Config...s::Sshd(3pm)

📦

version 2.10.2.1 · 5 versions

2.10.2.1 · ubuntu resolute, ubuntu stonking (latest)
2.8.7.1 · ubuntu jammy
2.9.4.1 · ubuntu noble
2.9.0.2 · debian bookworm
2.9.9.1 · debian trixie, ubuntu questing

💅 Config::Model::models::Sshd (3pm) — User Contributed Perl Documentation 💅

okay bestie, so this is the manual page for Config::Model::models::Sshd, and it's giving very official system-manual energy. Buckle up, we're doing all of it. 🫶

✨ NAME ✨

Config::Model::models::Sshd — Configuration class Sshd

🌸 DESCRIPTION 🌸

These are the configuration classes used by Config::Model. 💖

This configuration class was generated from the sshd_system documentation, by parse-man.pl — you can find it at:

https://github.com/dod38fr/config-model-openssh/contrib/parse-man.pl

(ngl, auto-generated config classes are such a mood. the script did the homework so you don't have to. ✨)

👑 Elements 👑

this is the main character section, bestie. every element below is one thing you can configure in sshd_config. no skipping, we're listing ALL of them. 🎀

🌈 AcceptEnv 🌈

Specifies what environment variables sent by the client will be copied into the session's environ(7). 👀 For how to set up the client side of this, go look at SendEnv and SetEnv in ssh_config5. And heads up: the TERM environment variable is always accepted whenever the client requests a pseudo-terminal, because the protocol literally requires it. no takebacks. 💅

Variables are specified by name, and the name may contain the wildcard characters '*' and '?'. Multiple environment variables may be separated by whitespace, or you can spread them across multiple AcceptEnv directives — either way works, slay.

🚨 WARNING, for real: some environment variables could be used to bypass restricted user environments. Because of that, you should be careful when you use this directive. The default is to not accept any environment variables at all.

Optional. Type: list of uniline.

🌈 AllowAgentForwarding 🌈

Specifies whether ssh-agent1 forwarding is permitted. The default is yes. ✅

💀 real talk: disabling agent forwarding does not improve security unless users are also denied shell access, because they can always just install their own forwarders. so like… it's not the flex you think it is.

Optional. Type: boolean.

upstream_default value :
    yes

🌈 AllowGroups 🌈

This keyword can be followed by a list of group name patterns, separated by spaces. If you specify it, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. 🎯

Only group names are valid — a numerical group ID is not recognized. By default, login is allowed for all groups.

The allow/deny groups directives are processed in this order:

DenyGroups
AllowGroups

See PATTERNS in ssh_config5 for more info on patterns. This keyword may appear multiple times in sshd_config, and each instance appends to the list (it doesn't overwrite, it stacks, we love to see it).

Optional. Type: list of uniline.

🌈 AllowStreamLocalForwarding 🌈

Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 🧦

The available options are:

yes (the default) or all — allow StreamLocal forwarding
no — prevent all StreamLocal forwarding
local — allow local (from the perspective of ssh(1)) forwarding only
remote — allow remote forwarding only

💀 same caveat as before: disabling StreamLocal forwarding does not improve security unless users are also denied shell access, because they can always install their own forwarders. periodt.

Optional. Type: enum. choice: 'all', 'local', 'no', 'remote', 'yes'.

upstream_default value :
    yes

🌈 AllowTcpForwarding 🌈

Specifies whether TCP forwarding is permitted. 🔌

The available options are:

yes (the default) or all — allow TCP forwarding
no — prevent all TCP forwarding
local — allow local (from the perspective of ssh(1)) forwarding only
remote — allow remote forwarding only

💀 and yes, same thing again: disabling TCP forwarding does not improve security unless users are also denied shell access, since they can always install their own forwarders. iykyk.

Optional. Type: enum. choice: 'all', 'local', 'no', 'remote', 'yes'.

upstream_default value :
    yes

🌈 AllowUsers 🌈

This keyword can be followed by a list of user name patterns, separated by spaces. If specified, login is allowed only for user names that match one of the patterns. 🫶

Only user names are valid — a numerical user ID is not recognized. By default, login is allowed for all users.

