ssh_config

package module
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Published: Mar 24, 2021 License: MIT Imports: 13 Imported by: 0

README

ssh_config

This is a Go parser for ssh_config files. Importantly, this parser attempts to preserve comments in a given file, so you can manipulate a ssh_config file from a program, if your heart desires.

It's designed to be used with the excellent x/crypto/ssh package, which handles SSH negotiation but isn't very easy to configure.

The ssh_config Get() and GetStrict() functions will attempt to read values from $HOME/.ssh/config and fall back to /etc/ssh/ssh_config. The first argument is the host name to match on, and the second argument is the key you want to retrieve.

port := ssh_config.Get("myhost", "Port")

You can also load a config file and read values from it.

var config = `
Host *.test
  Compression yes
`

cfg, err := ssh_config.Decode(strings.NewReader(config))
fmt.Println(cfg.Get("example.test", "Port"))

Some SSH arguments have default values - for example, the default value for KeyboardAuthentication is "yes". If you call Get(), and no value for the given Host/keyword pair exists in the config, we'll return a default for the keyword if one exists.

Manipulating SSH config files

Here's how you can manipulate an SSH config file, and then write it back to disk.

f, _ := os.Open(filepath.Join(os.Getenv("HOME"), ".ssh", "config"))
cfg, _ := ssh_config.Decode(f)
for _, host := range cfg.Hosts {
    fmt.Println("patterns:", host.Patterns)
    for _, node := range host.Nodes {
        // Manipulate the nodes as you see fit, or use a type switch to
        // distinguish between Empty, KV, and Include nodes.
        fmt.Println(node.String())
    }
}

// Print the config to stdout:
fmt.Println(cfg.String())

Spec compliance

Wherever possible we try to implement the specification as documented in the ssh_config manpage. Unimplemented features should be present in the issues list.

Notably, the Match directive is currently unsupported.

Errata

This is the second comment-preserving configuration parser I've written, after an /etc/hosts parser. Eventually, I will write one for every Linux file format.

Donating

Donations free up time to make improvements to the library, and respond to bug reports. You can send donations via Paypal's "Send Money" feature to kev@inburke.com. Donations are not tax deductible in the USA.

Documentation

Overview

Package ssh_config provides tools for manipulating SSH config files.

Importantly, this parser attempts to preserve comments in a given file, so you can manipulate a `ssh_config` file from a program, if your heart desires.

The Get() and GetStrict() functions will attempt to read values from $HOME/.ssh/config, falling back to /etc/ssh/ssh_config. The first argument is the host name to match on ("example.com"), and the second argument is the key you want to retrieve ("Port"). The keywords are case insensitive.

port := ssh_config.Get("myhost", "Port")

You can also manipulate an SSH config file and then print it or write it back to disk.

f, _ := os.Open(filepath.Join(os.Getenv("HOME"), ".ssh", "config"))
cfg, _ := ssh_config.Decode(f)
for _, host := range cfg.Hosts {
	fmt.Println("patterns:", host.Patterns)
	for _, node := range host.Nodes {
		fmt.Println(node.String())
	}
}

// Write the cfg back to disk:
fmt.Println(cfg.String())

BUG: the Match directive is currently unsupported; parsing a config with a Match directive will trigger an error.

Index

Examples

Constants

This section is empty.

Variables

View Source
var DefaultUserSettings = &UserSettings{
	IgnoreErrors:       false,
	SystemConfigFinder: SystemConfigFinder,
	UserConfigFinder:   UserConfigFinder,
}

DefaultUserSettings is the default UserSettings and is used by Get and GetStrict. It checks both $HOME/.ssh/config and /etc/ssh/ssh_config for keys, and it will return parse errors (if any) instead of swallowing them.

View Source
var ErrDepthExceeded = errors.New("ssh_config: max recurse depth exceeded")

ErrDepthExceeded is returned if too many Include directives are parsed. Usually this indicates a recursive loop (an Include directive pointing to the file it contains).

Functions

func Default

func Default(keyword string) string

Default returns the default value for the given keyword, for example "22" if the keyword is "Port". Default returns the empty string if the keyword has no default, or if the keyword is unknown. Keyword matching is case-insensitive.

Default values are provided by OpenSSH_7.4p1 on a Mac.

Example
package main

import (
	"fmt"

	"github.com/dimandzhi/ssh_config"
)

func main() {
	fmt.Println(ssh_config.Default("Port"))
	fmt.Println(ssh_config.Default("UnknownVar"))
}
Output:

22

func Get

func Get(alias, key string) string

Get finds the first value for key within a declaration that matches the alias. Get returns the empty string if no value was found, or if IgnoreErrors is false and we could not parse the configuration file. Use GetStrict to disambiguate the latter cases.

The match for key is case insensitive.

Get is a wrapper around DefaultUserSettings.Get.

func GetStrict

func GetStrict(alias, key string) (string, error)

GetStrict finds the first value for key within a declaration that matches the alias. If key has a default value and no matching configuration is found, the default will be returned. For more information on default values and the way patterns are matched, see the manpage for ssh_config.

error will be non-nil if and only if a user's configuration file or the system configuration file could not be parsed, and u.IgnoreErrors is false.

GetStrict is a wrapper around DefaultUserSettings.GetStrict.

func SystemConfigFinder

func SystemConfigFinder() string

func UserConfigFinder

func UserConfigFinder() string

Types

type Config

type Config struct {
	// A list of hosts to match against. The file begins with an implicit
	// "Host *" declaration matching all hosts.
	Hosts []*Host
	// contains filtered or unexported fields
}

Config represents an SSH config file.

func Decode

func Decode(r io.Reader) (*Config, error)

Decode reads r into a Config, or returns an error if r could not be parsed as an SSH config file.

Example
package main

import (
	"fmt"
	"strings"

	"github.com/dimandzhi/ssh_config"
)

func main() {
	var config = `
Host *.example.com
  Compression yes
`

	cfg, _ := ssh_config.Decode(strings.NewReader(config))
	val, _ := cfg.Get("test.example.com", "Compression")
	fmt.Println(val)
}
Output:

yes

func (*Config) Get

func (c *Config) Get(alias, key string) (string, error)

Get finds the first value in the configuration that matches the alias and contains key. Get returns the empty string if no value was found, or if the Config contains an invalid conditional Include value.

The match for key is case insensitive.

func (Config) MarshalText

func (c Config) MarshalText() ([]byte, error)

func (Config) String

func (c Config) String() string

String returns a string representation of the Config file.

type Empty

type Empty struct {
	Comment string
	// contains filtered or unexported fields
}

Empty is a line in the config file that contains only whitespace or comments.

func (*Empty) Pos

func (e *Empty) Pos() Position

Pos returns e's Position.

func (*Empty) String

func (e *Empty) String() string

String prints e as it was parsed in the config file.

type Host

type Host struct {
	// A list of host patterns that should match this host.
	Patterns []*Pattern
	// A Node is either a key/value pair or a comment line.
	Nodes []Node
	// EOLComment is the comment (if any) terminating the Host line.
	EOLComment string
	// contains filtered or unexported fields
}

Host describes a Host directive and the keywords that follow it.

func (*Host) Matches

func (h *Host) Matches(alias string) bool

Matches returns true if the Host matches for the given alias. For a description of the rules that provide a match, see the manpage for ssh_config.

Example
package main

import (
	"fmt"

	"github.com/dimandzhi/ssh_config"
)

func main() {
	pat, _ := ssh_config.NewPattern("test.*.example.com")
	host := &ssh_config.Host{Patterns: []*ssh_config.Pattern{pat}}
	fmt.Println(host.Matches("test.stage.example.com"))
	fmt.Println(host.Matches("othersubdomain.example.com"))
}
Output:

true
false

func (*Host) String

func (h *Host) String() string

String prints h as it would appear in a config file. Minor tweaks may be present in the whitespace in the printed file.

type Include

type Include struct {
	// Comment is the contents of any comment at the end of the Include
	// statement.
	Comment string
	// contains filtered or unexported fields
}

