assh

Overview

A transparent wrapper that adds support for regex, aliases, gateways, dynamic hostnames, graphviz, json output, yaml configuration, and more to SSH.

lib-ssh wraps assh as a ProxyCommand; it means that it works seamlessly with:

• ssh
• scp
• rsync
• git
• Desktop applications depending on lib-ssh or ssh (i.e., Tower, Atom.io, SSH Tunnel Manager)

For specific examples, see 3rd Party Integration

Features

Configuration features

• regex support
• aliases gate -> gate.domain.tld
• gateways -> transparent ssh connection chaining
• includes: split configuration in multiple files, note that OpenSSH as of v7.3 has native support for this
• local command execution: finally the reverse of RemoteCommand
• templates: equivalent to host but you can't connect directly to a template, perfect for inheritance
• inheritance: make hosts inherits from host hosts or templates
• variable expansion: resolve variables from the environment
• smart proxycommand: RAW tcp connection when possible with netcat and socat as default fallbacks
• rate limit: configure a per-host or global rate-limiting
• JSON output
• Graphviz: graphviz reprensentation of the hosts

Using Gateway from command line

assh can use the ProxyCommand with netcat feature of OpenSSH transparently and without the pain of using extended configuration.

Connect to hosta using hostb as a gateway.

flowchart 
    direction TB

    y[you]
    a[hosta]
    b[hostb]
    fw((firewall))

    style fw fill:#f00,color:#fff

    y ==x fw
    fw .-> a
    
    y --> b
    b --> a

$ ssh hosta/hostb
user@hosta $

Equivalent to ssh -o ProxyCommand="ssh hostb nc %h %p" hosta

---

Connect to hosta using hostb as a gateway using hostc as a gateway.

flowchart 
    direction TB

    y[you]
    a[hosta]
    b[hostb]
    c[hostc]
    fw((firewall))

    style fw fill:#f00,color:#fff

    y ==x fw
    fw ..-> a
    
    y --> c
    c --> b
    b --> a

$ ssh hosta/hostb/hostc
user@hosta $

Equivalent to ssh -o ProxyCommand="ssh -o ProxyCommand='ssh hostc nc %h %p' hostb nc %h %p" hosta

Using Gateways from configuration file

You can define an equivalent of the "ProxyCommand with netcat" feature of OpenSSH, with a simpler syntax, more advanced workflows, and a unique fallback feature.

Let's consider the following assh.yml file

hosts:
  hosta:
    Hostname: 1.2.3.4

  hostb:
    Hostname: 5.6.7.8
    Gateways: hosta

  hostc:
    Hostname: 9.10.11.12
    Gateways: hostb

  hostd:
    Hostname: 13.14.15.16
    GatewayConnectTimeout: 2
    Gateways:
    - direct
    - hosta

• ssh hosta -> ssh 1.2.3.4
• ssh hostb -> ssh -o ProxyCommand="ssh hostb nc %h %p" hosta
• ssh hostc -> ssh -o ProxyCommand="ssh -o ProxyCommand='ssh hostc nc %h %p' hostb nc %h %p" hosta
• ssh hostd ->
  • assh will try to ssh 13.14.15.16
  • then, fallback on ssh -o ProxyCommand="ssh hostd nc %h %p" hosta
  • this method allows you to have the best performances when it is possible, but ensure your commands will work if you are outside of your company for instance

Under the hood features

• Automatically regenerates ~/.ssh/config file when needed
• Inspect parent process to determine log level (if you use ssh -vv, assh will automatically run in debug mode)
• Automatically creates ControlPath directories so you can use slashes in your ControlPath option, can be enabled with the ControlMasterMkdir: true configuration in host or globally.

Hooks

Events

BeforeConnect

BeforeConnect is called just before assh tries to connect to the remote SSH port.

Note: BeforeConnect will be called for each SSH connection; if you use multiple gateways, it will be called for each gateways until one succeed to connect.

---

Example of Golang template variables:

// Host: https://pkg.go.dev/moul.io/assh/v2/pkg/config#Host
{{.Host.Name}}                                  //  localhost
{{.Host.HostName}}                              //  127.0.0.1
{{.Host.Port}}                                  //  22
{{.Host.User}}                                  //  moul
{{.Host.Prototype}}                             //  moul@127.0.0.1:22
{{.Host}}                                       //  {"HostName":"localhost","Port":22","User":"moul","ControlPersist":"yes",...}
{{printf "%s:%s" .Host.HostName .Host.Port}}    //  localhost:22

OnConnect

OnConnect is called as soon as assh is connected to the remote SSH port.

