sbin/ping/ping.8

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.\"     @(#)ping.8	8.2 (Berkeley) 12/11/93
.\"
.Dd December 11, 1993
.Dt PING 8
.Os BSD 4.3
.Sh NAME
.Nm ping
.Nd send
.Tn ICMP ECHO_REQUEST
packets to network hosts
.Sh SYNOPSIS
.Nm ping
.Op Fl dfnqrvR
.Op Fl c Ar count
.Op Fl i Ar wait
.Op Fl l Ar preload
.Op Fl p Ar pattern
.Op Fl s Ar packetsize
.Sh DESCRIPTION
.Nm Ping
uses the
.Tn ICMP
protocol's mandatory
.Tn ECHO_REQUEST
datagram to elicit an
.Tn ICMP ECHO_RESPONSE
from a host or gateway.
.Tn ECHO_REQUEST
datagrams (``pings'') have an IP and
.Tn ICMP
header,
followed by a
.Dq struct timeval
and then an arbitrary number of ``pad'' bytes used to fill out the
packet.
The options are as follows:
.Bl -tag -width Ds
.It Fl c Ar count
Stop after sending (and receiving)
.Ar count
.Tn ECHO_RESPONSE
packets.
.It Fl d
Set the
.Dv SO_DEBUG
option on the socket being used.
.It Fl f
Flood ping.
Outputs packets as fast as they come back or one hundred times per second,
whichever is more.
For every
.Tn ECHO_REQUEST
sent a period ``.'' is printed, while for every
.Tn 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.
.Bf -emphasis
This can be very hard on a network and should be used with caution.
.Ef
.It Fl i Ar wait
Wait
.Ar wait
seconds
.Em between sending each packet .
The default is to wait for one second between each packet.
This option is incompatible with the
.Fl f
option.
.It Fl l Ar preload
If
.Ar preload
is specified,
.Nm ping
sends that many packets as fast as possible before falling into its normal
mode of behavior.
.It Fl n
Numeric output only.
No attempt will be made to lookup symbolic names for host addresses.
.It Fl p Ar 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,
.Dq Li \-p ff
will cause the sent packet to be filled with all
ones.
.It Fl q
Quiet output.
Nothing is displayed except the summary lines at startup time and
when finished.
.It Fl R
Record route.
Includes the
.Tn RECORD_ROUTE
option in the
.Tn 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.
.It Fl 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
.Xr routed 8 ) .
.It Fl s Ar packetsize
Specifies the number of data bytes to be sent.
The default is 56, which translates into 64
.Tn ICMP
data bytes when combined
with the 8 bytes of
.Tn ICMP
header data.
.It Fl v
Verbose output.
.Tn ICMP
packets other than
.Tn ECHO_RESPONSE
that are received are listed.
.El
.Pp
When using
.Nm 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 calculation, 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
.Dv SIGINT ,
a brief summary is displayed.
.Pp
This program is intended for use in network testing, measurement and
management.
Because of the load it can impose on the network, it is unwise to use
.Nm ping
during normal operations or from automated scripts.
.Sh ICMP PACKET DETAILS
An IP header without options is 20 bytes.
An
.Tn ICMP
.Tn ECHO_REQUEST
packet contains an additional 8 bytes worth
of
.Tn ICMP
header followed by an arbitrary amount of data.
When a
.Ar 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
.Tn ICMP
.Tn ECHO_REPLY
will always be 8 bytes more than the requested data space
(the
.Tn ICMP
header).
.Pp
If the data space is at least eight bytes large,
.Nm 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.
.Sh DUPLICATE AND DAMAGED PACKETS
.Nm Ping
will report duplicate and damaged packets.
Duplicate packets should never occur, and seem to be caused by
inappropriate 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.
.Pp
Damaged packets are obviously serious cause for alarm and often
indicate broken hardware somewhere in the
.Nm ping
packet's path (in the network or in the hosts).
.Sh 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.
.Pp
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
.Fl p
option of
.Nm ping .
.Sh TTL DETAILS
The
.Tn 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
.Tn TTL
field by exactly one.
.Pp
The
.Tn TCP/IP
specification states that the
.Tn TTL
field for
.Tn TCP
packets should
be set to 60, but many systems use smaller values (4.3
.Tn BSD
uses 30, 4.2 used
15).
.Pp
The maximum possible value of this field is 255, and most Unix systems set
the
.Tn TTL
field of
.Tn ICMP ECHO_REQUEST
packets to 255.
This is why you will find you can ``ping'' some hosts, but not reach them
with
.Xr telnet 1
or
.Xr ftp 1 .
.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
.Tn TTL
field in its response:
.Bl -bullet
.It
Not change it; this is what Berkeley Unix systems did before the
.Bx 4.3 tahoe
release.
In this case the
.Tn TTL
value in the received packet will be 255 minus the
number of routers in the round-trip path.
.It
Set it to 255; this is what current Berkeley Unix systems do.
In this case the
.Tn TTL
value in the received packet will be 255 minus the
number of routers in the path
.Xr from
the remote system
.Em to
the
.Nm ping Ns Em ing
host.
.It
Set it to some other value.
Some machines use the same value for
.Tn ICMP
packets that they use for
.Tn TCP
packets, for example either 30 or 60.
Others may use completely wild values.
.El
.Sh BUGS
Many Hosts and Gateways ignore the
.Tn RECORD_ROUTE
option.
.Pp
The maximum IP header length is too small for options like
.Tn RECORD_ROUTE
to
be completely useful.
There's not much that that can be done about this, however.
.Pp
Flood pinging is not recommended in general, and flood pinging the
broadcast address should only be done under very controlled conditions.
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr ifconfig 8 ,
.Xr routed 8
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .