Documentation ¶
Overview ¶
Package ssh wraps the crypto/ssh package with a higher-level API for building SSH servers. The goal of the API was to make it as simple as using net/http, so the API is very similar.
You should be able to build any SSH server using only this package, which wraps relevant types and some functions from crypto/ssh. However, you still need to use crypto/ssh for building SSH clients.
ListenAndServe starts an SSH server with a given address, handler, and options. The handler is usually nil, which means to use DefaultHandler. Handle sets DefaultHandler:
ssh.Handle(func(s ssh.Session) { io.WriteString(s, "Hello world\n") }) log.Fatal(ssh.ListenAndServe(":2222", nil))
If you don't specify a host key, it will generate one every time. This is convenient except you'll have to deal with clients being confused that the host key is different. It's a better idea to generate or point to an existing key on your system:
log.Fatal(ssh.ListenAndServe(":2222", nil, ssh.HostKeyFile("/Users/progrium/.ssh/id_rsa")))
Although all options have functional option helpers, another way to control the server's behavior is by creating a custom Server:
s := &ssh.Server{ Addr: ":2222", Handler: sessionHandler, PublicKeyHandler: authHandler, } s.AddHostKey(hostKeySigner) log.Fatal(s.ListenAndServe())
This package automatically handles basic SSH requests like setting environment variables, requesting PTY, and changing window size. These requests are processed, responded to, and any relevant state is updated. This state is then exposed to you via the Session interface.
The one big feature missing from the Session abstraction is signals. This was started, but not completed. Pull Requests welcome!
Index ¶
- Variables
- func AgentRequested(sess Session) bool
- func DefaultSessionHandler(srv *Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx Context)
- func DirectTCPIPHandler(srv *Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx Context)
- func ForwardAgentConnections(l net.Listener, s Session)
- func Handle(handler Handler)
- func KeysEqual(ak, bk PublicKey) bool
- func ListenAndServe(addr string, handler Handler, options ...Option) error
- func NewAgentListener() (net.Listener, error)
- func NewPtyReadWriter(rw io.ReadWriter) io.ReadWriter
- func NewPtyWriter(w io.Writer) io.Writer
- func Serve(l net.Listener, handler Handler, options ...Option) error
- func SetAgentRequested(ctx Context)
- type BannerHandler
- type ChannelHandler
- type ConnCallback
- type ConnectionFailedCallback
- type Context
- type ForwardedTCPHandler
- type Handler
- type KeyboardInteractiveHandler
- type LocalPortForwardingCallback
- type Option
- func AllocatePty() Option
- func EmulatePty() Option
- func HostKeyFile(filepath string) Option
- func HostKeyPEM(bytes []byte) Option
- func KeyboardInteractiveAuth(fn KeyboardInteractiveHandler) Option
- func NoPty() Option
- func PasswordAuth(fn PasswordHandler) Option
- func PublicKeyAuth(fn PublicKeyHandler) Option
- func WrapConn(fn ConnCallback) Option
- type PasswordHandler
- type Permissions
- type Pty
- type PtyCallback
- type PtyHandler
- type PublicKey
- type PublicKeyHandler
- type RequestHandler
- type ReversePortForwardingCallback
- type Server
- func (srv *Server) AddHostKey(key Signer)
- func (srv *Server) Close() error
- func (srv *Server) Handle(fn Handler)
- func (srv *Server) HandleConn(newConn net.Conn)
- func (srv *Server) ListenAndServe() error
- func (srv *Server) Serve(l net.Listener) error
- func (srv *Server) SetOption(option Option) error
- func (srv *Server) Shutdown(ctx context.Context) error
- type ServerConfigCallback
- type Session
- type SessionRequestCallback
- type Signal
- type Signer
- type SubsystemHandler
- type Window
Examples ¶
Constants ¶
This section is empty.