Note: OnConnect is not aware of the authentication process and will always be raised.

---

Example of Golang template variables:

// Host: https://pkg.go.dev/moul.io/assh/v2/pkg/config#Host
{{.Host.Name}}                                  //  localhost
{{.Host.HostName}}                              //  127.0.0.1
{{.Host.Port}}                                  //  22
{{.Host.User}}                                  //  moul
{{.Host.Prototype}}                             //  moul@127.0.0.1:22
{{.Host}}                                       //  {"HostName":"localhost","Port":22","User":"moul","ControlPersist":"yes",...}
{{printf "%s:%s" .Host.HostName .Host.Port}}    //  localhost:22

// Stats: https://pkg.go.dev/moul.io/assh/v2/pkg/commands#ConnectionStats
{{.Stats.ConnectedAt}}                           //  2016-07-20 11:19:23.467900594 +0200 CEST

OnConnectError

OnConnectError is called when assh fails to open a new TCP connection.

---

Example of Golang template variables:

// Host: https://pkg.go.dev/moul.io/assh/v2/pkg/config#Host
{{.Host.Name}}                                  //  localhost
{{.Host.HostName}}                              //  127.0.0.1
{{.Host.Port}}                                  //  22
{{.Host.User}}                                  //  moul
{{.Host.Prototype}}                             //  moul@127.0.0.1:22
{{.Host}}                                       //  {"HostName":"localhost","Port":22","User":"moul","ControlPersist":"yes",...}
{{printf "%s:%s" .Host.HostName .Host.Port}}    //  localhost:22

// Error
{{.Error}}                                      //  dial tcp: lookup localhost: no such host

OnDisconnect

OnDisconnect is called as the assh socket is closed.

warning: if you don't see a notification when closing an SSH connection, then you probably have ControlMaster configured; OnDisconnect is not linked to the ssh program but to its socket which may stay alive even after exiting the ssh program.

---

Example of Golang template variables:

// Host: https://pkg.go.dev/moul.io/assh/v2/pkg/config#Host
{{.Host.Name}}                                  //  localhost
{{.Host.HostName}}                              //  127.0.0.1
{{.Host.Port}}                                  //  22
{{.Host.User}}                                  //  moul
{{.Host.Prototype}}                             //  moul@127.0.0.1:22
{{.Host}}                                       //  {"HostName":"localhost","Port":22","User":"moul","ControlPersist":"yes",...}
{{printf "%s:%s" .Host.HostName .Host.Port}}    //  localhost:22

// Stats: https://pkg.go.dev/moul.io/assh/v2/pkg/commands#ConnectionStats
{{.Stats.ConnectedAt}}                           //  2016-07-20 11:19:23.467900594 +0200 CEST
{{.Stats.WrittenBytes}}                          //  3613
{{.Stats.WrittenBytesHuman}}                     //  3.6kb
{{.Stats.DisconnectAt}}                          //  2016-07-20 11:19:29,520515792 +0200 CEST
{{.Stats.ConnectionDuration}}                    //  6.052615198s
{{.Stats.ConnectionDurationHuman}}               //  6s
{{.Stats.AverageSpeed}}                          //  596.933bps
{{.Stats.AverageSpeedHuman}}                     //  3.4kb/s

BeforeConfigWrite

BeforeConfigWrite is called just before assh rewrite the ~/.ssh/config file.

