Quickly check whether many network hosts are up-and-running

Question

Objective: check if a host is up-and-running on the network,
with a method that can be efficiently parallelized over many hosts.

Emphasis on a QUICK turnaround of the check.

Reason to emphasise speed: multiple checks (in the hundreds) need to be performed in rapid succession and return results reasonably quickly.

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Current method

The current script uses a simple ping command. The choice is not obligatory, on the contrary any suitable tool with equal or better reliability and speed may be used as replacement.

Current script

Something along these lines:

ping -c 1 -W 100 -q "$NETWORK_HOST" &> /dev/null

Obvious deficiency of this approach: responses are needed with a sub-second turn-around and this can wait up to a second, from experience.

Alternative considered

Forget about speed of sequential execution, run many ping commands in parallel using GNU parallel and collate the results at the end. This has been experimented with but seems even worse in practice.

Intuition about a better solution

It seems like ping may work just fine as a "polling health check", all it needs to be tweaked to do is to wait very little time and time-out in case of no response.

Assumption: the network is considered reliable and FAST, the hosts don't have either of the qualities necessarily.

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Question

How would you solve this? Which tool(s) would you use? Is this the right approach? Could you provide a code snippet?

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• Environment: OS X + Ubuntu hosts
• Scripting shell: Bash
• Can install additional software if needed.
• Can compile/install new code for an application not in the repository and use that.

Answer 1

Provided that ping is good enough for you, fping is an alternative that works in parallel right away.

This is simplified version of what I use. It works with a list of hosts (one per line) passed in via pipe:

probe_hosts() {
    local report
    fping 2>/dev/null | while read report;
    do
        local host=${report/ is *}
        local state=${report/* is }
        if [ "$state" == "alive" ];
        then
            echo $host
        else
            echo unreachable: $host >&2
        fi
    done
}

# this is how you use it:
cat list_of_hosts \
    | probe_hosts \
    | do_something_with_live_hosts

Answer 2

You can give nmap a range:

$ nmap -sn 138.0.0.0/24
$ nmap -sn 138.0.0.0-255

The -sn flag means to just ping the host and return (i.e. no port scan like nmap usually does).

Edit: After reading the comments I see that bahamat mentions arp. In fact, arp with no arguments runs faster than nmap for me, and finds everything connected to my LAN:

$ time arp
real    0m0.411s