Config...temSsh(3pm)

📦
version 2.10.2.1 · 5 versions
, in ubuntu resolute, ubuntu stonking

💖 Config::Model::models::SystemSsh(3pm) 💖

User Contributed Perl Documentation — and now, a vibe check

okay so bestie, this is a Perl module manpage from the Config::Model universe, and it's a whole thing. grab a snack, we're doing this together. 🍓

✨ NAME ✨

Config::Model::models::SystemSsh — Configuration class SystemSsh. yes that's the whole name. it's giving "my parents named me after a namespace". 💅

🦋 DESCRIPTION 🦋

These are the configuration classes used by Config::Model. specifically, it's the configuration class that Config::Model uses to edit or validate /etc/ssh/ssh_config — and yes, as root, so behave. 👑

🌸 Elements 🌸

okay this is the main character section. every keyword you can shove into your ssh_config lives here, one by one. let's go. 🧚

🌈 Host

This one restricts the following declarations — like, everything until the next Host or Match keyword — to only apply to hosts that match one of the patterns you give after the keyword. if you provide more than one pattern, separate them with whitespace. a single '*' as a pattern gives you global defaults for all hosts. the host is usually the hostname argument from the command line (check out the CanonicalizeHostname keyword for the exceptions, iykyk).

You can negate a pattern entry by prefixing it with an exclamation mark ('!'). and here's the tea: if a negated entry matches, the Host entry is ignored, regardless of whether any other patterns on the line match. so negated matches are your bestie when you need exceptions for wildcard matches. ✨

See PATTERNS for more info on patterns.

Optional. Type hash of node of class Ssh::HostElement.

🌟 AddKeysToAgent

Specifies whether keys should automatically get added to a running ssh-agent1. 🔑

  • 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-add1.
  • If this option is set to ask, ssh(1) will require confirmation using the SSH_ASKPASS program before adding a key (see ssh-add1 for details).
  • If this option is set to confirm, every use of the key must be confirmed, as if the -c option was specified to ssh-add1.
  • If this option is set to no, no keys are added to the agent.

Alternatively, you can specify this option as a time interval using the format described in the TIME FORMATS section of sshd_config5 — that sets the key's lifetime in ssh-agent1, after which it gets automatically removed. so the argument must be no (the default), yes, confirm (optionally followed by a time interval), ask, or a time interval.

Optional. Type enum. choice: 'ask', 'confirm', 'no', 'yes'.

upstream_default value:

no

⭐ AddressFamily

Specifies which address family to use when connecting. valid arguments are any (the default), inet (IPv4 only), or inet6 (IPv6 only). pick your fighter. 🌐

Optional. Type enum. choice: 'any', 'inet', 'inet6'.

upstream_default value:

any

🎀 BatchMode

If set to yes, user interaction like password prompts and host key confirmation requests gets disabled. on top of that, the ServerAliveInterval option will be set to 300 seconds by default — (Debian-specific, bestie, don't @ me). this option is a lifesaver in scripts and other batch jobs where there's literally no user around to interact with ssh(1), and where you want to detect a broken network swiftly. the argument must be yes or no (the default).

Optional. Type boolean.

upstream_default value:

no

🦄 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, so if you've got one lonely IP this does nothing for you. 🏠

Optional. Type uniline.

💜 BindInterface

Use the address of the specified interface on the local machine as the source address of the connection. same energy as BindAddress but make it interface-y. 📡

Optional. Type uniline.

🌸 CanonicalDomains

When CanonicalizeHostname is enabled, this option specifies the list of domain suffixes to search for the specified destination host. 🔍

Optional. Type uniline.

✨ 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.

Optional. Type boolean.

upstream_default value:

yes

👑 CanonicalizeHostname

Controls whether explicit hostname canonicalization is performed. the default, no, means no name rewriting at all — the system resolver handles all hostname lookups. If set to yes, then for connections that don't 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. slay. 💅

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.

Optional. Type enum. choice: 'always', 'no', 'none', 'yes'.

upstream_default value:

no

🔥 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). so basically one dot and you're done. 🫶

Optional. Type integer.

upstream_default value:

1

🦋 CanonicalizePermittedCNAMEs

Specifies rules to determine whether CNAMEs should be followed when canonicalizing hostnames. the rules consist of one or more arguments of source_domain_list:target_domain_list — where source_domain_list is a pattern-list of domains that may follow CNAMEs in canonicalization, and target_domain_list is a pattern-list of domains that they may resolve to.

