<refentry id="tracepath">
<refmeta>
<refentrytitle>tracepath</refentrytitle>
<manvolnum>8</manvolnum>
<refmiscinfo>iputils-&snapshot;</refmiscinfo>
</refmeta>
<refnamediv>
<refname>tracepath, tracepath6</refname>
<refpurpose>
traces path to a network host discovering MTU along this path</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>tracepath</command>
<arg choice="opt">-n</arg>
<arg choice="opt">-b</arg>
<arg choice="opt">-l <replaceable/pktlen/</arg>
<arg choice="opt">-m <replaceable/max_hops/</arg>
<arg choice="opt">-p <replaceable/port/</arg>
<arg choice="req"><replaceable/destination/</arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1><title>DESCRIPTION</title>
<para>
It traces path to <replaceable/destination/ discovering MTU along this path.
It uses UDP port <replaceable/port/ or some random port.
It is similar to <command/traceroute/, only does not require superuser
privileges and has no fancy options.
</para>
<para>
<command/tracepath6/ is good replacement for <command/traceroute6/
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><title>OPTIONS</title>
<variablelist>
<varlistentry>
<term><option/-n/</term>
<listitem><para>
Print primarily IP addresses numerically.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option/-b/</term>
<listitem><para>
Print both of host names and IP addresses.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option/-l/</term>
<listitem><para>
Sets the initial packet length to <replaceable/pktlen/ instead of
65535 for <command/tracepath/ or 128000 for <command/tracepath6/.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option/-m</term>
<listitem><para>
Set maximum hops (or maximum TTLs) to <replaceable/max_hops/
instead of 30.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option/-p/</term>
<listitem><para>
Sets the initial destination port to use.
</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1><title>OUTPUT</title>
<para>
<literallayout>
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>
</para>
<para>
The first column shows <literal/TTL/ of the probe, followed by colon.
Usually value of <literal/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 <literal/[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 <literal/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><title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle/traceroute/<manvolnum/8/</citerefentry>,
<link linkend="traceroute6">
<citerefentry><refentrytitle/traceroute6/<manvolnum/8/</citerefentry></link>,
<link linkend="ping">
<citerefentry><refentrytitle/ping/<manvolnum/8/</citerefentry></link>.
</para>
</refsect1>
<refsect1><title>AUTHOR</title>
<para>
<command/tracepath/ was written by
<ulink url="mailto:kuznet@ms2.inr.ac.ru">Alexey Kuznetsov
<kuznet@ms2.inr.ac.ru></ulink>.
</para>
</refsect1>
<refsect1><title>SECURITY</title>
<para>
No security issues.
</para>
<para>
This lapidary deserves to be elaborated.
<command/tracepath/ is not a privileged program, unlike
<command/traceroute/, <command/ping/ and other beasts of this kind.
<command/tracepath/ 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><title>AVAILABILITY</title>
<para>
<command/tracepath/ is part of <filename/iputils/ package
and the latest versions are available in source form at
<ulink url="http://www.skbuff.net/iputils/iputils-current.tar.bz2">
http://www.skbuff.net/iputils/iputils-current.tar.bz2</ulink>.
</para>
</refsect1>
<![IGNORE[
<refsect1><title>COPYING</title>
<para>
<literallayout>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License Version 2.
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
For more details see the file COPYING in the source
distribution of Linux kernel of version 2.4.
</literallayout>
</para>
</refsect1>
]]>
</refentry>