from Cliff Frost

[unix-history] / usr / src / sbin / ping / ping.8

.TH PING 8 "May 25, 1989"
.SH NAME
ping \- send ICMP ECHO_REQUEST packets to network hosts
.SH SYNOPSIS
.B /etc/ping
[
.B -dfnqrvR
][
.B -c
count][
.B -i
wait][
.B -l
preload][
.B -p
pattern][
.B -s
packetsize]
.B [-h]
.B host
.SH DESCRIPTION
The DARPA Internet is a large and complex aggregation of
network hardware, connected together by gateways.
Tracking a single-point hardware or software failure
can often be difficult.
.I 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 \fBstruct timeval\fR, 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 \fI-s\fR option.
Other options are:
.TP
.B \-d
Set the SO_DEBUG option on the socket being used.
.TP
.B \-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.
\fIThis can be very hard on a network and should be used with caution.\fR
.TP
.B \-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.
.TP
.B \-n
Numeric output only.  No attempt will be made to lookup symbolic
names for host addresses.
.TP
.B \-q
Quiet output.  Nothing is displayed except the summary lines at the
beginning and at the end.
.TP
.B \-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
.I routed(8C)
).
.TP
.B \-v
Verbose output.  ICMP packets other than ECHO_RESPONSE that are received
are listed.
.TP
.B \-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.
.TP
.B \-c \fIcount\fR
Stop after receiving \fIcount\fR ECHO_RESPONSE packets.
.TP
.B \-i \fIwait\fR
.br
Wait \fIwait\fR seconds between sending each packet.
The default is to wait for one second between each packet.
This option is incompatible with the \fB-f\fR option.
.TP
.B \-l \fIpreload\fR
If
.I preload
is given,
.I ping
sends that many packets as fast as possible before
falling into its normal mode of behavior.
.TP
.B \-p \fIpattern\fR
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.
.TP
.B \-s \fIpacketsize\fR
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.
.PP
When using \fIping\fR 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''.
\fIPing\fR sends one datagram per second (or per \fIwait\fR seconds), and
prints one line of output for every ECHO_RESPONSE returned.
If an optional
.I 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
.I count
specified), or if the program is terminated with a SIGINT, a brief
summary is displayed.
.PP
This program is intended for use in network testing, measurement
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
.I ping
during normal operations or from automated scripts.
.SH 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
.I 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).
.PP
If the data space is at least eight bytes large,
.I 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.
.SH Duplicate and Damaged packets
\fIPing\fR will report duplicate and damaged packets.
Duplicate packets should never occur, and seem to be caused by
inappropriate 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.
.PP
Damaged packets are obviously serious cause for alarm and most likely
indicate broken hardware somewhere in the \fIping\fR packet's path
(in the network or in the hosts).
.SH Trying Different Data Patterns
It should go without saying that the (inter)network layer 
\fIshould\fR 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 \fB-p 00\fR), 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.
.PP
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 \fI-p\fR option of \fIping\fR.
.SH 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.
.PP
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).
.PP
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 or ftp.
.PP
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:
.TP
.B (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.
.TP
.B (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 \fIfrom\fR the remote system \fIto\fR the
\fIping\fRing host.
.TP
.B (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.
.SH BUGS
Many Hosts and Gateways ignore the RECORD_ROUTE option.
.PP
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.
.PP
Flood pinging is not recommended in general, and flood pinging the broadcast
address should only be done under very controlled conditions.
.SH AUTHOR
Mike Muuss
.SH SEE ALSO
netstat(1),
ifconfig(8C)