For example, Qq *.a.example.com:*.b.example.com,*.c.example.com will allow hostnames matching Qq *.a.example.com to be canonicalized to names in the Qq *.b.example.com or Qq *.c.example.com domains.

A single argument of Qq none causes no CNAMEs to be considered for canonicalization — and that's the default behaviour, so if you do nothing, nothing happens. iconic. ✨

Optional. Type uniline.

💀 CASignatureAlgorithms

Specifies which algorithms are allowed for signing of certificates by certificate authorities (CAs). the default is: ssh-ed25519, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521, sk-ssh-ed25519@openssh.com, sk-ecdsa-sha2-nistp256@openssh.com, rsa-sha2-512, rsa-sha2-256

  • If the specified list begins with a '+' character, then the specified algorithms will be appended to the 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 default set instead of replacing them.

ssh(1) will not accept host certificates signed using algorithms other than those specified. so get this list right or get rejected. 😭

Optional. Type uniline.

🎀 CertificateFile

Specifies a file from which the user's certificate is read. a corresponding private key must be provided separately in order to use this certificate — either from an IdentityFile directive or the -i flag to ssh(1), via ssh-agent1, 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.

It's possible to have multiple certificate files specified in configuration files; these certificates will be tried in sequence. multiple CertificateFile directives will add to the list of certificates used for authentication. the more the merrier bestie. 💅

Optional. Type uniline.

🌈 ChannelTimeout

Specifies whether and how quickly ssh(1) should close inactive channels. timeouts are specified as one or more ''type=interval'' pairs separated by whitespace, where the ''type'' must be the special keyword ''global'' or a channel type name from the list below, optionally containing wildcard characters. 👀

The timeout value ''interval'' is specified in seconds, or may use any of the units documented in the TIME FORMATS section. for example, ''session=5m'' would cause interactive sessions to terminate after five minutes of inactivity. specifying a zero value disables the inactivity timeout.

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 will be 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-agent1.
  • 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.

Optional. Type uniline.

🦋 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 will 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 will not be executed.

Optional. Type boolean.

upstream_default value:

no

🌟 Ciphers

Specifies the ciphers allowed and their order of preference. 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: chacha20-poly1305@openssh.com, aes128-gcm@openssh.com, aes256-gcm@openssh.com, aes128-ctr, aes192-ctr, aes256-ctr

Also, lowkey the easiest way to see the whole cipher menu is just asking ssh itself: ssh -Q cipher. That's it, that's the list. Optional. Type uniline. 🦋

ClearAllForwardings 🧹

Tells everything — local, remote, and dynamic port forwardings — set up in your config files or on the command line to just… ✨vanish✨. Cleared. Gone. Not the vibe today.

Where does this actually eat? It's mainly a glow-up for the ssh(1) command line, so you can nuke port forwardings that snuck into your config files without editing anything. And fun fact, scp(1) and sftp(1) set this on their own automatically, no input from you.

Argument's gotta be yes or no (the default). Optional. Type boolean.

upstream_default value:

no

Compression 🗜️

Do we squeeze the data or not? Argument must be yes or no (the default). Optional. Type boolean.

upstream_default value:

no

ConnectionAttempts 🔁

How many tries (one per second, no dilly-dallying) do we make before we give up and exit. Argument must be an integer. Genuinely useful in scripts when your connection is being flaky and dramatic. The default is 1. Optional. Type integer.

upstream_default value:

1

ConnectTimeout ⏳

Sets the timeout (in seconds) used when connecting to the SSH server, instead of just letting the default system TCP timeout cook. And bestie, it's not just the initial connect — this timeout covers establishing the connection and the initial SSH protocol handshake and key exchange. Whole package. 💅

Also, heads up: SetupTimeOut is a Debian-specific compatibility alias for this option. Optional. Type integer.

ControlMaster 👑

This one's the moment: it enables sharing multiple sessions over a single network connection. Iconic.

