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

NAME
ping, ping6 - send ICMP ECHO_REQUEST to network hosts

SYNOPSIS
ping [ -LRUbdfnqrvVaAB] [ -c count] [ -m mark] [ -i interval] [ -l preload] [ -p pattern] [ -s packetsize] [ -t
ttl] [ -w deadline] [ -F flowlabel] [ -I interface] [ -M hint] [ -N nioption] [ -Q tos] [ -S sndbuf] [ -T time-
stamp 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.

ping6 can also send Node Information Queries (RFC4620).

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 net-
works 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.

-m mark
use mark to tag the packets going out. This is useful for variety of reasons within the kernel such as using pol-
icy routing to select specific outbound processing.

-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 ran-
dom 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 nor-
mally, 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 nioption
Send ICMPv6 Node Information Queries (RFC4620), instead of Echo Request.

name Queries for Node Names.

ipv6 Queries for IPv6 Addresses. There are several IPv6 specific flags.

ipv6-global
Request IPv6 global-scope addresses.

ipv6-sitelocal
Request IPv6 site-local addresses.

ipv6-linklocal
Request IPv6 link-local addresses.

ipv6-all
Request IPv6 addresses on other interfaces.

ipv4 Queries for IPv4 Addresses. There is one IPv4 specific flag.

ipv4-all
Request IPv4 addresses on other interfaces.

subject-ipv6=ipv6addr
IPv6 subject address.

subject-ipv4=ipv4addr
IPv4 subject address.

subject-name=nodename
Subject name. If it contains more than one dot, fully-qualified domain name is assumed.

subject-fqdn=nodename
Subject name. Fully-qualified domain name is always assumed.

-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-depen-
dent problems in a network. For example, -p ff will cause the sent packet to be filled with all ones.

-D Print timestamp (unix time + microseconds as in gettimeofday) before each line.

-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 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, relia-
bility: 0x04, throughput: 0x08, low delay: 0x10. Multiple 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), con-
sisting 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 dis-
card 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 inter-
face 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 (prohibit 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 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. 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 spec-
ified, 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 time-
stamp 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 inappro-
priate 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 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 ``transi-
tions'', 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 compli-
cated.

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 val-
ues (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 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 exam-
ple 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 conditions.