If the pattern takes the form USER@HOST then USER and HOST are checked separately, which restricts logins to particular users from particular hosts. 🎯 HOST criteria may additionally contain addresses to match in CIDR address/masklen format.

The allow/deny users directives are processed in this order:

DenyUsers
AllowUsers

See PATTERNS in ssh_config5 for more info on patterns. This keyword may appear multiple times in sshd_config, and each instance appends to the list.

Optional. Type: list of uniline.

🌈 AuthenticationMethods 🌈

Specifies the authentication methods that must be successfully completed for a user to be granted access. 🔐

This option must be followed by one or more lists of comma-separated authentication method names, or by the single string any, which means the default behaviour of accepting any single authentication method. If the default is overridden, then successful authentication requires completion of every method in at least one of these lists. okay so it's giving "choose your own adventure but every path is locked". 💅

For example:

"publickey, password publickey, keyboard-interactive"

would require the user to complete public key authentication, followed by either password or keyboard interactive authentication. Only methods that are next in one or more lists are offered at each stage, so for this example it would not be possible to attempt password or keyboard-interactive authentication before public key. no queue-jumping allowed. 🙈

For keyboard interactive authentication, it's also possible to restrict authentication to a specific device by appending a colon followed by the device identifier bsdauth or pam, depending on the server configuration. For example:

"keyboard-interactive:bsdauth"

would restrict keyboard interactive authentication to the bsdauth device. 🎀

If the publickey method is listed more than once, sshd(8) verifies that keys that have been used successfully are not reused for subsequent authentications. For example:

"publickey, publickey"

requires successful authentication using two different public keys. iconic, honestly. 🔥

Note that each authentication method listed should also be explicitly enabled in the configuration.

The available authentication methods are: "gssapi-with-mic", "hostbased", "keyboard-interactive", "none" (used for access to password-less accounts when PermitEmptyPasswords is enabled), "password" and "publickey".

Optional. Type: uniline.

🌈 AuthorizedKeysCommand 🌈

Specifies a program to be used to look up the user's public keys. 🔑

The program must be:

owned by root
not writable by group or others
specified by an absolute path

Arguments to AuthorizedKeysCommand accept the tokens described in the TOKENS section. If no arguments are specified, then the username of the target user is used.

The program should produce on standard output zero or more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). AuthorizedKeysCommand is tried after the usual AuthorizedKeysFile files and will not be executed if a matching key is found there. By default, no AuthorizedKeysCommand is run.

Optional. Type: uniline.

🌈 AuthorizedKeysCommandUser 🌈

Specifies the user under whose account the AuthorizedKeysCommand is run. 🙋

It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands — like, give it one job and one job only, main character energy but make it minimal. ✨

🚨 If AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser is not, then sshd(8) will refuse to start. do not test this at 3am.

Optional. Type: uniline.

🌈 AuthorizedKeysFile 🌈

Specifies the file that contains the public keys used for user authentication. 🗝️

The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Arguments to AuthorizedKeysFile may include wildcards and accept the tokens described in the TOKENS section. After expansion, AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. Multiple files may be listed, separated by whitespace.

Alternately, this option may be set to none to skip checking for user keys in files.

The default is:

".ssh/authorized_keys .ssh/authorized_keys2"

Optional. Type: list of uniline.

📝 Note: AuthorizedKeysFile values are migrated from '-AuthorizedKeysFile2'.

🌈 AuthorizedPrincipalsCommand 🌈

Specifies a program to be used to generate the list of allowed certificate principals, as per AuthorizedPrincipalsFile. 🎟️

The program must be:

owned by root
not writable by group or others
specified by an absolute path

Arguments to AuthorizedPrincipalsCommand accept the tokens described in the TOKENS section. If no arguments are specified, then the username of the target user is used.

