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<refentry id='tracepath8'>
<refmeta>
  <refentrytitle>TRACEPATH</refentrytitle>
  <manvolnum>8</manvolnum>
  <refmiscinfo class='manual'>iputils</refmiscinfo>
</refmeta>
<refnamediv>
  <refname>tracepath</refname>
  <refname>tracepath6</refname>
  <refpurpose>traces path to a network host discovering MTU along this path</refpurpose>
</refnamediv>
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<refsynopsisdiv id='synopsis'>
<cmdsynopsis>
  <command>tracepath</command>
    <arg choice='opt'>-4 </arg>
    <arg choice='opt'>-6 </arg>
    <arg choice='opt'>-n </arg>
    <arg choice='opt'>-b </arg>
    <arg choice='opt'>-l <replaceable>pktlen</replaceable></arg>
    <arg choice='opt'>-m <replaceable>max_hops</replaceable></arg>
    <arg choice='opt'>-p <replaceable>port</replaceable></arg>
    <arg choice='plain'><replaceable>destination</replaceable></arg>
    <sbr/>
  </cmdsynopsis>
</refsynopsisdiv>


<refsect1 id='description'>
  <title>DESCRIPTION</title>
  <para>It traces path to <emphasis remap='I'>destination</emphasis> discovering MTU along this path.
It uses UDP port <emphasis remap='I'>port</emphasis> or some random port.
It is similar to <emphasis remap='B'>traceroute</emphasis>, only does not require superuser
privileges and has no fancy options.</para>

  <para><emphasis remap='B'>tracepath6</emphasis> is good replacement for <emphasis remap='B'>traceroute6</emphasis>
and classic example of application of Linux error queues.
The situation with IPv4 is worse, because commercial
IP routers do not return enough information in ICMP error messages.
Probably, it will change, when they will be updated.
For now it uses Van Jacobson's trick, sweeping a range
of UDP ports to maintain trace history.</para>
</refsect1>

<refsect1 id='options'>
  <title>OPTIONS</title>
<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>-n</option></term>
  <listitem>
<para>Print primarily IP addresses numerically.</para>
  </listitem>
  </varlistentry>
  <varlistentry>
  <term><option>-b</option></term>
  <listitem>
<para>Print both of host names and IP addresses.</para>
  </listitem>
  </varlistentry>
  <varlistentry>
  <term><option>-l</option></term>
  <listitem>
<para>Sets the initial packet length to <emphasis remap='I'>pktlen</emphasis> instead of
65535 for <command>tracepath</command> or 128000 for <emphasis remap='B'>tracepath6</emphasis>.</para>
  </listitem>
  </varlistentry>
  <varlistentry>
  <term><option>-m</option></term>
  <listitem>
<para>Set maximum hops (or maximum TTLs) to <emphasis remap='I'>max_hops</emphasis>
instead of 30.</para>
  </listitem>
  </varlistentry>
  <varlistentry>
  <term><option>-p</option></term>
  <listitem>
<para>Sets the initial destination port to use.</para>
  </listitem>
  </varlistentry>
</variablelist>
</refsect1>

<refsect1 id='output'>
  <title>OUTPUT</title>
  <literallayout remap='.nf'>
root@mops:~ # tracepath6 3ffe:2400:0:109::2
 1?: [LOCALHOST]                              pmtu 1500
 1:  dust.inr.ac.ru                   0.411ms
 2:  dust.inr.ac.ru        asymm  1   0.390ms pmtu 1480
 2:  3ffe:2400:0:109::2               463.514ms reached
     Resume: pmtu 1480 hops 2 back 2
</literallayout> <!-- .fi -->

<para>The first column shows TTL of the probe, followed by colon.
Usually value of TTL is obtained from reply from network,
but sometimes reply does not contain necessary information and
we have to guess it. In this case the number is followed by ?.</para>

<para>The second column shows the network hop, which replied to the probe.
It is either address of router or word [LOCALHOST], if
the probe was not sent to the network.</para>

<para>The rest of line shows miscellaneous information about path to
the correspinding network hop. As rule it contains value of RTT.
Additionally, it can show Path MTU, when it changes.
If the path is asymmetric
or the probe finishes before it reach prescribed hop, difference
between number of hops in forward and backward direction is shown
following keyword async. This information is not reliable.
F.e. the third line shows asymmetry of 1, it is because the first probe
with TTL of 2 was rejected at the first hop due to Path MTU Discovery.</para>

<para>The last line summarizes information about all the path to the destination,
it shows detected Path MTU, amount of hops to the destination and our
guess about amount of hops from the destination to us, which can be
different when the path is asymmetric.</para>
</refsect1>

<refsect1 id='see_also'>
  <title>SEE ALSO</title>
  <para><citerefentry><refentrytitle>traceroute</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>traceroute6</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>ping</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
</refsect1>

<refsect1 id='author'>
  <title>AUTHOR</title>
  <para><command>tracepath</command> was written by
Alexey Kuznetsov &lt;kuznet@ms2.inr.ac.ru&gt;.</para>
</refsect1>

<refsect1 id='security'>
  <title>SECURITY</title>
  <para>No security issues.</para>

  <para>This lapidary deserves to be elaborated.
<command>tracepath</command> is not a privileged program, unlike
<emphasis remap='B'>traceroute</emphasis>, <emphasis remap='B'>ping</emphasis> and other beasts of this kind.
<command>tracepath</command> may be executed by everyone who has some access
to network, enough to send UDP datagrams to investigated destination
using given port.</para>
</refsect1>

<refsect1 id='availability'>
  <title>AVAILABILITY</title>
  <para><command>tracepath</command> is part of <emphasis remap='I'>iputils</emphasis> package.</para>
</refsect1>
</refentry>