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    • Testing for Remote Listener

    Posted on

    Anyone know a good method for testing whether a remote UDP service is reachable - preferably without the use of 

    netcat
    or similar tools? TCP-based listeners are easy enough to do a cursory test: TCP gives you a basic pass/fail on a socket open attempt. Thus, a test like:

          for HOST in ${SERVERLIST}
      do
         printf "Attempting service-connect to ${HOST}...\n"
         SOCKTEST=$(timeout 5 bash -c 'cat  /dev/null > \
                                   /dev/tcp/${HOST}/53')$?

         if [[ ${SOCKTEST} -eq 0 ]]
         then
            printf "Socket-test passed.\n"
         else
            printf "Socket-test failed.\n"
         fi

      done

    Is a fairly quick way to ensure that (in this case) the guys who provisioned a system pointed it to reachable DNS servers (didn't fat-finger any entries in the DHCP zone).

    Using the above method-type for UDP doesn't really work - you get false successes. The specific case I'm trying to verify is NTPD accessibility. Using 

    ntpdate
    would be fine, but I'd need to do my tests like 
    service ntpd stop ;  ; service ntpd start
    . Could also use 
    ntpq -c peers
    , but would really rather test servers individually rather than all at once. And, really, would overall prefer a generalizable method for UDP testing rather than having to do per-service test-definitions.

    Any ideas?

    by

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