The program should produce on standard output zero or more lines of AuthorizedPrincipalsFile output. If either AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is specified, then certificates offered by the client for authentication must contain a principal that is listed. 👑

By default, no AuthorizedPrincipalsCommand is run.

Optional. Type: uniline.

🌈 AuthorizedPrincipalsCommandUser 🌈

Specifies the user under whose account the AuthorizedPrincipalsCommand is run. 🙋

It is recommended to use a dedicated user that has no other role on the host than running authorized principals commands.

🚨 If AuthorizedPrincipalsCommand is specified but AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start. fr fr, don't skip this one.

Optional. Type: uniline.

🌈 AuthorizedPrincipalsFile 🌈

Specifies a file that lists principal names that are accepted for certificate authentication. 📜

When using certificates signed by a key listed in TrustedUserCAKeys, this file lists names, one of which must appear in the certificate for it to be accepted for authentication. Names are listed one per line, preceded by key options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). Empty lines and comments starting with '#' are ignored. 💅

Arguments to AuthorizedPrincipalsFile may include wildcards and accept the tokens described in the TOKENS section. After expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the user's home directory. The default is none, i.e. not to use a principals file — and in that case, the username of the user must appear in a certificate's principals list for it to be accepted.

Note that AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in TrustedUserCAKeys, and is not consulted for certification authorities trusted via ~/.ssh/authorized_keys — though the principals= key option offers a similar facility (see sshd(8) for details). 🧐

Optional. Type: uniline.

upstream_default value :
    none

The contents of the specified file are sent to the remote user before authentication is allowed. 📣

If the argument is none, then no banner is displayed. By default, no banner is displayed.

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 list you give starts with a + character, then your algorithms get appended to the default set instead of yeeting the defaults out 💖. If it starts with a - character, then those algorithms (wildcards included, bestie) get removed from the default set instead of replacing it. And if it starts with a ^ character, they get plopped at the head of the default set 👑.

Certificates signed using other algorithms will not be accepted for public key or host-based authentication. No exceptions, no cap.

Optional. Type uniline.

🕰️ ChannelTimeout ✨

Tells you whether — and how fast — sshd(8) should close inactive channels. Timeouts are given as one or more type=interval pairs separated by whitespace, where type has to be either the special keyword global or a channel type name from the list below, optionally with wildcard characters 🦋.

The timeout value interval is in seconds, or you can use any of the units documented in the TIME FORMATS section. For example, session=5m would make interactive sessions dip out 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 get closed 💀. Note that this global timeout is not matched by wildcards — you have to spell it out explicitly, fr fr.

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 that all resources associated with the session get removed — like, shell processes or X11 clients relating to the session may keep on executing 🙈.

Moreover, terminating an inactive channel or session doesn't necessarily close the SSH connection, and it doesn't stop 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 created right after 👀.

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

🔒 ChrootDirectory 💅

Specifies the pathname of a directory to chroot(2) to after authentication. At session startup sshd(8) checks that all components of the pathname are root-owned directories which are not writable by group or others 👑. After the chroot, sshd(8) changes the working directory to the user's home directory. Arguments to ChrootDirectory accept the tokens described in the TOKENS section.

The ChrootDirectory must contain the necessary files and directories to support the user's session. For an interactive session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), and tty(4) devices. For file transfer sessions using SFTP no additional configuration of the environment is necessary if the in-process sftp-server is used, though sessions which use logging may require /dev/log inside the chroot directory on some operating systems (see sftp-server8 for details) 🍓.

For safety, it is very important that the directory hierarchy be prevented from modification by other processes on the system (especially those outside the jail). Misconfiguration can lead to unsafe environments which sshd(8) cannot detect 😭. Like, it genuinely cannot catch this for you. Be careful.

The default is none, indicating not to chroot(2). Optional. Type uniline.

upstream_default value :
    none

💓 ClientAliveCountMax 🌸

Sets the number of client alive messages which may be sent without sshd(8) receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session 💀. It is important to note that the use of client alive messages is very different from TCPKeepAlive. The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable 😬. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive.

