SSH(1)

📦
version 9.6p1 · 4 versions
, in ubuntu noble

SSH 💖 (section 1 of the manual, bestie — the OpenSSH remote login client)

✨ NAME ✨

ssh — the OpenSSH remote login client. That's it, that's the whole vibe. 🦋

🌸 SYNOPSIS 🌸

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 ...]]
ssh [-Q query_option]

okay so look at that second form. ssh -Q query_option is its own whole little mode, and yes we will get to it, stay with me 💅

🫶 DESCRIPTION 🫶

ssh (the SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. Its whole job is giving you secure encrypted communications between two untrusted hosts over an insecure network. That's the flex. Not only that, but X11 connections, arbitrary TCP ports and Unix-domain sockets can also be forwarded over that secure channel. it's giving ✨magic tunnel✨.

ssh connects and logs into the specified destination, which you can hand over in either of these two forms:

  • [user@]hostname
  • 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, bestie, it's coming). 🔐

If a command is specified, it gets executed on the remote host instead of a login shell. A complete command line may be specified as command, or it may come with additional arguments. If supplied, those arguments get appended to the command, separated by spaces, before it's sent to the server to be executed. no cap, that's just how it goes 🎀

The options are as follows:

🎀 OPTIONS 🎀

  • -4 — Forces ssh to use IPv4 addresses only. four. that's the whole personality.
  • -6 — Forces ssh to use IPv6 addresses only. six. equally decisive.
  • -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). bestie I am begging you to read that twice. 🙈
  • -a — Disables forwarding of the authentication agent connection. (the anti--A. slay.)
  • -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. (yes, -B and -b are different, don't @ me, that's the manual's fault)
  • -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 one 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). 🗜️
  • -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. 🌈

  • -E log_file — Append debug logs to log_file instead of standard error. putting the receipts in a file, iconic. 🧾
  • -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/ssh/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. peek at the lore, then dip. 👀
  • -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. 🔑
  • -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, ~/.ssh/id_ed25519_sk and ~/.ssh/id_dsa. 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. 📜
  • -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. 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. 🐇🕳️
  • -K — Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server. 🎟️
  • -k — Disables forwarding (delegation) of GSSAPI credentials to the server.
  • -L [bind_address:]port:host:hostport
  • -L [bind_address:]port:remote_socket
  • -L local_socket:host:hostport
  • -L local_socket:remote_socket

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. 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. hi yes it's me 🫶
  • -M — Places the ssh client into “master” mode for connection sharing. Multiple -M options places ssh into “master” mode but with confirmation required using ssh-askpass(1) before each operation that changes the multiplexing state (e.g. opening a new session). Refer to the description of ControlMaster in ssh_config(5) for details. main character energy, honestly. 👑
  • -m mac_spec — A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference. See the MACs keyword in ssh_config(5) for more information. 💖
  • -N — okay bestie, this one is for when you literally do NOT want a remote command. It says "do not execute a remote command". Perfect for just forwarding ports. Peep the description of SessionType in ssh_config(5) for the deets. 💅
  • -n — Redirects stdin from /dev/null (actually, it prevents reading from stdin). You MUST use this when ssh is run in the background. A common trick is using it to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background. (This does not work if ssh needs to ask for a password or passphrase; see also the -f option.) Refer to the description of StdinNull in ssh_config(5) for details. 🦋
  • -O ctl_cmd — Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid commands are: “check” (check that the master process is running), “forward” (request forwardings without command execution), “cancel” (cancel forwardings), “exit” (request the master to exit), and “stop” (request the master to stop accepting further multiplexing requests). 👑
  • -o option — Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag. For full details of the options listed below, and their possible values, see ssh_config(5).
AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CASignatureAlgorithms
CertificateFile
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EnableEscapeCommandline
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForkAfterAuthentication
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
HashKnownHosts
Host
HostbasedAcceptedAlgorithms
HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
RequiredRSASize
SendEnv
ServerAliveInterval
ServerAliveCountMax
SessionType
SetEnv
StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation
  • -P tag — Specify 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 information. 🎀
  • -p port — Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file. 🌸
  • -Q query_option — Queries for the algorithms supported by one of the following 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), and 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. 🤫
  • -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.

This 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. 🚫
  • -t — Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty. 💅
  • -V — Display the version number and exit. ⭐
  • -v — Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3. 🐛
  • -W host:port — Requests that standard input and output on the client be forwarded to host on port over the secure channel. Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings, though these can be overridden in the configuration file or using -o command line options. 🔥
  • -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 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”. See also the Tunnel and TunnelDevice directives in ssh_config(5).