---

Example of Golang template variables:

{{.SSHConfigPath}}                               // ~/.ssh/config

Hooks drivers

Exec driver

Exec driver uses Golang's template system to execute a shell command

Usage: exec <binary> [args...]

defaults:
  Hooks:
    OnConnect: exec echo '{{.Host}}' | jq .
# executes: `echo '{"HostName":"localhost","Port":"22","User":"moul","ControlPersist":"yes",...}' | jq .
# which results in printing a pretty JSON of the host
# {
#   "HostName": "localhost",
#   "Port": "22",
#   "User": "moul",
#   "ControlPersist": "yes",
#   ...
# }

defaults:
  Hooks:
    OnConnect: exec echo 'New SSH connection to {{.Host.Prototype}}.' | mail -s "SSH connection journal" m+assh@42.am
# send an email with the connection prototype

defaults:
  Hooks:
    BeforeConfigWrite: exec cp {{.SSHConfigPath}} {{.SSHConfigPath}}.backup
# make a copy of ~/.ssh/config before being rewritten

defaults:
  Hooks:
    AfterConfigWrite: 'exec echo "# date: `date`" >> {{.SSHConfigPath}}'
# Append a comment with the compilation date to the generated ~/.ssh/config file

defaults:
  Hooks:
  AfterConfigWrite: 'exec cat /path/to/my/provider/generated/.ssh/config >> {{.SSHConfigPath}}'
# Append another .ssh/config file to the generated .ssh/config file

---

The exec commands are blocking, a new driver for background tasks is planned. For now, you can run a job in background like this:

defaults:
  Hooks:
    OnConnect:
    - exec sleep 60 &
# execute the `sleep 60` command in background (non-blocking)
# if you quit your ssh connection, the process will continue in background.

Write driver

Write driver uses Golang's template system to write out data to stdout

Usage: write <line:string...>

defaults:
  Hooks:
    OnConnect:
    - write New SSH connection to {{.Host.Prototype}}.
# writes: "New SSH connection to moul@127.0.0.1:22." on the terminal on connection

defaults:
  Hooks:
    OnDisconnect:
    - "write SSH connection to {{.Host.Name}} closed, {{ .Stats.WrittenBytes }} bytes written in {{ .Stats.ConnectionDuration }} ({{ .Stats.AverageSpeed }})"
# writes: SSH connection to localhost closed, 40 bytes written.

Notify driver

Notify driver uses Golang's template system to open Desktop notifications.

• Mac OS X: Built-in support
• Linux: Depends on gnotifier
• Windows: Not supported
• BSD: Not supported

Usage: notify <line:string...>

defaults:
  Hooks:
    OnConnect: notify New SSH connection to {{.Host.Prototype}}.

defaults:
  Hooks:
    OnDisconnect:
    - "notify SSH connection to {{.Host.Name}} closed, {{ .Stats.WrittenBytes }} bytes written in {{ .Stats.ConnectionDuration }} ({{ .Stats.AverageSpeed }})"

Configuration

assh now manages the ~/.ssh/config file, take care to keep a backup your ~/.ssh/config file.

~/.ssh/assh.yml is a YAML file containing:

• a hosts dictionary containing multiple HOST definitions
• a defaults section containing global flags
• and an includes section containing path to other configuration files

hosts:

  homer:
    # ssh homer ->  ssh 1.2.3.4 -p 2222 -u robert
    Hostname: 1.2.3.4
    User: robert
    Port: 2222

  bart:
    # ssh bart ->   ssh 5.6.7.8 -u bart           <- direct access
    #            or ssh 5.6.7.8/homer -u bart     <- using homer as a gateway
    Hostname: 5.6.7.8
    User: bart
    Gateways:
    - direct                   # tries a direct access first
    - homer                    # fallback on homer gateway

  maggie:
    # ssh maggie ->   ssh 5.6.7.8 -u maggie       <- direct access
    #              or ssh 5.6.7.8/homer -u maggie   <- using homer as a gateway
    User: maggie
    Inherits: bart             # inherits rules from "bart"

  bart-access:
    # ssh bart-access ->  ssh home.simpson.springfield.us -u bart
    Inherits:
    - bart-template
    - simpson-template

  lisa-access:
    # ssh lisa-access ->  ssh home.simpson.springfield.us -u lisa
    Inherits:
    - lisa-template
    - simpson-template

  marvin:
    # ssh marvin    -> ssh marvin    -p 23
    # ssh sad-robot -> ssh sad-robot -p 23
    # ssh bighead   -> ssh bighead   -p 23
    # aliases inherit everything from marvin, except hostname
    Port: 23
    Aliases:
    - sad-robot
    - bighead

  dolphin:
    # ssh dolphin   -> ssh dolphin -p 24
    # ssh ecco      -> ssh dolphin -p 24
    # same as above, but with fixed hostname
    Port: 24
    Hostname: dolphin
    Aliases: ecco
    RateLimit: 10M # 10Mbytes/second rate limiting

  schooltemplate:
    User: student
    IdentityFile: ~/.ssh/school-rsa
    ForwardX11: yes

  schoolgw:
    # ssh school ->   ssh gw.school.com -l student -o ForwardX11=no -i ~/.ssh/school-rsa
    Hostname: gw.school.com
    ForwardX11: no
    Inherits: schooltemplate

