man--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 flowlabel] [ -I interface] [ -M
hint] [ -Q tos] [ -S sndbuf] [ -T timestamp 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 effec-
tively 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 timeout 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 hundred
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 values 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 waiting 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 dec-
imal or hex number. Traditionally (RFC1349), these have been interpreted as: 0
for reserved (currently being redefined as congestion control), 1-4 for Type of
Service and 5-7 for Precedence. Possible settings for Type of Service are:
minimal cost: 0x02, reliability: 0x04, throughput: 0x08, low delay: 0x10. Mul-
tiple TOS bits should not be set simultaneously. Possible settings for special
Precedence range from priority (0x20) to net control (0xe0). You must be root
(CAP_NET_ADMIN capability) 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 Differentiated Services (DS),
consisting of: bits 0-1 of separate data (ECN will be used, here), and bits 2-7
of Differentiated Services 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 trans-
lates 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 (prohibit fragmenta-
tion, 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 net-
work 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 ver-
ify that the local network interface is up and running. Then, hosts and gateways fur-
ther and further away should be ‘‘pinged’’. Round-trip times and packet loss statis-
tics 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 cal-
culating the minimum/average/maximum round-trip time numbers. When the specified num-
ber of packets have been sent (and received) or if the program is terminated with a
SIGINT, a brief summary is displayed. Shorter current statistics can be obtained with-
out 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 possi-
ble 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 nor-
mal 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 hard-
ware 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 suffi-
cient ‘‘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 con-
trollers 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 com-
pletely 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).
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
at http://www.skbuff.net/iputils/iputils-current.tar.bz2.
iputils-071127 26 September 2013 PING(8)
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