githubmirror/iputils

doc/ping.xml
929 строк33 KB
Jan Tojnar
doc: Use namespace correctly
25 мар 2019, 23:31
25 мар 2019, 23:31c503834
<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>-aAbBdDfhLnOqrRUvV46</option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-c
        <replaceable>count</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>
    <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>
    <info>
      <title>OPTIONS</title>
    </info>
    <variablelist remap="TP">
      <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. Minimal interval is 200msec for not
          super-user. 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>-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>-f</option>
        </term>
        <listitem>
          <para>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.</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>-i</option>
          <emphasis remap="I">interval</emphasis>
        </term>
        <listitem>
          <para>Wait
          <emphasis remap="I">interval</emphasis> 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 than 0.2 seconds.</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, or an interface name. If
          <emphasis remap="I">interface</emphasis> is an address, it
          sets source address to specified interface address. If
          <emphasis remap="I">interface</emphasis> in an interface
          name, it sets source interface to specified interface.
          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.</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> (prohibit fragmentation,
          even local one),
          <emphasis remap="I">want</emphasis> (do PMTU discovery,
          fragment locally when packet size is large), 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 ICMPv6 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.</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.</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.</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.</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>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>
    <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="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
    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).</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="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 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).</para>
    <para>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
    <citerefentry>
      <refentrytitle>telnet</refentrytitle>
      <manvolnum>1</manvolnum>
    </citerefentry> or
    <citerefentry>
      <refentrytitle>ftp</refentrytitle>
      <manvolnum>1</manvolnum>
    </citerefentry>.</para>
    <para>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:</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 funcionality 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 2) if kernel does not support
    non-raw ICMP 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>