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|
<refentry xmlns="http://docbook.org/ns/docbook" version="5.0"
xml:id="man.ping">
<refentryinfo>
<title>ping</title>
<productname>iputils</productname>
</refentryinfo>
<refmeta>
<refentrytitle>
<application>ping</application>
</refentrytitle>
<manvolnum>8</manvolnum>
<refmiscinfo class='manual'>iputils</refmiscinfo>
</refmeta>
<refnamediv>
<refname>ping</refname>
<refpurpose>send ICMP ECHO_REQUEST to network
hosts</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis sepchar=" ">
<command>ping</command>
<arg choice="opt" rep="norepeat">
<option>-aAbBdCDfhHLnOqrRUvV346</option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-c
<replaceable>count</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-e
<replaceable>identifier</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-F
<replaceable>flowlabel</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-i
<replaceable>interval</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-I
<replaceable>interface</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-l
<replaceable>preload</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-m
<replaceable>mark</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-M
<replaceable>pmtudisc_option</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-N
<replaceable>nodeinfo_option</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-w
<replaceable>deadline</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-W
<replaceable>timeout</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-p
<replaceable>pattern</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-Q
<replaceable>tos</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-s
<replaceable>packetsize</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-S
<replaceable>sndbuf</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-t
<replaceable>ttl</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-T
<replaceable>timestamp option</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">hop...</arg>
<arg choice="req" rep="norepeat">destination</arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsection xml:id="description">
<info>
<title>DESCRIPTION</title>
</info>
<para>
<command>ping</command> 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.</para>
<para>
<command>ping</command> works with both IPv4 and IPv6. Using
only one of them explicitly can be enforced by specifying
<option>-4</option> or
<option>-6</option>.</para>
<para>
<command>ping</command> can also send IPv6 Node Information
Queries (RFC4620). Intermediate
<emphasis remap="I">hop</emphasis>s may not be allowed, because
IPv6 source routing was deprecated (RFC5095).</para>
</refsection>
<refsection xml:id="options">
<info>
<title>OPTIONS</title>
</info>
<variablelist remap="TP">
<varlistentry>
<term>
<option>-3</option>
</term>
<listitem>
<para>RTT precision (do not round up the result time).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-4</option>
</term>
<listitem>
<para>Use IPv4 only.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-6</option>
</term>
<listitem>
<para>Use IPv6 only.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-a</option>
</term>
<listitem>
<para>Audible ping.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-A</option>
</term>
<listitem>
<para>Adaptive ping. Interpacket interval adapts to
round-trip time, so that effectively not more than one
(or more, if preload is set) unanswered probe is present
in the network. The default interval is 2 ms,
for more info see option <option>-i</option>.
On networks with low RTT this mode is
essentially equivalent to flood mode.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-b</option>
</term>
<listitem>
<para>Allow pinging a broadcast address.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-B</option>
</term>
<listitem>
<para>Do not allow
<command>ping</command> to change source address of
probes. The address is bound to one selected when
<command>ping</command> starts.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-c</option>
<emphasis remap="I">count</emphasis>
</term>
<listitem>
<para>Stop after sending
<emphasis remap="I">count</emphasis> ECHO_REQUEST packets.
With
<emphasis remap="I">deadline</emphasis> option,
<command>ping</command> waits for
<emphasis remap="I">count</emphasis> ECHO_REPLY packets,
until the timeout expires.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-C</option>
</term>
<listitem>
<para>Call connect() syscall on socket creation.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-d</option>
</term>
<listitem>
<para>Set the SO_DEBUG option on the socket being used.
Essentially, this socket option is not used by Linux
kernel.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-D</option>
</term>
<listitem>
<para>Print timestamp (unix time + microseconds as in
gettimeofday) before each line.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-e</option>
<emphasis remap="I">identifier</emphasis>
</term>
<listitem>
<para>Set the identification field of ECHO_REQUEST.
Value 0 implies using <emphasis remap="I">raw socket</emphasis>
(not supported on <emphasis remap="I">ICMP datagram socket</emphasis>).