The default value is 3. If ClientAliveInterval is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds ⏱️. Setting a zero ClientAliveCountMax disables connection termination. Optional. Type integer.

upstream_default value :
    3

⏳ ClientAliveInterval 🦋

Sets a timeout interval in seconds after which, if no data has been received from the client, sshd(8) will send a message through the encrypted channel to request a response from the client 📨. The default is 0, indicating that these messages will not be sent to the client. Optional. Type integer.

upstream_default value :
    0

🚫 DenyGroups 🙅‍♀️

This keyword can be followed by a list of group name patterns, separated by spaces. Login is disallowed for users whose primary group or supplementary group list matches one of the patterns. Only group names are valid — a numerical group ID is not recognized, sorry bestie 🔥. By default, login is allowed for all groups. The allow/deny groups directives are processed in the following order:

DenyGroups
AllowGroups

See PATTERNS in ssh_config5 for more information on patterns. This keyword may appear multiple times in sshd_config, with each instance appending to the list. Optional. Type list of uniline.

🚫 DenyUsers 🙅‍♀️

This keyword can be followed by a list of user name patterns, separated by spaces. Login is disallowed for user names that match one of the patterns. Only user names are valid — a numerical user ID is not recognized 💅. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. HOST criteria may additionally contain addresses to match in CIDR address/masklen format 🎀. The allow/deny users directives are processed in the following order:

DenyUsers
AllowUsers

See PATTERNS in ssh_config5 for more information on patterns. This keyword may appear multiple times in sshd_config, with each instance appending to the list. Optional. Type list of uniline.

🛑 DisableForwarding ✋

Disables all forwarding features, including X11, ssh-agent1, TCP and StreamLocal. This option overrides all other forwarding-related options and may simplify restricted configurations. Optional. Type uniline.

🕵️ ExposeAuthInfo 🔍

Writes a temporary file containing a list of authentication methods and public credentials (e.g. keys) used to authenticate the user. The location of the file is exposed to the user session through the SSH_USER_AUTH environment variable 🌈. The default is no. Optional. Type boolean.

upstream_default value :
    no

💪 ForceCommand 👑

Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client and ~/.ssh/rc if present 😤. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside a Match block. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Specifying a command of internal-sftp will force the use of an in-process SFTP server that requires no support files when used with ChrootDirectory. The default is none. Optional. Type uniline.

upstream_default value :
    none

🚪 GatewayPorts 🌉

Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd(8) binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports 🛑. GatewayPorts can be used to specify that sshd should allow remote port forwardings to bind to non-loopback addresses, thus allowing other hosts to connect ✨. The argument may be:

no — force remote port forwardings to be available to the local host only.
yes — force remote port forwardings to bind to the wildcard address.
clientspecified — allow the client to select the address to which the forwarding is bound.

The default is no. Optional. Type enum. choice: 'clientspecified', 'no', 'yes'.

upstream_default value :
    no

🎫 GSSAPIAuthentication 🌟

Specifies whether user authentication based on GSSAPI is allowed. The default is no. Optional. Type boolean.

upstream_default value :
    no

🔑 HostbasedAcceptedAlgorithms 🧚

Specifies the signature algorithms that will be accepted for hostbased authentication as a list of comma-separated patterns. Alternately:

if the specified list begins with a + character, then the specified signature algorithms will be appended to the default set instead of replacing them 💖.
if the specified list begins with a - character, then the specified signature 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 signature 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 HostbasedAcceptedAlgorithms. This was formerly named HostbasedAcceptedKeyTypes. Optional. Type uniline.

🏠 HostbasedAuthentication 💜

Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (host-based authentication). The default is no. Optional. Type boolean.

upstream_default value :
    no

🦋 HostbasedUsesNameFromPacketOnly

okay so this one is all about vibes of identity during HostbasedAuthentication. It decides whether the server bothers to do a reverse name lookup when it's matching the name in your ~/.shosts, ~/.rhosts and /etc/hosts.equiv files.

