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PING(8)								       PING(8)

NAME
       ping - send ICMP ECHO_REQUEST packets to network hosts

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

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: 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.

       -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 run‐
       ning.  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  dis‐
       played.

       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	 indi‐
       cate  broken  hardware somewhere in the ping packet's path (in the net‐
       work or in the hosts).

TRYING DIFFERENT DATA PATTERNS
       The (inter)network layer should never treat packets differently depend‐
       ing  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 pat‐
       tern 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 trans‐
       mit 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 man‐
       age 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 decre‐
       ment 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
       or ftp.

       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(8), routed(8)

4.3 Berkeley Distribution	 June 24, 1990			       PING(8)
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