The value of the field may be printed with <option>-v</option> option.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-f</option>
</term>
<listitem>
<para>Flood ping. For every ECHO_REQUEST sent a period
“.” is printed, while for every 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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-F</option>
<emphasis remap="I">flow label</emphasis>
</term>
<listitem>
<para>IPv6 only. Allocate and set 20 bit flow label (in
hex) on echo request packets. If value is zero, kernel
allocates random flow label.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-h</option>
</term>
<listitem>
<para>Show help.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-H</option>
</term>
<listitem>
<para>Force DNS name resolution for the output. Useful for numeric
destination, or <option>-f</option> option, which by default do not
perform it. It can also help to workaround DNS resolution problems.
Override previously defined <option>-n</option> option.
See also <emphasis remap="I">IPUTILS_PING_PTR_LOOKUP</emphasis>
environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-i</option>
<emphasis remap="I">interval</emphasis>
</term>
<listitem>
<para>Wait
<emphasis remap="I">interval</emphasis> seconds between
sending each packet. Real number allowed with dot as
a decimal separator (regardless locale setup).
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 than 2 ms. Broadcast and multicast ping have even
higher limitation for regular user: minimum is 1 sec.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-I</option>
<emphasis remap="I">interface</emphasis>
</term>
<listitem>
<para>
<emphasis remap="I">interface</emphasis> is either an
address, an interface name or a VRF name. If
<emphasis remap="I">interface</emphasis> is an address, it
sets source address to specified interface address. If
<emphasis remap="I">interface</emphasis> is an interface
name, it sets source interface to specified interface.
If <emphasis remap="I">interface</emphasis> is a VRF
name, each packet is routed using the corresponding
routing table; in this case, the <option>-I</option> option
can be repeated to specify a source address.
NOTE: For IPv6, when doing ping to a link-local scope
address, link specification (by the '%'-notation in
<emphasis remap="I">destination</emphasis>, or by this
option) can be used but it is no longer required.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-l</option>
<emphasis remap="I">preload</emphasis>
</term>
<listitem>
<para>If
<emphasis remap="I">preload</emphasis> is specified,
<command>ping</command> sends that many packets not
waiting for reply. Only the super-user may select preload
more than 3.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-L</option>
</term>
<listitem>
<para>Suppress loopback of multicast packets. This flag
only applies if the ping destination is a multicast
address.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-m</option>
<emphasis remap="I">mark</emphasis>
</term>
<listitem>
<para>use
<emphasis remap="I">mark</emphasis> to tag the packets
going out. This is useful for variety of reasons within
the kernel such as using policy routing to select
specific outbound processing. CAP_NET_ADMIN or CAP_NET_RAW
(since Linux 5.17) capability is required, see
<citerefentry>
<refentrytitle>socket</refentrytitle>
<manvolnum>7</manvolnum>
</citerefentry>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-M</option>
<emphasis remap="I">pmtudisc_opt</emphasis>
</term>
<listitem>
<para>Select Path MTU Discovery strategy.
<emphasis remap="I">pmtudisc_option</emphasis> may be
either
<emphasis remap="I">do</emphasis> (set DF flag but
subject to PMTU checks by kernel, packets too large will
be rejected),
<emphasis remap="I">want</emphasis> (do PMTU discovery,
fragment locally when packet size is large),
<emphasis remap="I">probe</emphasis> (set DF flag and
bypass PMTU checks, useful for probing), or
<emphasis remap="I">dont</emphasis> (do not set DF
flag).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-N</option>
<emphasis remap="I">nodeinfo_option</emphasis>
</term>
<listitem>
<para>IPv6 only. Send IPv6 Node Information Queries
(RFC4620), instead of Echo Request. CAP_NET_RAW
capability is required.</para>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">help</emphasis>
</term>
<listitem>
<para>Show help for NI support.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">name</emphasis>
</term>
<listitem>
<para>Queries for Node Names.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv6</emphasis>
</term>
<listitem>
<para>Queries for IPv6 Addresses. There are several
IPv6 specific flags.</para>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv6-global</emphasis>
</term>
<listitem>
<para>Request IPv6 global-scope
addresses.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv6-sitelocal</emphasis>
</term>
<listitem>
<para>Request IPv6 site-local
addresses.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv6-linklocal</emphasis>
</term>
<listitem>
<para>Request IPv6 link-local
addresses.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv6-all</emphasis>
</term>
<listitem>
<para>Request IPv6 addresses on other
interfaces.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv4</emphasis>
</term>
<listitem>
<para>Queries for IPv4 Addresses. There is one IPv4
specific flag.</para>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">ipv4-all</emphasis>
</term>
<listitem>
<para>Request IPv4 addresses on other
interfaces.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">subject-ipv6=</emphasis><emphasis remap="I">ipv6addr</emphasis>
</term>
<listitem>
<para>IPv6 subject address.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">subject-ipv4=</emphasis><emphasis remap="I">ipv4addr</emphasis>
</term>
<listitem>
<para>IPv4 subject address.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">subject-name=</emphasis><emphasis remap="I">nodename</emphasis>
</term>
<listitem>
<para>Subject name. If it contains more than one
dot, fully-qualified domain name is assumed.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist remap="TP">
<varlistentry>
<term>
<emphasis remap="B">subject-fqdn=</emphasis><emphasis remap="I">nodename</emphasis>
</term>
<listitem>
<para>Subject name. Fully-qualified domain name is
always assumed.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-n</option>
</term>
<listitem>
<para>Numeric output only. No attempt will be made to lookup
symbolic names for host addresses (no reverse DNS resolution).