Variables ¶
var ( // ContextKeyUser is a context key for use with Contexts in this package. // The associated value will be of type string. ContextKeyUser = &contextKey{"user"} // ContextKeySessionID is a context key for use with Contexts in this package. // The associated value will be of type string. ContextKeySessionID = &contextKey{"session-id"} // ContextKeyPermissions is a context key for use with Contexts in this package. // The associated value will be of type *Permissions. ContextKeyPermissions = &contextKey{"permissions"} // ContextKeyClientVersion is a context key for use with Contexts in this package. // The associated value will be of type string. ContextKeyClientVersion = &contextKey{"client-version"} // ContextKeyServerVersion is a context key for use with Contexts in this package. // The associated value will be of type string. ContextKeyServerVersion = &contextKey{"server-version"} // ContextKeyLocalAddr is a context key for use with Contexts in this package. // The associated value will be of type net.Addr. ContextKeyLocalAddr = &contextKey{"local-addr"} // ContextKeyRemoteAddr is a context key for use with Contexts in this package. // The associated value will be of type net.Addr. ContextKeyRemoteAddr = &contextKey{"remote-addr"} // ContextKeyServer is a context key for use with Contexts in this package. // The associated value will be of type *Server. ContextKeyServer = &contextKey{"ssh-server"} // ContextKeyConn is a context key for use with Contexts in this package. // The associated value will be of type gossh.ServerConn. ContextKeyConn = &contextKey{"ssh-conn"} // ContextKeyPublicKey is a context key for use with Contexts in this package. // The associated value will be of type PublicKey. ContextKeyPublicKey = &contextKey{"public-key"} // ContextKeySession is a context key for use with Contexts in this package. // The associated value will be of type Session. ContextKeySession = &contextKey{"session"} )
var DefaultChannelHandlers = map[string]ChannelHandler{ "session": DefaultSessionHandler, }
var DefaultRequestHandlers = map[string]RequestHandler{}
var DefaultSubsystemHandlers = map[string]SubsystemHandler{}
var ErrServerClosed = errors.New("ssh: Server closed")
ErrServerClosed is returned by the Server's Serve, ListenAndServe, and ListenAndServeTLS methods after a call to Shutdown or Close.
var ErrUnsupported = errors.New("pty unsupported")
ErrUnsupported is returned when the platform does not support PTY.
Functions ¶
func AgentRequested ¶
AgentRequested returns true if the client requested agent forwarding.
func DefaultSessionHandler ¶
func DefaultSessionHandler(srv *Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx Context)
func DirectTCPIPHandler ¶
func DirectTCPIPHandler(srv *Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx Context)
DirectTCPIPHandler can be enabled by adding it to the server's ChannelHandlers under direct-tcpip.
func ForwardAgentConnections ¶
ForwardAgentConnections takes connections from a listener to proxy into the session on the OpenSSH channel for agent connections. It blocks and services connections until the listener stop accepting.
func ListenAndServe ¶
ListenAndServe listens on the TCP network address addr and then calls Serve with handler to handle sessions on incoming connections. Handler is typically nil, in which case the DefaultHandler is used.
Example ¶
package main import ( "io" "github.com/charmbracelet/ssh" ) func main() { ssh.ListenAndServe(":2222", func(s ssh.Session) { io.WriteString(s, "Hello world\n") }) }
Output:
func NewAgentListener ¶
NewAgentListener sets up a temporary Unix socket that can be communicated to the session environment and used for forwarding connections.
func NewPtyReadWriter ¶
func NewPtyReadWriter(rw io.ReadWriter) io.ReadWriter
NewPtyReadWriter return an io.ReadWriter that delegates the read to the given io.ReadWriter, and the writes to a ptyWriter.
func NewPtyWriter ¶
NewPtyWriter creates a writer that handles when the session has a active PTY, replacing the \n with \r\n.
func Serve ¶
Serve accepts incoming SSH connections on the listener l, creating a new connection goroutine for each. The connection goroutines read requests and then calls handler to handle sessions. Handler is typically nil, in which case the DefaultHandler is used.
func SetAgentRequested ¶
func SetAgentRequested(ctx Context)
SetAgentRequested sets up the session context so that AgentRequested returns true.
Types ¶
type BannerHandler ¶
BannerHandler is a callback for displaying the server banner.
type ChannelHandler ¶
type ChannelHandler func(srv *Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx Context)
type ConnCallback ¶
ConnCallback is a hook for new connections before handling. It allows wrapping for timeouts and limiting by returning the net.Conn that will be used as the underlying connection.
type ConnectionFailedCallback ¶
ConnectionFailedCallback is a hook for reporting failed connections Please note: the net.Conn is likely to be closed at this point
type Context ¶
type Context interface { context.Context sync.Locker // User returns the username used when establishing the SSH connection. User() string // SessionID returns the session hash. SessionID() string // ClientVersion returns the version reported by the client. ClientVersion() string // ServerVersion returns the version reported by the server. ServerVersion() string // RemoteAddr returns the remote address for this connection. RemoteAddr() net.Addr // LocalAddr returns the local address for this connection. LocalAddr() net.Addr // Permissions returns the Permissions object used for this connection. Permissions() *Permissions // SetValue allows you to easily write new values into the underlying context. SetValue(key, value interface{}) }
Context is a package specific context interface. It exposes connection metadata and allows new values to be easily written to it. It's used in authentication handlers and callbacks, and its underlying context.Context is exposed on Session in the session Handler. A connection-scoped lock is also embedded in the context to make it easier to limit operations per-connection.