Set it to yes and sshd(8) just trusts the name the client handed over — no attempt to resolve the name from the TCP connection itself. It's giving "I believe you bestie". 💅

Default is no. Which, honestly, fair.

Optional. Type: boolean.
upstream_default value: no

🎀 IgnoreRhosts

Should we ignore per-user .rhosts and .shosts files during HostbasedAuthentication? That's the whole question. And heads up: the system-wide /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used no matter what you set here. They are not part of this conversation, they're the main characters. 👑

Accepted values, bestie:

yes — the default, ignores all per-user files. bye.
shosts-only — allows the use of .shosts but ignores .rhosts.
no — allows both .shosts and rhosts. (yes, "rhosts", that's what the page says, don't @ me 😭)
Optional. Type: boolean.
upstream_default value: yes

🌸 Include

Include the specified configuration file(s). You can name multiple pathnames, and each pathname may contain glob(7) wildcards — those get expanded and processed in lexical order.

Files without absolute paths are assumed to be in /etc/ssh.

And yes, an Include directive may appear inside a Match block to do conditional inclusion. Sneaky. Iconic. 🫶

Optional. Type: list of uniline.

✨ IPQoS

Specifies the Differentiated Services Field Codepoint (DSCP) value for the connection. Networking queen behaviour fr fr.

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 use the operating system default.

This option may take one or two arguments, separated by whitespace:

one argument → it's used as the packet class unconditionally.
two values → the first is automatically selected for interactive sessions, 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 allow keyboard-interactive authentication. Default is yes. ✨

The argument to this keyword must be yes or no.

And ChallengeResponseAuthentication is a deprecated alias for this. it's giving old money, retired money. 💀

Optional. Type: boolean.
upstream_default value: yes

Note: KbdInteractiveAuthentication is migrated with '$old' and with: - $old => "- ChallengeResponseAuthentication"

🔥 KerberosAuthentication

Specifies whether the password provided by the user for PasswordAuthentication will be validated through the Kerberos KDC.

To use this option, the server needs a Kerberos servtab which allows the verification of the KDC's identity. No servtab, no party.

The default is no.

Optional. Type: boolean.
upstream_default value: no

👑 LogLevel

Gives the verbosity level that is used when logging messages from sshd(8). Turn the mic up or down, your call.

The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.

The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging output.

Logging with a DEBUG level violates the privacy of users and is not recommended. like, actually don't. 🙈

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

🍓 MaxAuthTries

Specifies the maximum number of authentication attempts permitted per connection. Once the number of failures reaches half this value, additional failures are logged. so like, halfway to the limit is when the receipts start. 🧾

The default is 6.

Optional. Type: integer.
upstream_default value: 6

🌈 MaxSessions

Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions permitted per network connection.

Multiple sessions may be established by clients that support connection multiplexing.

The tea on the numbers:

setting MaxSessions to 1 will effectively disable session multiplexing.
setting it to 0 will prevent all shell, login and subsystem sessions while still permitting forwarding. zero sessions but the forwarding stays slaying. 🔥

The default is 10.

Optional. Type: integer.
upstream_default value: 10

🎀 PAMServiceName

Specifies the service name used for Pluggable Authentication Modules (PAM) authentication, authorisation and session controls when UsePAM is enabled.

The default is sshd.

Optional. Type: uniline.

💜 PasswordAuthentication

Specifies whether password authentication is allowed. The default is yes.

Optional. Type: uniline.
upstream_default value: sshd

(and yes, bestie, the page really does say sshd right there. we report, we don't editorialise. 🫶)

🦋 PermitEmptyPasswords

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

The default is no. obviously. 😭

Optional. Type: boolean.
upstream_default value: no

🌸 PermitListen

Specifies the addresses/ports on which a remote TCP port forwarding may listen.

The listen specification must be one of the following forms:

PermitListen port
PermitListen host:port

Multiple permissions may be specified by separating them with whitespace.

an argument of any can be used to remove all restrictions and permit any listen requests.
an argument of none can be used to prohibit all listen requests.

