SSH_CONFIG(5)

📦

version 10.2p1 · 5 versions

🌈 SSH_CONFIG(5) 🌈

File Formats Manual. Bestie, we are doing the OpenSSH client config file today and honestly? It's giving main character energy. 💅

✨ NAME ✨

ssh_config — OpenSSH client configuration file

🦋 DESCRIPTION 🦋

okay so ssh(1) pulls its configuration data from a bunch of places, and the order matters, because first one wins. Here's the lineup, in order:

command-line options
user's configuration file (~/.ssh/config)
GSSAPI configuration file ($HOME/.ssh/config.gssapi)
Kerberos configuration file ($HOME/.ssh/config.krb)
system-wide configuration file (/etc/gsissh/ssh_config)

hpnssh(1) is fully compatible with OpenSSH's ssh and uses the same configuration format, directives, and options. hpnssh(1) can use the same configuration file as ssh(1) though the inverse is not necessarily true. That's because hpnssh(1) has some additional options which are discussed below. So basically, users should either not use hpnssh(1) specific directives in their default ssh_config files, or make sure they always use a custom config file. Pick your fighter. 🫶

Unless noted otherwise, for each parameter, the first obtained value will be used. The configuration files contain sections separated by Host specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is usually the one given on the command line (see the CanonicalizeHostname option for exceptions).

Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end. Yes bestie, that's the opposite of how your brain wants to do it. Trust. 😭

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. Configuration options may be separated by whitespace or optional whitespace and exactly one ‘=’; the latter format is useful to avoid the need to quote whitespace when specifying configuration options using the ssh, scp, and sftp -o option. ✨

The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):

Host — Restricts the following declarations (up to the next Host or Match keyword) to be only for those hosts that match one of the patterns given after the keyword. If more than one pattern is provided, they should be separated by whitespace. A single ‘*’ as a pattern can be used to provide global defaults for all hosts. The host is usually the hostname argument given on the command line (see the CanonicalizeHostname keyword for exceptions).

A pattern entry may be negated by prefixing it with an exclamation mark (‘!’). If a negated entry is matched, then the Host entry is ignored, regardless of whether any other patterns on the line match. Negated matches are therefore useful to provide exceptions for wildcard matches.

See “PATTERNS” for more information on patterns.

Match — Restricts the following declarations (up to the next Host or Match keyword) to be used only when the conditions following the Match keyword are satisfied. Match conditions are specified using one or more criteria or the single token all which always matches. The available criteria keywords are: canonical, final, exec, localnetwork, host, originalhost, tagged, command, user, localuser, and version. The all criteria must appear alone or immediately after canonical or final. Other criteria may be combined arbitrarily. All criteria but all, canonical, and final require an argument. Criteria may be negated by prepending an exclamation mark (‘!’).

The canonical keyword matches only when the configuration file is being re-parsed after hostname canonicalization (see the CanonicalizeHostname option). This may be useful to specify conditions that work with canonical host names only.

The final keyword requests that the configuration be re-parsed (regardless of whether CanonicalizeHostname is enabled), and matches only during this final pass. If CanonicalizeHostname is enabled, then canonical and final match during the same pass.

The exec keyword executes the specified command under the user's shell. If the command returns a zero exit status then the condition is considered true. Commands containing whitespace characters must be quoted. Arguments to exec accept the tokens described in the “TOKENS” section.

The localnetwork keyword matches the addresses of active local network interfaces against the supplied list of networks in CIDR format. This may be convenient for varying the effective configuration on devices that roam between networks. Note that network address is not a trustworthy criteria in many situations (e.g. when the network is automatically configured using DHCP) and so caution should be applied if using it to control security-sensitive configuration. 👀

The other keywords' criteria must be single entries or comma-separated lists and may use the wildcard and negation operators described in the “PATTERNS” section.

The criteria for the host keyword are matched against the target hostname, after any substitution by the Hostname or CanonicalizeHostname options. The originalhost keyword matches against the hostname as it was specified on the command-line.

The tagged keyword matches a tag name specified by a prior Tag directive or on the ssh(1) command-line using the -P flag. The command keyword matches the remote command that has been requested, or the subsystem name that is being invoked (e.g. "sftp" for an SFTP session). The empty string will match the case where a command or tag has not been specified, i.e. ‘Match tag ""’. The version keyword matches against the version string of ssh(1), for example “OpenSSH_10.0”.

The user keyword matches against the target username on the remote host. The localuser keyword matches against the name of the local user running ssh(1) (this keyword may be useful in system-wide ssh_config files).

Finally, the sessiontype keyword matches the requested session type, which may be one of shell for interactive sessions, exec for command execution sessions, subsystem for subsystem invocations such as sftp(1), or none for transport-only sessions, such as when ssh(1) is started with the -N flag.

AddKeysToAgent — Specifies whether keys should be automatically added to a running ssh-agent(1). If this option is set to yes and a key is loaded from a file, the key and its passphrase are added to the agent with the default lifetime, as if by ssh-add(1). If this option is set to ask, ssh(1) will require confirmation using the SSH_ASKPASS program before adding a key (see ssh-add(1) for details). If this option is set to confirm, each use of the key must be confirmed, as if the -c option was specified to ssh-add(1). If this option is set to no, no keys are added to the agent. Alternately, this option may be specified as a time interval using the format described in the “TIME FORMATS” section of sshd_config(5) to specify the key's lifetime in ssh-agent(1), after which it will automatically be removed. The argument must be no (the default), yes, confirm (optionally followed by a time interval), ask or a time interval. 🔑
AddressFamily — Specifies which address family to use when connecting. Valid arguments are any (the default), inet (use IPv4 only), or inet6 (use IPv6 only).
BatchMode — If set to yes, user interaction such as password prompts and host key confirmation requests will be disabled. This option is useful in scripts and other batch jobs where no user is present to interact with ssh(1). The argument must be yes or no (the default). 🤖
BindAddress — Use the specified address on the local machine as the source address of the connection. Only useful on systems with more than one address.
BindInterface — Use the address of the specified interface on the local machine as the source address of the connection.
CanonicalDomains — When CanonicalizeHostname is enabled, this option specifies the list of domain suffixes in which to search for the specified destination host.
CanonicalizeFallbackLocal — Specifies whether to fail with an error when hostname canonicalization fails. The default, yes, will attempt to look up the unqualified hostname using the system resolver's search rules. A value of no will cause ssh(1) to fail instantly if CanonicalizeHostname is enabled and the target hostname cannot be found in any of the domains specified by CanonicalDomains.
CanonicalizeHostname — Controls whether explicit hostname canonicalization is performed. The default, no, is not to perform any name rewriting and let the system resolver handle all hostname lookups. If set to yes then, for connections that do not use a ProxyCommand or ProxyJump, ssh(1) will attempt to canonicalize the hostname specified on the command line using the CanonicalDomains suffixes and CanonicalizePermittedCNAMEs rules. If CanonicalizeHostname is set to always, then canonicalization is applied to proxied connections too.

