FindMTU v0.9

Introduction

FindMTU is a tool that performs IPv6 path MTU discovery. You can use it
to debug network problems and to detect IPv6-in-IPv4 tunnels in the path
to a destination.

FindMTU only performs IPv6 path MTU discovery. It does not know about
IPv4.


Supported platforms

FindMTU has been tested on Linux 2.2 and 2.4 and on FreeBSD 4.6. Under
Linux, it uses Linux-specific interfaces to obtain socket error messages
as a normal user. Under FreeBSD, it uses raw sockets and must be run as
root. The raw sockets mode should run under other operating systems as
well, but this has not been tested.

NOTE: if you are planning to use FindMTU on FreeBSD, you must be aware
that FreeBSD (some versions at least) stores path MTU information in
cloned host routes that never expire, so you will see only get valid
results the first time you run FindMTU on a particular host, and all
subsequent invocations will return no information. To work around this,
delete the cloned host route before running FindMTU: see the file
README.freebsd for an example.


Compilation

To compile FindMTU, select the appropriate makefile (Makefile.linux
under Linux, Makefile.rawsocket otherwise), rename or symlink it to
Makefile, and type make.


Usage

To use FindMTU, just run:

./findmtu <hostname>
or
./findmtu <IPv6 address>

(Note that if you are not using Linux, you must be root.)

FindMTU outputs a single line. The first number is the path MTU to the
destination; the brackets provide details on what packets it received.
This includes both Packet Too Big errors (an IPv6 address followed by
the MTU) and other ICMP errors (an IPv6 address followed by a brief
description of the error; "reached" indicates the destination was
reached and replied with a port unreachable error).


Credits

FindMTU was written by Lorenzo Colitti <lorenzo@colitti.com>. Most of
the Linux code was lifted from Alexey Kuznetsov's tracepath6. Thanks
to Mark Santcroos <marks@ripe.net> for helping with the raw sockets.


License

FindMTU is distributed under the GPL. See the file COPYING for details.

                             FindMTU v0.9

Using FindMTU on FreeBSD

Some (all?) versions of FreeBSD store path MTU information by setting
up a cloned host route for every destination the kernel sends packets
to. This route is then used to cache path MTU information for the
destination host. Unfortunately, at least in FreeBSD 4.6, which is the
version we tested on, these cloned routes never expire (except after
a reboot, presumably).

This means that FindMTU will output valid results only the first time
it is run on a particular host, and all subsequent invocations will
return no information. The problem is componded by the fact that
there is no interface I know of to tell the kernel not to fragment an
IPv6 packet, so large packets will simply get fragmented by the kernel
when FindMTU attempts to send them. So FindMTU will receive no Packet
Too Big messages and wrongly presume that the MTU is the default value
of 1500.

Example:

# ./bin/findmtu 2001:760::6
1460 (2001:798:20:200::2 1480, 2001:798:20:200::2 1460, 2001:760::6 reached)
# ./bin/findmtu 2001:760::6
1500 (2001:760::6 reached)
#

To work around this, the cloned host route must be deleted. FindMTU
could do this itself by opening a routing socket, but currently it
doesn't, so you have to do it by hand. Something like this will do
fine:

	# Check there is no explicit route to the destination
	if netstat -rn -f inet6 | grep -qv $1; then
		# Delete host route. As it is not in the routing table,
		# we are sure that it is a cloned route and can be deleted
		# safely
		route delete -inet6 $i > /dev/null 2> /dev/null
	fi
	./findmtu $1