type ForwardedTCPHandler ¶
ForwardedTCPHandler can be enabled by creating a ForwardedTCPHandler and adding the HandleSSHRequest callback to the server's RequestHandlers under tcpip-forward and cancel-tcpip-forward.
func (*ForwardedTCPHandler) HandleSSHRequest ¶
type Handler ¶
type Handler func(Session)
Handler is a callback for handling established SSH sessions.
var DefaultHandler Handler
DefaultHandler is the default Handler used by Serve.
type KeyboardInteractiveHandler ¶
type KeyboardInteractiveHandler func(ctx Context, challenger gossh.KeyboardInteractiveChallenge) bool
KeyboardInteractiveHandler is a callback for performing keyboard-interactive authentication.
type LocalPortForwardingCallback ¶
type LocalPortForwardingCallback func(ctx Context, destinationHost string, destinationPort uint32) bool
LocalPortForwardingCallback is a hook for allowing port forwarding
type Option ¶
Option is a functional option handler for Server.
func AllocatePty ¶
func AllocatePty() Option
AllocatePty returns a functional option that allocates a PTY. Implementers who wish to use an actual PTY should use this along with the platform specific PTY implementation defined in pty_*.go.
func EmulatePty ¶
func EmulatePty() Option
EmulatePty returns a functional option that fakes a PTY. It uses PtyWriter underneath.
func HostKeyFile ¶
HostKeyFile returns a functional option that adds HostSigners to the server from a PEM file at filepath.
Example ¶
package main import ( "github.com/charmbracelet/ssh" ) func main() { ssh.ListenAndServe(":2222", nil, ssh.HostKeyFile("/path/to/host/key")) }
Output:
func HostKeyPEM ¶
HostKeyPEM returns a functional option that adds HostSigners to the server from a PEM file as bytes.
func KeyboardInteractiveAuth ¶
func KeyboardInteractiveAuth(fn KeyboardInteractiveHandler) Option
func NoPty ¶
func NoPty() Option
NoPty returns a functional option that sets PtyCallback to return false, denying PTY requests.
Example ¶
package main import ( "github.com/charmbracelet/ssh" ) func main() { ssh.ListenAndServe(":2222", nil, ssh.NoPty()) }
Output:
func PasswordAuth ¶
func PasswordAuth(fn PasswordHandler) Option
PasswordAuth returns a functional option that sets PasswordHandler on the server.
Example ¶
package main import ( "github.com/charmbracelet/ssh" ) func main() { ssh.ListenAndServe(":2222", nil, ssh.PasswordAuth(func(ctx ssh.Context, pass string) bool { return pass == "secret" }), ) }
Output:
func PublicKeyAuth ¶
func PublicKeyAuth(fn PublicKeyHandler) Option
PublicKeyAuth returns a functional option that sets PublicKeyHandler on the server.
Example ¶
package main import ( "os" "github.com/charmbracelet/ssh" ) func main() { ssh.ListenAndServe(":2222", nil, ssh.PublicKeyAuth(func(ctx ssh.Context, key ssh.PublicKey) bool { data, _ := os.ReadFile("/path/to/allowed/key.pub") allowed, _, _, _, _ := ssh.ParseAuthorizedKey(data) return ssh.KeysEqual(key, allowed) }), ) }
Output:
func WrapConn ¶
func WrapConn(fn ConnCallback) Option
WrapConn returns a functional option that sets ConnCallback on the server.
type PasswordHandler ¶
PasswordHandler is a callback for performing password authentication.
type Permissions ¶
type Permissions struct {
*gossh.Permissions
}
The Permissions type holds fine-grained permissions that are specific to a user or a specific authentication method for a user. Permissions, except for "source-address", must be enforced in the server application layer, after successful authentication.