Include holds the result of an Include directive, including the config files that have been parsed as part of that directive. At most 5 levels of Include statements will be parsed.

func NewInclude

func NewInclude(directives []string, hasEquals bool, pos Position, comment string, system bool, depth uint8) (*Include, error)

NewInclude creates a new Include with a list of file globs to include. Configuration files are parsed greedily (e.g. as soon as this function runs). Any error encountered while parsing nested configuration files will be returned.

func (*Include) Get

func (inc *Include) Get(alias, key string) string

Get finds the first value in the Include statement matching the alias and the given key.

func (*Include) Pos

func (i *Include) Pos() Position

Pos returns the position of the Include directive in the larger file.

func (*Include) String

func (inc *Include) String() string

String prints out a string representation of this Include directive. Note included Config files are not printed as part of this representation.

type KV

type KV struct {
	Key     string
	Value   string
	Comment string
	// contains filtered or unexported fields
}

KV is a line in the config file that contains a key, a value, and possibly a comment.

func (*KV) Pos

func (k *KV) Pos() Position

Pos returns k's Position.

func (*KV) String

func (k *KV) String() string

String prints k as it was parsed in the config file. There may be slight changes to the whitespace between values.

type Node

type Node interface {
	Pos() Position
	String() string
}

Node represents a line in a Config.

type Pattern

type Pattern struct {
	// contains filtered or unexported fields
}

Pattern is a pattern in a Host declaration. Patterns are read-only values; create a new one with NewPattern().

Example
package main

import (
	"fmt"

	"github.com/dimandzhi/ssh_config"
)

func main() {
	pat, _ := ssh_config.NewPattern("*")
	host := &ssh_config.Host{Patterns: []*ssh_config.Pattern{pat}}
	fmt.Println(host.Matches("test.stage.example.com"))
	fmt.Println(host.Matches("othersubdomain.any.any"))
}
Output:

true
true

func NewPattern

func NewPattern(s string) (*Pattern, error)

NewPattern creates a new Pattern for matching hosts. NewPattern("*") creates a Pattern that matches all hosts.

From the manpage, a pattern consists of zero or more non-whitespace characters, `*' (a wildcard that matches zero or more characters), or `?' (a wildcard that matches exactly one character). For example, to specify a set of declarations for any host in the ".co.uk" set of domains, the following pattern could be used:

Host *.co.uk

The following pattern would match any host in the 192.168.0.[0-9] network range:

Host 192.168.0.?

func (Pattern) String

func (p Pattern) String() string

String prints the string representation of the pattern.

type Position

type Position struct {
	Line int // line within the document
	Col  int // column within the line
}

Position of a document element within a SSH document.

Line and Col are both 1-indexed positions for the element's line number and column number, respectively. Values of zero or less will cause Invalid(), to return true.

func (Position) Invalid

func (p Position) Invalid() bool

Invalid returns whether or not the position is valid (i.e. with negative or null values)

func (Position) String

func (p Position) String() string

String representation of the position. Displays 1-indexed line and column numbers.

type UserSettings

type UserSettings struct {
	IgnoreErrors bool

	SystemConfigFinder configFinder

	UserConfigFinder configFinder
	// contains filtered or unexported fields
}

UserSettings checks ~/.ssh and /etc/ssh for configuration files. The config files are parsed and cached the first time Get() or GetStrict() is called.

func (*UserSettings) Get

func (u *UserSettings) Get(alias, key string) string

Get finds the first value for key within a declaration that matches the alias. Get returns the empty string if no value was found, or if IgnoreErrors is false and we could not parse the configuration file. Use GetStrict to disambiguate the latter cases.

The match for key is case insensitive.

func (*UserSettings) GetStrict

func (u *UserSettings) GetStrict(alias, key string) (string, error)

GetStrict finds the first value for key within a declaration that matches the alias. If key has a default value and no matching configuration is found, the default will be returned. For more information on default values and the way patterns are matched, see the manpage for ssh_config.

error will be non-nil if and only if a user's configuration file or the system configuration file could not be parsed, and u.IgnoreErrors is false.

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