script to check if I can access multiple servers - bash

Question

I made this script to check if I can connect to a list of many servers:
for SERVER in $(cat servers.txt); do
ssh root#$SERVER && echo OK $SERVER || echo ERR $SERVER
done
The problem is that if is the first time I’m connecting to the server, the server asks the classic question “The authenticity of host ‘x.x.x.x’ can't be established... bla bla bla” Then I have to respond yes or not and it losses the purpose of making it a script, is there any way to bypass that so I can add it to the script?
Also, There are some servers in which I don't have my keys in them but they have the option to enter a password. In that case it will wait until I try a password to continue with the script execution, so I was wondering if there is a way to improve this script so if the server asks for a password then set it to ERR $SERVER and continue with the script?
Thank you for your help.

Answer 1

Do you actually need to establish an SSH connection?
Or is simply opening a socket connection to the host on the SSH port enough for you to determine that the server is online?
servers.txt
hostA.example.com
hostB.example.com
hostC.example.com
port-probe.sh
#!/bin/bash
PORT=22
TIMEOUT=3
for SERVER in $(cat servers.txt); do
# Open a socket and send a char
echo "-" | nc -w $TIMEOUT $SERVER $PORT &> /dev/null
# Check exit code of NC
if [ $? -eq 0 ]; then
echo "$SERVER is Available."
else
echo "$SERVER is Unavailable."
fi
done

Answer 2

You can use the -o flag to set options in SSH:
for SERVER in $(cat servers.txt); do
ssh -o StrictHostKeyChecking=no -o BatchMode=yes root#$SERVER exit && echo OK $SERVER || echo ERR $SERVER
done
Check out the manpage for ssh_config: man ssh_config for all of the options available with the -o flag.

Answer 3

If you have quite a few servers what you may want to do is connect to all of them simultaneously. This way it only waits maximum 2 minutes (default TCP connect timeout) if any of the servers is unresponsive.
Connect to each server without allocating a terminal and execute echo . command, redirect the output into a named file. Issue these commands in a loop asynchronously. Then wait till all commands complete, iterate over the log files and check which ones have dots in it. Then report the servers whose log files do not have a dot in it.
E.g.:
#/bin/bash
servers="$#"
for server in $servers; do
ssh -o StrictHostKeyChecking=no -o PasswordAuthentication=no -Tn $server echo . >$server.log 2>$server.error.log &
done
wait # After 2 minutes all connection attempts timeout.
for server in $servers; do
[[ -s $server.log ]] || echo "Failed to connect to $server" >2
done