ping is one of the oldest IP utilities around. Simply put, ping asks another host if it is alive, and records the round-trip time between the request and the reply.
In this section, we'll look at several examples of the use of ping to test reachability, send a specified number of packets, suppress all but summary output, stress the network, record the route a packet takes, set the TTL, specify ToS, and specify the source IP.
The ping utility has a simple and elegant design. When run, it will craft a packet bound for the specified destination, send the packet, and record the time it took that packet to reach its destination. The generated packet is an ICMP packet known as an echo-request. If the destination host receives the packet, it should generate an echo-reply. The success or failure of this very simple operation can provide some insight into the state of a network or a series of networks.
In most cases, the ICMP echo-request packets and echo-reply packets, upon which ping's functionality relies, are allowed through routers and firewalls, however with the advent of trojans and distributed denial of service tools which transmit information within ICMP packets, some networks and network administrators block ICMP at their borders. For an example of such a trojan, see this dissection of the trinoo distributed denial of service tool. As a result of these nefarious uses of echo-request and echo-reply packets, some cautious network administrators block all non-essential ICMP at their border routers. See Section 4.10, “ICMP and Routing” for a more complete discussion of ICMP.
Thus, we can no longer assume (as perhaps we once could) that simply because a host is not answering our ping request, this host is down. There may be a device which has been configured to filter out this traffic.
If a host is reachable and answering our echo-requests, then we may also wish to believe that the round-trip times recorded by ping are an accurate representation of network conditions. This can be misleading. Some routers are configured to give ICMP diagnostic messages the lowest priority of any IP packets travelling through them, in which case that router may contribute significantly to the round trip time of any echo-request packet passing through it.
With knowledge of these two potential roadblocks to the successful use of ping as a network diagnostic tool, we can begin to explore how ping is useful. In most internal networks, and many public networks, there are no filters to block our echo-request packets.
In its simplest form, ping is used interactively on the command line to test reachability of a remote host. Again, you'll see in all of the examples below the use of the -n switch to suppress DNS lookups. Since the proper functioning of DNS relies on a properly configured network, and ping is one of your tools for diagnosing network problems, it makes sense to suppress all name lookup until you have verified that the IP layer is functioning properly.
Let's see first if the host morgan can reach its default gateway. This example is similar to the test we performed in Example 1.2 from tristan.
On many systems, ping can be used by non-root users, but there are some options and features to ping which require the user to have administrative privilege or root-level access to the box. Therefore, all examples below will be run as the root user. Please be aware, that many diagnostics can be run without this high a level of privilege.
Example G.1. Using ping to test reachability
[root@morgan]# ping -n 192.168.98.254 PING 192.168.98.254 (192.168.98.254) from 192.168.98.82 : 56(84) bytes of data. 64 bytes from 192.168.98.254: icmp_seq=0 ttl=255 time=231 usec 64 bytes from 192.168.98.254: icmp_seq=1 ttl=255 time=179 usec 64 bytes from 192.168.98.254: icmp_seq=2 ttl=255 time=215 usec <ctrl-C> --- 192.168.98.254 ping statistics --- 3 packets transmitted, 3 packets received, 0% packet loss round-trip min/avg/max/mdev = 0.179/0.208/0.231/0.024 ms
We have verified from morgan that its default gateway, branch-router is reachable. The first line of output tells us what the source and destination addresses (and names, if using DNS) are. Additionally, we learn the size of the data segment of the ping packet, 56 bytes, and the size of the entire outbound IP packet 84 bytes.
Each subsequent line of output before the summary is a record of the receipt of a reply from the destination (and what IP address sent the reply). Because ping needs to keep track of the number of bytes it has sent, and the round-trip time, each time you run ping, it creates a sequence number inside the data of the ping packet and reports the sequence number on any packets which return. By analyzing the timestamps on the returned packets, ping can determine the round trip time of the journey and reports this as the final field in each line of output.
At the end of the run, ping summarizes the number of replies, and performs some calculations on the round-trip times. As with much data collection, you need a large sample set of data to draw conclusions about your network. You can usually conclude that something is quite wrong if you cannot reach a remote host, but you should be cautious when concluding that your Ethernet card is bad simply because round-trip times to a destination on the LAN is high. It is more likely that there's another problem. Collecting ping data from a number of hosts to a number of destinations can help you determine if the problem is a localized to a single machine.
