Testing active ssh keys on the local network - linux

Question

I am trying currently to achieve a bash script that will validate if SSH keys on a server are still linked to known hosts that are active on the local area network. You can find below the beginning of my bash script to achieve this:
#!/bin/bash
# LAN SSH KEYS DISCOVERY SCRIPT
# TRYING TO FIND THOSE SSH KEYS NOW
cat /etc/passwd | grep /bin/bash > bash_users
cat bash_users | cut -d ":" -f 6 > cutted.bash_users_home_dir
for bash_users in $(cat cutted.bash_users_home_dir)
do
ls -al $bash_users/.ssh/*id_* >> ssh-keys.txt
done
# DISCOVERING THE KNOWN_HOSTS NOW
for known_hosts in $(cat cutted.bash_users_home_dir)
do
cat $bash_users/.ssh/known_hosts | awk '{print $1}' | sort -u >>
hosts_known.txt
sleep 2
done
hosts_known=$(wc -l hosts_known.txt)
echo "We have $hosts_known known hosts that could be still active via SSH
keys"
# TIME TO TEST WHICH SSH servers are still active with the SSH keys
# AND THIS IS WHERE I AM FROZEN...
# Would love to have bash script that could
# ssh -l $users_that_have_/bin/bash -i $ssh_keys $ssh_servers
# Would also be very nice if it could save active
# SSH servers with the valid keys in output.txt in the format
# username:local-IP:/path/to/SSH_key
Please feel very comfortable to edit/modify the bash script above if it can serve better the goals described.
Any help would be very appreciated,
Thanks

Answer 1

The following works cool:
</etc/passwd \
grep /bin/bash |
cut -d: -f6 |
sudo xargs -i -- sh -c '
[ -e "$1" ] && cat "$1"
' -- {}/.ssh/known_hosts |
cut -d' ' -f1 |
tr ',' '\n' |
sed '
/^\[/{
s/\[\(.*\)\]:\(.*\)/\1 \2/;
t;
};
s/$/ 22/;
' |
sort -u |
xargs -l1 -- sh -c '
if echo "~" | nc -q1 -w3 "$1" "$2" | grep -q "^SSH"; then
echo "#### SUCCESS $1 $2";
else
echo "#### ERROR $1 $2";
fi
' --
So:
Start with /etc/passwd
Filter all "bash_users" as you call them
Filter user home directories only cut -d: -f6
For each user home directory sudo xargs -i -- run
Check if the file .ssh/known_hosts inside the user home directory exists
If it does, print it
Filter only hosts names
Multiple hosts signatures may share same key and are separated by a comma. Replace comma for newline
Now a sed script:
If a line starts with a [ that means it has a format of [host]:port and I want to replace it with host port
If the line does not start with a [ I add 22 to the end of the line so it's host 22
Then I sort -u
Now for each line:
I get the ssh version from ssh echo "~" | nc hostname port returns smth like "SSH-2.0-OpenSSH_6.0" + newline + "Protocol mismatch".
So if the line returned by nc hostname port starts with SSH that means there is ssh running on the other side
I added timeout for unresponsive hosts, but I think nc -w timeout option may also be used. Probably also nc -q 1 should be specified.
Now the real fun is, when you add the max-procs option to the last xargs line, you can check all hosts simultaneously. On my host I have 47 unique addresses and xargs -P30 checks them ALL in like 2 seconds.
But really there are some problems. The script needs root to read from all users known_hosts. But worse, the known_hosts may be hashed. It would be better to firstly know the list of hosts on your network, and then generate known_hosts from it. It would look like ssh-keyscan -f list_of_hosts > ~/.ssh/known_hosts or similar. Generaly ssh-keygen -F hostname should be used if a host exists in known_hosts, sadly there is no listing command. known_hosts file format may be found in ssh documentation.