SSH(1)

💖 SSH(1) 💖 — the OpenSSH remote login client, but make it a group chat

okay so bestie, welcome to ssh(1) — straight out of the General Commands Manual section. This is the remote login client, and yes, it's about to become your whole personality. Sit down, hydrate, let's go 🫶

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✨ NAME ✨

ssh — the OpenSSH remote login client 💅

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✨ SYNOPSIS ✨

Like, the vibe in its purest form:

ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
    [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
    [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
    [-J destination] [-L address] [-l login_name] [-m mac_spec]
    [-O ctl_cmd] [-o option] [-P tag] [-p port] [-R address]
    [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
    [command [argument ...]]

and also, when you're just nosy:

ssh [-Q query_option]

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✨ DESCRIPTION ✨

ssh (the SSH client, hi) is a program for logging into a remote machine and for executing commands on a remote machine. Its whole deal is providing secure encrypted communications between two untrusted hosts over an insecure network — like, it's giving bodyguard energy fr fr.

X11 connections, arbitrary TCP ports and Unix-domain sockets can also be forwarded over the secure channel. No cap. 🔥

ssh is binary compatible with the more well known OpenSSH and uses the same configuration directives, methods, and keywords except where noted — with the exception of the default port. HPN-SSH servers, by default, use port 2222 for connection, and as such, ssh clients attempt to connect on that port. If the connection attempt on port 2222 fails it will fallback to port 22. Please see the -p switch for more information. (Yes, it tries the spicy port first and then quietly knocks on the classic door. Iconic behaviour honestly.)

ssh connects and logs into the specified destination, which may be specified as either [user@]hostname or a URI of the form ssh://[user@]hostname[:port]. The user must prove their identity to the remote machine using one of several methods (see below). Prove it, bestie. Show your ID. 🎀

If a command is specified, it will be executed on the remote host instead of a login shell. A complete command line may be specified as command, or it may have additional arguments. If supplied, the arguments will be appended to the command, separated by spaces, before it is sent to the server to be executed.

The options are as follows:

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🎀 OPTIONS 🎀

• -4 — Forces ssh to use IPv4 addresses only. Old reliable. 🧚
• -6 — Forces ssh to use IPv6 addresses only. Future-forward queen. ✨
• -A — Enables forwarding of connections from an authentication agent such as ssh-agent(1). This can also be specified on a per-host basis in a configuration file.

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. A safer alternative may be to use a jump host (see -J).

So like… it slays but it's also lowkey a risk. Read the warning, bestie 😭

• -a — Disables forwarding of the authentication agent connection. The opposite vibe of -A, respectfully.
• -B bind_interface — Bind to the address of bind_interface before attempting to connect to the destination host. This is only useful on systems with more than one address.
• -b bind_address — Use bind_address on the local machine as the source address of the connection. Only useful on systems with more than one address. (Twins, basically.)
• -C — Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and Unix-domain connections). The compression algorithm is the same used by gzip(1). Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option in ssh_config(5). Squish it 🍓
• -c cipher_spec — Selects the cipher specification for encrypting the session. cipher_spec is a comma-separated list of ciphers listed in order of preference. See the Ciphers keyword in ssh_config(5) for more information.
• -D [bind_address:]port — Specifies a local “dynamic” application-level port forwarding. This works by allocating a socket to listen to port on the local side, optionally bound to the specified bind_address. Whenever a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.

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

(the way this one turns your laptop into a whole SOCKS server… main character energy, no notes)

• -E log_file — Append debug logs to log_file instead of standard error. Receipts, but saved to a file 💜
• -e escape_char — Sets the escape character for sessions with a pty (default: ‘~’). The escape character is only recognized at the beginning of a line. The escape character followed by a dot (‘.’) closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to “none” disables any escapes and makes the session fully transparent. 🦋
• -F configfile — Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/gsissh/ssh_config) will be ignored. The default for the per-user configuration file is ~/.ssh/config. If set to “none”, no configuration files will be read.
• -f — 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 -n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm.