The host name may contain wildcards as described in the PATTERNS section in ssh_config5. The wildcard '*' can also be used in place of a port number to allow all ports.

By default all port forwarding listen requests are permitted.

Note that the GatewayPorts option may further restrict which addresses may be listened on.

Note also that ssh(1) will request a listen host of 'localhost' if no listen host was specifically requested, and this name is treated differently to explicit localhost addresses of '127.0.0.1' and '::1'. yes that's a whole plot twist. 💀

Optional. Type: list of uniline.

⭐ PermitOpen

Specifies the destinations to which TCP port forwarding is permitted.

The forwarding specification must be one of the following forms:

PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port

Multiple forwards may be specified by separating them with whitespace.

an argument of any can be used to remove all restrictions and permit any forwarding requests.
an argument of none can be used to prohibit all forwarding requests.

The wildcard '*' can be used for host or port to allow all hosts or ports respectively. Otherwise, no pattern matching or address lookups are performed on supplied names.

By default all port forwarding requests are permitted.

Optional. Type: list of uniline.

👑 PermitRootLogin

Specifies whether root can log in using ssh(1).

The argument must be yes, prohibit-password, forced-commands-only or no. The default is prohibit-password.

Breaking down the options, because this one is important bestie:

If this option is set to prohibit-password (or its deprecated alias, without-password), password and keyboard-interactive authentication are disabled for root.
If this option is set to forced-commands-only, root login with public key authentication will be allowed, but only if the command option has been specified (which may be useful for taking remote backups even if root login is normally not allowed). All other authentication methods are disabled for root.
If this option is set to no, root is not allowed to log in. periodt.
Optional. Type: enum. choice: 'forced-commands-only', 'no', 'prohibit-password', 'yes'.

💅 PermitTTY

Specifies whether pty(4) allocation is permitted. The default is yes.

Optional. Type: boolean.
upstream_default value: yes

🧚 PermitTunnel

Specifies whether tun(4) device forwarding is allowed.

The argument must be yes, point-to-point (layer 3), ethernet (layer 2), or no.

Specifying yes permits both point-to-point and ethernet. The default is no.

Independent of this setting, the permissions of the selected tun(4) device must allow access to the user. that part is not optional, that part is the law. ⚖️

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

🎀 PermitUserRC

Specifies whether any ~/.ssh/rc file is executed. The default is yes.

Optional. Type: boolean.
upstream_default value: yes

✨ PubkeyAcceptedAlgorithms

Specifies the signature algorithms that will be accepted for public key authentication as a list of comma-separated patterns. this is the guest list, and the bouncer is strict. 🚪

The prefix lore:

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 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.

🔥 PubkeyAuthOptions

Sets one or more public key authentication options.

The supported keywords are: none (the default; indicating no additional options are enabled), touch-required and verify-required.

The touch-required option causes public key authentication using a FIDO authenticator algorithm (i.e. ecdsa-sk or ed25519-sk) to always require the signature to attest that a physically present user explicitly confirmed the authentication (usually by touching the authenticator). By default, sshd(8) requires user presence unless overridden with an authorized_keys option. The touch-required flag disables this override.
The verify-required option requires a FIDO key signature attest that the user was verified, e.g. via a PIN. 🔐

Neither the touch-required or verify-required options have any effect for other, non-FIDO, public key types. they simply do not apply, they're not the target audience. 💅

Optional. Type: enum. choice: 'none', 'touch-required', 'verify-required'.

🌈 PubkeyAuthentication

Specifies whether public key authentication is allowed. The default is yes.

Optional. Type: boolean.
upstream_default value: yes

💀 RefuseConnection

Indicates that sshd(8) should unconditionally terminate the connection. no explanation, no negotiation, just door. 🚪

Additionally, a refuseconnection penalty may be recorded against the source of the connection if PerSourcePenalties are enabled.

This option is only really useful in a Match block.

