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NAME    [Toc]    [Back]

     ping -- send ICMP ECHO_REQUEST packets to network hosts

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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: Other options are:

     -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 ever
	     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 net-
	     work 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.	Only
	     the super-user may use this option.

     -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.

     If ping does not receive any reply packets at all it will exit with code
     1.  On error it exits with code 2. Otherwise it exits with code 0. This
     makes it possible to use the exit code to see if a host is alive or not.

     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    [Toc]    [Back]

     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.


     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).


     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    [Toc]    [Back]

     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:

     +o	 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

     +o	 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

     +o	 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    [Toc]    [Back]

     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    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

     The ping command appeared in 4.3BSD.

Linux NetKit 0.09		August 30, 1996 	     Linux NetKit 0.09
[ Back ]
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