If the ExitOnForwardFailure configuration option is set to “yes”, then a client started with -f will wait for all remote port forwards to be successfully established before placing itself in the background. Refer to the description of ForkAfterAuthentication in ssh_config(5) for details.

• -G — Causes ssh to print its configuration after evaluating Host and Match blocks and exit. Just showing you the fit before it leaves ✨
• -g — Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.
• -I pkcs11 — Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing keys for user authentication. Hardware keys, fancy 🔑
• -i identity_file — Selects a file from which the identity (private key) for public key authentication is read. You can also specify a public key file to use the corresponding private key that is loaded in ssh-agent(1) when the private key file is not present locally. The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519 and ~/.ssh/id_ed25519_sk. Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files). If no certificates have been explicitly specified by the CertificateFile directive, ssh will also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.

multiple -i options?? she contains multitudes 💖

• -J destination — Connect to the target host by first making an ssh connection to the jump host described by destination and then establishing a TCP forwarding to the ultimate destination from there. Multiple jump hops may be specified separated by comma characters. IPv6 addresses can be specified by enclosing the address in square brackets. This is a shortcut to specify a ProxyJump configuration directive. Note that configuration directives supplied on the command-line generally apply to the destination host and not any specified jump hosts. Use ~/.ssh/config to specify configuration for jump hosts. Hop, hop, hop, slay 🐇
• -K — Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.
• -k — Disables forwarding (delegation) of GSSAPI credentials to the server. (-K vs -k is literally the most sibling-coded option pair in here 😭)
• -L — Specifies that connections to the given TCP port or Unix socket on the local (client) host are to be forwarded to the given host and port, or Unix socket, on the remote side. Four flavours, pick your fighter:

-L [bind_address:]port:host:hostport
-L [bind_address:]port:remote_socket
-L local_socket:host:hostport
-L local_socket:remote_socket

This works by allocating a socket to listen to either a TCP port on the local side, optionally bound to the specified bind_address, or to a Unix socket. Whenever a connection is made to the local port or socket, the connection is forwarded over the secure channel, and a connection is made to either host port hostport, or the Unix socket remote_socket, from the remote machine.

Port forwardings can also be specified in the configuration file. Only the superuser can forward privileged ports. IPv6 addresses can be specified by enclosing the address 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.

• -l login_name — Specifies the user to log in as on the remote machine. This also may be specified on a per-host basis in the configuration file. Who are we today bestie 🎀

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and that's just the first stretch of the alphabet, there is SO much more coming — don't go anywhere 💅

• -M — bestie, this drops the ssh client into "master" mode for connection sharing. And if you stack multiple -M options? It still goes "master" mode but now it wants confirmation before every operation that changes the multiplexing state (think: opening a new session), and it asks via ssh-askpass(1). Full deets live in the ControlMaster description in ssh_config(5). 💅
• -m mac_spec — a comma-separated list of MAC (message authentication code) algorithms, and the order you write them in = your preference order, so put your fave first. Check the MACs keyword in ssh_config(5) for more. 🔐
• -N — do not execute a remote command. Literally the "I'm only here for the port forwarding" flag, iconic behaviour honestly. Refer to the SessionType description in ssh_config(5) for details. 🚇
• -n — redirects stdin from /dev/null (okay so technically it prevents reading from stdin, let's be accurate). You must use this when ssh is running in the background. The classic trick is using it to run X11 programs on a remote machine. Like:

ssh -n shadows.cs.hut.fi emacs &

That'll start an emacs on shadows.cs.hut.fi, the X11 connection gets automatically forwarded over an encrypted channel, and the ssh program goes into the background. ✨

Heads up bestie: this does not work if ssh needs to ask for a password or passphrase. See also the -f option. And refer to the StdinNull description in ssh_config(5) for details.