If this option is enabled, then the configuration files are processed again using the new target name to pick up any new configuration in matching Host and Match stanzas. A value of none disables the use of a ProxyJump host. 👑

CanonicalizeMaxDots — Specifies the maximum number of dot characters in a hostname before canonicalization is disabled. The default, 1, allows a single dot (i.e. hostname.subdomain).
CanonicalizePermittedCNAMEs — this one is all about ✨rules✨ for deciding whether CNAMEs get followed when we're canonicalizing hostnames. You give it one or more arguments shaped like source_domain_list:target_domain_list — where source_domain_list is a pattern-list of domains that are allowed to follow CNAMEs during canonicalization, and target_domain_list is a pattern-list of domains they're allowed to resolve to. 💫

Like, for example, "*.a.example.com:*.b.example.com,*.c.example.com" will let hostnames matching "*.a.example.com" get canonicalized to names over in the "*.b.example.com" or "*.c.example.com" domains. Cute, right?

And if you pass a single argument of "none", then literally no CNAMEs are considered for canonicalization. That's the default behaviour, bestie. 🤷♀️

CASignatureAlgorithms — okay so the default here is handled system-wide by crypto-policies(7). Info about the defaults, how to change them, and how to customize existing policies with sub-policies all lives in the manual page update-crypto-policies(8). 📖

It specifies which algorithms are allowed for signing of certificates by certificate authorities (CAs). If your list starts with a + character, then the algorithms you listed get appended to the default set instead of replacing them. If it starts with a - character, then those algorithms (wildcards included 👀) get removed from the default set instead of replacing them.

And ssh(1) will not accept host certificates signed using algorithms other than those you specified. No cap. 🚫

CertificateFile — points at a file from which the user's certificate gets read. You've gotta provide a corresponding private key separately to actually use this certificate — either from an IdentityFile directive or the -i flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider or SecurityKeyProvider. 🔑

Arguments to CertificateFile may use the tilde syntax to refer to a user's home directory, the tokens described in the “TOKENS” section, and environment variables as described in the “ENVIRONMENT VARIABLES” section. ✨

And yes you can have multiple certificate files specified in config files — they get tried in sequence. Multiple CertificateFile directives just add to the list of certificates used for authentication. Stack 'em up. 💅

ChannelTimeout — specifies whether and how quickly ssh(1) should close inactive channels. 🕰️ Timeouts are given as one or more “type=interval” pairs separated by whitespace, where the “type” has to be the special keyword “global” or a channel type name from the list below, optionally containing wildcard characters.

The timeout value “interval” is in seconds, or you can use any of the units documented in the “TIME FORMATS” section. So like, “session=5m” would make interactive sessions terminate after five minutes of inactivity. Specifying a zero value disables the inactivity timeout entirely. 🫶

The special timeout “global” applies to all active channels, taken together. Traffic on any active channel will reset the timeout, but when the timeout expires then all open channels get closed. Note that this global timeout is not matched by wildcards and must be specified explicitly. 👑

The available channel type names include:

agent-connection — open connections to ssh-agent(1).
direct-tcpip, direct-streamlocal@openssh.com — open TCP or Unix socket (respectively) connections that have been established from an ssh(1) local forwarding, i.e. LocalForward or DynamicForward.
forwarded-tcpip, forwarded-streamlocal@openssh.com — open TCP or Unix socket (respectively) connections that have been established to an sshd(8) listening on behalf of an ssh(1) remote forwarding, i.e. RemoteForward.
session — the interactive main session, including shell session, command execution, scp(1), sftp(1), etc.
tun-connection — open TunnelForward connections.
x11-connection — open X11 forwarding sessions.

Note that in all the above cases, terminating an inactive session does not guarantee to remove all resources associated with the session, e.g. shell processes or X11 clients relating to the session may continue to execute. 😭

Moreover, terminating an inactive channel or session does not necessarily close the SSH connection, nor does it prevent a client from requesting another channel of the same type. In particular, expiring an inactive forwarding session does not prevent another identical forwarding from being subsequently created. 💀

The default is not to expire channels of any type for inactivity.

CheckHostIP — if set to yes, ssh(1) will additionally check the host IP address in the known_hosts file. 🔍 This lets it detect if a host key changed due to DNS spoofing, and it'll add addresses of destination hosts to ~/.ssh/known_hosts in the process — regardless of the setting of StrictHostKeyChecking. If the option is set to no (the default), the check just won't be executed. Simple as that.
Ciphers — the default is handled system-wide by crypto-policies(7). Info about defaults, how to modify them, and how to customize existing policies with sub-policies lives in the manual page update-crypto-policies(8). 📖

This specifies the ciphers allowed and their order of preference. Multiple ciphers must be comma-separated. If your list starts with a + character, the ciphers you listed get appended to the built-in openssh default set instead of replacing them. If it starts with a - character, those ciphers (wildcards included 👀) get removed from the built-in openssh default set instead of replacing them. And if it starts with a ^ character, those ciphers get placed at the head of the built-in openssh default set. Iconic behaviour honestly. ✨

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

You can also get the list of available ciphers with ssh -Q cipher. Slay. 💅

ClearAllForwardings — says that all local, remote, and dynamic port forwardings specified in the configuration files or on the command line get cleared. 🧹 This option is primarily useful when used from the ssh(1) command line to clear port forwardings set in configuration files, and it's automatically set by scp(1) and sftp(1). The argument must be yes or no (the default).
Compression — specifies whether to use compression. The argument must be yes or no (the default). 🗜️
ConnectionAttempts — specifies the number of tries (one per second) to make before exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails. The default is 1. 🔁
ConnectTimeout — specifies the timeout (in seconds) used when connecting to the SSH server, instead of using the default system TCP timeout. This timeout is applied both to establishing the connection and to performing the initial SSH protocol handshake and key exchange. ⏳
ControlMaster — enables the sharing of multiple sessions over a single network connection. 🎀 When set to yes, ssh(1) will listen for connections on a control socket specified using the ControlPath argument. Additional sessions can connect to this socket using the same ControlPath with ControlMaster set to no (the default). These sessions will try to reuse the master instance's network connection rather than initiating new ones, but will fall back to connecting normally if the control socket does not exist, or is not listening. 💫

Setting this to ask will cause ssh(1) to listen for control connections, but require confirmation using ssh-askpass(1). If the ControlPath cannot be opened, ssh(1) will continue without connecting to a master instance.

