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pingman.txt
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1995-03-03
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PING(8) PING(8)
NAME
ping - send ICMP ECHO_REQUEST packets to network hosts
SYNOPSIS
/etc/ping [ -dfnqrvR ][ -c count][ -i wait][ -l preload][
-p pattern][ -s packetsize] [-h] host
DESCRIPTION
The DARPA Internet is a large and complex aggregation of
network hardware, connected together by gateways. Track-
ing a single-point hardware or software failure can often
be difficult. 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. Default datagram length is 64 bytes, but
this may be changed using the -s option. Other options
are:
-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 dis-
play of how many packets are being dropped. This
can be very hard on a network and should be used
with caution.
-h You may use this flag to specify the host, or leave
it off if the host is the last token on the command
line.
-n Numeric output only. No attempt will be made to
lookup symbolic names for host addresses.
-q Quiet output. Nothing is displayed except the sum-
mary lines at the beginning and at the end.
-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(8C) ).
-v Verbose output. ICMP packets other than
ECHO_RESPONSE that are received are listed.
-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.
-c count
Stop after receiving count ECHO_RESPONSE packets.
-i wait
Wait wait 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 preload
If preload is given, ping sends that many packets
as fast as possible before falling into its normal
mode of behavior.
-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.
-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.
-w seconds
Specifies the number of seconds to wait for a
response, default is 10.
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''. Ping sends
one datagram per second (or per wait seconds), and prints
one line of output for every ECHO_RESPONSE returned. If
an optional count is given, only that number of requests
is sent. Round-trip times and packet loss statistics are
computed. If duplicate packets are received, they are not
included in the packet loss calculation, although the
round trip time of these packets is used in calculating
the minimun/average/maximum round-trip time numbers. When
all responses have been received or the program times out
(with a count specified), or if the program is terminated
with a SIGINT, a brief summary is displayed.
This program is intended for use in network testing, mea-
surement and management. It should be used primarily for
manual fault isolation. Because of the load it could
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 packetsize is given, this indicated the size of
this extra blob 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.
This explains why if less than eight bytes of pad are
requested, 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. The author has seen
duplicates in many situations and has never known them to
be a good thing, although the presence of low levels of
duplicates may not always be cause for alarm. Network
maintainers ignore them at their own risk as they have
been known to be harbingers of severe network problems.
Damaged packets are obviously serious cause for alarm and
most likely indicate broken hardware somewhere in the ping
packet's path (in the network or in the hosts).
Trying Different Data Patterns
It should go without saying that 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 "enough" 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 (as in -p 00), 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 have to be prepared 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 num-
ber 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 says 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 pack-
ets to 255. This is why you will find you can "ping" some
hosts, but not reach them with telnet or ftp.
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:
(1) Not change it; this is what Berkeley Unix systems
did until 4.3 BSD tahoe level releases. In this
case the TTL value in the received packet will be
255 minus the number of routers in the round-trip
path.
(2) Set it to 255; this is what Berkeley Unix systems
have done since the 4.3 tahoe release. 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.
(3) 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 we can do about that however.
Flood pinging is not recommended in general, and flood
pinging the broadcast address should only be done under
very controlled conditions.
AUTHOR
Mike Muuss
SEE ALSO
netstat(1), ifconfig(8C)
May 25, 1989 4