• -O ctl_cmd — controls an active connection multiplexing master process. When -O is specified, the ctl_cmd argument gets interpreted and passed along to the master process. The valid commands are:
  • check — check that the master process is running
  • forward — request forwardings without command execution
  • cancel — cancel forwardings
  • proxy — connect to a running multiplexing master in proxy mode
  • exit — request the master to exit
  • stop — request the master to stop accepting further multiplexing requests
• -o option — gives you options in the same format used in the configuration file. Super useful for specifying options that don't have their own separate command-line flag. For the full details on the options listed below and their possible values, go read ssh_config(5). 📖

AddKeysToAgent
AddressFamily
BatchMode
BindAddress
BindInterface
CASignatureAlgorithms
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CertificateFile
ChannelTimeout
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectTimeout
ConnectionAttempts
ControlMaster
ControlPath
ControlPersist
DisableMTAES*
DynamicForward
EnableEscapeCommandline
EnableSSHKeysign
EscapeChar
ExitOnForwardFailure
FallbackPort
FingerprintHash
ForkAfterAuthentication
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GatewayPorts
GlobalKnownHostsFile
HappyEyes
HappyDelay
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
HashKnownHosts
Host
HostKeyAlgorithms
HostKeyAlias
HostbasedAcceptedAlgorithms
HostbasedAuthentication
Hostname
HPNDisabled*
IPQoS
IdentitiesOnly
IdentityAgent
IdentityFile
IgnoreUnknown
Include
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel
LogVerbose
MACs
Metrics
MetricsInterval
MetricsPath
NoHostAuthenticationForLocalhost
NoneCipherEnabled*
NoneEnabled*
NoneMacEnabled*
NumberOfPasswordPrompts
ObscureKeystrokeTiming
PKCS11Provider
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
RequiredRSASize
RevokedHostKeys
SecurityKeyProvider
SendEnv
ServerAliveCountMax
ServerAliveInterval
SessionType
SetEnv
StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
SyslogFacility
TCPKeepAlive
TcpRcvBufPoll*
Tag
Tunnel
TunnelDevice
UpdateHostKeys
UseMPTCP
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation

* Hpnssh specific configuration option.

• -P tag — specifies a tag name that may be used to select configuration in ssh_config(5). Refer to the Tag and Match keywords in ssh_config(5) for more info. 🏷️
• -p port — the port to connect to on the remote host. You can also set this on a per-host basis in the configuration file. HPN-SSH uses a default port of 2222. It will automatically fallback to use the SSH standard port 22 if it cannot connect on port 2222. That fallback behaviour can be modified with the FallbackPort option.

Real talk bestie: if outbound port 2222 is blocked, it may appear that the hpnssh client is non-responsive. In that event, either specify the correct port or use the ConnectTimeout option to trigger the port fallback more quickly. ⏰

• -Q query_option — queries for the algorithms supported by one of these features:
  • cipher — supported symmetric ciphers
  • cipher-auth — supported symmetric ciphers that support authenticated encryption
  • help — supported query terms for use with the -Q flag
  • mac — supported message integrity codes
  • kex — key exchange algorithms
  • kex-gss — GSSAPI key exchange algorithms
  • key — key types
  • key-ca-sign — valid CA signature algorithms for certificates
  • key-cert — certificate key types
  • key-plain — non-certificate key types
  • key-sig — all key types and signature algorithms
  • protocol-version — supported SSH protocol versions
  • sig — supported signature algorithms

Alternatively, any keyword from ssh_config(5) or sshd_config(5) that takes an algorithm list may be used as an alias for the corresponding query_option. 🧠

• -q — quiet mode. Causes most warning and diagnostic messages to be suppressed. Shhh. 🤫
• -R [bind_address:]port:host:hostport
• -R [bind_address:]port:local_socket
• -R remote_socket:host:hostport
• -R remote_socket:local_socket
• -R [bind_address:]port

Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the local side. 🔁

It works by allocating a socket to listen to either a TCP port or to a Unix socket on the remote side. Whenever a connection is made to this port or Unix socket, the connection is forwarded over the secure channel, and a connection is made from the local machine to either an explicit destination specified by host port hostport, or local_socket, or — if no explicit destination was specified — ssh will act as a SOCKS 4/5 proxy and forward connections to the destinations requested by the remote SOCKS client. 🧦

Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in square brackets.

By default, TCP listening sockets on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address. An empty bind_address, or the address *, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config(5)).