Frequently, you'll want to use ping in a script, or you'll want to specify that ping should only run for a few cycles. Fortunately, this is trivial (and I'll use the count option many times further below in this section). The -c restricts the number of packets which ping will send (or receive). It can be combined with some of the other options for a variety of diagnostic purposes.
Example G.2. Using ping to specify number of packets to send
[root@morgan]# ping -c 10 -n 192.168.100.17 PING 192.168.100.17 (192.168.100.17) from 192.168.98.82 : 56(84) bytes of data. 64 bytes from 192.168.100.17: icmp_seq=0 ttl=251 time=39.568 msec 64 bytes from 192.168.100.17: icmp_seq=1 ttl=251 time=38.529 msec 64 bytes from 192.168.100.17: icmp_seq=2 ttl=251 time=38.214 msec 64 bytes from 192.168.100.17: icmp_seq=3 ttl=251 time=38.173 msec 64 bytes from 192.168.100.17: icmp_seq=4 ttl=251 time=38.652 msec 64 bytes from 192.168.100.17: icmp_seq=5 ttl=251 time=38.278 msec 64 bytes from 192.168.100.17: icmp_seq=6 ttl=251 time=38.472 msec 64 bytes from 192.168.100.17: icmp_seq=7 ttl=251 time=38.481 msec 64 bytes from 192.168.100.17: icmp_seq=8 ttl=251 time=38.248 msec 64 bytes from 192.168.100.17: icmp_seq=9 ttl=251 time=38.188 msec --- 192.168.100.17 ping statistics --- 10 packets transmitted, 10 packets received, 0% packet loss round-trip min/avg/max/mdev = 38.173/38.480/39.568/0.423 ms
In this example, we see a very regular 38 millisecond round trip time between morgan (192.168.98.82) and isolde (192.168.100.17). After sending 10 echo request packets and receiving the replies, ping summarizes the data for us and exits.
Occasionally, either in a script, or on the command line, you may not care about the output of each individual line. In this case, you can suppress everything except the summary data with the -q switch. In the following example, we are again testing reachability of isolde (192.168.100.17) though we only care about the summary output.
Example G.3. Using ping to specify number of packets to send
[root@morgan]# ping -q -c 10 -n 192.168.100.17 PING 192.168.100.17 (192.168.100.17) from 192.168.98.82 : 56(84) bytes of data. --- 192.168.100.17 ping statistics --- 10 packets transmitted, 10 packets received, 0% packet loss round-trip min/avg/max/mdev = 37.853/38.370/39.320/0.430 ms
Here, we see only the output from ping as it begins to send packets to the destination, and the summary output when it has completed its run.
These are some simple examples of the use of ping to gather and present statistics on reachability of destination hosts, packet loss, and round trip times. Some other diagnostics information can be gathered with ping, too. Let's look at the use of ping to test reachability as aggressively as possible.
Occasionally, you'll want to stress the network to test how many packets you can squeeze through a link, and how gracefully performance on that link degrades. Fortunately, ping, when run with the -f switch can perform exactly this kind of test for you.
Example G.4. Using ping to stress a network
[root@morgan]# ping -c 400 -f -n 192.168.99.254 PING 192.168.99.254 (192.168.99.254) from 192.168.98.82 : 56(84) bytes of data. ............ --- 192.168.99.254 ping statistics --- 411 packets transmitted, 400 packets received, 2% packet loss round-trip min/avg/max/mdev = 37.840/62.234/97.807/12.946 ms
In this example, we have used the default packet size and sent 411 packets, receiving only 400 back from the remote host for a mere 2% packet loss. By increasing the packet size of the packet we are sending across the link we can get a sense for how quickly performance degrades on this link. If we use a much larger packet size (still smaller than Ethernet's 1500 byte frame), we see even more packet loss. We'll specify a packet size of 512 bytes with the -s option.
