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