Docu review done: Tue 17 Oct 2023 10:49:43 AM CEST

Table of content

• Commands
• SSH Options
• Unresponsive Session
• Run command in background via ssh and no open ssh session
  • Via tmux
  • Via screen
• SSH pubkey signing
• Portforwarding
• SSH Agent Hijacking
• Find out which ips are used in a ip range (e.g. vlan) for undocumented vips
• Query OpenSSH server

Commands

SSH Options

Unresponsive Session

Sometimes it happens that you stay connected to a server while you do something else or walk away. Then it can happen, the when you return to your terminal where you executed the ssh command, that it got stuck and does not respond any more. Of course you could now close just the terminal and forget about it, but what if you have done other things in that one too and want to keep working in that one.

Well there is a easy way to do so, you just have to press the following keys one after the other and it will kill the session and return you to your old shell session of your terminal.

1. Enter
2. ~ Tilda
3. . Dot

After doing so you will see something like this:

myremoteuser@remotehost1:~$
myremoteuser@remotehost1:~$ Connection to remotehost1 closed
mylocaluser@localhost:~$

Returncode will be 255 for this action

Run command in background via ssh and no open ssh session

Via tmux

Make sure that you dont have remain-on-exit is not set in the tmux config This would keep the tmux session open till the user manually terminates it

$ ssh mydestinationhost "tmux myfancytmuxepidamicname -d \"sleep 10\""

Via screen

Make sure that you dont have zombie cr is not set in the screen config This would keep the screen session open till the user manually terminates it

$ ssh mydestinationhost screen -d -m "sleep 10"

SSH PUBKEY SIGNING

Generate CA for signing pubkeys It will ask your for a pwd, please use a good one ;)

$ ssh-keygen -f <caname>_ca

Now you will find two files in the directory: <caname>_ca and <caname>_ca.pub To sign now the pubkeys from the other hosts you should have them local available.

$ ssh-keygen -s <caname>_ca. -I <key_identifier> -h -n <host_name> host_pub_key_file #

Optional you can add a expire with -V e.g. -V +52w

Sample:

$ sudo ssh-keygen -s /home/suchademon/VersionControl/ssh-sign/we-are-the-sons-of-sparda_ca -I host_sparda -h -n sparda /etc/ssh/ssh_host_ed25519_key.pub
    [sudo] password for suchademon:
    Enter passphrase:
    Signed host key /etc/ssh/ssh_host_ed25519_key-cert.pub: id "host_sparda" serial 0 for sparda valid forever

Deploy new signed pub key to host and restart ssh daemon

Portforwarding

Forward multi ports from source host to destination in one ssh connect

$ ssh -R<SRCPORT>:<DESTIP>:<DESTPORT> -R<SRCPORT>:<DESTIP>:<DESTPORT>... -l root <SSHDESTINATION>

Sample:

$ ssh -R9999:10.0.7.4:9999 -R8081:192.168.0.2:8081 -R8140:192.168.0.2:8140  -R389:10.0.9.5:389 -l root
$ ssh -R27:192.168.0.1:22 -l root 192.168.1.2

SSH Agent Hijacking

First check if an addtional use is loged-in and check the user name

$ w
 14:08:38 up 29 days,  4:19,  2 users,  load average: 4.03, 1.60, 1.23
USER        TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
SOMEONEELSE pts/1    10.10.10.100     14:00    7.00s  0.03s  0.03s -bash
ISTME       pts/2    10.10.10.101     14:08    0.00s  0.04s  0.00s w

Become root

$ su -

Get process of the ssh session

$ pstree -p SOMEONEELSE
sshd(110863)───bash(110864)

Shortest way is to check the tmp dir, and search for agent.

$ find /tmp/ -name "agent.110863" | grep ssh
/tmp/ssh-TE6SgmexKR/agent.110863

Now you can just easily check the ssh agent

$ SSH_AUTH_SOCK=/tmp/ssh-TE6SgmexKR/agent.110863 ssh-add -l
256 ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab SOMEONEELSE@SOMETHING (ED25519)
16384 ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab:ab .ssh/id_rsa (RSA)

So you know that there are keys loaded and can use them ;)

$ SSH_AUTH_SOCK=/tmp/ssh-TE6SgmexKR/agent.110863 ssh SOMEONEELSE@HOST2

Find out which ips are used in a ip range (e.g. vlan) for undocumented vips

a="10.70.44."; for i in {150..152}; do echo "${a}${i}: $(ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o ConnectTimeout=1 -o ConnectionAttempts=1 -q ${a}${i} hostname 2>/dev/null)"; done

Query OpenSSH server

You can fetch informations like ciphers, mak and so on from running OpenSSH serivce by using ssh -Q This will return you the list of resultes.

For example quering security configuration from a server:

$ for f in cipher mac kex key ; do echo "$f:" ;  ssh -Q $f 10.42.42.1 ; echo ; echo ; echo ; done