ping



PING(8)                System Manager’s Manual: iputils                PING(8)




NAME

       ping, ping6 - send ICMP ECHO_REQUEST to network hosts


SYNOPSIS

       ping  [ -LRUbdfnqrvVaAB]  [ -c count]  [ -i interval]  [ -l preload]  [
       -p pattern]  [ -s packetsize]  [ -t ttl]  [ -w deadline]  [ -F  flowla-
       bel]  [ -I interface]  [ -M hint]  [ -Q tos]  [ -S sndbuf]  [ -T times-
       tamp option]  [ -W timeout]  [ hop ...]  destination



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.


OPTIONS

       -a     Audible ping.

       -A     Adaptive ping. Interpacket interval adapts to  round-trip  time,
              so  that  effectively  not more than one (or more, if preload is
              set) unanswered probes present in the network. Minimal  interval
              is  200msec  for  not super-user.  On networks with low rtt this
              mode is essentially equivalent to flood mode.

       -b     Allow pinging a broadcast address.

       -B     Do not allow ping to  change  source  address  of  probes.   The
              address is bound to one selected when ping starts.

       -c count
              Stop  after  sending  count  ECHO_REQUEST packets. With deadline
              option, ping waits for count ECHO_REPLY packets, until the time-
              out expires.

       -d     Set  the SO_DEBUG option on the socket being used.  Essentially,
              this socket option is not used by Linux kernel.

       -F flow label
              Allocate and set 20 bit flow  label  on  echo  request  packets.
              (Only  ping6).  If  value  is zero, kernel allocates random flow
              label.

       -f     Flood ping. 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.   If  interval is not given, it sets interval to
              zero and outputs packets as fast as they come back or  one  hun-
              dred  times  per second, whichever is more.  Only the super-user
              may use this option with zero interval.

       -i interval
              Wait interval seconds between sending each packet.  The  default
              is  to  wait for one second between each packet normally, or not
              to wait in flood mode. Only super-user may set interval to  val-
              ues less 0.2 seconds.

       -I interface address
              Set  source address to specified interface address. Argument may
              be numeric IP address or name of device. When pinging IPv6 link-
              local address this option is required.

       -l preload
              If  preload is specified, ping sends that many packets not wait-
              ing for reply.  Only the super-user may select preload more than
              3.

       -L     Suppress  loopback of multicast packets.  This flag only applies
              if the ping destination is a multicast address.

       -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 tos Set Quality of Service -related bits in ICMP datagrams.  tos can
              be either decimal or hex number.  Traditionally (RFC1349), these
              have been interpreted as: 0 for reserved (currently being  rede-
              fined  as  congestion  control), 1-4 for Type of Service and 5-7
              for Precedence.  Possible settings for Type of Service are: min-
              imal cost: 0x02, reliability: 0x04, throughput: 0x08, low delay:
              0x10.  Multiple TOS bits should not be set simultaneously.  Pos-
              sible settings for special Precedence range from priority (0x20)
              to net control (0xe0).  You must be root (CAP_NET_ADMIN capabil-
              ity) to use Critical or higher precedence value.  You cannot set
              bit 0x01 (reserved) unless ECN has been enabled in  the  kernel.
              In RFC2474, these fields has been redefined as 8-bit Differenti-
              ated Services (DS), consisting of: bits  0-1  of  separate  data
              (ECN  will  be  used, here), and bits 2-7 of Differentiated Ser-
              vices Codepoint (DSCP).

       -q     Quiet output.  Nothing is displayed except the summary lines  at
              startup time and when finished.

       -R     Record  route.  (IPv4  only) 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 interface.  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 provided the option -I is also used.

       -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.

       -S sndbuf
              Set  socket  sndbuf.  If not specified, it is selected to buffer
              not more than one packet.

       -t ttl Set the IP Time to Live.

       -T timestamp option
              Set special IP  timestamp  options.   timestamp  option  may  be
              either  tsonly  (only  timestamps),  tsandaddr  (timestamps  and
              addresses) or tsprespec host1 [host2 [host3 [host4]]] (timestamp
              prespecified hops).

       -M hint
              Select Path MTU Discovery strategy.  hint may be either do (pro-
              hibit fragmentation, even local one), want (do  PMTU  discovery,
              fragment locally when packet size is large), or dont (do not set
              DF flag).

       -U     Print full user-to-user latency (the  old  behaviour).  Normally
              ping prints network round trip time, which can be different f.e.
              due to DNS failures.

       -v     Verbose output.

       -V     Show version and exit.

       -w deadline
              Specify a timeout, in seconds, before ping exits  regardless  of
              how  many  packets have been sent or received. In this case ping
              does not stop after count packet are sent, it waits  either  for
              deadline  expire  or until count probes are answered or for some
              error notification from network.

       -W timeout
              Time to wait for a response, in seconds. The option affects only
              timeout  in  absense  of any responses, otherwise ping waits for
              two RTTs.

       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.  Shorter current statistics can be obtained without termination
       of process with signal SIGQUIT.

       If ping does not receive any reply packets at all  it  will  exit  with
       code  1.  If  a packet count and deadline are both specified, and fewer
       than count packets are received by the time the deadline  has  arrived,
       it  will  also  exit with code 1.  On other 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

       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 of size of struct timeval ping  uses  the
       beginning  bytes  of this space to include a timestamp which it uses in
       the computation of round trip times.  If the data space is shorter,  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 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 tel-
       net(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 condi-
         tions.


SEE ALSO

       netstat(1), ifconfig(8).


HISTORY

       The ping command appeared in 4.3BSD.

       The version described here is its descendant specific to Linux.


SECURITY

       ping requires CAP_NET_RAWIO capability to be executed. It may  be  used
       as set-uid root.


AVAILABILITY

       ping  is part of iputils package and the latest versions are  available
       in  source  form   for   anonymous   ftp   ftp://ftp.inr.ac.ru/ip-rout-
       ing/iputils-current.tar.gz.



iputils-020927                 27 September 2002                       PING(8)

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