If the port argument is 0, the listen port will be dynamically allocated on the server and reported to the client at run time. When used together with -O forward, the allocated port will be printed to the standard output. 👑

• -S ctl_path — specifies the location of a control socket for connection sharing, or the string none to disable connection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config(5) for details. 🔌
• -s — may be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH as a secure transport for other applications (e.g. sftp(1)). The subsystem is specified as the remote command. Refer to the description of SessionType in ssh_config(5) for details. 🛰️
• -T — disable pseudo-terminal allocation. 🚫

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• -t — bestie, this one forces pseudo-terminal allocation. It's the move when you gotta run arbitrary screen-based programs on a remote machine — so useful, like when you're implementing menu services, iykyk. Stack multiple -t options and it'll force tty allocation even if ssh has no local tty. periodt.
• -V — shows you the version number and dips. that's it, that's the flag 💅
• -v — verbose mode ✨ makes ssh spill debugging messages about its progress. literally the girl you want when you're debugging connection, authentication, and configuration problems. Stack multiple -v options to crank the verbosity — the maximum is 3. no more than that, she said what she said.
• -W host:port — asks that standard input and output on the client get forwarded to host on port over the secure channel. It implies -N, -T, ExitOnForwardFailure and ClearAllForwardings, though these can be overridden in the configuration file or by using -o command line options. (yes bestie, you can fight the defaults, the config file and -o win.)
• -w local_tun[:remote_tun] — requests tunnel device forwarding with the specified tun(4) devices between the client (local_tun) and the server (remote_tun).

The devices can be specified by numerical ID or by the keyword “any”, which uses the next available tunnel device. If remote_tun isn't specified, it defaults to “any”. See also the Tunnel and TunnelDevice directives in ssh_config(5).

Heads up: if the Tunnel directive is unset, it gets set to the default tunnel mode, which is “point-to-point”. If you want a different Tunnel forwarding mode, you gotta specify it before -w. order matters, bestie 🫶

• -X — enables X11 forwarding ✨ You can also set this per-host in a configuration file.

okay so this one needs caution. Users who can bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker could then do stuff like keystroke monitoring 😭

For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default. Go look at the ssh -Y option and the ForwardX11Trusted directive in ssh_config(5) for the full tea.

• -x — disables X11 forwarding. the off switch, the anti--X 💀
• -Y — enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls. (that's the whole difference, bestie, don't sleep on it.)
• -y — send log information using the syslog(3) system module. By default this information goes to stderr instead. 🍓

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ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. The file format and configuration options are described in ssh_config(5).

💖 AUTHENTICATION 💖

The OpenSSH SSH client supports SSH protocol 2.

The methods available for authentication are: GSSAPI-based authentication, host-based authentication, public key authentication, keyboard-interactive authentication, and password authentication. They get tried in the order listed above, though PreferredAuthentications can be used to change the default order. (yes, you can reorder the lineup. main character energy.)

Host-based authentication works like this: if the machine the user logs in from is listed in /etc/hosts.equiv or /etc/gsissh/shosts.equiv on the remote machine, the user is non-root, and the user names are the same on both sides — or if the files ~/.rhosts or ~/.shosts exist in the user's home directory on the remote machine and contain a line with the name of the client machine and the name of the user on that machine — then the user is considered for login. On top of that, the server has to be able to verify the client's host key (see the description of /etc/gsissh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing, and routing spoofing.

[Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.] 👑

Public key authentication works like this: the scheme is based on public-key cryptography, using cryptosystems where encryption and decryption are done using separate keys, and it is unfeasible to derive the decryption key from the encryption key. The idea is that each user creates a public/private key pair for authentication purposes. The server knows the public key, and only the user knows the private key. ssh implements the public key authentication protocol automatically, using one of the ECDSA, Ed25519 or RSA algorithms.

The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells the server which key pair it would like to use for authentication. The client proves that it has access to the private key, and the server checks that the corresponding public key is authorized to accept the account. 🔐

The server may inform the client of errors that prevented public key authentication from succeeding after authentication completes using a different method. These can be viewed by increasing the LogLevel to DEBUG or higher (e.g. by using the -v flag).

