How to check if I can log in to server via ssh?

Question

I have a list of servers:

cat list.txt
10.10.10.10 servera
10.11.10.10 serverb

How can I check that I can log in via ssh to them or not? I mean by default I should be able to log in via ssh key auth.., so in short, I need a solution that sorts the lines (servers) in the list.txt like this:

• servers that I can log in via ssh key
• servers that prompts for password (of course password is unknown..)
• servers that are unreachable

Answer 1

You can do that with a combination of the BatchMode option and "parsing" the output. (ssh always returns 255 if it fails to connect for whatever reason, so you can't use the return code to distinguish between types of failures.)

With BatchMode on, no password prompt or other interaction is attempted, so a connect that requires a password will fail. (I also put a ConnectTimeout in there which should be adjusted to fit your needs. And picked really bad filenames.)

#! /bin/bash

rm good no_auth other
while read ip host ; do
    status=$(ssh -o BatchMode=yes -o ConnectTimeout=5 $ip echo ok 2>&1)
    if [[ $status == ok ]] ; then
        echo $ip $host >> good
    elif [[ $status == "Permission denied"* ]] ; then
        echo $ip $host $status >> no_auth
    else
        echo $ip $host $status >> other
    fi
done < list.txt

You could detect other types of errors (like missing server public key) if you need more detailed classification. If you need the results in a single, sorted file, just cat the various output files together as you see fit.

Answer 2

servers that I can log in via ssh key
servers that prompts for password (of course password is unknown..)

expect will provide input to your interactive commands. The ssh client with the -v flag will tell you what auth methods the server accepts. If you get to the interactive prompt, make it quit. Do what you wan, you have all you need.

servers that are unreachable

Same, one command to rule them all, and in the darkness, bind them... Hum.

Answer 3

Look up the various tools that automate running commands on multiple hosts through ssh. For example, with Mussh:

mussh -H hosts.txt -o ConnectTimeout=5 -P -d -c 'echo `hostname` is alive'

Massage the output as needed.

Side note: why are you storing IP addresses in list.txt? Server names are enough. If the names you want to use aren't the DNS names, use Host directives in ~/.ssh/config.