path: root/sbin/ping/ping.8

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.\"     @(#)ping.8	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: July 3 2010 $
.Dt PING 8
.Os
.Sh NAME
.Nm ping
.Nd send ICMP ECHO_REQUEST packets to network hosts
.Sh SYNOPSIS
.Nm ping
.Bk -words
.Op Fl DdEefLnqRrv
.Op Fl c Ar count
.Op Fl I Ar ifaddr
.Op Fl i Ar wait
.Op Fl l Ar preload
.Op Fl p Ar pattern
.Op Fl s Ar packetsize
.Op Fl T Ar tos
.Op Fl t Ar ttl
.Op Fl V Ar rtable
.Op Fl w Ar maxwait
.Ar host
.Ek
.Sh DESCRIPTION
.Nm
uses the ICMP protocol's mandatory
.Dv ECHO_REQUEST
datagram to elicit an ICMP
.Dv ECHO_REPLY
from a host or gateway.
.Dv ECHO_REQUEST
datagrams
.Pq Dq pings
have an IP and ICMP header,
followed by a
.Dq struct timeval
and then an arbitrary number of
.Dq pad
bytes used to fill out the packet.
The options are as follows:
.Bl -tag -width Ds
.It Fl c Ar count
Stop sending after
.Ar count
.Dv ECHO_REQUEST
packets have been sent.
If
.Ar count
is 0, send an unlimited number of packets.
.It Fl D
Set the
.Dv Don't Fragment
bit.
.It Fl d
Set the
.Dv SO_DEBUG
option on the socket being used.
.It Fl E
Emit an audible beep (by sending an ASCII BEL character to the
standard error output) when no packet is received before the next
packet is transmitted.
To cater for round-trip times that are longer than the interval between
transmissions, further missing packets cause a bell only if the maximum
number of unreceived packets has increased.
This option is disabled for flood pings.
.It Fl e
Emit an audible beep (by sending an ASCII BEL character to the
standard error output) after each non-duplicate response is received.
This option is disabled for flood pings.
.It Fl f
Flood ping.
Outputs packets as fast as they come back or one hundred times per second,
whichever is more.
For every
.Dv ECHO_REQUEST
sent, a period
.Sq \&.
is printed, while for every
.Dv 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 option.
.Bf -emphasis
This can be very hard on a network and should be used with caution.
.Ef
.It Fl I Ar ifaddr
Specify the interface address to transmit from
on machines with multiple interfaces.
For unicast and multicast pings.
.It Fl i Ar wait
Wait
.Ar 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
.Fl f
option.
.It Fl L
Disable the loopback, so the transmitting host doesn't see the ICMP
requests.
For multicast pings.
.It Fl l Ar preload
If
.Ar preload
is specified,
.Nm
sends that many packets as fast as possible before falling into its normal
mode of behavior.
Only root may set a preload value.
.It Fl n
Numeric output only.
No attempt will be made to look up symbolic names for host addresses.
.It Fl p Ar pattern
You may specify up to 16
.Dq pad
bytes to fill out the packet you send.
This is useful for diagnosing data-dependent problems in a network.
For example,
.Dq -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
.Dv RECORD_ROUTE
option in the
.Dv 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,
.Nm
will print the route list and then truncate it at the correct spot.
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.
.It Fl s Ar 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.
.It Fl T Ar tos
Use the specified type of service.
.It Fl t Ar ttl
Use the specified time-to-live.
.It Fl V Ar rtable
Set the routing table to be used for outgoing packets.
The default is 0.
.It Fl v
Verbose output.
ICMP packets other than
.Dv ECHO_REPLY
that are received are listed.
.It Fl w Ar maxwait
Specifies the maximum number of seconds to wait for responses
after the last request has been sent.
The default is 10.
.El
.Pp
When using
.Nm
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
.Dq pinged .
.Pp
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 and
the standard deviation.
.Pp
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.
The summary information can also be displayed while
.Nm
is running by sending it a
.Dv SIGINFO
signal (see the
.Cm status
argument of
.Xr stty 1
for more information).
.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
during normal operations or from automated scripts.
.Pp
.Nm
exits 0 if at least one reply is received,
and \*(Gt0 if no reply is received or an error occurred.
.Sh ICMP PACKET DETAILS
An IP header without options is 20 bytes.
An ICMP
.Dv ECHO_REQUEST
packet contains an additional 8 bytes worth of
ICMP header followed by an arbitrary amount of data.
When a
.Ar 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
.Dv 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,
.Nm
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
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
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
.Dq 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 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 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).
.Pp
The maximum possible value of this field is 255, and most
.Ux
systems set the TTL field of ICMP
.Dv ECHO_REQUEST
packets to 255.
This is why you will find you can
.Dq ping
some hosts, but not reach them
with
.Xr telnet 1
or
.Xr ftp 1 .
.Pp
In normal operation,
.Nm
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:
.Bl -bullet
.It
Not change it; this is what Berkeley
.Ux
systems did before the
.Bx 4.3 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.
.It
Set it to 255; this is what current Berkeley
.Ux
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.
.It
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.
.El
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr ifconfig 8 ,
.Xr ping6 8 ,
.Xr spray 8
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .
.Sh BUGS
Many hosts and gateways ignore the
.Dv RECORD_ROUTE
option.
.Pp
The maximum IP header length is too small for options like
.Dv RECORD_ROUTE
to
be completely useful.
There's not much 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.