previous articles we’ve built some useful servers. But this time let’s take a
look at one of the most powerful network troubleshooting tools ever written:
the mighty “ping” command.
may have used this program and wonder why I’d devote an entire article to it,
but if you don’t know how to ping, you definitely
need to keep reading.
was developed by a fellow named Mike Muuss back in 1983 for the express purpose
of troubleshooting fundamental IP network issues. The name is borrowed from
sonar systems: You “ping” a host and then wait for a reply, much as a submarine
might ping an object in the water, then wait for the echo.
uses the Internet Control Message Protocol or ICMP, which operates at a
relatively low level. You can imagine that it’s underneath any server software
running on the PC.
has implications: If someone yells, “Our Web server is down,” I’ll try “ping
www.crawfordbroadcasting.com.” If I can ping it, that tells me that the server
machine itself is powered on and that the network connection is OK. I need to
check the Apache Web server for a problem. But if I can’t ping it, I’ll call
Denver and ask my colleague Amanda Alexander to go make sure no one turned it
off or unplugged the network cable by accident.
is useful because it tests everything in your network settings. It checks the
physical connection as well. The sequence of a typical ping goes something like
- If you use a domain name, a Domain
Name Server is used to determine the IP address.
- If you use an IP address (or
once the IP address has been fetched from a Domain Name Server), the netmask is
used to determine if it’s on your local network.
- If the address is local,
Address Resolution Protocol is used to find the physical address of the local
- If the address is external,
ARP is used to find the physical address of your gateway (this is typically the
“default gateway” in your network settings).
- Now that the target has been
located, an ICMP ECHO_REQUEST is sent to it.
- The target responds with an
ICMP ECHO reply. The ping command calculates the time needed for the round trip
and displays it.
for a ping to work, all of your network settings must be correct, and
your network connection must be good. This makes it a complete, one-stop test
of the network in one go.
CHECK YOUR FIREWALLS
Some firewalls block pings, which
actually violates published standards. RFC1122 states that support for ICMP ECHO_REQUEST
and ICMP ECHO — which are what a ping uses — is “mandatory.” In spite of this,
some public servers block pings to avoid denial-of-service attacks. Some
hackers will send a flood of ping requests at the target to bog it down. Other
security experts recommend that ping replies be disabled because of the way
that some crackers search for computers to exploit: They’ll ping an entire
block of network addresses and then take a look at the responses for desirable
on your own internal network, I strongly recommend that every machine should
respond to ping requests. Most network-capable appliances, from printers to
Web-capable remote controls, do not block ping echoes; they’re ready to
respond out of the box. But Windows, in particular, will
block ICMP by default. You need to either enable it in the firewall, or enable
file and printer sharing. Do a Web search on your specific OS; for example, go
to Google and try “opensuse linux ping firewall” or something like that.
The bottom line is to ensure that
everything on your internal, local network(s) can respond to a ping.
TROUBLE TO USE
Ping is easy. Simply open a command
prompt and type in “ping,” followed by the IP address or domain name that
you’re trying to reach. The various ping programs included with various
operating systems behave a bit differently. For example, Windows automatically
stops after four ping attempts, while on Linux, by default, the ping will keep
repeating until you press CTRL-C to stop it.
Ping Test of a Printer on the Local Network
Fig. 1 shows a typical response; I’ve
pinged an HP printer on our network.
The positive result tells me that the
printer has a working connection on our local network, that my connection is
configured properly (at least for the local network) and that the target
printer is powered up and is running.
If I see this, and yet I still can’t
print, I’ll check my PC’s printer settings, and then go ensure that the printer
hasn’t experienced an error.
Fig. 1 also shows the response times in
milliseconds; this is for the complete round trip from your PC to the target
and then back. The first response usually takes the longest, because the target
has to be located. Once it has been found, subsequent pings are a bit faster.
We’ll return to that in a moment; now let’s ping a remote site (see Fig. 2).
try Google, because it does reply to pings (they have so many servers distributed
around the globe that I guess they figure, “Good luck trying to swamp us!”).
This adds another useful test: “google.com” must be translated to an IP
address, which means that our ping command is going to use our DNS settings.
Because it’s a remote IP address, it’ll use a gateway. This tells me that my
laptop’s DNS and default gateway are correct.
Ping Test of a Remote
This one-two procedure (ping someone on
the local network, then ping Google) is a standard test that we use daily. It’s
much faster than pulling up a Web browser to see if we can get onto the
Internet, then waiting for a timeout if there’s a problem. As a general rule,
if a ping doesn’t respond within one second, unless you’re trying a host on the
other side of the planet, just press “CTRL-C” to stop the test. You’ve got a
problem that needs to be fixed.
TIMES AND LATENCY
The response times are interesting,
too, and here’s a super tip: When transporting audio-over-IP, if you have
access to more than one Internet service or microwave link, move your connection
around and do some pings to see which is the fastest. Audio-over-IP needs low
latency; you want virtually no delay between send and receive. Note that if you
test encrypted links you’ll also confirm what you’ve probably suspected:
Encrypted links can be significantly slower, because you have the overhead of
encryption and decryption.
isn’t the best tool for checking network quality; a hardware analyzer or the
analysis software built into many microwave links nowadays is better. But you
can still see gross errors and dropouts on a remote connection. You’d see
wildly varying response times — the first might be 50 ms, the next is 400 ms,
and so on — or even missed replies entirely.
On a microwave data link, this is
commonly caused by interference. Here’s the bottom line: if you get even one
ping reply, the remote host is connected and powered up. It’s trying to
respond. If you have wildly different ping response times and/or dropped pings,
you’ve got a problem with network quality.
Again, imagine the flow of the signal
as you troubleshoot. You’re sending out an ICMP ECHO_REQUEST and the remote
host should return the corresponding ECHO reply within one second if everything
is OK.There are three possible
If you get “Timed Out” — i.e., the ping
never completes at all — this means just what it says. Ping sent the echo
request but timed out waiting for a reply. If you are certain that the remote
target is powered up and is working, you should troubleshoot on your end.
If the ping returns “Network
Unreachable,” first check your physical network connection. Next, if you’re
trying to reach the local network, check your netmask and IP address. If you
can’t ping the Internet, check your default gateway.
If you see “Unknown Host” or something
like that, check your DNS settings. Your PC is unable to translate names like
“google.com” into the appropriate IP address for the ping.
On that last one, it’s useful to keep handy a couple of known
IP addresses on the Internet that you can ping. For example, I know that our
Web server in Denver is at 220.127.116.11; if I can ping that IP address, but
“ping www.crawfordbroadcasting.com” doesn’t work, I’ve got a DNS problem. If
you can’t ping either, either your Internet connection is down or your gateway
is set incorrectly.
the results in Figs. 1 and 2 will vary from one OS to the next. Learn how your
version of ping responds to different problems. Do a ping with your network
cable unplugged and note the result; then do a ping with the target unplugged.
Learn to recognize what the various replies mean and you can troubleshoot
network issues much more quickly and easily.
If you want more information, do a Web
search on “ping,” or read the Wikipedia article.There are more advanced versions of ping available on the
Internet, too, some which will allow you to scan an entire network and do other
fancy tricks. But the default ping program included with Windows, Mac OS and
Linux is still plenty good enough. Learn to use it!
M. Poole, CBRE-AMD, CBNT, is chief engineer of Crawford Broadcasting in
Birmingham, Ala. Find past Radio IT Management columns at radioworld.com under
the Business tab.