This is the default for numeric destination or <option>-f</option>
option. Override previously defined <option>-H</option> option.
See also <emphasis remap="I">IPUTILS_PING_PTR_LOOKUP</emphasis>
environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-O</option>
</term>
<listitem>
<para>Report outstanding ICMP ECHO reply before sending
next packet. This is useful together with the timestamp
<option>-D</option> to log output to a diagnostic file and
search for missing answers.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-p</option>
<emphasis remap="I">pattern</emphasis>
</term>
<listitem>
<para>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,
<option>-p ff</option> will cause the sent packet to be
filled with all ones.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-q</option>
</term>
<listitem>
<para>Quiet output. Nothing is displayed except the
summary lines at startup time and when finished.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-Q</option>
<emphasis remap="I">tos</emphasis>
</term>
<listitem>
<para>Set Quality of Service -related bits in ICMP
datagrams.
<emphasis remap="I">tos</emphasis> can be decimal
(<command>ping</command> only) or hex number.</para>
<para>In RFC2474, these fields are interpreted as 8-bit
Differentiated Services (DS), consisting of: bits 0-1 (2
lowest bits) of separate data, and bits 2-7 (highest 6
bits) of Differentiated Services Codepoint (DSCP). In
RFC2481 and RFC3168, bits 0-1 are used for ECN.</para>
<para>Historically (RFC1349, obsoleted by RFC2474), these
were interpreted as: bit 0 (lowest bit) for reserved
(currently being redefined as congestion control), 1-4
for Type of Service and bits 5-7 (highest bits) for
Precedence.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-r</option>
</term>
<listitem>
<para>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
<option>-I</option> is also used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-R</option>
</term>
<listitem>
<para>
<command>ping</command> only. 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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-s</option>
<emphasis remap="I">packetsize</emphasis>
</term>
<listitem>
<para>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.
The maximum allowed value is 65507 for IPv4
(65467 when <option>-R</option> or <option>-T</option>
or Intermediate <emphasis remap="I">hop</emphasis>s)
or 65527 for IPv6, but most systems
limit this to a smaller, system-dependent number.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-S</option>
<emphasis remap="I">sndbuf</emphasis>
</term>
<listitem>
<para>Set socket sndbuf. If not specified, it is selected
to buffer not more than one packet.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-t</option>
<emphasis remap="I">ttl</emphasis>
</term>
<listitem>
<para>
<command>ping</command> only. Set the IP Time to
Live.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-T</option>
<emphasis remap="I">timestamp option</emphasis>
</term>
<listitem>
<para>Set special IP timestamp options.
<emphasis remap="I">timestamp option</emphasis> may be
either
<emphasis remap="I">tsonly</emphasis> (only timestamps),
<emphasis remap="I">tsandaddr</emphasis> (timestamps and
addresses) or
<emphasis remap="I">tsprespec host1 [host2 [host3
[host4]]]</emphasis> (timestamp prespecified hops).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-U</option>
</term>
<listitem>
<para>Print full user-to-user latency (the old
behaviour). Normally
<command>ping</command> prints network round trip time,
which can be different f.e. due to DNS failures.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-v</option>
</term>
<listitem>
<para>Verbose output. Do not suppress DUP replies when pinging
multicast address.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-V</option>
</term>
<listitem>
<para>Show version and exit.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-w</option>
<emphasis remap="I">deadline</emphasis>
</term>
<listitem>
<para>Specify a timeout, in seconds, before
<command>ping</command> exits regardless of how many
packets have been sent or received. In this case
<command>ping</command> does not stop after
<emphasis remap="I">count</emphasis> packet are sent, it
waits either for
<emphasis remap="I">deadline</emphasis> expire or until
<emphasis remap="I">count</emphasis> probes are answered
or for some error notification from network.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-W</option>
<emphasis remap="I">timeout</emphasis>
</term>
<listitem>
<para>Time to wait for a response, in seconds. The option
affects only timeout in absence of any responses,
otherwise
<command>ping</command> waits for two RTTs.
