ping LG #56

#

#### "Linux Gazette...making Linux just a little more fun!"

# ping

#### By Pat Eyler

#

## An Overview of ping

Ping is an diagnostic tool for verifying connectivity between two hosts on a network. It sends ICMP Echo Request packets to a remote IP address and watches for ICMP responses. The author of the initial version of the ping program that we use today was Mike Muss. Many other people have tweaked, rewritten, and variously abused ping since then.

The name ping itself is somewhat colorful. Some people will claim that it is an acronym standing for the Packet INternet Groper, this is not the case. Ping was named after the sound of a sonar tracking system. There is even a story claiming that a system administrator wrote a script which repeatedly pinged a host on the network and made an audible "pinging" alert for each success. The system administrator was then able to methodically go through his network checking BNC connectors until he found the dodgy connector that had been plaguing his network — when the noises stopped, he'd found his culprit.

Ping used to be a very good indicator of a machines ability to receive and send IP packets in general. If you could ping a host, you could also make an ftp or http connection. With the wider advent of packet filtering for security, this is becoming less true. Many firewalls explicitly disallow ICMP packets on the twin grounds that,

1) people don't need to know what your internal network looks like,
2) and, any protocol can be used to launch an attack, even ICMP.

Deciding whether to let ICMP through your firewall is a tough call to make. There are certainly good uses for ICMP, but there are also attacks based on ICMP (e.g., the "Ping of Death", which used oversized ping packets to overload the IP stack of the target — with often spectacular results). If you choose to allow ICMP into your network make sure you've thought about the repercussions.

There are additional flavors of the ping command that have been written for other purposes. Among the most common is the fping command. Which was written to ping a range of addresses, and is commonly used in network scanners and monitors like saint and mon (both of which are covered in this chapter). Another variation is the Net::Ping module, which provides a perl implementation of Ping functionality that can easily be used from within a script without calling an external program. You might use the script something like this:

Example 1. Using Net::Perl

    #!/usr/bin/perl -w

    use strict;
    use Net::Ping;

    my $host = $ARGV[0];

    my $p = Net::Ping->new("icmp");

    if ($p->ping($host)) {
        print "$host is alive.\n";
    } else {
        print "$host is not reachable.\n";
    }

## ping at Work

Ping is most often used without additional arguments and shut off with a Ctrl–c. The results look like this:

    [pate@cherry pate]$ ping mango
    PING mango (192.168.1.1) from 192.168.1.10 : 56(84) bytes of data.
    64 bytes from mango (192.168.1.1): icmp_seq=0 ttl=255 time=0.5 ms
    64 bytes from mango (192.168.1.1): icmp_seq=1 ttl=255 time=0.3 ms
    64 bytes from mango (192.168.1.1): icmp_seq=2 ttl=255 time=0.3 ms
    64 bytes from mango (192.168.1.1): icmp_seq=3 ttl=255 time=0.3 ms
    64 bytes from mango (192.168.1.1): icmp_seq=4 ttl=255 time=0.3 ms
    64 bytes from mango (192.168.1.1): icmp_seq=5 ttl=255 time=0.3 ms

    --- mango ping statistics ---
    6 packets transmitted, 6 packets received, 0% packet loss
    round-trip min/avg/max = 0.3/0.3/0.5 ms
    [pate@cherry pate]$

This output can split into three sections. The first section, the single line starting with the word "PING", shows an overview of the command. The second section, the lines beginning with "64 bytes" show a running tally of the responses received. The third section, everything after the line "--- mango ping statistics ---", shows a summary of the results. In this case, the results are good, none of the packets were dropped and they were all passed fairly quickly.

This example also shows another important point, you should not rely on a single packet to diagnose your network. A series of five or ten is much better, as you can count up to 40% data loss as congestion on a network, and even a single packet dropped can be attributed to a busy host on the other end.

