NAME

     ping — send ICMP ECHO_REQUEST packets to network hosts

SYNOPSIS

     ping [-Rdfnqrv] [-c count] [-i wait] [-l preload] [-p pattern] [-s packetsize] host

DESCRIPTION

     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.  The options are as follows:

     -c count
             Stop after sending (and receiving) count ECHO_RESPONSE packets.

     -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 every 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.  This can be very hard on a network and should be used with caution.

     -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 specified, ping sends that many packets as fast as possible before
             falling into its normal mode of behavior.

     -n      Numeric output only.  No attempt will be made to lookup 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.  Many hosts ignore or discard 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.

     -v      Verbose output.  ICMP packets other than ECHO_RESPONSE that are received are listed.

     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
     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 SIGINT, a brief summary is
     displayed.

     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 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 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 PACKETS

     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.

     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 practice 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 receives.  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.

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 that can be done about this, however.

     Flood pinging is not recommended in general, and flood pinging the broadcast address should
     only be done under very controlled conditions.

SEE ALSO

     netstat(1), ifconfig(1), routed(8)

HISTORY

     The ping command appeared in 4.3BSD.