.\" -*-nroff-*- .\" .\" Documentation for pathmtu .\" .\" (c) 2008 Straylight/Edgeware. .\" .\"----- Licensing notice --------------------------------------------------- .\" .\" This file is part of Trivial IP Encryption (TrIPE). .\" .\" TrIPE is free software; you can redistribute it and/or modify .\" it under the terms of the GNU General Public License as published by .\" the Free Software Foundation; either version 2 of the License, or .\" (at your option) any later version. .\" .\" TrIPE 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. .\" .\" You should have received a copy of the GNU General Public License .\" along with TrIPE; if not, write to the Free Software Foundation, .\" Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. . .\"-------------------------------------------------------------------------- .so ../common/defs.man \" @@@PRE@@@ . .\"-------------------------------------------------------------------------- .TH pathmtu 1 "29 December 2008" "Straylight/Edgeware" "TrIPE: Trivial IP Encryption" . .\"-------------------------------------------------------------------------- .SH "NAME" . pathmtu \- discover path MTU to a given host . .\"-------------------------------------------------------------------------- .SH "SYNOPSIS" . .B pathmtu .RB [ \-t .IR timeout ] .RB [ \-H .IR header ] .I host .RI [ port ] . .\"-------------------------------------------------------------------------- .SH "DESCRIPTION" . The .B pathmtu program discovers the size of the largest IP packet which can be sent to a given .I host (specified as a dotted-quad IP address or host name) without being fragmented. This is useful information, particularly when setting up VPN tunnel interfaces. .PP The program works by sending UDP packets and finding out whether they get fragmented. The packets are sent to a specified .I port (specified as a number or service name) on the destination host. The destination does not need to be listening on the given port \(en indeed, it doesn't matter if the port is firewalled. The default port is 7 (echo), chosen because if it is active, we'll get an answer. .PP If the local host or some intermediate router is configured to drop ICMP fragmentation-required errors then the discovery attempt will silently fail. It is likely that TCP connections with the destination host will fail in unexpected ways if this is the case. Don't drop fragmentation-required errors! .PP Command-line options are as follows. .TP .B "\-h, \-\-help" Writes a brief description of the command-line options available to standard output and exits with status 0. .TP .B "\-v, \-\-version" Writes tripe's version number to standard output and exits with status 0. .TP .B "\-u, \-\-usage" Writes a brief usage summary to standard output and exits with status 0. .TP .BI "\-t, \-\-timeout=" timeout Sets the time to wait for a reply, in seconds. If no reply or error is received within the timeout, it is assumed that the attempt to send a packet was successful. The timeout can be fractional; the default is five seconds. .TP .BI "\-H, \-\-header=" header Sets the packet header, in hexadecimal. If you set an explicit port number, it may be worth setting the packet header too, so as not to alarm anything which might be listening on that port. The default packet contents are a fixed pseudorandomly-generated block of data. . .\"-------------------------------------------------------------------------- .SH "BUGS" . The .B pathmtu program currently only works on Linux. Code for other operating systems is welcome. . .\"-------------------------------------------------------------------------- .SH "AUTHOR" . Mark Wooding, . .\"----- That's all, folks --------------------------------------------------