The user creates their key pair by running ssh-keygen(1). This stores the private key in:

• ~/.ssh/id_ecdsa (ECDSA)
• ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA)
• ~/.ssh/id_ed25519 (Ed25519)
• ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519)
• ~/.ssh/id_rsa (RSA)

and stores the public key in:

• ~/.ssh/id_ecdsa.pub (ECDSA)
• ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA)
• ~/.ssh/id_ed25519.pub (Ed25519)
• ~/.ssh/id_ed25519_sk.pub (authenticator-hosted Ed25519)
• ~/.ssh/id_rsa.pub (RSA)

in the user's home directory. The user should then copy the public key to ~/.ssh/authorized_keys in their home directory on the remote machine. The authorized_keys file corresponds to the conventional ~/.rhosts file, and has one key per line, though the lines can be very long. After this, the user can log in without giving the password. slay ✨

A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private keys, signed certificates are used. This has the advantage that a single trusted certification authority can be used in place of many public/private keys. See the CERTIFICATES section of ssh-keygen(1) for more information.

The most convenient way to use public key or certificate authentication may be with an authentication agent. See ssh-agent(1) and (optionally) the AddKeysToAgent directive in ssh_config(5) for more information. 🦋

Keyboard-interactive authentication works like this: the server sends an arbitrary "challenge" text and prompts for a response, possibly multiple times. Examples of keyboard-interactive authentication include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD systems).

Finally, if other authentication methods fail, ssh prompts the user for a password. The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone listening on the network. 🎀

ssh automatically maintains and checks a database containing identification for all hosts it has ever been used with. Host keys are stored in ~/.ssh/known_hosts in the user's home directory. Additionally, the file /etc/gsissh/ssh_known_hosts is automatically checked for known hosts. Any new hosts are automatically added to the user's file. If a host's identification ever changes, ssh warns about this and disables password authentication to prevent server spoofing or man-in-the-middle attacks, which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to control logins to machines whose host key is not known or has changed. 😭 stay safe out there

When the user's identity has been accepted by the server, the server either executes the given command in a non-interactive session or, if no command has been specified, logs into the machine and gives the user a normal shell as an interactive session. All communication with the remote command or shell will be automatically encrypted. 🔥

If an interactive session is requested, ssh by default will only request a pseudo-terminal (pty) for interactive sessions when the client has one. The flags -T and -t can be used to override this behaviour.

If a pseudo-terminal has been allocated, the user may use the escape characters noted below.

If no pseudo-terminal has been allocated, the session is transparent and can be used to reliably transfer binary data. On most systems, setting the escape character to “none” will also make the session transparent even if a tty is used. (iykyk, this is the scp-ish vibe.)

The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed.

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🌈 ESCAPE CHARACTERS 🌈

When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.

A single tilde character can be sent as ~~ or by following the tilde by a character other than those described below. The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files using the EscapeChar configuration directive, or on the command line by the -e option.

The supported escapes (assuming the default ‘~’) are:

• ~. — Disconnect. 💀
• ~^Z — Background ssh.
• ~# — List forwarded connections.
• ~& — Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.
• ~? — Display a list of escape characters.
• ~B — Send a BREAK to the remote system (only useful if the peer supports it).
• ~C — Open command line. Currently this allows the addition of port forwardings using the -L, -R and -D options (see above). It also allows the cancellation of existing port-forwardings with -KL[bind_address:]port for local, -KR[bind_address:]port for remote and -KD[bind_address:]port for dynamic port-forwardings. !command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config(5). Basic help is available, using the -h option. ✨
• ~R — Request rekeying of the connection (only useful if the peer supports it).
• ~V — Decrease the verbosity (LogLevel) when errors are being written to stderr.
• ~v — Increase the verbosity (LogLevel) when errors are being written to stderr.