  • 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 try to reuse the master instance's network connection rather than initiating new ones, but they'll fall back to connecting normally if the control socket doesn't exist, or isn't listening. 🫶

Setting this to ask will cause ssh(1) to listen for control connections, but require confirmation using ssh-askpass1. And if the ControlPath can't be opened, ssh(1) will just continue on without connecting to a master instance. No drama.

X11 and ssh-agent1 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. One master, one display, one agent. That's the rule.

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. Those options are: auto and autoask. The latter requires confirmation like the ask option.

Optional. Type enum. choice: 'ask', 'auto', 'autoask', 'no', 'yes'.

upstream_default value:

no

ControlPath 🛣️

Specify the path to the control socket used for connection sharing — the one described up in the ControlMaster section, if you skipped it, go back 👀 — or the string none to disable connection sharing entirely.

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.

Real talk bestie: 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. 🎀

Optional. Type uniline.

ControlPersist 💤

Used in conjunction with ControlMaster, this specifies that the master connection should remain open in the background (waiting for future client connections, like a little gremlin 🧚) after the initial client connection has been closed.

  • If set to no (the default), the master connection will not be placed into the background, and will close as soon as the initial client connection is closed.
  • If set to yes or 0, the master connection will remain in the background indefinitely — until killed or closed via a mechanism such as the ssh -O exit.
  • If set to a time in seconds, or a time in any of the formats documented in sshd_config5, then the backgrounded master connection will automatically terminate after it has remained idle (with no client connections) for the specified time. ✨

Optional. Type uniline.

DynamicForward 🌀

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. Sneaky. Clever. Obsessed.

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. 👑

Optional. Type list of uniline.

EnableEscapeCommandline ⌨️

Enables the command line option in the EscapeChar menu for interactive sessions (default '~C'). By default, the command line is disabled. Optional. Type uniline.

EnableSSHKeysign 🔑

Setting this option to yes in the global client configuration file /etc/ssh/ssh_config enables the use of the helper program ssh-keysign8 during HostbasedAuthentication.

The argument must be yes or no (the default). This option should be placed in the non-hostspecific section. See ssh-keysign8 for more information. Optional. Type boolean.

upstream_default value:

no

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 — which makes the connection transparent for binary data. 🫧 Optional. Type uniline.

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.

Important nuance: 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). Optional. Type boolean.

upstream_default value:

no

FingerprintHash 🔍

Specifies the hash algorithm used when displaying key fingerprints. Valid options are: md5 and sha256 (the default). Optional. Type enum. choice: 'md5', 'sha256'.

upstream_default value:

sha256

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). Optional. Type boolean.

upstream_default value:

no

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. 😭

Optional. Type boolean.

upstream_default value:

no

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. 💀

Optional. Type boolean.

upstream_default value:

no

ForwardX11Timeout ⏲️

Specify a timeout for untrusted X11 forwarding using the format described in the TIME FORMATS section of sshd_config5. 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. 🌸 Optional. Type integer.

ForwardX11Trusted 🤝

  • If this option is set to yes (the Debian-specific default), remote X11 clients will have full access to the original X11 display.
  • If this option is set to no (the upstream 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. Optional. Type boolean.

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). Optional. Type boolean.

upstream_default value:

no

GlobalKnownHostsFile

Okay so this one is the global host key database — the big shared one, not your personal lil file. You can list one or more files, separated by whitespace (yes bestie, spaces count).

The default is /etc/ssh/ssh_known_hosts /etc/ssh/ssh_known_hosts2.

Optional. Type uniline.

  • default value: /etc/ssh/ssh_known_hosts /etc/ssh/ssh_known_hosts2

GSSAPIAuthentication

Do we allow user authentication that's based on GSSAPI? That's the whole question. Default is no.

Optional. Type boolean.

  • upstream_default value: no

GSSAPIClientIdentity

If you set this, it tells ssh which GSSAPI client identity to use when it's connecting to the server. Leave it unset (the default) and the default identity gets used instead. 💅

Optional. Type uniline.

GSSAPIDelegateCredentials

Forward — i.e. delegate — credentials to the server. Default is no.

Optional. Type boolean.