X11 and ssh-agent(1) forwarding is supported over these multiplexed connections, however the display and agent forwarded will be the one belonging to the master connection — i.e. it is not possible to forward multiple displays or agents. 🙈

Two additional options allow for opportunistic multiplexing: try to use a master connection but fall back to creating a new one if one does not already exist. These options are auto and autoask. The latter requires confirmation like the ask option. 👑

ControlPath — specify the path to the control socket used for connection sharing as described in the ControlMaster section above, or the string none to disable connection sharing. Arguments to ControlPath may use the tilde syntax to refer to a user's home directory, the tokens described in the “TOKENS” section, and environment variables as described in the “ENVIRONMENT VARIABLES” section. ✨

It is recommended that any ControlPath used for opportunistic connection sharing include at least %h, %p, and %r (or alternatively %C) and be placed in a directory that is not writable by other users. This ensures that shared connections are uniquely identified. 🔒

ControlPersist — okay so when you use this with ControlMaster, it says the master connection should stay open in the background (just vibing, waiting for future client connections) after the initial client connection has been closed. If you set it to no (the default), then the master connection will not be placed into the background, and will close as soon as the initial client connection is closed. If you set it to yes or 0, then the master connection will remain in the background indefinitely (until killed or closed via a mechanism such as ssh -O exit). If you set it to a time in seconds, or a time in any of the formats documented in sshd_config(5), then the backgrounded master connection will automatically terminate after it has remained idle (with no client connections) for the specified time. 💅
DynamicForward — this specifies that a TCP port on the local machine be forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. ✨

The argument must be [bind_address:]port. IPv6 addresses can be specified by enclosing addresses in square brackets. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of localhost indicates that the listening port be bound for local use only, while an empty address or * indicates that the port should be available from all interfaces. 🦋

Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh(1) will act as a SOCKS server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports. 👑

EnableEscapeCommandline — enables the command line option in the EscapeChar menu for interactive sessions (default ~C). By default, the command line is disabled. 🎀
EnableSSHKeysign — setting this option to yes in the global client configuration file /etc/gsissh/ssh_config enables the use of the helper program ssh-keysign(8) during HostbasedAuthentication. The argument must be yes or no (the default). This option should be placed in the non-hostspecific section. See ssh-keysign(8) for more information. ssh-keysign should be installed explicitly. 🔑
EscapeChar — sets the escape character (default: ~). The escape character can also be set on the command line. The argument should be a single character, ^ followed by a letter, or none to disable the escape character entirely (making the connection transparent for binary data). 💫
ExitOnForwardFailure — specifies whether ssh(1) should terminate the connection if it cannot set up all requested dynamic, tunnel, local, and remote port forwardings, (e.g. if either end is unable to bind and listen on a specified port). Note that ExitOnForwardFailure does not apply to connections made over port forwardings and will not, for example, cause ssh(1) to exit if TCP connections to the ultimate forwarding destination fail. The argument must be yes or no (the default). 🚪
FallbackPort — specifies the port hpnssh should try to connect to if it fails connecting to the default hpnsshd port of 2222. The default is port 22. HPN-SSH only. 🌀
FingerprintHash — specifies the hash algorithm used when displaying key fingerprints. Valid options are: md5 and sha256 (the default). 🫶
ForkAfterAuthentication — requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies the StdinNull configuration option being set to "yes". The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm, which is the same as ssh host xterm if the ForkAfterAuthentication configuration option is set to "yes". 🌸

If the ExitOnForwardFailure configuration option is set to "yes", then a client started with the ForkAfterAuthentication configuration option being set to "yes" will wait for all remote port forwards to be successfully established before placing itself in the background. The argument to this keyword must be yes (same as the -f option) or no (the default). ✨

ForwardAgent — specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument may be yes, no (the default), an explicit path to an agent socket or the name of an environment variable (beginning with $) in which to find the path. 🧚

Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent. ⚠️

ForwardX11 — specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be yes or no (the default). 🌈

X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X11 authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring if the ForwardX11Trusted option is also enabled. 👀

ForwardX11Timeout — specify a timeout for untrusted X11 forwarding using the format described in the “TIME FORMATS” section of sshd_config(5). X11 connections received by ssh(1) after this time will be refused. Setting ForwardX11Timeout to zero will disable the timeout and permit X11 forwarding for the life of the connection. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed. ⏳
ForwardX11Trusted — if this option is set to yes, remote X11 clients will have full access to the original X11 display. 💖

If this option is set to no (the default), remote X11 clients will be considered untrusted and prevented from stealing or tampering with data belonging to trusted X11 clients. Furthermore, the xauth(1) token used for the session will be set to expire after 20 minutes. Remote clients will be refused access after this time. 🚫

See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients. 📜

GatewayPorts — specifies whether remote hosts are allowed to connect to local forwarded ports. By default, ssh(1) binds local port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify that ssh should bind local port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be yes or no (the default). 🌐
GlobalKnownHostsFile — specifies one or more files to use for the global host key database, separated by whitespace. The default is /etc/gsissh/ssh_known_hosts, /etc/gsissh/ssh_known_hosts2. 📁
GSSAPIAuthentication — specifies whether user authentication based on GSSAPI is allowed. The default is yes. 🎟️
GSSAPIClientIdentity — if set, specifies the GSSAPI client identity that ssh should use when connecting to the server. The default is unset, which means that the default identity will be used. 🪪
GSSAPIDelegateCredentials — forward (delegate) credentials to the server. The default is yes. 🫱
GSSAPIKeyExchange — specifies whether key exchange based on GSSAPI may be used. When using GSSAPI key exchange the server need not have a host key. The default is "yes". 🔄
GSSAPIRenewalForcesRekey — if set to "yes" then renewal of the client's GSSAPI credentials will force the rekeying of the ssh connection. With a compatible server, this will delegate the renewed credentials to a session on the server. 🔁

Checks are made to ensure that credentials are only propagated when the new credentials match the old ones on the originating client and where the receiving server still has the old set in its cache. 🧠

The default is "no". 🙅

For this to work GSSAPIKeyExchange needs to be enabled in the server and also used by the client. 🤝

GSSAPIServerIdentity — if set, specifies the GSSAPI server identity that ssh should expect when connecting to the server. The default is unset, which means that the expected GSSAPI server identity will be determined from the target hostname. 🏷️
GSSAPITrustDns — set to "yes" to indicate that the DNS is trusted to securely canonicalize the name of the host being connected to. If "no", the hostname entered on the command line will be passed untouched to the GSSAPI library. The default is "no". 📡
GSSAPIKexAlgorithms — the default is handled system-wide by crypto-policies(7). Information about defaults, how to modify the defaults and how to customize existing policies with sub-policies are present in manual page update-crypto-policies(8). 📚

The list of key exchange algorithms that are offered for GSSAPI key exchange. Possible values are:

gss-gex-sha1-
gss-group1-sha1-
gss-group14-sha1-
gss-group14-sha256-
gss-group16-sha512-
gss-nistp256-sha256-
gss-curve25519-sha256-

This option only applies to connections using GSSAPI. 🔐

The list of supported key exchange algorithms may also be obtained using "ssh -Q GSSAPIKexAlgorithms". 💻

