (usually by touching the authenticator). By default, sshd(8) requires user presence unless overridden with an authorized_keys option. The touch-required
flag disables this override.
The verify-required
option requires a FIDO key signature attest that the user was verified, e.g. via a PIN.
Neither the touch-required
or verify-required
options have any effect for other, non-FIDO, public key types.
PubkeyAuthentication
yes
.RekeyLimit
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.RevokedKeys
none
to not use one. 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).RDomain
%D
, then the domain in which the incoming connection was received will be applied.SecurityKeyProvider
SetEnv
SetEnv
override the default environment and any variables specified by the user via AcceptEnv
or PermitUserEnvironment
.StreamLocalBindMask
The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files.
StreamLocalBindUnlink
StreamLocalBindUnlink
is not enabled, sshd
will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-domain socket file. The argument must be yes
or no
. The default is no
.
StrictModes
yes
. Note that this does not apply to ChrootDirectory
, whose permissions and ownership are checked unconditionally.Subsystem
The command sftp-server
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.
SyslogFacility
TCPKeepAlive
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
none
to not use one. 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 using TrustedUserCAKeys
. For more details on certificates, see the CERTIFICATES section in ssh-keygen(1).UseDNS
If this option is set to no
(the default) then only addresses and not host names may be used in ~/.ssh/authorized_keysfrom
and sshd_config
Match
Host
directives.
VersionAddendum
none
.X11DisplayOffset
X11Forwarding
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
), 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 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.
X11UseLocalhost
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
none
to not use one. The default is /usr/X11R6/bin/xauth.Overview
The X Window System (also known as X11, or just X) is a software package and network protocol that lets you interact locally, using your personal computer's display, mouse, and keyboard, with the graphical user interface (GUI) of an application running on a remote networked computer.
You can use X forwarding in an SSH session on your personal computer to securely run graphical applications (X clients) installed on the Indiana University research supercomputers.
Requirements
For X forwarding in SSH to work, your personal computer must be running an X server program. The X server program manages the interaction between the remote application (the X client) and your computer's hardware.
Most Linux distributions have the X server installed, but if your personal computer is running Windows or macOS, you will most likely need to install and run an X server application. For example:
- For Windows, download and install Xming. For X forwarding to work, you'll need to start Xming before connecting to the remote system with your SSH client (for example, PuTTY).
- For macOS, download and install XQuartz. For X forwarding to work, you'll need to start XQuartz before making an SSH connection to the remote system. Once XQuartz launches, you can use X forwarding with SSH from the Terminal or from the
xterm
application in XQuartz.
Additionally, your personal computer's SSH terminal application must have X11 forwarding enabled:
- In Linux, the SSH terminal supports X forwarding by default.
- In macOS, you may need to edit your
ssh_config
file (typically found at/etc/ssh/ssh_config
or~/.ssh/config
) if you have trouble using X forwarding. Ifssh_config
includes#X11Forwardingno
(or justX11Forwarding no
), uncomment out the line (remove the leading#
), and change it toX11Forwarding yes
. - In PuTTY for Windows, you can enable X forwarding in new or saved SSH sessions by selecting Enable X11 forwarding in the 'PuTTY Configuration' window (Connection > SSH > X11).
Also, the remote computer's SSH application must be configured to accept X server connections. The IU research supercomputers all have SSH configured to allow X forwarding (trusted mode only).
Use SSH with X forwarding
Linux or macOS
To use SSH with X forwarding on your Linux or macOS personal computer to run an X client application installed on an IU research supercomputer:
- Open your SSH terminal client.
- On the command line, enter (replacing
username
with your IU username):- For Big Red 3:
- For Carbonate:
- Log in with your IU passphrase.
To test if X forwarding is working, try running xclock
; on the command line, enter:
If X forwarding is working, the xclock
graphical clock will appear on your personal computer's desktop.
PuTTY for Windows
To use SSH with X forwarding in PuTTY for Windows:
- Launch your X server application (for example, Xming).
- Make sure your connection settings for the remote system have Enable X11 forwarding selected; in the 'PuTTY Configuration' window, see Connection > SSH > X11.
- Open an SSH session to the desired remote system:
System Hostname Big Red 3 bigred3.uits.iu.edu
Carbonate carbonate.uits.iu.edu
- Log in normally with your IU username and passphrase.
To test if X forwarding is working, try running xclock
; on the command line, enter:
If X forwarding is working, the xclock
graphical clock will appear on your personal computer's desktop.
Use X forwarding for interactive sessions
On Carbonate
On Carbonate, if your interactive session requires:
- Less than 20 minutes of processor time: Connect via SSH with X forwarding enabled, and then launch the X client from the command line.
- More than 20 minutes of processor time: Submit a request for an interactive job via the batch system; for help, see the Interactive jobs section of Use Slurm to submit and manage jobs on high performance computing systems.
On Big Red 3
On Big Red 3, submit a request for an interactive job with X11 forwarding in the debug or general partition, and then launch your X application from a compute node:
- SSH to Big Red 3 with X forwarding enabled.
- On the command line, use the Slurm
srun
command with the--x11
flag to request an X11-enabled interactive session in the debug or general partition; for example:For more about using the Slurm
srun
command, see Use Slurm to submit and manage jobs on high performance computing systems. - Once you are placed on a compute node, you can launch graphical X applications (for example,
xterm
), as well as your own binaries, from the command line. Depending on the application and your~/.modules
file, you may need to load the module for the desired X client before launching the application.
Get help
Research computing support at IU is provided by the Research Technologies division of UITS. To ask a question or get help regarding Research Technologies services, including IU's research supercomputers and research storage systems, and the scientific, statistical, and mathematical applications available on those systems, contact UITS Research Technologies. For service-specific support contact information, see Research computing support at IU.