Real number allowed with dot as a decimal separator
(regardless locale setup).
0 means infinite timeout.</para>
</listitem>
</varlistentry>
</variablelist>
<para>When using
<command>ping</command> 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/mdev round-trip time
numbers.</para>
<para>Population standard deviation (mdev), essentially an
average of how far each ping RTT is from the mean RTT. The
higher mdev is, the more variable the RTT is (over time).
With a high RTT
variability, you will have speed issues with bulk transfers
(they will take longer than is strictly speaking necessary, as
the variability will eventually cause the sender to wait for
ACKs) and you will have middling to poor VoIP quality.</para>
<para>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.</para>
<para>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
<command>ping</command> during normal operations or from
automated scripts.</para>
</refsection>
<refsection xml:id="environment">
<info>
<title>ENVIRONMENT</title>
</info>
<emphasis remap="I">IPUTILS_PING_PTR_LOOKUP</emphasis> environment
variable set to 0 disable reverse DNS resolution (PTR lookup) by default.
It will be overrided by <option>-H</option> or <option>-n</option> option.
</refsection>
<refsection xml:id="exit_status">
<info>
<title>EXIT STATUS</title>
</info>
<para>If
<command>ping</command> does not receive any reply packets at
all it will exit with code 1. If a packet
<emphasis remap="I">count</emphasis> and
<emphasis remap="I">deadline</emphasis> are both specified, and
fewer than
<emphasis remap="I">count</emphasis> packets are received by the
time the
<emphasis remap="I">deadline</emphasis> 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.</para>
</refsection>
<refsection xml:id="ipv6_link_local_destinations">
<info>
<title>IPV6 LINK-LOCAL DESTINATIONS</title>
</info>
<para>For IPv6, when the destination address has link-local scope and
<command>ping</command> is using <emphasis remap="I">ICMP datagram sockets</emphasis>,
the output interface must be specified.
When <command>ping</command> is using <emphasis remap="I">raw sockets</emphasis>,
it is not strictly necessary to specify the output interface but it should be done
to avoid ambiguity when there are multiple possible output interfaces.</para>
<para>There are two ways to specify the output interface:</para>
<variablelist remap="TP">
<varlistentry>
<term>
• using the
<emphasis remap="I">% notation</emphasis>
</term>
<listitem>
<para>The destination address is postfixed with
<emphasis remap="I">%</emphasis>
and the output interface name or ifindex, for example:</para>
<para><command>ping fe80::5054:ff:fe70:67bc%eth0</command></para>
<para><command>ping fe80::5054:ff:fe70:67bc%2</command></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
• using the
<emphasis remap="I">-I option</emphasis>
</term>
<listitem>
<para>When using <emphasis remap="I">ICMP datagram sockets</emphasis>,
this method is supported since the following kernel versions: 5.17,
5.15.19, 5.10.96, 5.4.176, 4.19.228, 4.14.265.
Also it is not supported on musl libc.</para>
</listitem>
</varlistentry>
</variablelist>
</refsection>
<refsection xml:id="icmp_packet_details">
<info>
<title>ICMP PACKET DETAILS</title>
</info>
<para>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
<emphasis remap="I">packetsize</emphasis> is given, this
indicates 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).</para>
<para>If the data space is at least of size of struct timeval
<command>ping</command> 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.</para>
</refsection>
<refsection xml:id="duplicate_and_damaged_packets">
<info>
<title>DUPLICATE AND DAMAGED PACKETS</title>
</info>
<para>
<command>ping</command> 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.</para>
<para>Damaged packets are obviously serious cause for alarm and
often indicate broken hardware somewhere in the
<command>ping</command> packet's path (in the network or in the
hosts).</para>
</refsection>
<refsection xml:id="id_collisions">
<info>
<title>ID COLLISIONS</title>
</info>
<para>
Unlike TCP and UDP, which use port to uniquely identify the recipient to
deliver data, ICMP uses identifier field (ID) for identification.