HashKnownHosts — tells ssh(1) to hash host names and addresses when they get added to ~/.ssh/known_hosts. 🫶 These hashed names still work totally normally with ssh(1) and sshd(8), but if somebody ever peeks inside the file's contents, they won't be able to visually clock any identifying info. Default is no. Heads up though: names and addresses already chillin' in your known hosts files will not get converted automatically — you can hash them by hand with ssh-keygen(1) if you want. 💅
HappyEyes — set this to yes and the client uses the Happy Eyeballs connection method, as described by RFC 8305. ✨ So when you're connecting to a target that supports both IPv4 and IPv6, the client tries IPv6 first, waits a set time (250ms by default), and then tries IPv4. Whichever socket comes back first wins, periodt. 🌈 Note: this process may, rarely, produce false positives in some monitoring tools, because of the multiple connection attempts that happen before authentication. The default is no.
HappyDelay — how long the HappyEyes connection method should wait before trying IPv4 after that initial IPv6 connection. 🎀 This value is a positive integer of milliseconds, and the default is 250ms.
HostbasedAcceptedAlgorithms — the default here is handled system-wide by crypto-policies(7). 🧚 Info about the defaults, how to modify them, and how to customize existing policies with sub-policies all lives in the manual page update-crypto-policies(8).

This one specifies the signature algorithms used for hostbased authentication, as a comma-separated list of patterns. 💖 And the wild little trick: if the list you give begins with a + character, those signature algorithms get appended to the built-in openssh default set instead of replacing it. If it begins with a - character, those signature algorithms (wildcards included) get removed from the built-in openssh default set instead of replacing it. And if it begins with a ^ character, those signature algorithms get placed at the head of the built-in openssh default set. 👑

The -Q option of ssh(1) can list the supported signature algorithms for you. Also FYI, this used to be named HostbasedKeyTypes. 🔥

HostbasedAuthentication — specifies whether to try rhosts based authentication with public key authentication. The argument must be yes or no (the default). 🦋
HostKeyAlgorithms — the host key signature algorithms the client wants to use, in order of preference. 💜 Same little prefix magic as above: if the list begins with +, those signature algorithms are appended to the default set instead of replacing it. If it begins with -, those signature algorithms (wildcards included) are removed from the default set instead of replacing it. If it begins with ^, those signature algorithms are placed at the head of the default set. 🌸

The default for this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256

If hostkeys are known for the destination host, then this default gets modified to prefer their algorithms. 🫶

You can also grab the list of available signature algorithms with "ssh -Q HostKeyAlgorithms".

And ngl there's a whole dependency web here: the proposed HostKeyAlgorithms during KEX are limited to the set of algorithms defined in PubkeyAcceptedAlgorithms, so they're indirectly affected by system-wide crypto_policies(7). crypto_policies(7) can not handle the list of host key algorithms directly, because doing so would break the order given by the known_hosts file. 😭

HostKeyAlias — an alias to use instead of the real host name when looking up or saving the host key in the host key database files, and when validating host certificates. 🌟 Super useful for tunneling SSH connections, or for when multiple servers are running on a single host. 👑
Hostname — the real host name to log into. This lets you set up nicknames or abbreviations for hosts. 💖 Arguments to Hostname accept the tokens described in the “TOKENS” section. Numeric IP addresses are also permitted (both on the command line and in Hostname specifications). The default is the name given on the command line.
HPNDisabled — okay so in some situations, like transfers on a local area network, the impact of the HPN code actually produces a net decrease in performance. 💀 In those cases it's helpful to disable the HPN functionality. By default HPNDisabled is set to no. HPNSSH only. 🧚
IdentitiesOnly — tells ssh(1) to only use the configured authentication identity and certificate files (either the default files, or the ones explicitly configured in the ssh_config files or passed on the ssh(1) command-line), even if ssh-agent(1) or a PKCS11Provider or SecurityKeyProvider is offering more identities. 🙈 The argument to this keyword must be yes or no (the default). This option is intended for situations where ssh-agent offers many different identities. 💅
IdentityAgent — the Unix-domain socket used to communicate with the authentication agent. 🦋

This option overrides the SSH_AUTH_SOCK environment variable, and it can be used to select a specific agent. Setting the socket name to none disables the use of an authentication agent. If the string "SSH_AUTH_SOCK" is specified, the location of the socket will be read from the SSH_AUTH_SOCK environment variable. Otherwise, if the specified value begins with a $ character, then it's treated as an environment variable containing the location of the socket. ✨

Arguments to IdentityAgent may use the tilde syntax to refer to a user's home directory, the tokens described in the “TOKENS” section, and environment variables as described in the “ENVIRONMENT VARIABLES” section. 🌸

IdentityFile — specifies a file from which the user's ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 🔑 You can also specify a public key file, so that the corresponding private key loaded in ssh-agent(1) gets used when the private key file isn't present locally. The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519 and ~/.ssh/id_ed25519_sk. Additionally, any identities represented by the authentication agent will be used for authentication unless IdentitiesOnly is set. And if no certificates have been explicitly specified by CertificateFile, ssh(1) will try to load certificate information from the filename you get by appending -cert.pub to the path of a specified IdentityFile. 💜

Arguments to IdentityFile may use the tilde syntax to refer to a user's home directory, or the tokens described in the “TOKENS” section. Alternately, an argument of none may be used to indicate that no identity files should be loaded. 🎀

And yes you can have multiple identity files specified in configuration files — all these identities will be tried in sequence. Multiple IdentityFile directives add to the list of identities tried (which, btw, differs from how other configuration directives behave). 👑

IdentityFile may be used together with IdentitiesOnly to select which identities in an agent are offered during authentication. IdentityFile may also be used together with CertificateFile in order to provide any certificate also needed for authentication with the identity. 🫶

The authentication identity can also be specified in the form of a PKCS#11 URI starting with the string pkcs11:. There's support for a subset of the PKCS#11 URI as defined in RFC 7512 (implemented path arguments id, manufacturer, object, token and query arguments module-path and pin-value). The URI can not be in quotes. ⭐

IgnoreUnknown — a pattern-list of unknown options to be ignored if they're encountered during configuration parsing. 🔥 This can suppress errors if ssh_config contains options that are unrecognised by ssh(1). It's recommended that IgnoreUnknown be listed early in the configuration file, because it will not be applied to unknown options that appear before it. 💀
Include — include the specified configuration file(s). 🌈 You can specify multiple pathnames, and each pathname may contain glob(7) wildcards, tokens as described in the “TOKENS” section, environment variables as described in the “ENVIRONMENT VARIABLES” section and, for user configurations, shell-like ~ references to user home directories. Wildcards will be expanded and processed in lexical order. Files without absolute paths are assumed to be in ~/.ssh if included in a user configuration file, or /etc/ssh if included from the system configuration file. And the Include directive may appear inside a Match or Host block to perform conditional inclusion. ✨
IPQoS — sets the Differentiated Services Field Codepoint (DSCP) value for your connections 💅. Accepted values are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6, cs7, ef, le, a numeric value, or none to just use whatever your operating system default is.