There are several useful options to the ping command. These are summarized in the following table:

Table 1. Ping Command Options

Switch | Description
--- | ---
`-c` `count` | Stop sending and receiving packets after `count` packets.
`-d` | Set the SO_DEBUG on the socket used.
`-f` | Send the packets as fast as possible. (flood)
`-i` `wait` | Set an interval of `wait` seconds between packets.
`-I` `〈device〉` | Sets the output interface.
`-l` `preload` | Sends `preload` packets as fast as possible, then drops back to normal mode.
`-n` | Don't look up hostnames, just give IP addresses. (numeric)
`-p` `pattern` | Specify up to 16 bytes of "pad data" to be sent with the packet.
`-q` | Output only summary lines. (quiet)
`-r` | Don't use routing tables to send the packet, just drop it out the local interface.
`-R` | Set the Record Route option.
`-s` `packetsize` | Set the number of data bytes sent to `packetsize`.
`-T tsonly` | Sends a ping with the timestamp option.
`-T tsandaddr` | Collects timestamps and addresses
`-T tsprespec` `[host1 [host2 [host3 [host4]]]]` | Collects timestamps and addresses from prespecified hops.

These options can be combined to make ping even more helpful. One thing that you cannot see is that the ping command used in the previous section is likely to take several seconds to run and report back. Using the `-f` switch will reduce the time spent waiting for the command. Combining this with the `-c 10` and the `-q` switches will give you quick results and easier output to read:

    	[root@cherry /root]# ping -c 10 -fq mango
    PING mango (192.168.1.1) from 192.168.1.10 : 56(84) bytes of data.

    --- mango ping statistics ---
    10 packets transmitted, 10 packets received, 0% packet loss
    round-trip min/avg/max = 0.2/0.2/0.9 ms
    [root@cherry /root]#

Note: The -f and -l switches can only be used by root, as they can cause serious network degradation if misused.

It might be of some benefit to test larger packets, using `ping -c10 -s 1024 -qf` will send larger packets for you. This can be especially useful where you suspect problems with fragmented packets.

To see the route that your packets are traversing, you can use `ping -c10 -R`. This command produces the following output:

    	PING tbr.nailed.org (206.66.240.72) from 192.168.1.10 : 56(124) bytes of data.
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=0 ttl=239 time=217.2 ms
    RR: 	192.168.1.10
    	216.41.39.90
    	serial0.mmgw32.bos1.Level3.net (209.244.39.25)
    	208.218.130.22
    	166.90.184.2
    	so-6-0-0.mp2.NewYork1.level3.net (209.247.10.45)
    	137.39.52.10
    	180.ATM7-0.BR2.NYC9.ALTER.NET (152.63.22.229)
    	lo0.XR2.NYC9.ALTER.NET (137.39.4.175)

    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=1 ttl=239 time=1940.8 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=2 ttl=239 time=250.6 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=3 ttl=239 time=230.3 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=4 ttl=239 time=289.8 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=5 ttl=239 time=1261.4 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=6 ttl=239 time=469.4 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=7 ttl=239 time=1272.3 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=8 ttl=239 time=353.1 ms	(same route)
    64 bytes from bigfun.whirlycott.com (206.66.240.72): icmp_seq=9 ttl=239 time=1281.1 ms	(same route)

    --- tbr.nailed.org ping statistics ---
    10 packets transmitted, 10 packets received, 0% packet loss
    round-trip min/avg/max = 217.2/756.6/1940.8 ms

Note: The record route option specified by the `-R` switch is not honored by all routers and hosts. Further, it contains only a limited space to hold router addresses, traceroute may be a better tool for identifying the path packets follow through a network.

The ping command is a very useful tool for your troubleshooting kit, and should not be overlooked.

This article is copyright 2000, Pat Eyler and New Riders Publishing. It is presented under the Open Publication License, with no additional terms applied. It is a draft version of a section of the book Networking Linux: A Practical Guide to TCP/IP, which will be published by New Riders Publishing in the winter.

##### Copyright © 2000, Pat Eyler and New Riders Publishing
Published in Issue 56 of Linux Gazette, August 2000