✨ TCP FORWARDING ✨

okay bestie so you can tunnel arbitrary TCP connections over a secure channel 💅 and you get to choose how — either straight from the command line or from a config file. no wrong answers. some iconic use cases: a secure connection to a mail server, or the classic — slipping through firewalls like it's nothing 🧚

so here's the vibe in the example below: we're encrypting communication for an IRC client, even though the IRC server it connects to does not directly support encrypted communication. rude of it, but we move. here's how it goes down: the user connects to the remote host using ssh, specifying the ports to be used to forward the connection. after that you can just start the program locally, and ssh will encrypt and forward the connection to the remote server. main character energy honestly 👑

the following example tunnels an IRC session from the client to an IRC server at server.example.com, joining channel #users, nickname pinky, using the standard IRC port, 6667:

$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
$ irc -c '#users' pinky IRC/127.0.0.1

the -f option backgrounds ssh, and the remote command sleep 10 is specified to allow an amount of time (10 seconds, in the example) to start the program which is going to use the tunnel. and lowkey important: if no connections are made within the time specified, ssh will exit. bye girl 💀

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🦋 X11 FORWARDING 🦋

if the ForwardX11 variable is set to yes (or, y'know, see the description of the -X, -x, and -Y options above 💖) and the user is using X11 (meaning the DISPLAY environment variable is set), then the connection to the X11 display is automatically forwarded to the remote side — in such a way that any X11 programs started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. so the whole thing is encrypted, slay.

bestie do NOT manually set DISPLAY. that's not your job, that's ssh's job. let her cook 🍓

forwarding of X11 connections can be configured on the command line or in configuration files, whichever you vibe with.

the DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. this is normal, and happens because ssh creates a “proxy” X server on the server machine for forwarding the connections over the encrypted channel. no cap, that's just the design 🌈

ssh will also automatically set up Xauthority data on the server machine. for this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. and here's the tea: the real authentication cookie is never sent to the server machine (and no cookies are sent in the plain). periodt 🔥

if the ForwardAgent variable is set to yes (or see the description of the -A and -a options above), and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side. little agent bestie going on a field trip ✨

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👑 VERIFYING HOST KEYS 👑

when connecting to a server for the first time, a fingerprint of the server's public key is presented to the user (unless the option StrictHostKeyChecking has been disabled). so you get a lil vibe check before you commit. fingerprints can be determined using ssh-keygen(1):

$ ssh-keygen -l -f /etc/gsissh/ssh_host_rsa_key

if the fingerprint is already known, it can be matched and the key can be accepted or rejected. and if only legacy (MD5) fingerprints for the server are available, the ssh-keygen(1) -E option may be used to downgrade the fingerprint algorithm to match. retro, but make it functional 💅

because of the difficulty of comparing host keys just by looking at fingerprint strings (they all kinda look like alphabet soup fr fr 😭), there is also support to compare host keys visually, using random art. by setting the VisualHostKey option to yes, a small ASCII graphic gets displayed on every login to a server, no matter if the session itself is interactive or not. by learning the pattern a known server produces, a user can easily find out that the host key has changed when a completely different pattern is displayed. iconic little art piece honestly 🎀

okay but real talk — because these patterns are not unambiguous, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaranteed proof. it's giving "probably your bestie" not "confirmed by the government" 🫶

to get a listing of the fingerprints along with their random art for all known hosts, the following command line can be used:

$ ssh-keygen -lv -f ~/.ssh/known_hosts

if the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. an additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to match the fingerprint with that of the key presented. very futuristic of them ⭐

in this example, we are connecting a client to a server, host.example.com. the SSHFP resource records should first be added to the zonefile for host.example.com:

$ ssh-keygen -r host.example.com.

the output lines will have to be added to the zonefile. to check that the zone is answering fingerprint queries:

$ dig -t SSHFP host.example.com

finally the client connects:

$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?

see the VerifyHostKeyDNS option in ssh_config(5) for more information. iykyk 🌸

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🌈 SSH-BASED VIRTUAL PRIVATE NETWORKS 🌈

ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network pseudo-device, allowing two networks to be joined securely. two networks, one encrypted situationship 💜 the sshd_config(5) configuration option PermitTunnel controls whether the server supports this, and at what level (layer 2 or layer 3 traffic).

the following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway to the remote network, at 192.168.1.15, allows it.