This one's a lil extra — it can take one or two arguments, separated by whitespace.

One argument? It's used as the packet class unconditionally, periodt.
Two values? The first is automatically selected for interactive sessions and the second for non-interactive sessions.

The default is ef (Expedited Forwarding) for interactive sessions and none (the operating system default) for non-interactive sessions. If using the HappyEyes option these default to lowlatency and respectively.

KbdInteractiveAuthentication — specifies whether to use keyboard-interactive authentication. The argument to this keyword must be yes (the default) or no. And heads up bestie: ChallengeResponseAuthentication is a deprecated alias for this, so like… use the new name 🫶.
KbdInteractiveDevices — the list of methods to use in keyboard-interactive authentication. Multiple method names must be comma-separated. The default is to use the server specified list. The methods available vary depending on what the server supports — for an OpenSSH server, it may be zero or more of: bsdauth and pam.
KexAlgorithms — the default here is handled system-wide by crypto-policies(7). Info about defaults, how to modify the defaults, and how to customize existing policies with sub-policies all lives in manual page update-crypto-policies(8).

This one specifies the permitted KEX (Key Exchange) algorithms that will be used and their preference order ✨. The selected algorithm will be the first algorithm in this list that the server also supports. Multiple algorithms must be comma-separated.

And the prefix girlies:

If the specified list begins with a + character, then the specified methods will be appended to the built-in openssh default set instead of replacing them.
If the specified list begins with a - character, then the specified algorithms (including wildcards) will be removed from the built-in openssh default set instead of replacing them.
If the specified list begins with a ^ character, then the specified algorithms will be placed at the head of the built-in openssh default set.

The list of supported key exchange algorithms may also be obtained using "ssh -Q kex".

KnownHostsCommand — a command to use to obtain a list of host keys, in addition to those listed in UserKnownHostsFile and GlobalKnownHostsFile. This command is executed after the files have been read. It may write host key lines to standard output in identical format to the usual files (described in the “VERIFYING HOST KEYS” section in ssh(1)). Arguments to KnownHostsCommand accept the tokens described in the “TOKENS” section.

It's a busy lil queen — the command may be invoked multiple times per connection:

once when preparing the preference list of host key algorithms to use,
again to obtain the host key for the requested host name, and
if CheckHostIP is enabled, one more time to obtain the host key matching the server's address.

If the command exits abnormally or returns a non-zero exit status then the connection is terminated. No vibe check passed, no connection, sorry 💀.

LocalCommand — a command to execute on the local machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. Arguments to LocalCommand accept the tokens described in the “TOKENS” section.

The command is run synchronously and does not have access to the session of the ssh(1) that spawned it. It should not be used for interactive commands. This directive is ignored unless PermitLocalCommand has been enabled.

LocalForward — specifies that a TCP port or Unix-domain socket on the local machine be forwarded over the secure channel to the specified host and port (or Unix-domain socket) from the remote machine 🦋. For a TCP port, the first argument must be [bind_address:]port or a Unix domain socket path. The second argument is the destination and may be host:hostport or a Unix domain socket path if the remote host supports it.

IPv6 addresses can be specified by enclosing addresses in square brackets.

If either argument contains a / in it, that argument will be interpreted as a Unix-domain socket (on the corresponding host) rather than a TCP port.

Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports 👑. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address:

bind_address of localhost indicates that the listening port be bound for local use only.
An empty address or * indicates that the port should be available from all interfaces.

Unix domain socket paths may use the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section.

LogLevel — gives the verbosity level that is used when logging messages from ssh(1). 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 verbose output. Spicy levels of yapping, truly 📢.
LogVerbose — specify one or more overrides to LogLevel. An override consists of one or more pattern lists that matches the source file, function and line number to force detailed logging for. For example, an override pattern of:

kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

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 default is handled system-wide by crypto-policies(7). Information about defaults, how to modify the defaults and how to customize existing policies with sub-policies are present in manual page update-crypto-policies(8).

This specifies the MAC (message authentication code) algorithms in order of preference. The MAC algorithm is used for data integrity protection 🔒. Multiple algorithms must be comma-separated.

If the specified list begins with a + character, then the specified algorithms will be appended to the built-in openssh default set instead of replacing them.
If the specified list begins with a - character, then the specified algorithms (including wildcards) will be removed from the built-in openssh default set instead of replacing them.
If the specified list begins with a ^ character, then the specified algorithms will be placed at the head of the built-in openssh default set.

The algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended ✨.

The list of available MAC algorithms may also be obtained using "ssh -Q mac".

Metrics — indicate that the ssh client should gather and write TCP stack metrics from the TCP_INFO socket structure on compliant operating systems to a text file. These metrics will be collected from both the local and remote hosts if metrics are supported by the version of SSH and TCP_INFO is supported by the operating system. Currently operating system support is limited to Linux version 3.7 and greater, NetBSD, and FreeBSD (version 6 or greater). If one of the hosts does not support the Metrics option then only data from the the host that does will be reported. See the HPN-README file for more detail. The argument to the keyword must be yes or no (the default). See MetricsInterval and MetricsPath for more metrics options. HPNSSH only.
MetricsInterval — the frequency, in seconds, with which the TCP_INFO struct will be polled. The default value is 5 seconds. Metrics must be set to yes for this option to have any effect. HPNSSH only.
MetricsPath — the file path where Metrics should write the TCP stack metrics. The default is ./ssh_stack_metrics.local and ./ssh_stack_metrics.remote. The user may specify any other filepath and filename that they have write access to. E.g. MetricsPath=/tmp/my_metrics. The suffix of .local and .remote will be appended to this path. Files are not overwritten by subsequent metrics sessions — instead, data from new sessions will be appended if the file already exists. Metrics must be set to yes for this option to have any effect. HPNSSH only.
NoHostAuthenticationForLocalhost — disable host authentication for localhost (loopback addresses). The argument to this keyword must be yes or no (the default).
NoneEnabled — enable or disable the use of the None cipher.

Care must always be used when enabling this as it will allow users to send data in the clear 😭. However, it is important to note that authentication information remains encrypted even if this option is enabled.

Default is no. HPNSSH only.

NoneMacEnabled — enable or disable the use of the None MAC.

When this is enabled ssh will *not* provide data integrity of any data being transmitted between hosts. Use with caution as it, unlike just using NoneEnabled, doesn't provide data integrity and protection against man-in-the-middle attacks 💀.

As with NoneEnabled all authentication remains encrypted and integrity is ensured. Default is no. HPNSSH only.