  "expanded-host[0-7]*":
    # ssh somehost2042 ->       ssh somehost2042.some.zone
    Hostname: "%h.some.zone"

  vm-*.school.com:
    # ssh vm-42.school.com ->   ssh vm-42.school.com/gw.school.com -l student -o ForwardX11=yes -i ~/.ssh/school-rsa
    Gateways: schoolgw
    Inherits: schooltemplate
    # do not automatically create `ControlPath` -> may result in error
    ControlMasterMkdir: true

  "*.shortcut1":
    ResolveCommand: /bin/sh -c "echo %h | sed s/.shortcut1/.my-long-domain-name.com/"

  "*.shortcut2":
    ResolveCommand: /bin/sh -c "echo $(echo %h | sed s/.shortcut2//).my-other-long-domain-name.com"

  "*.scw":
    # ssh toto.scw -> 1. dynamically resolves the IP address
    #                 2. ssh {resolved ip address} -u root -p 22 -o UserKnownHostsFile=null -o StrictHostKeyChecking=no
    # requires github.com/scaleway/scaleway-cli
    ResolveCommand: /bin/sh -c "scw inspect -f {{.PublicAddress.IP}} server:$(echo %h | sed s/.scw//)"
    User: root
    Port: 22
    UserKnownHostsFile: /dev/null
    StrictHostKeyChecking: no

  my-env-host:
    User: user-$USER
    Hostname: ${HOSTNAME}${HOSTNAME_SUFFIX}

templates:
  # Templates are similar to Hosts; you can inherit from them
  # but you cannot ssh to a template
  bart-template:
    User: bart
  lisa-template:
    User: lisa
  simpson-template:
    Host: home.simpson.springfield.us

defaults:
  # Defaults are applied to each hosts
  ControlMaster: auto
  ControlPath: ~/tmp/.ssh/cm/%h-%p-%r.sock
  ControlPersist: yes
  Port: 22
  User: bob
  Hooks:
    # Automatically backup ~/.ssh/config
    BeforeConfigWrite:
      - 'exec set -x; cp {{.SSHConfigPath}} {{.SSHConfigPath}}.bkp'

    AfterConfigWrite:
      # Concat another `ssh_config` file with the one just generated by `assh`
      - 'exec cat ~/.ssh/my-heroku-generated-config >> {{.SSHConfigPath}}'

      # Alert me with a Desktop notification
      - notify "{{.SSHConfigPath}} has been rewritten"

    OnConnect:
      # Log internal information to a file
      - exec printf '{{.}}' | jq . >> ~/.ssh/last_connected_host.txt

      # Alert me with a Desktop notification
      - notify New SSH connection to {{.Host.Prototype}} at {{.Stats.ConnectedAt}}

      # Write the host prototype to the terminal stderr
      - write New SSH connection to {{.Host.Prototype}}

    OnDisconnect:
      # write on terminal and in a Desktop notification some statistics about the finished connection
      - "write  SSH connection to {{.Host.HostName}} closed, {{.Stats.WrittenBytes }} bytes written in {{.Stats.ConnectionDuration}} ({{.Stats.AverageSpeed}}bps)"
      - "notify SSH connection to {{.Host.HostName}} closed, {{.Stats.WrittenBytes }} bytes written in {{.Stats.ConnectionDuration}} ({{.Stats.AverageSpeed}}bps)"

includes:
- ~/.ssh/assh.d/*.yml
- /etc/assh.yml
- $ENV_VAR/blah-blah-*/*.yml

ASSHBinaryPath: ~/bin/assh  # optionally set the path of assh

For further inspiration, these assh.yml files on public GitHub projects can educate you on how people are using assh

Usage

assh usage

NAME:
   assh - advanced ssh config

USAGE:
   assh [global options] command [command options] [arguments...]

