PING(8) BSD System Manager's Manual PING(8)NAMEping - send ICMP ECHO_REQUEST packets to network hosts
SYNOPSISping [-DdfLnqRrv] [-c count] [-I ifaddr] [-i wait] [-l preload]
[-p pattern] [-s packetsize] [-T tos] [-t ttl] [-w maxwait] host
DESCRIPTIONping uses the ICMP protocol's mandatory ECHO_REQUEST datagram to elicit
an ICMP ECHO_REPLY 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 count
Stop after sending count ECHO_REQUEST packets.
-D Set the Don't Fragment bit.
-d Set the SO_DEBUG option on the socket being used.
-f Flood ping. Outputs packets as fast as they come back or one hun-
dred 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 superuser may use this op-
tion. This can be very hard on a network and should be used with
caution.
-I ifaddr
Specify the interface address to transmit from on machines with
multiple interfaces. For unicast and multicast pings.
-i wait
Wait wait seconds between sending each packet. The default is to
wait for one second between each packet. The wait time may be
fractional, but only the superuser may specify a value less than
one second. This option is incompatible 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
root may set a preload value.
-n Numeric output only. No attempt will be made to look up 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. If more routes come back than should, such as due to
an illegal spoofed packet, ping will print the route list and
then truncate it at the correct spot. Many hosts ignore or dis-
card 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. If the -D or -T options are specified,
or the -t option to a unicast destination, a raw socket will be
used and the 8 bytes of header data are included in packetsize.
-T tos Use the specified type of service.
-t ttl Use the specified time-to-live.
-v Verbose output. ICMP packets other than ECHO_REPLY that are re-
ceived are listed.
-w maxwait
Specifies the maximum number of seconds to wait for a response to
a packet before transmitting the next one. The default is 10.
In addition, the following option may be used for multicast pings:
-L Disable the loopback, so the transmitting host doesn't see the
ICMP requests.
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 and the standard
deviation.
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.
The summary information can also be displayed while ping is running by
sending it a SIGINFO signal (see the status argument of stty(1) for more
information).
This program is intended for use in network testing, measurement and
management. Because of the load it can impose on the network, it is un-
wise 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 indicates 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 PACKETSping will report duplicate and damaged packets. Duplicate packets should
never occur, and seem to be caused by inappropriate link-level re-
transmissions. 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 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 prac-
tice 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:
+ 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.
+ 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.
+ 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.
SEE ALSOnetstat(1), ifconfig(8), routed(8), spray(8)HISTORY
The ping command appeared in 4.3BSD.
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 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.
MirOS BSD #10-current December 11, 1993 3