NoneSwitch — switches the encryption cipher over to the None cipher after authentication has happened. Bestie, listen: NoneEnabled has to be turned on on both sides — client and server — or this goes nowhere. The second the connection flips to the NONE cipher, a warning gets sent to STDERR, so you'll know. And if a client asks for a NoneSwitch from a server that does not explicitly have NoneEnabled set to yes, the connection attempt just fails with an error. 💀

Note: the NONE cipher cannot be used in interactive (shell) sessions, and it will fail silently if you try. Set to no by default. HPNSSH only.

Note: this option cannot actually be set in the ssh_config file. It must be set on the command line using -oNoneEnabled. This is to stop it from being used by accident. It's included here for completeness, because neither NoneMacEnabled nor NoneCipherEnabled have any effect at all without this option. 🙈

NumberOfPasswordPrompts — how many password prompts you get before ssh gives up on you. The argument to this keyword must be an integer. The default is 3.
ObscureKeystrokeTiming — tells ssh(1) whether it should try to hide your inter-keystroke timings from passive observers snooping on network traffic. If it's enabled, then for interactive sessions ssh(1) will send keystrokes at fixed intervals of a few tens of milliseconds, and will keep sending fake keystroke packets for a little while after you stop typing. The argument to this keyword must be yes, no, or an interval specifier of the form interval:milliseconds (e.g. interval:80 for 80 milliseconds). The default is to obscure keystrokes using a 20ms packet interval. Note that smaller intervals will result in higher fake keystroke packet rates. 👑
PasswordAuthentication — whether to use password authentication at all. The argument to this keyword must be yes (the default) or no.
PermitLocalCommand — lets you run local commands via the LocalCommand option, or by using the !command escape sequence in ssh(1). The argument must be yes or no (the default).
PermitRemoteOpen — says which destinations remote TCP port forwarding is allowed to reach when RemoteForward is being used as a SOCKS proxy. The forwarding specification must be one of these forms, fr fr:

PermitRemoteOpen host:port
PermitRemoteOpen IPv4_addr:port
PermitRemoteOpen [IPv6_addr]:port

You can stack multiple forwards by separating them with whitespace. An argument of any removes all restrictions and permits any forwarding request whatsoever. An argument of none prohibits 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 the supplied names.

PKCS11Provider — which PKCS#11 provider to use, or none to say "no provider, thanks" (that's the default). The argument to this keyword is a path to the PKCS#11 shared library that ssh(1) should use to talk to a PKCS#11 token providing keys for user authentication. 🔑
Port — the port number to connect on the remote host. The default is 22.
PreferredAuthentications — the order in which the client should try authentication methods. This lets a client prefer one method (e.g. keyboard-interactive) over another (e.g. password). The default is:

gssapi-keyex,gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password

ProxyCommand — the command to use to connect to the server. The command string extends all the way to the end of the line, and it's executed using the user's shell ‘exec’ directive so you don't get a lingering shell process hanging around being messy.

Arguments to ProxyCommand accept the tokens described in the “TOKENS” section. The command can be basically anything, and it should read from its standard input and write to its standard output. It should eventually connect to an sshd(8) server running on some machine, or execute sshd -i somewhere. Host key management will be done using the Hostname of the host being connected (defaulting to the name typed by the user). Setting the command to none disables this option entirely. Note that CheckHostIP is not available for connects with a proxy command. 🫶

This directive is useful in conjunction with nc(1) and its proxy support. For example, the following directive would connect via an HTTP proxy at 192.0.2.0:

ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p

ProxyJump — one or more jump proxies, written as either [user@]host[:port] or an ssh URI. Multiple proxies may be separated by comma characters and will be visited sequentially. Setting this option will cause ssh(1) to connect to the target host by first making an ssh(1) connection to the specified ProxyJump host and then establishing a TCP forwarding to the ultimate target from there. Setting the host to none disables this option entirely.

Note that this option will compete with the ProxyCommand option — whichever is specified first will prevent later instances of the other from taking effect. 😭

Note also that the configuration for the destination host (either supplied via the command-line or the configuration file) is not generally applied to jump hosts. ~/.ssh/config should be used if specific configuration is required for jump hosts.

ProxyUseFdpass — says that ProxyCommand will pass a connected file descriptor back to ssh(1) instead of continuing to execute and pass data. The default is no.
PubkeyAcceptedAlgorithms — the default here is handled system-wide by crypto-policies(7). Information about defaults, how to modify the defaults, and how to customize existing policies with sub-policies lives in the manual page update-crypto-policies(8).

This one specifies the signature algorithms that will be used for public key authentication, as a comma-separated list of patterns. If the specified list begins with a ‘+’ character, then the algorithms after it will be appended to the built-in openssh default instead of replacing it. If the specified list begins with a ‘-’ character, then the specified algorithms (including wildcards) will be removed from the built-in openssh default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified algorithms will be placed at the head of the built-in openssh default set. ✨

The list of available signature algorithms may also be obtained using "ssh -Q PubkeyAcceptedAlgorithms".

This option affects also HostKeyAlgorithms.

PubkeyAuthentication — whether to try public key authentication. The argument to this keyword must be yes (the default), no, unbound or host-bound. The final two options enable public key authentication while respectively disabling or enabling the OpenSSH host-bound authentication protocol extension required for restricted ssh-agent(1) forwarding.
RefuseConnection — lets a connection be refused straight up by the configuration file. If this option is specified, then ssh(1) will terminate immediately before attempting to connect to the remote host, display an error message that contains the argument to this keyword, and return a non-zero exit status. This option may be useful to express reminders or warnings to the user via ssh_config. 🎀
RekeyLimit — the maximum amount of data that may be transmitted or received before the session key gets renegotiated, optionally followed by 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 of sshd_config(5). 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.
RemoteCommand — a command to execute on the remote machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. Arguments to RemoteCommand accept the tokens described in the “TOKENS” section.
RemoteForward — says that a TCP port or Unix-domain socket on the remote machine gets forwarded over the secure channel. The remote port may either be forwarded to a specified host and port or Unix-domain socket from the local machine, or it may act as a SOCKS 4/5 proxy that lets a remote client connect to arbitrary destinations from the local machine. The first argument is the listening specification and may be [bind_address:]port or, if the remote host supports it, a Unix domain socket path. If you're forwarding to a specific destination then the second argument must be host:hostport or a Unix domain socket path — otherwise, if no destination argument is specified, then the remote forwarding will be established as a SOCKS proxy. When acting as a SOCKS proxy, the destination of the connection can be restricted by PermitRemoteOpen. 🌸

IPv6 addresses can be specified by enclosing addresses in square brackets.