If the Tunnel directive is unset, it will be set to the default tunnel mode, which is “point-to-point”. If a different Tunnel forwarding mode is desired, then it should be specified before -w. 🌸

  • -X — Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file. ✨

X11 forwarding should be enabled with caution. 💅 Okay so here's the tea: anyone who can bypass file permissions on the remote host (like, for your X authorization database) can reach your local X11 display through that forwarded connection. And then?? An attacker could do stuff like keystroke monitoring. 😭 Not cute.

For this reason, X11 forwarding gets subjected to X11 SECURITY extension restrictions by default. Want the full lore? Go look at the ssh -Y option and the ForwardX11Trusted directive in ssh_config(5). 🫶

(Debian-specific: X11 forwarding is not subjected to X11 SECURITY extension restrictions by default, because too many programs currently crash in this mode. Set the ForwardX11Trusted option to “no” to restore the upstream behaviour. This may change in future depending on client-side improvements.)
  • -x — Disables X11 forwarding. 🚫✨
  • -Y — Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls. 👑
(Debian-specific: In the default configuration, this option is equivalent to -X, since ForwardX11Trusted defaults to “yes” as described above. Set the ForwardX11Trusted option to “no” to restore the upstream behaviour. This may change in future depending on client-side improvements.)
  • -y — Send log information using the syslog(3) system module. By default this information is sent to stderr. 📝

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. Authentication methods are tried in the order specified above, though PreferredAuthentications can be used to change the default order. 🔥

Host-based authentication works as follows: If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/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 containing the name of the client machine and the name of the user on that machine, the user is considered for login. Additionally, the server must be able to verify the client's host key (see the description of /etc/ssh/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 as follows: 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 public key authentication protocol automatically, using one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) (on non-OpenBSD systems, see http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) contains a brief discussion of the DSA and 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 may 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_dsa (DSA)
  • ~/.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_dsa.pub (DSA)
  • ~/.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. 🎉

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 as follows: 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/ssh/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. 😳

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. ✨

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

🌈 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 yeet arbitrary TCP connections over a secure channel — either straight from the command line or from a config file, dealer's choice 💅. Classic uses? A secure connection to a mail server, or ✨getting through firewalls✨ (the firewall never even knows, iykyk).

Let's take the example where we encrypt communication for an IRC client — even though the IRC server it connects to does not directly support encrypted communication. Here's the whole vibe:

  • you connect to the remote host with ssh, specifying the ports to be used to forward the connection
  • after that, you can start the program locally, and ssh will encrypt and forward the connection to the remote server 🦋

So this 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 that's gonna use the tunnel. If no connections are made within the time specified, ssh will exit 👋.

🎀 X11 FORWARDING 🎀

If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above 💖) and the user is using X11 (like, 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 gets made from the local machine. Periodt.

🚨 bestie do NOT manually set DISPLAY. Just don't. Let ssh handle it.

Forwarding of X11 connections can be configured on the command line or in configuration files, obviously.

The DISPLAY value that ssh sets will point to the server machine, but with a display number greater than zero. This is normal, and it happens because ssh creates a “proxy” X server on the server machine for forwarding the connections over the encrypted channel ✨.

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
  • verify that any forwarded connections carry this cookie, and replace it by the real cookie when the connection is opened

The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain, no cap 🔥).

And 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, then the connection to the agent is automatically forwarded to the remote side too 💅.

👑 VERIFYING HOST KEYS 👑

When you connect to a server for the first time, a fingerprint of the server's public key is presented to you (unless the option StrictHostKeyChecking has been disabled 🙈). Fingerprints can be determined using ssh-keygen(1):

$ ssh-keygen -l -f /etc/ssh/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.

Because comparing host keys just by staring at fingerprint strings is hard 😭, there's also support for comparing 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 whether the session itself is interactive or not. By learning the pattern a known server produces, you can easily find out that the host key has changed when a completely different pattern shows up. But ⚠️: because these patterns are not unambiguous, a pattern that looks similar to the pattern you remembered only gives a good probability that the host key is the same — not guaranteed proof. Not me trusting vibes as cryptography 💀.

To get a listing of the fingerprints along with their random art for all known hosts, use this command line:

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

If the fingerprint is unknown, there's an alternative method of verification 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.

In this example, we're 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 🫶.

🧚 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 🔐. 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 🛰️. More permanent VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8). Lowkey iconic advice, fr fr 💜.