  • upstream_default value: no

GSSAPIKeyExchange

Can we use key exchange based on GSSAPI? And here's the cute part: when you're doing GSSAPI key exchange, the server doesn't even need a host key. The default is no.

Optional. Type uniline.

GSSAPIRenewalForcesRekey

Set this to yes and renewal of the client's GSSAPI credentials will force the rekeying of the ssh connection. And with a compatible server? It delegates the renewed credentials to a session on the server. Iconic behavior honestly.

Checks are made to make sure 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. So it's careful, it's not just out here vibing.

The default is no.

For this to actually work, GSSAPIKeyExchange needs to be enabled in the server and also used by the client. No half-measures, bestie.

Optional. Type uniline.

GSSAPIServerIdentity

If set, this specifies the GSSAPI server identity that ssh should expect when connecting to the server. Default is unset — which means the expected GSSAPI server identity gets determined from the target hostname.

Optional. Type uniline.

GSSAPITrustDns

Set it to yes to say "yeah, the DNS is trusted to securely canonicalize the name of the host we're connecting to." Set it to no and the hostname you typed on the command line gets passed untouched to the GSSAPI library. Default is no.

Optional. Type uniline.

GSSAPIKexAlgorithms

The list of key exchange algorithms 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-.

The default is gss-group14-sha256-, gss-group16-sha512-, gss-nistp256-sha256-, gss-curve25519-sha256-, gss-gex-sha1-, gss-group14-sha1-.

This option only applies to connections using GSSAPI, just so you know. Optional. Type uniline.

HashKnownHosts

This tells ssh(1) to hash host names and addresses when they get added to ~/.ssh/known_hosts. The hashed names still work totally normally with ssh(1) and sshd(8) — they just don't visually spill identifying info if somebody discloses the file's contents. Privacy queen. 👑

Default is no.

Heads up: existing names and addresses already in your known hosts files will not be converted automatically. You can hash them manually with ssh-keygen1 though.
Also, using this option may break facilities like tab-completion that rely on being able to read unhashed host names from ~/.ssh/known_hosts. So like… trade-offs, bestie.

Optional. Type boolean.

  • upstream_default value: no

HostbasedAcceptedAlgorithms

Specifies the signature algorithms that get used for hostbased authentication, as a comma-separated list of patterns. And there's a whole lil grammar here, so listen up:

  • If the list starts with a '+' character, the specified signature algorithms get appended to the default set instead of replacing them.
  • If it starts with a '-' character, the specified signature algorithms (wildcards included) get removed from the default set instead of replacing them.
  • If it starts with a '^' character, the specified signature algorithms get 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-ssh-ed25519@openssh.com, sk-ecdsa-sha2-nistp256@openssh.com, rsa-sha2-512, rsa-sha2-256

The -Q option of ssh(1) may be used to list supported signature algorithms. This was formerly named HostbasedKeyTypes.

Optional. Type uniline.

Note: HostbasedAcceptedAlgorithms is migrated with '$old' and with: - $old => "- HostbasedKeyTypes"

HostbasedAuthentication

Do we try rhosts based authentication with public key authentication? The argument must be yes or no (the default).

Optional. Type boolean.

  • upstream_default value: no

HostKeyAlgorithms

Specifies the host key signature algorithms that the client wants to use, in order of preference. And same lil grammar as before, bestie:

  • List starts with '+' → those signature algorithms get appended to the default set instead of replacing them.
  • List starts with '-' → those signature algorithms (wildcards included) get removed from the default set instead of replacing them.
  • List starts with '^' → those signature algorithms get 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. Respectfully adapting, love that for us.

The list of available signature algorithms may also be obtained using:

ssh -Q HostKeyAlgorithms

Optional. Type uniline.

HostKeyAlias

Specifies an alias that should be used 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.

This option is useful for tunneling SSH connections, or for multiple servers running on a single host. So like, one physical host, many little identities. 🦋

Optional. Type uniline.

Hostname

Specifies the real host name to log into. This is how you get to use nicknames or abbreviations for hosts — main character energy, honestly.

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.

Optional. Type uniline.

IdentitiesOnly

This says ssh(1) should only use the configured authentication identity and certificate files — either the default files, or those explicitly configured in the ssh_config files, or passed on the ssh(1) command-line — even if ssh-agent1 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. We've all been there. 😭

Optional. Type boolean.

