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. 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:
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
path.
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
host.
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
o Many Hosts and Gateways ignore the RECORD_ROUTE option.
o 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.
o 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)