Therefore, if on the same machine, at the same time, two ping processes
use the same ID, echo reply can be delivered to a wrong recipient.
This is a known problem due to the limited size of the 16-bit ID field.
That is a historical limitation of the protocol that cannot be fixed
at the moment unless we encode an ID into the ping packet payload.
<command>ping</command> prints <emphasis remap="I">DIFFERENT ADDRESS</emphasis>
error and packet loss is negative.
</para>
<para>
<command>ping</command> uses PID to get unique number. The default value of
<emphasis remap="I">/proc/sys/kernel/pid_max</emphasis> is 32768.
On the systems that use ping heavily and with <emphasis remap="I">pid_max</emphasis>
greater than 65535 collisions are bound to happen.
</para>
</refsection>
<refsection xml:id="trying_different_data_patterns">
<info>
<title>TRYING DIFFERENT DATA PATTERNS</title>
</info>
<para>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
“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 transmit can be
complicated.</para>
<para>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
<option>-p</option> option of
<command>ping</command>.</para>
</refsection>
<refsection xml:id="ttl_details">
<info>
<title>TTL DETAILS</title>
</info>
<para>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.</para>
<para>The TTL field for TCP packets may take various values.
The maximum possible value of this field is 255, a recommended
initial value is 64. For more information, see the TCP/Lower-Level
Interface section of RFC9293.</para>
<para>In normal operation <command>ping</command> 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:</para>
<variablelist remap="TP">
<varlistentry>
<listitem>
<para>• 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.</para>
</listitem>
</varlistentry>
<varlistentry>
<listitem>
<para>• 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
<emphasis remap="B">from</emphasis> the remote system
<emphasis remap="B">to</emphasis> the
<command>ping</command>ing host.</para>
</listitem>
</varlistentry>
<varlistentry>
<listitem>
<para>• 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.</para>
</listitem>
</varlistentry>
</variablelist>
</refsection>
<refsection xml:id="bugs">
<info>
<title>BUGS</title>
</info>
<variablelist remap="TP">
<varlistentry>
<listitem>
<para>• Many Hosts and Gateways ignore the RECORD_ROUTE
option.</para>
</listitem>
</varlistentry>
<varlistentry>
<listitem>
<para>• The maximum IP header length is too small for
options like RECORD_ROUTE to be completely useful.
There's not much that can be done about this,
however.</para>
</listitem>
</varlistentry>
<varlistentry>
<listitem>
<para>• Flood pinging is not recommended in general, and
flood pinging the broadcast address should only be done
under very controlled conditions.</para>
</listitem>
</varlistentry>
</variablelist>
</refsection>
<refsection xml:id="see_also">
<info>
<title>SEE ALSO</title>
</info>
<para>
<citerefentry>
<refentrytitle>ip</refentrytitle>
<manvolnum>8</manvolnum>
</citerefentry>,
<citerefentry>
<refentrytitle>ss</refentrytitle>
<manvolnum>8</manvolnum>
</citerefentry>.</para>
</refsection>
<refsection xml:id="history">
<info>
<title>HISTORY</title>
</info>
<para>The
<command>ping</command> command appeared in 4.3BSD.</para>
<para>The version described here is its descendant specific to
Linux.</para>
<para>As of version s20150815, the
<emphasis remap="B">ping6</emphasis> binary doesn't exist
anymore. It has been merged into
<command>ping</command>. Creating a symlink named
<emphasis remap="B">ping6</emphasis> pointing to
<command>ping</command> will result in the same functionality as
before.</para>
</refsection>
<refsection xml:id="security">
<info>
<title>SECURITY</title>
</info>
<para>
<command>ping</command> requires CAP_NET_RAW capability to be
executed 1) if the program is used for non-echo queries (see
<option>-N</option> option) or when the identification field set to 0
for ECHO_REQUEST (see <option>-e</option>), or
2) if kernel does not support ICMP datagram sockets, or
3) if the user is not allowed to create an ICMP echo socket.
The program may be used as set-uid root.
</para>
</refsection>
<refsection xml:id="availability">
<info>
<title>AVAILABILITY</title>
</info>
<para>
<command>ping</command> is part of
<emphasis remap="I">iputils</emphasis> package.</para>
</refsection>
</refentry>
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