  • upstream_default value: no

IdentityAgent

Specifies the UNIX socket used to communicate with the authentication agent. 🫶

This option overrides the SSH_AUTH_SOCK environment variable and can be used to select a specific agent. Setting the socket name to none disables the use of an authentication agent entirely.

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 will be 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.

Optional. Type uniline.

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 to use the corresponding private key that's loaded in ssh-agent1 when the private key file isn't present locally. We love a flexible queen. ✨

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.

If no certificates have been explicitly specified by CertificateFile, ssh(1) will try to load certificate information from the filename obtained 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.

It's possible to have multiple identity files specified in configuration files — all these identities will be tried in sequence. Multiple IdentityFile directives will add to the list of identities tried. (Yeah, this behaviour differs from that of other configuration directives. She's special.)

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

Optional. Type list of uniline.

IgnoreUnknown

Specifies a pattern-list of unknown options to be ignored if they're encountered during configuration parsing. This may be used to suppress errors if ssh_config contains options that are unrecognised by ssh(1). Very forgiving of you. 💜

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. Order matters, bestie.

Optional. Type uniline.

💖 Include

Include the specified config file(s), bestie — yes, you can pull in multiple pathnames. Each pathname can have glob(7) wildcards, tokens like the ones in the TOKENS section, environment variables like the ones in the ENVIRONMENT VARIABLES section and, for user configs, shell-ish ~ references to home directories. Wildcards get expanded and processed in lexical order (alphabetical, basically). Files without absolute paths are assumed to live in ~/.ssh if they came from a user config file, or /etc/ssh if they came from the system config file. The Include directive can also chill inside a Match or Host block to do conditional inclusion. Optional. Type list of uniline.

🌈 IPQoS

Specifies the Differentiated Services Field Codepoint (DSCP) value for 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 the operating system defaults to. This option can take one or two arguments, separated by whitespace. One argument = used as the packet class unconditionally, no notes. Two values = the first one gets auto-picked 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. Optional. Type uniline.

🎀 KbdInteractiveAuthentication

Specifies whether to use keyboard-interactive authentication. The argument has to be yes (the default) or no. Oh and heads up — ChallengeResponseAuthentication is a deprecated alias for this one. Optional. Type boolean.

upstream_default value :
    yes

🦋 KbdInteractiveDevices

Specifies the list of methods to use in keyboard-interactive authentication. Multiple method names have to be comma-separated. The default is to use the list the server itself specifies. The methods available vary depending on what your server supports. For an OpenSSH server, it may be zero or more of: bsdauth and pam. Optional. Type list of uniline.

✨ KexAlgorithms

Specifies the permitted KEX (Key Exchange) algorithms that will be used and their preference order. The chosen algorithm is the first one in this list that the server also supports. Multiple algorithms have to be comma-separated.

The list-starting-characters lore, because it's iconic:

  • If the list begins with a '+' character, the specified algorithms get appended to the default set instead of replacing them.
  • If the list begins with a '-' character, the specified algorithms (wildcards included) get removed from the default set instead of replacing them.
  • If the list begins with a '^' character, the specified algorithms get placed at the head of the default set.

The default is: mlkem768x25519-sha256, sntrup761x25519-sha512, sntrup761x25519-sha512@openssh.com, curve25519-sha256, curve25519-sha256@libssh.org, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group14-sha256

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

🫶 KnownHostsCommand

Specifies a command to use to obtain a list of host keys, in addition to the ones listed in UserKnownHostsFile and GlobalKnownHostsFile. This command runs after the files have been read. It may write host key lines to standard output in the exact same format as the usual files (described in the VERIFYING HOST KEYS section in ssh(1)). Arguments to KnownHostsCommand accept the tokens described in the TOKENS section. 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 vibes, just gone. 😭

Optional. Type uniline.

💅 LocalCommand

Specifies 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 it's 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. Periodt.

Optional. Type uniline.

🔥 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. Slash = socket, iykyk.

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. 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. Unix domain socket paths may use the tokens described in the TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section. Optional. Type list of node of class Ssh::PortForward.