Optional. Type: uniline.

RekeyLimit

Okay so this one is all about when do we switch up the session key, bestie. 💅 It sets the maximum amount of data that can be sent or received before the session key gets renegotiated, and you can optionally tack on a maximum amount of time that may pass before rekeying too.

The first argument is in bytes and can wear a little suffix glow-up: 'K', 'M' or 'G' for Kilobytes, Megabytes or Gigabytes respectively. ✨
The default is somewhere between '1G' and '4G', depending on the cipher. iykyk.
That optional second value is in seconds and can use any of the units documented in the TIME FORMATS section.

The default value for RekeyLimit is default none, which basically means rekeying happens after the cipher's default amount of data has been sent or received, and no time based rekeying is done. Optional. Type uniline.

RevokedKeys

This one is the block list energy we love. 🚫 It specifies the revoked public keys file, or none to not use one. Any keys listed in this file will be refused for public key authentication.

Not me warning you: if this file is not readable, then public key authentication will be refused for all users. So like, keep it readable, bestie. 😭

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 info on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen1. Optional. Type uniline.

SetEnv

This is where you just set the vibes, literally. 🌸 It specifies one or more environment variables to set in child sessions started by sshd(8) as ''NAME=VALUE''. The environment value may be quoted (e.g. if it contains whitespace characters).

Environment variables set by SetEnv override the default environment and any variables specified by the user via AcceptEnv or PermitUserEnvironment. Optional. Type uniline.

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. Optional. Type uniline.

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, sshd will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-domain socket file.

The argument must be yes or no. The default is no. Optional. Type boolean.

upstream_default value:

no

TrustedUserCAKeys

Specifies a file containing public keys of certificate authorities that are trusted to sign user certificates for authentication, or none to not use one. 🎀

Keys are listed one per line; empty lines and comments starting with '#' are allowed.
If a certificate is presented for authentication and has its signing CA key listed in this file, then it may be used for authentication for any user listed in the certificate's principals list.

Heads up though: certificates that lack a list of principals will not be permitted for authentication using TrustedUserCAKeys.

For more details on certificates, see the CERTIFICATES section in ssh-keygen1. Optional. Type uniline.

UnusedConnectionTimeout

Specifies whether and how quickly sshd(8) should close client connections with no open channels. 👀

Open channels include:

active shell, command execution or subsystem sessions
connected network, socket, agent or X11 forwardings

Forwarding listeners, such as those from the ssh(1) -R flag, are not considered as open channels and do not prevent the timeout. The timeout value is specified in seconds or may use any of the units documented in the TIME FORMATS section.

Note that this timeout starts when the client connection completes user authentication but before the client has an opportunity to open any channels.

Caution should be used when using short timeout values, as they may not provide sufficient time for the client to request and open its channels before terminating the connection. 💀

The default none is to never expire connections for having no open channels. This option may be useful in conjunction with ChannelTimeout. Optional. Type uniline.

X11DisplayOffset

Specifies the first display number available for sshd(8)Ns's X11 forwarding. This prevents sshd from interfering with real X11 servers. The default is 10. Optional. Type uniline.

X11Forwarding

Specifies whether X11 forwarding is permitted. The argument must be yes or no. The default is no.

When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd(8) proxy display is configured to listen on the wildcard address (see X11UseLocalhost), though this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side.

The security risk of using X11 forwarding is that the client's X11 display server may be exposed to attack when the SSH client requests forwarding (see the warnings for ForwardX11 in ssh_config5). A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a no setting.

Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders. 🙈

Optional. Type boolean.

upstream_default value:

no

X11UseLocalhost

Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or to the wildcard address. 🌈

By default, sshd binds the forwarding server to the loopback address and sets the hostname part of the DISPLAY environment variable to localhost. This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. X11UseLocalhost may be set to no to specify that the forwarding server should be bound to the wildcard address.

The argument must be yes or no. The default is yes. Optional. Type boolean.

upstream_default value:

yes

AuthorizedKeysFile2

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