VERSION:
2.8.0 (HEAD)

AUTHOR(S):
   Manfred Touron <https://github.com/moul/assh>

COMMANDS:
   ping          Send packets to the SSH server and display statistics
   info          Display system-wide information
   config        Manage ssh and assh configuration
   sockets       Manage control sockets
   help, h       Shows a list of commands or help for one command

GLOBAL OPTIONS:
  --config value, -c value       Location of config file (default: "~/.ssh/assh.yml") [$ASSH_CONFIG]
  --debug, -D                    Enable debug mode [$ASSH_DEBUG]
  --verbose, -V                  Enable verbose mode
  --help, -h                     show help
  --version, -v                  print the version

Usage examples

assh config build

Rewrites and replaces the existing ~/.ssh/config file.

This action is automatically done by assh when detecting configuration changes. Running this command is useful to set up assh or repair the configuration file.

$ assh config build > ~/.ssh/config

assh config list

List hosts and options.

$ assh config list
Listing entries

    *.scw -> root@[hostname_not_specified]:22
        StrictHostKeyChecking=no [custom options]
        UserKnownHostsFile=/dev/null [custom options]

    *.shortcut1 -> bob@[hostname_not_specified]:22

    *.shortcut2 -> bob@[hostname_not_specified]:22

    bart -> bart@5.6.7.8:22

    bart-access -> bob@[hostname_not_specified]:22

    dolphin -> bob@dolphin:24

    expanded-host[0-7]* -> bob@%h.some.zone:22

    homer -> robert@1.2.3.4:2222

    lisa-access -> bob@[hostname_not_specified]:22

    maggie -> maggie@[hostname_not_specified]:22

    marvin -> bob@[hostname_not_specified]:23

    my-env-host -> user-moul@[hostname_not_specified]:22

    schoolgw -> bob@gw.school.com:22
        ForwardX11=no [custom options]

    schooltemplate -> student@[hostname_not_specified]:22
        ForwardX11=yes [custom options]
        IdentityFile=~/.ssh/school-rsa [custom options]

    vm-*.school.com -> bob@[hostname_not_specified]:22

    (*) General options:
        ControlMaster: auto
        ControlPath: ~/tmp/.ssh/cm/%h-%p-%r.sock
        ControlPersist: yes
        Port: 22
        User: bob

assh config graphviz

Generate a graphviz graph of the hosts

$ assh config graphviz | dot -Tpng > assh-hosts.png

assh config search <keyword>

Search for <keyword> in hosts and host options.

$ assh config search bart
Listing results for bart:
    bart -> bart@5.6.7.8:22
    bart-access -> moul@[hostname_not_specified]:22

assh info

Display system-wide information.

$ assh info
Debug mode (client): false
CLI Path: /path/to/assh
Go version: go1.6.2
OS/Arch: darwin/amd64

RC files:
- ~/.ssh/assh.yml
- ~/.ssh/assh.d/hosts.yml
- ~/.ssh/assh.d/moul.yml
- ~/.ssh/assh.d/test.yml

Statistics:
- 299 hosts
- 2 templates
- 4 included files

assh sockets list

List active control sockets.

$ assh sockets list
4 active control sockets in "~/.ssh/cm/":

- bart/homer/lisa-22-root.sock (14 minutes)
- bart/homer-22-root.sock (14 minutes)
- bart-22-root.sock (14 minutes)
- marge-22-bart.sock (1 hour)

assh sockets flush

Close active control sockets.

$ assh sockets flush
Closed 4 control sockets.

assh sockets master

Create a master control sockets.

$ assh sockets master

assh ping

Send packets to the SSH server and display stats.