on the client:

# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
# route add 10.0.99.0/24 10.1.1.2

on the server:

# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
# route add 10.0.50.0/24 10.1.1.1

client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the PermitRootLogin server option. the following entry would permit connections on tun(4) device 1 from user jane and on tun device 2 from user john, if PermitRootLogin is set to forced-commands-only:

tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups, such as for wireless VPNs. so like, cute for a quick thing, not a forever thing 🙈 more permanent VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8). give those girls the long-term gig 💖

🌿 ENVIRONMENT 🌿

okay so ssh is out here just setting env vars for you like a good bestie 💖 normally it sets these:

• DISPLAY — this one tells everyone where the X11 server is at 🖥️ ssh sets it automatically to a value shaped like “hostname:n”, where “hostname” is the host where the shell runs and ‘n’ is an integer ≥ 1. ssh uses this special little value to forward X11 connections over the secure channel ✨ and bestie, please do not set DISPLAY yourself — that literally renders the X11 connection insecure (and then you'd have to manually copy any required authorization cookies, which is giving unpaid labor 😭).
• HOME — set to the path of the user's home directory 🏠
• LOGNAME — a synonym for USER, kept around for compatibility with systems that use this variable 🫶
• MAIL — set to the path of the user's mailbox 📬
• PATH — set to the default PATH, as specified when compiling ssh 🛤️
• SSH_ASKPASS — if ssh needs a passphrase, it reads it from the current terminal if it was run from a terminal. But if ssh has no terminal associated with it and both DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase 🪟 this is highkey useful when you're calling ssh from a .xsession or related script. (heads up: on some machines you may need to redirect the input from /dev/null to make this work 👀)
• SSH_ASKPASS_REQUIRE — lets you control the askpass program even more:
  • if it's set to “never” then ssh will never attempt to use one 🙅
  • if it's set to “prefer”, then ssh will prefer to use the askpass program instead of the TTY when requesting passwords 💅
  • and finally if it's set to “force”, then the askpass program will be used for all passphrase input regardless of whether DISPLAY is set 🔥
• SSH_AUTH_SOCK — identifies the path of a Unix-domain socket used to communicate with the agent 🔌
• SSH_CONNECTION — identifies the client and server ends of the connection. It contains four space-separated values: client IP address, client port number, server IP address, and server port number 🌐
• SSH_ORIGINAL_COMMAND — contains the original command line if a forced command is executed. You can use it to extract the original arguments 🕵️
• SSH_TTY — set to the name of the tty (path to the device) associated with the current shell or command. If the current session has no tty, this variable is just… not set 🫥
• SSH_TUNNEL — optionally set by sshd(8) to contain the interface names assigned if tunnel forwarding was requested by the client 🚇
• SSH_USER_AUTH — optionally set by sshd(8), this variable may contain a pathname to a file that lists the authentication methods successfully used when the session was established, including any public keys that were used 🔑
• TZ — set to indicate the present time zone if it was set when the daemon was started (i.e. the daemon passes the value on to new connections) ⏰
• USER — set to the name of the user logging in 🙋
• X509_CERT_DIR — used for GSI authentication. Specifies a non-standard location for the CA certificates directory 📁
• X509_USER_CERT — used for GSI authentication. Specifies a non-standard location for the certificate to be used for authentication to the server 📜
• X509_USER_KEY — used for GSI authentication. Specifies a non-standard location for the private key to be used for authentication to the server 🗝️
• X509_USER_PROXY — used for GSI authentication. Specifies a non-standard location for the proxy credential to be used for authentication to the server 🎭

and additionally ✨ ssh reads ~/.ssh/environment, and adds lines of the format “VARNAME=value” to the environment if the file exists and users are allowed to change their environment. For more info, go look at the PermitUserEnvironment option in sshd_config(5) 📖