Example G.5. Using ping to stress a network with large packets
[root@morgan]# ping -s 512 -c 400 -f -n 192.168.99.254 PING 192.168.99.254 (192.168.99.254) from 192.168.98.82 : 512(540) bytes of data. ............................................................................ ................................................................ --- 192.168.99.254 ping statistics --- 551 packets transmitted, 400 packets received, 27% packet loss round-trip min/avg/max/mdev = 47.854/295.711/649.595/153.345 ms
Flooding a low bandwidth link, like the ISDN link between morgan and masq-gw can be detrimental to other traffic on that link, so it is wise to use the -f with restraint. Although ping is a versatile tool for network diagnostics, it is not intended as a network performance measurement tool. For this sort of task, try netperf or collect some data with SNMP to analyze with MRTG.
As you can see, the use of ping floods is a good way to stress the network to which you are connected, and can be a good diagnostic tool. Be careful to stress the network for short periods of time if possible, or in a carefully controlled setting. Unless you want to alienate coworkers and anger your network administrator, you shouldn't start a ping flood and go home for the night.
The options we have outlined above are common options to ping, but now, let's look at some of the less common options. Occasionally, you may find yourself on a linux box without traceroute or mtr. Perhaps it's an embedded linux host, or a minimal installation with ping. There is an almost unknown option for recording the route a packet takes. By comparison to the more sophisticated tools for tracing network paths, ping with the record route option (-R) doesn't convey the information in as visually an appealing way, but it can get the job done.
Example G.6. Recording a network route with ping
[root@morgan]# ping -c 2 -n -R 192.168.99.35 PING 192.168.99.35 (192.168.99.35) from 192.168.98.82 : 56(124) bytes of data. 64 bytes from 192.168.99.35: icmp_seq=0 ttl=253 time=56.311 msec RR: 192.168.98.82 192.168.98.254 192.168.99.1 192.168.99.35 192.168.99.35 192.168.99.1 192.168.98.254 192.168.98.82 64 bytes from 192.168.99.35: icmp_seq=1 ttl=253 time=47.893 msec (same route) --- 192.168.99.35 ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/mdev = 47.893/52.102/56.311/4.209 ms
As always, ping summarizes the output after it has completed its run, but let's examine the new section. By using the record route option, we are asking all routers along the way to include their IPs in the header. Although some routers may not observe this courtesy, many do. Unfortunately, there is only room to record 8 different hops (FIXME--verify this!), so the use of ping -R is mostly useful only in smaller networks.
The first IP we hit is our own IP on the way out our Ethernet interface, 192.168.98.82. Then it is a palindromic journey through the network stacks of each of the following hosts in order: branch-router, isdn-router, tristan, and back again in reverse order.
ping is even nice enough to report to us that a subsequent journey took the same route as the first packet. If you have a statically routed internal network, any subsequent packets should look exactly like the second packet. If dynamic routing is in use on your internal network, you may find that the routes change occasionally.
Now, frankly, I'm not sure of a practical use for the following option to ping, however, you can specify the TTL for an outbound echo requust packet. By setting the TTL you are specifying the maximum number of hops this packet will travel before it will be dropped. Conventionally, the TTL is set by the kernel to a reasonable number of hops (like 64). The -t provides us the capability to force the TTL for our echo requests. Now that we know it takes four hops to get to tristan from morgan we should be able to test whether setting the TTL makes any difference.
Example G.7. Setting the TTL on a ping packet
[root@morgan]# ping -c 1 -n -t 4 192.168.99.35 tcpdump: listening on eth0 02:02:04.679152 192.168.98.82 > 192.168.99.35: icmp: echo request (DF) 02:02:04.711474 192.168.99.35 > 192.168.98.82: icmp: echo reply [root@morgan]# ping -c 1 -n -t 3 192.168.99.35 tcpdump: listening on eth0 02:01:50.810567 192.168.98.82 > 192.168.99.35: icmp: echo request (DF) 02:01:50.841917 192.168.99.1 > 192.168.98.82: icmp: time exceeded in-transit
Clearly, we are able to reach tristan if we set the TTL on our echo requests to 4, but as soon as we drop the TTL to 3, we get a reply from the third hop (isdn-router), telling us that our packet was too old to be forwarded to its destination. If you are unclear on the rationale for TTL, I'd suggest reviewing some of the general IP documentation available in Section I.1.3, “General IP Networking Resources”.
