I’ve written something about SSH and here are more, on authentication, tunnelling and some other.

Configuration

system-wide client: /etc/ssh/ssh_config

system-wide daemon: /etc/ssh/sshd_config

User: ~/.ssh/config

Format of ~/.ssh/config (more on man ssh_config):

Host my-server.com
User admin
IdentityFile ~/.ssh/admin.id_dsa
BatchMode yes
EscapeChar none

Host mm
User matt
HostName might.net
IdentityFile ~/.ssh/matt.id_dsa

Host *.lab.ucaprica.edu
User u8193

Key-based authentication

Create key pair:

$ ssh-keygen -t dsa

this will create private key in ~/.ssh/id_dsa and public key in ~/.ssh/id_dsa/pub. We can specify -f path/to/key if we want to specify the path for the newly generated key. We should set appropriate permission to the private key.

Then append public key to authorized_keys:

$ cat ~/.ssh/id_dsa.pub | ssh remote_host `cat >> ~/.ssh/authorized_keys`

We can also use ssh-copy-id command to do the above:

$ ssh-copy-id remote_server
The authenticity of host 'name-of-remote-server (144.144.144.144)' can't be established.
ECDSA key fingerprint is 9f:1e:ab:b6:ff:71:88:a9:98:7a:8d:f1:42:7d:8c:20.
Are you sure you want to continue connecting (yes/no)? yes
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
Password:

Using a specified key other than the default ~/.ssh/id_rsa:

$ ssh -i ~/.ssh/myprivatekey hostname

passphase

ssh-keygen will ask for a passphase when a key is generated. We can change the passphase of an existing key by ssh-keygen -p

using ssh-agent

To run ssh-agent:

$ eval $(ssh-agent)

which in turn is something similar to the following:

SSH_AUTH_SOCK=/var/folders/y6/5fd4y7sj71g9v5d43fkpvn940000gn/T//ssh-ZMjJ69ERT1hc/agent.73328; export SSH_AUTH_SOCK;
SSH_AGENT_PID=73329; export SSH_AGENT_PID;
echo Agent pid 73329;

Or, for putting into ~/.bashrc or ~/.bash_profile (source):

ssh-add -l &>/dev/null
if [ "$?" == 2 ]; then
  test -r ~/.ssh-agent && \
    eval "$(<~/.ssh-agent)" >/dev/null

  ssh-add -l &>/dev/null
  if [ "$?" == 2 ]; then
    (umask 066; ssh-agent -t 3600 > ~/.ssh-agent)
    eval "$(<~/.ssh-agent)" >/dev/null
    ssh-add
  fi
fi

After ssh-agent is running, we can list all the keys available to the ssh-agent:

$ ssh-add -L

or add a key to the agent:

$ ssh-add ~/.ssh/path/to/privatekey

Omitting the path to a particular key will add all default key files in ~/.ssh. If there are too many keys in use, it is recommended to specify the correct key for a host in the ssh config file:

Host yourserver
HostName yourserver.tld
IdentityFile ~/.ssh/path/to/privatekey
IdentitiesOnly yes
User user

Agents can also be forwarded if:

  • ForwardAgent yes in client’s ssh_config file, or using -A in ssh command
  • AllowAgentForwarding yes in each server’s /etc/ssh/sshd_config file

Because these are required at every login and therefore it should be included in the .bashrc script, such as the following:

if [ -z "$SSH_AUTH_SOCK" ] ; then
    eval `ssh-agent`
    ssh-add # or ssh-add -K if in macOS
fi

In macOS Sierra 10.12.2+, we can load keys automatically and make use of the macOS keychain to hold the key’s passphase by adding the following to ~/.ssh/config:

Host *
  AddKeysToAgent yes
  UseKeychain yes
  IdentityFile ~/.ssh/id_rsa
  IdentityFile ~/.ssh/id_dsa

and macOS’s way to add a key to ssh-agent using the passphase from keychain is the -K option:

$ ssh-add -K ~/.ssh/path/to/privatekey

Tunneling

local port forwarding (port listened locally)

At host A, connect to a port at host B by way of host C:

hostA$ ssh hostC -L localport:hostB:remoteport

hostA$ telnet localhost:localport

If ssh goes with -g (option GatewayPorts=yes), the localport is bind to public instead of local loopback. Or we can explicitly specify the binding address:

hostA$ ssh hostC -L localaddress:localport:hostB:remoteport

remote port forwarding (port listened at remote host)

Give remote host B access to host C by way of the local machine A:

hostA$ ssh hostB -R remoteport:hostC:targetport

hostB$ telnet localhost:remoteport

SOCKS proxy

$ ssh-D localport remote_host

then a SOCKS5 proxy is created at localhost port localport and traffic will be tunneled to remote_host. In Firefox, DNS will not be tunneled through SOCKS proxy unless network.proxy.socks_remote_dns=True

ProxyJump

From host A connect to host B by way of host C. As if we first SSH into host C and then from there SSH into host B

hostA$ ssh -J userC@hostC:22 userB@hostB

Multiple jump hops may be specified by commas

hostA$ ssh -J user1@host1:port1,user2@host2:post2 -p port3 user3@host3

The -J option can also be specified in the config file as ProxyJump directive. Older version of SSH does not provide ProxyJump directive and we have to use ProxyCommand directive instead:

ProxyCommand ssh user@proxyhost -W %h:%p

create a layer-3 tun device between hosts A and B

Must be root at both hosts in order to create tun0 virtual network devices. Also PermitTunnel=yes and PermitRootLogin=yes in sshd_config on both ends.

hostA# ssh -f -w 0:0 root@hostB true
hostA# ifconfig tun0 192.168.1.101 netmask 255.255.255.0

hostB# ifconfig tun0 192.168.1.102 netmask 255.255.255.0

create a layer-2 tap device between hosts A and B

First we have to create the tap device on both hosts by either of the following two commands

# tunctl -t tap0
# tuntap add devtap0 mode tap

Then set up addresses on both hosts:

hostA# ifconfig tap0 192.168.1.101 netmask 255.255.255.0

hostB# ifconfig tap0 192.168.1.102 netmask 255.255.255.0

Lastly set up the SSH tunnel:

hostA# ssh -o Tunnel=ethernet -f -w 0:0 root@hostB true

we can check the tunnel working by ping on remote address, or the command ethtool tap0 should give Link detected: yes

To shutdown, send SIGTERM to the ssh process.

other useful arguments to ssh in tunneling

  • -f to drop ssh to background
  • -N to run no command at remote server (-f -N = daemonized mode)
  • -o ServerAliveInterval=30 to send something over the TCP every 30 seconds to keep the tunnel alive. Similar options are ClientAliveInterval and TCPKeepAlive

Miscellaneous ssh usage

Copy file:

cat file | ssh -c -e none remote_host 'cat > file'

-e none is to disable escape sequences. -c enable compression (optional)

SSH banner: Create a text file somewhere and edit sshd_config for:

Banner /etc/mybanner.txt

Time lock against brute force SSH attack in iptables:

iptables -A INPUT -p tcp -m state --syn --state NEW --dport 22 -m limit --limit 1/minute --limit-burst 1 -j ACCEPT
iptables -A INPUT -p tcp -m state --syn --state NEW --dport 22 -j DROP

Reference

  • http://matt.might.net/articles/ssh-hacks/
  • https://www.taos.com/advanced-ssh-tunneling/
  • https://dzone.com/articles/advanced-secure-shell-6-things-you-can-do-with-ssh
  • http://rabexc.org/posts/using-ssh-agent