⭐ 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. Optional. Type enum. choice: 'DEBUG', 'DEBUG1', 'DEBUG2', 'DEBUG3', 'ERROR', 'FATAL', 'INFO', 'QUIET', 'VERBOSE'.

upstream_default value :
    INFO

🧚 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. Optional. Type uniline.

🍓 MACs

Specifies the MAC (message authentication code) algorithms

in order of preference. The MAC algorithm is used for data integrity protection. Multiple algorithms have to be comma-separated. If the specified list begins with a '+' character, then the specified algorithms will be appended to the 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 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 default set.

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

The default is: umac-64-etm@openssh.com, umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com, hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, umac-64@openssh.com, umac-128@openssh.com, hmac-sha2-256, hmac-sha2-512, hmac-sha1

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

💜 NoHostAuthenticationForLocalhost

Disable host authentication for localhost (loopback addresses). The argument to this keyword must be yes or no (the default). Optional. Type boolean.

upstream_default value :
    no

🎀 NumberOfPasswordPrompts

Specifies the number of password prompts before giving up. The argument to this keyword must be an integer. The default is 3. Optional. Type integer.

upstream_default value :
    3

🙈 ObscureKeystrokeTiming

Specifies whether ssh(1) should try to obscure inter-keystroke timings from passive observers of network traffic. If enabled, then for interactive sessions, ssh(1) will send keystrokes at fixed intervals of a few tens of milliseconds and will send fake keystroke packets for some time after typing ceases. 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. Optional. Type uniline.

upstream_default value :
    interval:20

🌸 PasswordAuthentication

Specifies whether to use password authentication. The argument to this keyword must be yes (the default) or no. Optional. Type boolean.

upstream_default value :
    yes

🦋 PermitLocalCommand

Allow local command execution via the LocalCommand option or using the ! command escape sequence in ssh(1). The argument must be yes or no (the default). Optional. Type boolean.

upstream_default value :
    no

🌈 PermitRemoteOpen

Specifies the destinations to which remote TCP port forwarding is permitted when RemoteForward is used as a SOCKS proxy. The forwarding specification must be one of the following forms:

PermitRemoteOpen host:port
PermitRemoteOpen IPv4_addr:port
PermitRemoteOpen [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. Optional. Type list of uniline.

💖 PKCS11Provider

Specifies which PKCS#11 provider to use or none to indicate that no provider should be used (the default). The argument to this keyword is a path to the PKCS#11 shared library ssh(1) should use to communicate with a PKCS#11 token providing keys for user authentication. Optional. Type uniline.

✨ Port ✨

Specifies the port number to connect on the remote host. The default is 22. Optional. Type uniline. 💅

🌸 PreferredAuthentications 🌸

Specifies the order in which the client should try authentication methods. This lets a client prefer one method (e.g. keyboard-interactive) over another method (e.g. password). The default is:

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

Optional. Type list of uniline. ✨

🧚 ProxyCommand 🧚

Specifies the command to use to connect to the server. The command string extends to the end of the line, and it's executed using the user's shell exec directive to avoid a lingering shell process. No cap.

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 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

Optional. Type uniline. 🦋

👑 ProxyJump 👑

Specifies one or more jump proxies 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. Optional. Type list of uniline. 💜

🌈 ProxyUseFdpass 🌈

Specifies that ProxyCommand will pass a connected file descriptor back to ssh(1) instead of continuing to execute and pass data. The default is no. Optional. Type boolean.

upstream_default value:

no

💖 PubkeyAcceptedAlgorithms 💖

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 default instead of replacing it.
  • If the specified list begins with a '-' character, then the specified algorithms (including wildcards) will be removed from the 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 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-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com, rsa-sha2-512, rsa-sha2-256

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

Note: PubkeyAcceptedAlgorithms is migrated with '$old' and with: - $old => "- PubkeyAcceptedKeyTypes"

🎀 PubkeyAuthentication 🎀

Specifies 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-agent1 forwarding. Optional. Type enum. choice: 'host-bound', 'no', 'unbound', 'yes'.

upstream_default value:

yes

💀 RefuseConnection 💀

Allows a connection to be refused 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. Optional. Type uniline. 🙈

🔥 RekeyLimit 🔥

Specifies the maximum amount of data that may be transmitted or received before the session key is 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_config5. 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. Optional. Type uniline. ✨

🦋 RemoteCommand 🦋

Specifies 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. Optional. Type uniline. 💜

🌸 RemoteForward 🌸

Specifies that a TCP port or Unix-domain socket on the remote machine be 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 may act as a SOCKS 4/5 proxy that allows a remote client to 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 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 additional forwardings can be given on the command line. Privileged ports can be forwarded only when logging in as root on the remote machine. 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. Slay.

