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SSH Cheat Sheet

OpenSSH Commands

Copy Keys

ssh-copy-id [-i keyfile] [email protected]

100% non-interactive SSH: What parameters to use to avoid any interaction.

ssh -i my_priv_key -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no 
-o PreferredAuthentications=publickey [email protected] -n "/bin/ls"

Using SSH Agent

eval $(ssh-agent)       # Start agent on demand

ssh-add -l                      # List keys
ssh-add                         # Add default key
ssh-add ~/.ssh/id_rsa           # Add specific key
ssh-add -t 3600 ~/.ssh/id_rsa   # Add with timeout
ssh-add -D                      # Drop keys

ssh -A ...          # Enforce agent forwarding

Transparent Multi-Hop

ssh -A -t host1 ssh -A -t host2 ssh -A -t host3 ...

SOCKS proxy

Start a SOCKS proxy with

ssh -D 44444 <remote host>

For a reverse SOCKS proxy run

ssh -f -N -D 44444 localhost           # Start proxy locally
ssh myserver -R44445:localhost:44444   # Connect it via remote port-forwarding

export HTTPS_PROXY=socks5://localhost:4445/   # Configure remote host to use the SOCKS forwarding
export HTTP_PROXY=socks5://localhost:4445/

ssh_keygen

Extract public key from private key using ssh-keygen

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

Enfore no passphrase

ssh-keygen -t rsa -N ""

ssh_config

Read the authorized_keys HowTo to learn about syntax and options…

Per Host Keys

Host example.com
IdentityFile ~/.ssh/example.com_id_rsa

Agent Forwarding

Agent Forwarding explained with pictures! Configured in /etc/ssh_config with

Host *
ForwardAgent yes

Multiplexing Connections

This is done using a “ControlMaster”. This means the first SSH sessions connection will be used for all following ones saving you the connection overhead. Note: when you kill the first connection, all connections will die! Also the first connection won’t terminate even if you request it to.

Create ~/.ssh/tmp before using below snippet

ControlMaster auto
ControlPath /home/<user name>/.ssh/tmp/%h_%p_%r

If you are using such an SSH configuration and want a real new connection add “-S” to the ssh invocation.

Use Gateway/Jumphost

You can configure jumphosts using ProxyCommand and netcat:

Host unreachable_host
  ProxyCommand ssh -e none gateway_host exec nc %h %p

Automatic Jump Host Proxying

Host <your jump host>
  ForwardAgent yes
  Hostname <your jump host>
  User <your user name on jump host>

# Note the server list can have wild cards, e.g. "webserver-* database*"
Host <server list>
  ForwardAgent yes
  User <your user name on all these hosts>
  ProxyCommand ssh -q <your jump host> nc -q0 %h 22

Automatic Port Knocking

Host myserver
   User myuser
   Host myserver.com
   ProxyCommand bash -c '/usr/bin/knock %h 1000 2000 3000 4000; sleep 1; exec /bin/nc %h %p'

Troubleshooting

Pseudo-terminal will not be allocated…

This happens when piping shell commands through SSH. Try adding “-T” or “-t -t” when doing sudo.

Misc

  • SFTP chroot with umask: How to enforce a umask with SFTP

    Subsystem sftp /usr/libexec/openssh/sftp-server -u 0002
    
  • Parallel SSH on Debian

    apt-get install pssh
    

    and use it like this

    pssh -h host_list.txt <command>
    pssh -i -t 60 -h host_list.txt -- <command>   # 60s timeout, list output
    
  • Clustered SSH on Debian

    apt-get install clusterssh
    

    and use it like this

    cssh server1 server2
    
  • Vim Remote File Editing:

    vim scp:[email protected]//some/directory/file.txt
    
  • MonkeySphere: Use GPG keys with SSH agent

    monkeysphere subkey-to-ssh-agent -t 3600
    

Port Knocking

Setup server:

apt-get install knockd iptables-persistent

# Change sequence numbers in /etc/knockd.conf
# Default is sequence    = 7000,8000,9000

# set START_KNOCKD=1 in /etc/default/knockd

service knockd start

Use from client

knock <server> <sequence>

e.g.

knock example.com 7000 8000 9000

“Secret” Hot Keys

SSH Escape Key: Pressing “~?” (directly following a newline) gives a menu for escape sequences:

Supported escape sequences:
  ~.  - terminate connection (and any multiplexed sessions)
  ~B  - send a BREAK to the remote system
  ~C  - open a command line
  ~R  - Request rekey (SSH protocol 2 only)
  ~^Z - suspend ssh
  ~#  - list forwarded connections
  ~&  - background ssh (when waiting for connections to terminate)
  ~?  - this message
  ~~  - send the escape character by typing it twice
(Note that escapes are only recognized immediately after newline.)

SSHFS

To mount a remote home dir

 sshfs [email protected]: /mnt/home/user/

Unmount again with

fuserumount -u /mnt/home/user