Ping

Ping

ping – send ICMP ECHO_REQUEST packets to network hosts

SYNOPSIS

ping [-dfnqrvR] [-c count] [-i wait] [-l preload] [-p pattern] [-s
packetsize]

DESCRIPTION

Ping uses the ICMP protocol’s mandatory ECHO_REQUEST datagram to elicit
an ICMP ECHO_RESPONSE from a host or gateway. ECHO_REQUEST datagrams
(« pings ») have an IP and ICMP header, followed by a « struct timeval »
and then an arbitrary number of « pad » bytes used to fill out the pack-
et. The options are as follows: Other options are:

-c count
Stop after sending (and receiving) count ECHO_RESPONSE packets.

-d Set the SO_DEBUG option on the socket being used.

-f Flood ping. Outputs packets as fast as they come back or one
hundred times per second, whichever is more. For every
ECHO_REQUEST sent a period « . » is printed, while for ever
ECHO_REPLY received a backspace is printed. This provides a
rapid display of how many packets are being dropped. Only the
super-user may use this option. This can be very hard on a net-
work and should be used with caution.

-i wait
Wait wait seconds between sending each packet. The default is to
wait for one second between each packet. This option is incom-
patible with the -f option.

-l preload
If preload is specified, ping sends that many packets as fast as
possible before falling into its normal mode of behavior. Only
the super-user may use this option.

-n Numeric output only. No attempt will be made to lookup symbolic
names for host addresses.

-p pattern
You may specify up to 16 « pad » bytes to fill out the packet you
send. This is useful for diagnosing data-dependent problems in a
network. For example, « -p ff » will cause the sent packet to be
filled with all ones.

-q Quiet output. Nothing is displayed except the summary lines at
startup time and when finished.

-R Record route. Includes the RECORD_ROUTE option in the
ECHO_REQUEST packet and displays the route buffer on returned
packets. Note that the IP header is only large enough for nine
such routes. Many hosts ignore or discard this option.

-r Bypass the normal routing tables and send directly to a host on
an attached network. If the host is not on a directly-attached
network, an error is returned. This option can be used to ping a
local host through an interface that has no route through it
(e.g., after the interface was dropped by routed(8)).

-s packetsize
Specifies the number of data bytes to be sent. The default is
56, which translates into 64 ICMP data bytes when combined with
the 8 bytes of ICMP header data.

-v Verbose output. ICMP packets other than ECHO_RESPONSE that are
received are listed.

When using ping for fault isolation, it should first be run on the local
host, to verify that the local network interface is up and running.
Then, hosts and gateways further and further away should be « pinged ».
Round-trip times and packet loss statistics are computed. If duplicate
packets are received, they are not included in the packet loss calcula-
tion, although the round trip time of these packets is used in calculat-
ing the minimum/average/maximum round-trip time numbers. When the speci-
fied number of packets have been sent (and received) or if the program is
terminated with a SIGINT, a brief summary is displayed.

If ping does not receive any reply packets at all it will exit with code
1. On error it exits with code 2. Otherwise it exits with code 0. This
makes it possible to use the exit code to see if a host is alive or not.

This program is intended for use in network testing, measurement and man-
agement. Because of the load it can impose on the network, it is unwise
to use ping during normal operations or from automated scripts.

ICMP PACKET DETAILS

An IP header without options is 20 bytes. An ICMP ECHO_REQUEST packet
contains an additional 8 bytes worth of ICMP header followed by an arbi-
trary amount of data. When a packetsize is given, this indicated the
size of this extra piece of data (the default is 56). Thus the amount of
data received inside of an IP packet of type ICMP ECHO_REPLY will always
be 8 bytes more than the requested data space (the ICMP header).

If the data space is at least eight bytes large, ping uses the first
eight bytes of this space to include a timestamp which it uses in the
computation of round trip times. If less than eight bytes of pad are
specified, no round trip times are given.

DUPLICATE AND DAMAGED PACKETS

Ping will report duplicate and damaged packets. Duplicate packets should
never occur, and seem to be caused by inappropriate link-level retrans-
missions. Duplicates may occur in many situations and are rarely (if ev-
er) a good sign, although the presence of low levels of duplicates may
not always be cause for alarm.

Damaged packets are obviously serious cause for alarm and often indicate
broken hardware somewhere in the ping packet’s path (in the network or in
the hosts).

TRYING DIFFERENT DATA PATTERNS

The (inter)network layer should never treat packets differently depending
on the data contained in the data portion. Unfortunately, data-dependent
problems have been known to sneak into networks and remain undetected for
long periods of time. In many cases the particular pattern that will
have problems is something that doesn’t have sufficient « transitions »,
such as all ones or all zeros, or a pattern right at the edge, such as
almost all zeros. It isn’t necessarily enough to specify a data pattern
of all zeros (for example) on the command line because the pattern that
is of interest is at the data link level, and the relationship between
what you type and what the controllers transmit can be complicated.

This means that if you have a data-dependent problem you will probably
have to do a lot of testing to find it. If you are lucky, you may manage
to find a file that either can’t be sent across your network or that
takes much longer to transfer than other similar length files. You can
then examine this file for repeated patterns that you can test using the
-p option of ping.

TTL DETAILS

The TTL value of an IP packet represents the maximum number of IP routers
that the packet can go through before being thrown away. In current
practice you can expect each router in the Internet to decrement the TTL
field by exactly one.

The TCP/IP specification states that the TTL field for TCP packets should
be set to 60, but many systems use smaller values (4.3 BSD uses 30, 4.2
used 15).

The maximum possible value of this field is 255, and most Unix systems
set the TTL field of ICMP ECHO_REQUEST packets to 255. This is why you
will find you can « ping » some hosts, but not reach them with telnet(1)
or ftp(1).

In normal operation ping prints the ttl value from the packet it re-
ceives. When a remote system receives a ping packet, it can do one of
three things with the TTL field in its response:

+o Not change it; this is what Berkeley Unix systems did before the
4.3BSD-Tahoe release. In this case the TTL value in the received
packet will be 255 minus the number of routers in the round-trip
path.

+o Set it to 255; this is what current Berkeley Unix systems do. In
this case the TTL value in the received packet will be 255 minus the
number of routers in the path from the remote system to the pinging
host.

+o Set it to some other value. Some machines use the same value for
ICMP packets that they use for TCP packets, for example either 30 or
60. Others may use completely wild values.

BUGS

Many Hosts and Gateways ignore the RECORD_ROUTE option.

The maximum IP header length is too small for options like RECORD_ROUTE
to be completely useful. There’s not much that that can be done about
this, however.

Flood pinging is not recommended in general, and flood pinging the broad-
cast address should only be done under very controlled conditions.

SEE ALSO

netstat(1), ifconfig(8), routed(8)

HISTORY

The ping command appeared in 4.3BSD.

EOF

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