2.11BSD/man/cat8/ping.0
PING(8) UNIX Programmer's Manual PING(8)
NAME
ping - send ICMP ECHO_REQUEST packets to network hosts
SYNOPSIS
ping [-dfnqrvR] [-c _c_o_u_n_t] [-i _w_a_i_t] [-l _p_r_e_l_o_a_d] [-p _p_a_t_-
_t_e_r_n] [-s _p_a_c_k_e_t_s_i_z_e]
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
packet. The options are as follows:
-c _c_o_u_n_t Stop after sending (and receiving) _c_o_u_n_t
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
every 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
-i _w_a_i_t Wait _w_a_i_t seconds between sending each
packet. The default is to wait for one
second between each packet. This option is
incompatible with the -f option.
-l _p_r_e_l_o_a_d If _p_r_e_l_o_a_d is specified, ping sends that many
packets as fast as possible before falling
into its normal mode of behavior.
-n Numeric output only. No attempt will be made
to lookup symbolic names for host addresses.
-p _p_a_t_t_e_r_n 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.
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-R Record route. Includes the RECORD_ROUTE
option in the ECHO_REQUEST packet and
displays the route buffer on returned pack-
ets. 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 _p_a_c_k_e_t_s_i_z_e 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 inter-
face 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 cal-
culation, although the round trip time of these packets is
used in calculating the minimum/average/maximum round-trip
time numbers. When the specified number of packets have
been sent (and received) or if the program is terminated
with a SIGINT, a brief summary is displayed.
This program is intended for use in network testing, meas-
urement and management. 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 arbitrary amount of data. When a
_p_a_c_k_e_t_s_i_z_e 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
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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 inap-
propriate link-level retransmissions. Duplicates may occur
in many situations and are rarely (if ever) 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 dif-
ferently depending on the data contained in the data por-
tion. 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 relation-
ship 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 (4.3BSD 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
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hosts, but not reach them with telnet(1) or ftp(1).
In normal operation ping prints the ttl value from the
packet it receives. 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 sys-
tems 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 _t_o the ping'ing 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 ping-
ing the broadcast 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.
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