Perl Net::Ping not working properly with ICMP by default

If you tried to ping a host with Perl Net::Ping using the ICMP protocol and that failed, even though the “ping” command-line utility can ping the host, you’re not alone :) I had the same problem and it turned out to be due to the fact that Net::Ping by default sends no DATA in the ICMP request and thus its requests are rather short and non-standard. Here are some tcpdump examples:

  • Net::Ping with ICMP protocol, everything else is defaults: “$p = new Net::Ping(‘icmp’)“, no replies from remote host, note that the length is just 8 bytes:
    12:29:02.898083 IP source-addr > source-addr: ICMP echo request, id 2194, seq 41, length 8
    12:29:03.711595 IP source-addr > dest-addr: ICMP echo request, id 2194, seq 42, length 8
    
  • Linux “ping” command-line utility, remote host replies accordingly, the length is 64 bytes total:
    12:30:18.278865 IP source-addr > dest-addr: ICMP echo request, id 2488, seq 1, length 64
    12:30:18.289922 IP dest-addr > source-addr: ICMP echo reply, id 2488, seq 1, length 64
    12:30:18.790610 IP source-addr > dest-addr: ICMP echo request, id 2488, seq 2, length 64
    12:30:18.811029 IP dest-addr > source-addr: ICMP echo reply, id 2488, seq 2, length 64
    
  • Net::Ping with ICMP protocol with user-defined length, “$p = new Net::Ping(‘icmp’, 1, 56)“, remote host replies accordingly, the length is 64 bytes total:
    12:30:48.377496 IP source-addr > dest-addr: ICMP echo request, id 2488, seq 6, length 64
    12:30:48.433690 IP dest-addr > source-addr: ICMP echo reply, id 2488, seq 6, length 64
    12:30:48.934310 IP source-addr > dest-addr: ICMP echo request, id 2488, seq 7, length 64
    12:30:48.946152 IP dest-addr > source-addr: ICMP echo reply, id 2488, seq 7, length 64
    

Bottom line is that if you are going to use Net::Ping with ICMP, specify 56 for the “bytes” parameter when creating an instance of the Net::Ping object. This way you will be sending standard ICMP requests with total length of 64 bytes.

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