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_config5). Optional. Type list of node of class Ssh::PortForward. 🫶

⭐ 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). Optional. Type enum. choice: 'auto', 'force', 'no', 'yes'. 💅

🔒 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. Optional. Type integer.

upstream_default value:

1024

💜 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-keygen1. For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen1. 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. Optional. Type uniline.

🎀 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. Optional. Type uniline. 🦋

🌈 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_config5 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. Optional. Type list of uniline. 💖

ServerAliveCountMax

Okay so this one sets the number of server alive messages (peep the next one for what those even are 💌) that can get sent without ssh(1) hearing anything back from the server. If you hit that threshold while those messages are going out, ssh just dips — it disconnects and kills the session. Bye 👋

Now here's the thing you actually need to know: server alive messages are NOT the same as TCPKeepAlive (which is right below, bestie, keep reading). The server alive messages travel through the encrypted channel, so they are not spoofable. The TCP keepalive option that TCPKeepAlive turns on is spoofable. The server alive mechanism really shines when the client or the server genuinely needs to know when a connection has gone unresponsive.

The default value is 3. So like, say ServerAliveInterval (below!) is 15 and you left ServerAliveCountMax at the default — if the server goes unresponsive, ssh will disconnect after approximately 45 seconds. Math slay ✨

Optional. Type integer.

upstream_default value :

3

ServerAliveInterval

Sets a timeout interval in seconds. After that much time with no data received from the server, ssh(1) sends a message through the encrypted channel to request a response from the server. The default is 0, meaning these messages just... don't get sent. Unless BatchMode is set, in which case it's 300 (Debian-specific, bestie 🐧).

Oh and ProtocolKeepAlives is a Debian-specific compatibility alias for this option. Vintage. Optional. Type integer.

upstream_default value :

0

SessionType

Use this to either request invocation of a subsystem on the remote system, or to prevent the execution of a remote command entirely. That second one is chef's kiss 🫶 when you're only forwarding ports. The argument to this keyword must be none (same as the -N option), subsystem (same as the -s option) or default (shell or command execution). Optional. Type enum. choice: 'default', 'none', 'subsystem'.

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 has to be prepared to accept the environment variable. Like, you can't just barge in, you need the server's blessing 💅

The ''VALUE'' may use the tokens described in the TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section. Optional. Type uniline.

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) or no (the default). Optional. Type boolean.

upstream_default value :

no

StreamLocalBindMask

Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file for local or remote port forwarding. Heads up, this option is only used for port forwarding to a Unix-domain socket file. No Unix socket, no opinion 🎀

The default value is 0177, which creates a Unix-domain socket file that's readable and writable only by the owner. And a lil warning: not all operating systems honor the file mode on Unix-domain socket files. Optional. Type uniline.

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). Optional. Type boolean.

upstream_default value :

no

StrictHostKeyChecking

This is the big one, bestie. Pay attention 💀

If this flag is set to yes, ssh(1) will never automatically add host keys to the ~/.ssh/known_hosts file, and it refuses to connect to hosts whose host key has changed. This gives you maximum protection against man-in-the-middle (MITM) attacks 🛡️ — though it can be super annoying when the /etc/ssh/ssh_known_hosts file is poorly maintained, or when you're frequently connecting to new hosts. This option basically forces the user to manually add all new hosts. It's giving "trust issues but make it security."