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📂 FILES 📂

• ~/.rhosts — this file is used for host-based authentication (see above 👆). On some machines this file may need to be world-readable if the user's home directory is on an NFS partition, because sshd(8) reads it as root 😤 additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permission for most machines is read/write for the user, and not accessible by others 🔒
• ~/.shosts — used in exactly the same way as .rhosts, but it allows host-based authentication without permitting login with rlogin/rsh 🚫
• ~/.ssh/ — the default location for all user-specific configuration and authentication information 🗂️ there's no general requirement to keep the entire contents of this directory secret, but the recommended permissions are read/write/execute for the user, and not accessible by others 💅
• ~/.ssh/authorized_keys — lists the public keys (ECDSA, Ed25519, RSA) that can be used for logging in as this user 🔑 the format of this file is described in the sshd(8) manual page. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others ✨
• ~/.ssh/config — the per-user configuration file ⚙️ the file format and configuration options are described in ssh_config(5). Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not writable by others. no cap 🚨
• ~/.ssh/environment — contains additional definitions for environment variables; see “ENVIRONMENT”, above 🌱
• ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk, ~/.ssh/id_rsa — contains the private key for authentication 🗝️ these files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). ssh will simply ignore a private key file if it is accessible by others 😭 it's possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using AES-128 🔐
• ~/.ssh/id_ecdsa.pub, ~/.ssh/id_ecdsa_sk.pub, ~/.ssh/id_ed25519.pub, ~/.ssh/id_ed25519_sk.pub, ~/.ssh/id_rsa.pub — contains the public key for authentication 🌈 these files are not sensitive and can (but need not) be readable by anyone 🫶
• ~/.ssh/known_hosts — contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys 📋 see sshd(8) for further details of the format of this file.
• ~/.ssh/rc — commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started 🏃 see the sshd(8) manual page for more information.
• /etc/hosts.equiv — this file is for host-based authentication (see above 👆). It should only be writable by root 👑
• /etc/gsissh/shosts.equiv — used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh 🚫
• /etc/gsissh/ssh_config — systemwide configuration file ⚙️ the file format and configuration options are described in ssh_config(5).
• /etc/gsissh/ssh_host_ecdsa_key, /etc/gsissh/ssh_host_ed25519_key, /etc/gsissh/ssh_host_rsa_key — these files contain the private parts of the host keys and are used for host-based authentication 🗝️
• /etc/gsissh/ssh_known_hosts — systemwide list of known host keys 📋 this file should be prepared by the system administrator to contain the public host keys of all machines in the organization. It should be world-readable 👀 see sshd(8) for further details of the format of this file.
• /etc/gsissh/sshrc — commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started 🏃 see the sshd(8) manual page for more information.

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🚪 EXIT STATUS 🚪

ssh exits with the exit status of the remote command, or with 255 if an error occurred 💀 so like, if you see 255, something went wrong fr fr.

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🌐 IPV6 🌐

IPv6 address can be used everywhere where IPv4 address 🌍 in all entries the IPv6 address must be enclosed in square brackets. Note: the square brackets are metacharacters for the shell and must be escaped in shell ⚠️

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👀 SEE ALSO 👀

scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)

the whole squad, honestly 💖✨

📜 STANDARDS 📜

bestie the paperwork is REAL and honestly? slay of them to write it all down. every single one of these is a whole document that says what SSH is allowed to do, so here they are, all of them, no skips 💅

• S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, January 2006.
• T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January 2006.
• T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January 2006.
• T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.
• T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.
• J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
• F. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, January 2006.
• J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, January 2006.
• M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, January 2006.
• B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, January 2006.
• M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
• J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.
• D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, December 2009.
• A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security, 1999, International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).

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👑 AUTHORS 👑

okay so who do we thank for this absolute icon of a program? 🌸

OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Then Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH — iconic behaviour, honestly. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. And Chris Rapier, Michael Stevens, Ben Bennet, and Mike Tasota developed the HPN extensions at the Pittsburgh Supercomuting Center with grants from Cisco, the National Library of Medicine, and the National Science Foundation. group project that ATE fr fr 🫶

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and that's the whole page, bestie!! 🦋 you made it to the end, which is lowkey a flex 💪 this one came from Debian and it's dated December 4, 2024, filed under SSH(1). now go close your laptop and hydrate, you've earned it 💖✨🎀