If either argument contains a / in it, that argument will be interpreted as a Unix-domain socket (on the corresponding host) rather than a TCP port. 🦋

Multiple forwardings may be specified, and you can also just yeet extra forwardings on the command line. ✨ Privileged ports? Only forwardable when you're logging in as root on the remote machine, bestie, them's the rules. Unix domain socket paths may use the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. 💅

If the port argument is 0, the listen port will be dynamically allocated on the server and reported to the client at run time. lowkey magic ✨

If the bind_address is not specified, the default is to only bind to loopback addresses. If the bind_address is ‘*’ or an empty string, then the forwarding is requested to listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config(5)). 👑

RequestTTY — specifies whether to request a pseudo-tty for the session. The argument may be one of:
- no — never request a TTY 🙅‍♀️
- yes — always request a TTY when standard input is a TTY
- force — always request a TTY 💪
- auto — request a TTY when opening a login session

This option mirrors the -t and -T flags for ssh(1). 🎀

RequiredRSASize — specifies the minimum RSA key size (in bits) that ssh(1) will accept. User authentication keys smaller than this limit will be ignored. Servers that present host keys smaller than this limit will cause the connection to be terminated 💀. The default is 1024 bits. Note that this limit may only be raised from the default — you can't lower it, bestie, it's giving minimum-viable-security. 🔥
RevokedHostKeys — specifies revoked host public keys. Keys listed in this file will be refused for host authentication. Note that if this file does not exist or is not readable, then host authentication will be refused for all hosts. 😭 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). Arguments to RevokedHostKeys may use the tilde syntax to refer to a user's home directory, the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. ✨
SecurityKeyProvider — specifies a path to a library that will be used when loading any FIDO authenticator-hosted keys, overriding the default of using the built-in USB HID support. 🔑

If the specified value begins with a ‘$’ character, then it will be treated as an environment variable containing the path to the library. 💅

SendEnv — specifies what variables from the local environ(7) should be sent to the server. The server must also support it, and the server must be configured to accept these environment variables. Note that the TERM environment variable is always sent whenever a pseudo-terminal is requested as it is required by the protocol. Refer to AcceptEnv in sshd_config(5) for how to configure the server. Variables are specified by name, which may contain wildcard characters. Multiple environment variables may be separated by whitespace or spread across multiple SendEnv directives. 🌸

See “PATTERNS” for more information on patterns.

It is possible to clear previously set SendEnv variable names by prefixing patterns with -. The default is not to send any environment variables. 🙈

ServerAliveCountMax — sets the number of server alive messages (see below) which may be sent without ssh(1) receiving any messages back from the server. If this threshold is reached while server alive messages are being sent, ssh will disconnect from the server, terminating the session. 💀 It is important to note that the use of server alive messages is very different from TCPKeepAlive (below). The server alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The server alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive. 👑

The default value is 3. If, for example, ServerAliveInterval (see below) is set to 15 and ServerAliveCountMax is left at the default, if the server becomes unresponsive, ssh will disconnect after approximately 45 seconds. ⏳

ServerAliveInterval — sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel to request a response from the server. The default is 0, indicating that these messages will not be sent to the server. 🦋
SessionType — may be used to either request invocation of a subsystem on the remote system, or to prevent the execution of a remote command at all. The latter is useful for just forwarding ports. The argument to this keyword must be:
- none — same as the -N option
- subsystem — same as the -s option
- default — shell or command execution
SetEnv — directly specify one or more environment variables and their contents to be sent to the server in the form “NAME=VALUE”. Similarly to SendEnv, with the exception of the TERM variable, the server must be prepared to accept the environment variable. 💖

The “VALUE” may use the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. ✨

StdinNull — redirects stdin from /dev/null (actually, prevents reading from stdin). Either this or the equivalent -n option must be used when ssh is run in the background. The argument to this keyword must be:
- yes — same as the -n option
- no — the default
StreamLocalBindMask — sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file for local or remote port forwarding. This option is only used for port forwarding to a Unix-domain socket file. 🧚

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 — specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and StreamLocalBindUnlink is not enabled, ssh 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).

StrictHostKeyChecking — okay bestie, this is the big one, pay attention 👀

If this flag is set to yes, ssh(1) will never automatically add host keys to the ~/.ssh/known_hosts file, and refuses to connect to hosts whose host key has changed. This provides maximum protection against man-in-the-middle (MITM) attacks, though it can be annoying when the /etc/gsissh/ssh_known_hosts file is poorly maintained or when connections to new hosts are frequently made. This option forces the user to manually add all new hosts. 💀

If this flag is set to accept-new then ssh will automatically add new host keys to the user's known_hosts file, but will not permit connections to hosts with changed host keys.

If this flag is set to no or off, ssh will automatically add new host keys to the user known hosts files and allow connections to hosts with changed hostkeys to proceed, subject to some restrictions. 🙈

If this flag is set to ask (the default), new host keys will be added to the user known host files only after the user has confirmed that is what they really want to do, and ssh will refuse to connect to hosts whose host key has changed.

The host keys of known hosts will be verified automatically in all cases. ✨

SyslogFacility — gives the facility code that is used when logging messages from ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is USER. 📋
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. 😤

The default is yes (to send TCP keepalive messages), and the client will notice if the network goes down or the remote host dies. This is important in scripts, and many users want it too. 💅

To disable TCP keepalive messages, the value should be set to no. See also ServerAliveInterval for protocol-level keepalives. 🦋

Tag — specify a configuration tag name that may be later used by a Match directive to select a block of configuration. 🏷️
TcpRcvBufPoll — enable of disable the polling of the tcp receive buffer through the life of the connection. You would want to make sure that this option is enabled for systems making use of autotuning kernels (linux 2.4.24+, 2.6, MS Vista). Default is yes. HPNSSH only. 🔥
Tunnel — request tun(4) device forwarding between the client and the server. The argument must be yes, point-to-point (layer 3), ethernet (layer 2), or no (the default). Specifying yes requests the default tunnel mode, which is point-to-point. 🌈
TunnelDevice — specifies the tun(4) devices to open on the client (local_tun) and the server (remote_tun). 🎀

The argument must be local_tun[:remote_tun]. The devices may be specified by numerical ID or the keyword any, which uses the next available tunnel device. If remote_tun is not specified, it defaults to any. The default is any:any. ✨

UpdateHostKeys — okay so this one decides whether ssh(1) is allowed to accept notifications of extra hostkeys from the server that show up after authentication is done, and then slide them into UserKnownHostsFile. The argument has to be yes, no or ask. Why do we care? Because it lets you learn alternate hostkeys for a server, and it supports graceful key rotation — the server can yeet you replacement public keys before the old ones get removed. Glow-up for key management, honestly. ✨

Extra hostkeys are only accepted if all of these hold:

the key that authenticated the host was already trusted or was explicitly accepted by the user,
the host was authenticated via UserKnownHostsFile (so like, not GlobalKnownHostsFile),
and the host was authenticated using a plain key, not a certificate.

UpdateHostKeys is enabled by default if you haven't overridden the default UserKnownHostsFile setting and haven't enabled VerifyHostKeyDNS. Otherwise it gets set to no.