Type of Service (ToS) is increasingly in use on backbones across the Internet which has brought with it Service Level Agreements (SLA). If you have an SLA with your provider, you may find the use of ping -Q to set the IP packet ToS flags will help you to determine if your provider is holding up their end of the bargain.
In Example G.8 we'll set the ToS flag and verify with tcpdump that the ToS flag on the outbound packets have actually been set. Let's assume that we have an SLA with a backbone provider for our link between our German office (18.104.22.168) and our North American office (22.214.171.124). We'll send two test packets to the remote end, and observe the data on the wire.
Example G.8. Setting ToS for a diagnostic ping
[root@wan-gw]# ping -c 2 -Q 8 -n 126.96.36.199 PING 188.8.131.52 (184.108.40.206) from 220.127.116.11 : 56(84) bytes of data. 64 bytes from 18.104.22.168: icmp_seq=0 ttl=252 time=51.633 msec 64 bytes from 22.214.171.124: icmp_seq=1 ttl=252 time=36.323 msec --- 126.96.36.199 ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/mdev = 36.323/43.978/51.633/7.655 ms [root@wan-gw]# tcpdump -nni wan0 icmp tcpdump: listening on wan0 21:55:37.983149 10.10.14.2 > 10.10.22.254: icmp: echo request (DF) [tos 0x8] 21:55:38.034770 10.10.22.254 > 10.10.14.2: icmp: echo reply [tos 0x8] 21:55:38.982277 10.10.14.2 > 10.10.22.254: icmp: echo request (DF) [tos 0x8] 21:55:39.018588 10.10.22.254 > 10.10.14.2: icmp: echo reply [tos 0x8]
Naturally, ping reports to us the round-trip times, the source and destination IPs, and that there was no packet loss. And our tcpdump output shows that the ToS flags were properly set on the packet. With all of this information, we can collect data about the reliability of the network between our two offices.
Occasionally, you'll find yourself on a heavily packet filtered host, or a host which employs conditional routing for packets with certain source addresses. Such packet filtering can prevent or conflict with the use of ping. Fortunately, ping allows the user to specify the source address of an outbound packet, thus allowing traversal of packet filters and conditional routing tables.
My classic example of a need for specifying source address on a ping is a VPN connected network. Let's assume masq-gw has a CIPEpeer in another city. Let's assume the internal IP on the peer is 192.168.70.254. If masq-gw sends a packet to the peer with a source address of 188.8.131.52, the peer might drop the inbound packet on a VPN interface from the public IP of the peer . In this case, the peer should still accept traffic from masq-gw if the originating IP is inside the private network IP range.
In the Example G.9 we'll use ping to check reachability of the inside interface of the CIPE peer of masq-gw.
Example G.9. Specifying a source address for ping
[root@masq-gw]# ping -c 2 -n -I 192.168.99.254 192.168.70.254 PING 192.168.70.254 (192.168.70.254) from 192.168.99.254 : 56(84) bytes of data. 64 bytes from 192.168.70.254: icmp_seq=0 ttl=254 time=69.285 msec 64 bytes from 192.168.70.254: icmp_seq=1 ttl=254 time=53.976 msec --- 192.168.70.254 ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/mdev = 53.976/61.630/69.285/7.658 ms
By forcing the echo request packet to use the IP bound to one of our internal interfaces as the source address with the -I we are able to send traffic through the CIPE tunnel to the other side, and back.
As you can see, ping is a versatile tool in the network administrator's toolkit, and can be used for a wide range of tests beyond the simple reachability test. For a brief and humourous introduction to the program itself, see The Story of Ping.
Now that we have a good idea of the uses of the ping utility, let's move on to some other tools which can provide us other diagnostic data about our networks.
 If the admin controls both sides of the link, it is a matter of choice and preference whether traffic from the outside IP of the peer VPN endpoint should be allowed. I'll argue that traffic from the peer endpoint should not be allowed, but this is opinion only.