<refentry xmlns="http://docbook.org/ns/docbook" version="5.0"
xml:id="man.tracepath">
<refentryinfo>
<title>tracepath</title>
<productname>iputils</productname>
</refentryinfo>
<refmeta>
<refentrytitle><application>tracepath</application></refentrytitle>
<manvolnum>8</manvolnum>
<refmiscinfo class='manual'>iputils</refmiscinfo>
</refmeta>
<refnamediv>
<refname><application>tracepath</application></refname>
<refpurpose>traces path to a network host discovering MTU along
this path</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis sepchar=" ">
<command>tracepath</command>
<arg choice="opt" rep="norepeat">
<option>-4</option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-6</option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-n</option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-b</option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-l
<replaceable>pktlen</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-m
<replaceable>max_hops</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-p
<replaceable>port</replaceable></option>
</arg>
<arg choice="opt" rep="norepeat">
<option>-V</option>
</arg>
<arg choice="req" rep="norepeat">destination</arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsection xml:id="description">
<info>
<title>DESCRIPTION</title>
</info>
<para>It traces the network 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>. However, it does not
require superuser privileges and has no fancy options.</para>
<para>
<emphasis remap='B'>tracepath -6</emphasis> is a 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 are updated. For now it uses Van Jacobson's
trick, sweeping a range of UDP ports to maintain trace
history.</para>
</refsection>
<refsection xml:id="options">
<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>-n</option>
</term>
<listitem>
<para>Print primarily IP addresses numerically.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-b</option>
</term>
<listitem>
<para>Print both: 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
<emphasis remap='B'>IPv4</emphasis> or 128000 for
<emphasis remap='B'>IPv6</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>
<varlistentry>
<term>
<option>-V</option>
</term>
<listitem>
<para>Print version and exit.</para>
</listitem>
</varlistentry>
</variablelist>
</refsection>
<refsect1 id='output'>
<title>OUTPUT</title>
<literallayout remap='.nf'>
root@mops:~ # tracepath -6 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>
<para>The first column shows the TTL of the probe, followed by
colon. Usually the value of TTL is obtained from the reply from
the network, but sometimes it does not contain the 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 the address of the router or the word
[LOCALHOST], if the probe was not sent to the network.</para>
<para>The rest of the line shows miscellaneous information about
the path to the corresponding network hop. It contains
the value of RTT, and additionally it can show Path MTU when it
changes. If the path is asymmetric or the probe finishes before
it reaches the prescribed hop, the number of hops in return
direction is shown next to the keyword "asymm". This information
is not reliable, e.g. the third line shows asymmetry of 1. This
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 paths
to the destination. It shows detected Path MTU, amount of hops
to the destination and our guess about the number of hops from
the destination to us, which can be different when the path is
asymmetric.</para>
</refsect1>
<refsect1 id='handling_errors'>
<title>HANDLING ERRORS</title>
<para>In case of errors <command>tracepath</command> prints short
error code.</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Output</entry>
<entry>Code</entry>
<entry>Meaning</entry>
</row>
</thead>
<tfoot>
<row>
<entry>!A</entry>
<entry>EACCES</entry>
<entry>Communication administratively prohibited</entry>
</row>
<row>
<entry>!H</entry>
<entry>EHOSTUNREACH</entry>
<entry>Destination host unreachable</entry>
</row>
<row>
<entry>!N</entry>
<entry>ENETUNREACH</entry>
<entry>Destination network unreachable</entry>
</row>
<row>
<entry>!P</entry>
<entry>EPROTO</entry>
<entry>Destination protocol unreachable</entry>
</row>
<row>
<entry>pmtu N</entry>
<entry>EMSGSIZE</entry>
<entry>Message too long</entry>
</row>
<row>
<entry>reached</entry>
<entry>ECONNREFUSED</entry>
<entry>Connection refused</entry>
</row>
<row>
<entry></entry>
<entry>ETIMEDOUT</entry>
<entry>Connection timed out</entry>
</row>
<row>
<entry>NET ERROR N</entry>
<entry></entry>
<entry>Any other error</entry>
</row>
</tfoot>
</tgroup>
</informaltable>
</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
<kuznet@ms2.inr.ac.ru>.</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
<command>traceroute</command>,
<command>ping</command> and other beasts of their kind.
<command>tracepath</command> may be executed by everyone who has
enough access to the network to send UDP datagrams to the
desired destination using the 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>