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 get added to the user known host files only after the user has confirmed that's really what they 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. Optional. Type enum. choice: 'yes', 'accept-new', 'no', 'off', 'ask'.

upstream_default value :

ask

SyslogFacility

Gives the facility code 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. Optional. Type enum. choice: 'AUTH', 'DAEMON', 'LOCAL0', 'LOCAL1', 'LOCAL2', 'LOCAL3', 'LOCAL4', 'LOCAL5', 'LOCAL6', 'LOCAL7', 'USER'.

upstream_default value :

USER

TCPKeepAlive

Specifies whether the system should send TCP keepalive messages to the other side. If they get sent, death of the connection or a crash of one of the machines gets properly noticed. 👀

But real talk — this option only uses TCP keepalives (as opposed to ssh-level keepalives), so it takes a long time to notice when the connection dies. Because of that, you probably want the ServerAliveInterval option too, bestie. However — and this is the catch 😭 — it means connections will die if the route goes down temporarily, and some people find that super 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 honestly many users want it too.

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

upstream_default value :

yes

Tag

Specify a configuration tag name that may be later used by a Match directive to select a block of configuration. Label it now, match it later 🏷️ Optional. Type uniline.

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. Optional. Type enum. choice: 'ethernet', 'no', 'point-to-point', 'yes'.

upstream_default value :

no

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. Optional. Type uniline.

upstream_default value :

any:any

UpdateHostKeys

Specifies whether ssh(1) should accept notifications of additional hostkeys from the server sent after authentication has completed, and add them to UserKnownHostsFile. The argument must be yes, no or ask. This option allows learning alternate hostkeys for a server, and supports graceful key rotation by letting a server send replacement public keys before the old ones are removed. Key rotation, but make it chill 🔄✨

Additional hostkeys are only accepted if:

  • the key used to authenticate the host was already trusted or explicitly accepted by the user,
  • the host was authenticated via UserKnownHostsFile (i.e. not GlobalKnownHostsFile),
  • and the host was authenticated using a plain key and not a certificate.

UpdateHostKeys is enabled by default if the user has not overridden the default UserKnownHostsFile setting and has not enabled VerifyHostKeyDNS — otherwise UpdateHostKeys will be 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: confirmation is currently incompatible with ControlPersist and will be disabled if it is enabled. 😬

Presently, only sshd(8) from OpenSSH 6.8 and greater support the Qq hostkeys@openssh.com protocol extension used to inform the client of all the server's hostkeys. Optional. Type enum. choice: 'ask', 'no', 'yes'.

User

Specifies the user to log in as. This is lowkey a lifesaver when a different user name is used on different machines — saves you the trouble of remembering 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. Optional. Type uniline.

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. Optional. Type list of uniline.

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 a 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). Optional. Type enum. choice: 'ask', 'no', 'yes'.

upstream_default value :

no

VersionAddendum

Optionally specifies additional text to append to the SSH protocol banner sent by the client upon connection. The default is none. Optional. Type uniline.

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. Optional. Type uniline.

upstream_default value :

no

🌸 WarnWeakCrypto 🌸

okay so this one is the vibe check on your crypto 💅 it controls whether the user gets 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. Optional. Type uniline.

✨ XAuthLocation ✨

Specifies the full pathname of the xauth(1) program. The default is /usr/bin/xauth. Optional. Type uniline.

  • upstream_default value: /usr/bin/xauth

💀 FallBackToRsh 💀

This parameter is now ignored by Ssh. Deprecated Optional. Type uniline.

💀 HostbasedKeyTypes 💀

This parameter is now ignored by Ssh. Deprecated Optional. Type uniline.

💀 PubkeyAcceptedKeyTypes 💀

This parameter is now ignored by Ssh. Deprecated Optional. Type uniline.

💀 UseRsh 💀

This parameter is now ignored by Ssh. Deprecated Optional. Type uniline.

👑 Match 👑

bestie, this is the one that gates everything 💅 it 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. Optional. Type hash of node of class Ssh::HostElement.

🦋 SEE ALSO 🦋

  • cme
  • Config::Model::models::Ssh::HostElement
  • Config::Model::models::Ssh::PortForward

💖 AUTHOR 💖

Dominique Dumont

2013 Dominique Dumont

🎀 LICENSE 🎀

LGPL2

and that's a wrap on this page, bestie 🫶 no cap, you read the whole thing — main character energy fr fr 💅 this beauty ships with perl v5.40.1, dated 2025-10-26, and it's the Config...temSsh(3pm) page — go forth and configure, icon 👑✨

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