🌈 ENVIRONMENT 🌈

ok so like, ssh sets a whole lil wardrobe of environment variables for you bestie, and here's the full lineup:

  • DISPLAY — tells everyone where the X11 server is at. ssh sets this itself to a value shaped like hostname:n, where hostname is the host where the shell runs and n is an integer ≥ 1. and it uses this ✨special✨ value to forward X11 connections over the secure channel. bestie please don't set DISPLAY yourself, because that makes the X11 connection insecure AND you'd have to manually copy any required authorization cookies. it's giving unnecessary labor. 🙈
  • HOME — set to the path of the user's home directory.
  • LOGNAME — a synonym for USER, set 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 run the program specified by SSH_ASKPASS and pop an X11 window to read the passphrase. this is so useful when you're calling ssh from a .xsession or a related script. (heads up: on some machines you may need to redirect the input from /dev/null to make this work. iykyk.)
  • SSH_ASKPASS_REQUIRE — more control over the askpass program situation. set it to never and ssh will never try to use one. set it to prefer and ssh will prefer the askpass program over the TTY when asking for passwords. and if you set it to force, then the askpass program gets used for all passphrase input, regardless of whether DISPLAY is set. periodt. 💅
  • SSH_AUTH_SOCK — the path of a Unix-domain socket used to talk to the agent. 🫶
  • SSH_CONNECTION — identifies the client and server ends of the connection. it holds four space-separated values: client IP address, client port number, server IP address, and server port number. four. count them. 👑
  • SSH_ORIGINAL_COMMAND — this one holds the original command line if a forced command is executed. super handy for extracting 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 just... isn't set. not invited to the party. 😭
  • 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. receipts, basically. 🧾✨
  • 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.

and ALSO, bonus round: ssh reads ~/.ssh/environment and adds lines of the format VARNAME=value to the environment — but only if the file exists and users are allowed to change their environment. for more info, go peek at the PermitUserEnvironment option in sshd_config(5). 💖

🎀 FILES 🎀

ok bestie, this is the whole friend group of files ssh fw with. every single one, no skips:

  • ~/.rhosts — 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. also, this file must be owned by the user, and must not have write permissions for anyone else. 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. the upgrade, honestly.
  • ~/.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 (DSA, 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 isn't 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). and because of the potential for abuse, this file must have strict permissions: read/write for the user, and not writable by others. it may be group-writable provided that the group in question contains only the user. strict queen energy. 👑
  • ~/.ssh/environment — contains additional definitions for environment variables; see “ENVIRONMENT”, above. ⬆️
  • ~/.ssh/id_dsa
  • ~/.ssh/id_ecdsa
  • ~/.ssh/id_ecdsa_sk
  • ~/.ssh/id_ed25519
  • ~/.ssh/id_ed25519_sk
  • ~/.ssh/id_rsa

these contain the private key for authentication. they hold sensitive data and should be readable by the user but not accessible by others (read/write/execute). and fr fr, ssh will simply ignore a private key file if it is accessible by others. you can specify a passphrase when generating the key, which will be used to encrypt the sensitive part of this file using AES-128. 💀 don't leave these out in the open.

  • ~/.ssh/id_dsa.pub
  • ~/.ssh/id_ecdsa.pub
  • ~/.ssh/id_ecdsa_sk.pub
  • ~/.ssh/id_ed25519.pub
  • ~/.ssh/id_ed25519_sk.pub
  • ~/.ssh/id_rsa.pub

these contain the public key for authentication. they are not sensitive and can (but need not) be readable by anyone. share freely, bestie. 🫶✨

  • ~/.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/ssh/shosts.equiv — used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.
  • /etc/ssh/ssh_config — the systemwide configuration file. the file format and configuration options are described in ssh_config(5).
  • /etc/ssh/ssh_host_key
  • /etc/ssh/ssh_host_dsa_key
  • /etc/ssh/ssh_host_ecdsa_key
  • /etc/ssh/ssh_host_ed25519_key
  • /etc/ssh/ssh_host_rsa_key

these files contain the private parts of the host keys and are used for host-based authentication. 🔥

  • /etc/ssh/ssh_known_hosts — the 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/ssh/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.

💀 EXIT STATUS 💀

ok so ssh exits with the exit status of the remote command — or with 255 if an error occurred. that's it, that's the whole vibe check. 🌸

👑 SEE ALSO 👑

go read these next bestie, they're all connected:

scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-argv0(1), ssh-keygen(1), ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)

✨ STANDARDS ✨

okay bestie, the SSH protocol is literally built on a whole stack of RFCs and papers, and the way that every single one of them is listed here? iconic. here's the full receipts, no cap:

  • 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).

👑 AUTHORS 👑

so OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen — shoutout to the OG fr fr. 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. and Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. absolute legends, every single one. 💅✨

okay that's the whole page bestie! this manual is from the Debian package, dated October 11, 2023, and it's literally just called SSH(1). go forth and securely shell into things, ily! 🫶💖🎀

📦 where this page comes from