type Pty ¶
type Pty struct { // Term is the TERM environment variable value. Term string // Window is the Window sent as part of the pty-req. Window Window // Modes represent a mapping of Terminal Mode opcode to value as it was // requested by the client as part of the pty-req. These are outlined as // part of https://datatracker.ietf.org/doc/html/rfc4254#section-8. // // The opcodes are defined as constants in golang.org/x/crypto/ssh (VINTR,VQUIT,etc.). // Boolean opcodes have values 0 or 1. // // Note: golang.org/x/crypto/ssh currently (2022-03-12) doesn't have a // definition for opcode 42 "iutf8" which was introduced in https://datatracker.ietf.org/doc/html/rfc8160. Modes gossh.TerminalModes // contains filtered or unexported fields }
Pty represents a PTY request and configuration.
func (*Pty) Start ¶
Start starts a *exec.Cmd attached to the Session. If a PTY is allocated, it will use that for I/O. On Windows, the process execution lifecycle is not managed by Go and has to be managed manually. This means that c.Wait() won't work. See https://github.com/charmbracelet/x/blob/main/exp/term/conpty/conpty_windows.go
type PtyCallback ¶
PtyCallback is a hook for handling PTY allocation requests.
type PtyHandler ¶
PtyHandler is a callback for handling PTY allocation requests.
type PublicKey ¶
PublicKey is an abstraction of different types of public keys.
func ParseAuthorizedKey ¶
func ParseAuthorizedKey(in []byte) (out PublicKey, comment string, options []string, rest []byte, err error)
ParseAuthorizedKey parses a public key from an authorized_keys file used in OpenSSH according to the sshd(8) manual page.
func ParsePublicKey ¶
ParsePublicKey parses an SSH public key formatted for use in the SSH wire protocol according to RFC 4253, section 6.6.
type PublicKeyHandler ¶
PublicKeyHandler is a callback for performing public key authentication.
type RequestHandler ¶
type ReversePortForwardingCallback ¶
ReversePortForwardingCallback is a hook for allowing reverse port forwarding
type Server ¶
type Server struct { Addr string // TCP address to listen on, ":22" if empty Handler Handler // handler to invoke, ssh.DefaultHandler if nil HostSigners []Signer // private keys for the host key, must have at least one Version string // server version to be sent before the initial handshake Banner string // server banner BannerHandler BannerHandler // server banner handler, overrides Banner KeyboardInteractiveHandler KeyboardInteractiveHandler // keyboard-interactive authentication handler PasswordHandler PasswordHandler // password authentication handler PublicKeyHandler PublicKeyHandler // public key authentication handler PtyCallback PtyCallback // callback for allocating and allowing PTY sessions, ssh.EmulatePtyCallback if nil PtyHandler PtyHandler // pty allocation handler, ssh.emulatePtyHandler if nil ConnCallback ConnCallback // optional callback for wrapping net.Conn before handling LocalPortForwardingCallback LocalPortForwardingCallback // callback for allowing local port forwarding, denies all if nil ReversePortForwardingCallback ReversePortForwardingCallback // callback for allowing reverse port forwarding, denies all if nil ServerConfigCallback ServerConfigCallback // callback for configuring detailed SSH options SessionRequestCallback SessionRequestCallback // callback for allowing or denying SSH sessions ConnectionFailedCallback ConnectionFailedCallback // callback to report connection failures IdleTimeout time.Duration // connection timeout when no activity, none if empty MaxTimeout time.Duration // absolute connection timeout, none if empty // ChannelHandlers allow overriding the built-in session handlers or provide // extensions to the protocol, such as tcpip forwarding. By default only the // "session" handler is enabled. ChannelHandlers map[string]ChannelHandler // RequestHandlers allow overriding the server-level request handlers or // provide extensions to the protocol, such as tcpip forwarding. By default // no handlers are enabled. RequestHandlers map[string]RequestHandler // SubsystemHandlers are handlers which are similar to the usual SSH command // handlers, but handle named subsystems. SubsystemHandlers map[string]SubsystemHandler // contains filtered or unexported fields }
Server defines parameters for running an SSH server. The zero value for Server is a valid configuration. When both PasswordHandler and PublicKeyHandler are nil, no client authentication is performed.
func (*Server) AddHostKey ¶
AddHostKey adds a private key as a host key. If an existing host key exists with the same algorithm, it is overwritten. Each server config must have at least one host key.
func (*Server) Close ¶
Close immediately closes all active listeners and all active connections.