If UpdateHostKeys is set to ask, then the user is asked to confirm the modifications to the known_hosts file. Heads up though: confirmation is currently incompatible with ControlPersist, and it will be disabled if ControlPersist is enabled. 😭

Also, presently, only sshd(8) from OpenSSH 6.8 and greater supports the "hostkeys@openssh.com" protocol extension that's used to tell the client about all the server's hostkeys. So old servers? They're not invited to this party. 💅

UseMPTCP — if you set it to yes, this enables Multipath TCP (MPTCP) instead of TCP. Only works on Linux, bestie. The default is no.
User — specifies the user to log in as. Super handy when a different user name is used on different machines, because it saves you the trouble of having to remember to give the user name on the command line. Arguments to User may use the tokens described in the “TOKENS” section (with the exception of %r and %C) and environment variables as described in the “ENVIRONMENT VARIABLES” section.
UserKnownHostsFile — specifies one or more files to use for the user host key database, separated by whitespace. Each filename may use tilde notation to refer to the user's home directory, the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. A value of none causes ssh(1) to ignore any user-specific known hosts files. The default is ~/.ssh/known_hosts, ~/.ssh/known_hosts2.
VerifyHostKeyDNS — specifies whether to verify the remote key using DNS and SSHFP resource records. If this option is set to yes, the client will implicitly trust keys that match a secure fingerprint from DNS. Insecure fingerprints will be handled as if this option was set to ask. If this option is set to ask, information on fingerprint match will be displayed, but the user will still need to confirm new host keys according to the StrictHostKeyChecking option. The default is no.

See also “VERIFYING HOST KEYS” in ssh(1).

VersionAddendum — optionally specifies additional text to append to the SSH protocol banner sent by the client upon connection. The default is none.
VisualHostKey — if this flag is set to yes, an ASCII art representation of the remote host key fingerprint is printed in addition to the fingerprint string at login and for unknown host keys. If this flag is set to no (the default), no fingerprint strings are printed at login, and only the fingerprint string will be printed for unknown host keys. ASCII art fingerprint?? it's giving hacker chic 🎀
WarnWeakCrypto — controls whether the user is warned when the cryptographic algorithms negotiated for the connection are weak or otherwise recommended against. Warnings may be disabled by turning off a specific warning or by disabling all warnings. Warnings about connections that don't use a post-quantum key exchange may be disabled using the no-pq-kex flag. no will disable all warnings. The default, equivalent to yes, is to enable all warnings.
XAuthLocation — specifies the full pathname of the xauth(1) program. The default is /usr/bin/xauth.

🌸 PATTERNS 🌸

A pattern consists of zero or more non-whitespace characters, ‘*’ (a wildcard that matches zero or more characters), or ‘?’ (a wildcard that matches exactly one character). So basically, wildcard soup, and we love her. 🍓

For example, to specify a set of declarations for any host in the ".co.uk" set of domains, the following pattern could be used:

Host *.co.uk

The following pattern would match any host in the 192.168.0.[0-9] network range:

Host 192.168.0.?

A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark (‘!’). For example, to allow a key to be used from anywhere within an organization except from the "dialup" pool, the following entry (in authorized_keys) could be used:

from="!*.dialup.example.com,*.example.com"

Note that a negated match will never produce a positive result by itself. Like, no cap. For example, attempting to match "host3" against the following pattern-list will fail:

from="!host1,!host2"

The solution here is to include a term that will yield a positive match, such as a wildcard:

from="!host1,!host2,*"

🦋 TOKENS 🦋

Arguments to some keywords can make use of tokens, which are expanded at runtime:

%% — A literal ‘%’.
%C — Hash of %l%h%p%r%j.
%d — Local user's home directory.
%f — The fingerprint of the server's host key.
%H — The known_hosts hostname or address that is being searched for.
%h — The remote hostname.
%I — A string describing the reason for a KnownHostsCommand execution: either ADDRESS when looking up a host by address (only when CheckHostIP is enabled), HOSTNAME when searching by hostname, or ORDER when preparing the host key algorithm preference list to use for the destination host.
%i — The local user ID.
%j — The contents of the ProxyJump option, or the empty string if this option is unset.
%K — The base64 encoded host key.
%k — The host key alias if specified, otherwise the original remote hostname given on the command line.
%L — The local hostname.
%l — The local hostname, including the domain name.
%n — The original remote hostname, as given on the command line.
%p — The remote port.
%r — The remote username.
%T — The local tun(4) or tap(4) network interface assigned if tunnel forwarding was requested, or "NONE" otherwise.
%t — The type of the server host key, e.g. ssh-ed25519.
%u — The local username.

CertificateFile, ControlPath, IdentityAgent, IdentityFile, Include, KnownHostsCommand, LocalForward, Match exec, RemoteCommand, RemoteForward, RevokedHostKeys, UserKnownHostsFile and VersionAddendum accept the tokens %%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p, %r, and %u.

KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.

Hostname accepts the tokens %% and %h.

LocalCommand accepts all tokens. 👑

ProxyCommand and ProxyJump accept the tokens %%, %h, %n, %p, and %r.

Note that some of these directives build commands for execution via the shell. Because ssh(1) performs no filtering or escaping of characters that have special meaning in shell commands (e.g. quotes), it is the user's responsibility to ensure that the arguments passed to ssh(1) do not contain such characters and that tokens are appropriately quoted when used. This one is serious, bestie — no escaping for you, it's all on you. 💀

🌈 ENVIRONMENT VARIABLES 🌈

Arguments to some keywords can be expanded at runtime from environment variables on the client by enclosing them in ${}, for example ${HOME}/.ssh would refer to the user's .ssh directory. If a specified environment variable does not exist then an error will be returned and the setting for that keyword will be ignored.

The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile, Include, KnownHostsCommand, and UserKnownHostsFile support environment variables. The keywords LocalForward and RemoteForward support environment variables only for Unix domain socket paths.

💜 FILES 💜

~/.ssh/config — This is the per-user configuration file. The format of this file is described above. This file is used by the SSH client. Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not writable by others.

Fr fr, chmod that file. If other people can write to it, that's a whole villain arc. 🙈

/etc/gsissh/ssh_config — Systemwide configuration file. This file provides defaults for those values that are not specified in the user's configuration file, and for those users who do not have a configuration file. This file must be world-readable.

🎀 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. Chris Rapier, Michael Stevens, Ben Bennet, and Mike Tasota developed the HPN extensions at the Pittsburgh Supercomuting Center with grants from Cisco, the National Library of Medicine, and the National Science Foundation.

And that's the whole page, bestie!! 🫶 This vibe check came from the Debian crew, dated October 4, 2025, and it's the SSH_CONFIG(5) manual page. Go forth and configure responsibly, you absolute icon. 💅👑

📦 where this page comes from

fedora 44: gsi-openssh-clients 10.2p1-4.fc44 · BSD-3-Clause AND BSD-2-Clause AND ISC AND SSH-OpenSSH AND ssh-keyscan AND snprintf AND LicenseRef-Fedora-Public-Domain AND X11-distribute-modifications-variant
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-18: a language model's rewrite of the original, under its terms