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, “Testing reachability of a locally connected host with
ping”
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
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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
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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.
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
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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
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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
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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
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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, “Setting ToS for a diagnostic ping” 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 (195.73.22.45) and our North American office (205.254.209.73). 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
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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 205.254.211.179, the peer might
drop the inbound packet on a VPN interface from the public IP of the
peer
[59].
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, “Specifying a source address for ping” 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
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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.
[59] 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.