$ assh ping -c 4 localhost
PING localhost (127.0.0.1) PORT 22 (ssh) PROTO tcp
Connected to 127.0.0.1: seq=0 time=321µs protocol=tcp port=22
Connected to 127.0.0.1: seq=1 time=501µs protocol=tcp port=22
Connected to 127.0.0.1: seq=2 time=550µs protocol=tcp port=22
Connected to 127.0.0.1: seq=3 time=641µs protocol=tcp port=22

--- localhost assh ping statistics ---
4 packets transmitted, 4 packets received, 0.00% packet loss
round-trip min/avg/max = 321µs/503.25µs/641µs

Install

Get the latest version using GO (recommended way):

go install moul.io/assh/v2@latest

note: tested with Go1.7 or above

---

Get the latest released version using homebrew (Mac OS X):

brew install assh

Build the latest version

brew install assh --HEAD

---

Get a released version on: https://github.com/moul/assh/releases

---

Install with asdf-vm:

asdf plugin add assh
asdf install assh latest
asdf global assh latest

---

Register the wrapper (optional)

To improve experience when using advanced pattern matching, add the following at the end of your .bashrc / .zshrc / config.fish:

alias ssh="assh wrapper ssh --"

This step is not mandatory but highly recommended.

---

Note: ssh does not understand advanced patterns; To bypass this limitation, assh maintains a list of known hosts and regenerate the ~/.ssh/config with all those expanded known hosts.

Without the wrapper, the ~/.ssh/config risks to be outdated when connecting to a new host for the first time and you will need to launch the command again.

With the wrapper, ssh will always be called with an updated ~/.ssh/config file.

Getting started

1. Backup your old ~/.ssh/config: cp ~/.ssh/config ~/.ssh/config.backup
2. Create a new ~/.ssh/assh.yml file
3. Run assh config build > ~/.ssh/config to validate the syntax of your ~/.ssh/assh.yml file and automatically build your ~/.ssh/config file
4. You are ready!

Docker

Experimental: assh may run in Docker, however you will have limitations:

• The assh containers does not have any binaries except assh, you can't use ProxyCommand, ResolveCommand...
• Docker may run on another host, ssh localhost will ssh to Docker host

docker run -it --rm -v ~/.ssh:/.ssh moul/assh --help

assh in Docker is slower and has more limitations, but it may be useful for testing or if you plan to use a Docker host as a remote Gateway

Alternative version

• v1 (2009-2015) - The original implementation. It worked quite well, but was a lot slower, less portable, harder to install for the user and harder to work on to develop new features and fix bugs

Troubleshooting

I can't use gateways

assh uses the built-in netcat mode of OpenSSH (shipped with OpenSSH 5.4) by default. If your ssh client doesn't support this feature, you can configure a custom ProxyCommand configuration, i.e.,

hosts:
  myserver:
    host: 1.2.3.4
    gateways: mygateway
    # configure a custom proxycommand
    proxycommand: /bin/nc %h %p

  mygateway:
    host: 5.6.7.8

---

You can configure this rule globally:

defaults:
  proxycommand: nc %h %p

---

Also, be sure to have netcat installed on your system, or use an alternative proxy binary, i.e., socat.

How to Configure resolver to parse /etc/hosts and/or handle mDNS requests

assh resolves hostnames using the system built-in resolver, depending on the OS, you can enable new features and/or change modules order.

• Linux - nsswitch documentation
• Linux - mDNS support (nss-mdns)
• Mac OS X - /etc/resolv.conf documentation

unix_listener: "/Users/.../.ssh/cm/..." too long for Unix domain socket

Starting with OpenSSH v6.7 the socket name can be shortened by configuring %C for the name expansion.

defaults:
  ControlPath: ~/tmp/.ssh/cm/%C.sock

%C is a unique identifier based on a hash of the tuple of (local host, remote user, hostname, port).

How to disable the automatic configuration rewrite

Each time you call ssh, assh will check if the generated ~/.ssh/config file is outdated.

By default, it will transparently regenerate the configuration file if needed.

You can disable this behavior by generating the configuration file like this:

assh config build --no-automatic-rewrite

3rd Party Integration

Ansible

In your ansible.cfg under ssh_connection, make sure you have the following, changing the path to your assh:

[ssh_connection]
ansible_ssh_executable = '/usr/local/bin/assh wrapper ssh'

3rd Party Projects

• ansible-dotfiles-assh: Ansible - Configure SSH with ASSH
• appflow: Multitenant environment automation

Contributors

Code Contributors

This project exists thanks to all the people who contribute. [Contribute].

Financial Contributors

Become a financial contributor and help us sustain our community. [Contribute]

Individuals

Organizations

Support this project with your organization. Your logo will show up here with a link to your website. [Contribute]

Stargazers over time

License

© 2009-2026 Manfred Touron - MIT License