1                             FindMTU v0.9
2
3Introduction
4
5FindMTU is a tool that performs IPv6 path MTU discovery. You can use it
6to debug network problems and to detect IPv6-in-IPv4 tunnels in the path
7to a destination.
8
9FindMTU only performs IPv6 path MTU discovery. It does not know about
10IPv4.
11
12
13Supported platforms
14
15FindMTU has been tested on Linux 2.2 and 2.4 and on FreeBSD 4.6. Under
16Linux, it uses Linux-specific interfaces to obtain socket error messages
17as a normal user. Under FreeBSD, it uses raw sockets and must be run as
18root. The raw sockets mode should run under other operating systems as
19well, but this has not been tested.
20
21NOTE: if you are planning to use FindMTU on FreeBSD, you must be aware
22that FreeBSD (some versions at least) stores path MTU information in
23cloned host routes that never expire, so you will see only get valid
24results the first time you run FindMTU on a particular host, and all
25subsequent invocations will return no information. To work around this,
26delete the cloned host route before running FindMTU: see the file
27README.freebsd for an example.
28
29
30Compilation
31
32To compile FindMTU, select the appropriate makefile (Makefile.linux
33under Linux, Makefile.rawsocket otherwise), rename or symlink it to
34Makefile, and type make.
35
36
37Usage
38
39To use FindMTU, just run:
40
41./findmtu <hostname>
42or
43./findmtu <IPv6 address>
44
45(Note that if you are not using Linux, you must be root.)
46
47FindMTU outputs a single line. The first number is the path MTU to the
48destination; the brackets provide details on what packets it received.
49This includes both Packet Too Big errors (an IPv6 address followed by
50the MTU) and other ICMP errors (an IPv6 address followed by a brief
51description of the error; "reached" indicates the destination was
52reached and replied with a port unreachable error).
53
54
55Credits
56
57FindMTU was written by Lorenzo Colitti <lorenzo@colitti.com>. Most of
58the Linux code was lifted from Alexey Kuznetsov's tracepath6. Thanks
59to Mark Santcroos <marks@ripe.net> for helping with the raw sockets.
60
61
62License
63
64FindMTU is distributed under the GPL. See the file COPYING for details.
65