Close returns any error returned from closing the Server's underlying Listener(s).
func (*Server) HandleConn ¶
func (*Server) ListenAndServe ¶
ListenAndServe listens on the TCP network address srv.Addr and then calls Serve to handle incoming connections. If srv.Addr is blank, ":22" is used. ListenAndServe always returns a non-nil error.
func (*Server) Serve ¶
Serve accepts incoming connections on the Listener l, creating a new connection goroutine for each. The connection goroutines read requests and then calls srv.Handler to handle sessions.
Serve always returns a non-nil error.
func (*Server) Shutdown ¶
Shutdown gracefully shuts down the server without interrupting any active connections. Shutdown works by first closing all open listeners, and then waiting indefinitely for connections to close. If the provided context expires before the shutdown is complete, then the context's error is returned.
type ServerConfigCallback ¶
type ServerConfigCallback func(ctx Context) *gossh.ServerConfig
ServerConfigCallback is a hook for creating custom default server configs
type Session ¶
type Session interface { gossh.Channel // User returns the username used when establishing the SSH connection. User() string // RemoteAddr returns the net.Addr of the client side of the connection. RemoteAddr() net.Addr // LocalAddr returns the net.Addr of the server side of the connection. LocalAddr() net.Addr // Environ returns a copy of strings representing the environment set by the // user for this session, in the form "key=value". Environ() []string // Exit sends an exit status and then closes the session. Exit(code int) error // Command returns a shell parsed slice of arguments that were provided by the // user. Shell parsing splits the command string according to POSIX shell rules, // which considers quoting not just whitespace. Command() []string // RawCommand returns the exact command that was provided by the user. RawCommand() string // Subsystem returns the subsystem requested by the user. Subsystem() string // PublicKey returns the PublicKey used to authenticate. If a public key was not // used it will return nil. PublicKey() PublicKey // Context returns the connection's context. The returned context is always // non-nil and holds the same data as the Context passed into auth // handlers and callbacks. // // The context is canceled when the client's connection closes or I/O // operation fails. Context() Context // Permissions returns a copy of the Permissions object that was available for // setup in the auth handlers via the Context. Permissions() Permissions // EmulatedPty returns true if the session is emulating a PTY using PtyWriter. EmulatedPty() bool // Pty returns PTY information, a channel of window size changes, and a boolean // of whether or not a PTY was accepted for this session. Pty() (Pty, <-chan Window, bool) // Signals registers a channel to receive signals sent from the client. The // channel must handle signal sends or it will block the SSH request loop. // Registering nil will unregister the channel from signal sends. During the // time no channel is registered signals are buffered up to a reasonable amount. // If there are buffered signals when a channel is registered, they will be // sent in order on the channel immediately after registering. Signals(c chan<- Signal) // Break regisers a channel to receive notifications of break requests sent // from the client. The channel must handle break requests, or it will block // the request handling loop. Registering nil will unregister the channel. // During the time that no channel is registered, breaks are ignored. Break(c chan<- bool) }
Session provides access to information about an SSH session and methods to read and write to the SSH channel with an embedded Channel interface from crypto/ssh.
When Command() returns an empty slice, the user requested a shell. Otherwise the user is performing an exec with those command arguments.
TODO: Signals
type SessionRequestCallback ¶
SessionRequestCallback is a callback for allowing or denying SSH sessions.
type Signal ¶
type Signal string
const ( SIGABRT Signal = "ABRT" SIGALRM Signal = "ALRM" SIGFPE Signal = "FPE" SIGHUP Signal = "HUP" SIGILL Signal = "ILL" SIGINT Signal = "INT" SIGKILL Signal = "KILL" SIGPIPE Signal = "PIPE" SIGQUIT Signal = "QUIT" SIGSEGV Signal = "SEGV" SIGTERM Signal = "TERM" SIGUSR1 Signal = "USR1" SIGUSR2 Signal = "USR2" )
POSIX signals as listed in RFC 4254 Section 6.10.
type SubsystemHandler ¶
type SubsystemHandler func(s Session)
type Window ¶
type Window struct { // Width is the number of columns. // It overrides WidthPixels. Width int // Height is the number of rows. // It overrides HeightPixels. Height int // WidthPixels is the drawable width of the window, in pixels. WidthPixels int // HeightPixels is the drawable height of the window, in pixels. HeightPixels int }
Window represents the size of a PTY window.
From https://datatracker.ietf.org/doc/html/rfc4254#section-6.2
Zero dimension parameters MUST be ignored. The character/row dimensions override the pixel dimensions (when nonzero). Pixel dimensions refer to the drawable area of the window.