pathmtu/pathmtu.c: Document the correct short option for `--version'.

[tripe] / pathmtu / pathmtu.1.in

.\" -*-nroff-*-
.\"
.\" Documentation for pathmtu
.\"
.\" (c) 2008 Straylight/Edgeware.
.\"
.
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.\"--------------------------------------------------------------------------
.so ../common/defs.man \" @@@PRE@@@
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.\"--------------------------------------------------------------------------
.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 [ \-H
.IR header ]
.RB [ \-m
.IR method ]
.br
	\c
.RB [ \-r
.IR retransmit ]
.RB [ \-g
.IR factor ]
.RB [ \-t
.IR timeout ]
.br
	\c
.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
The
.B pathmtu
program attempts to find a correct answer even if ICMP
fragmentation-required packets are suppressed.  It distinguishes between
the remote host dropping packets and an intermediate router failing to
report fragmentation-needed errors by sending a minimum-size packet and
seeing whether it gets any response to that.
.PP
The
.B pathmtu
program (currently) contains two different methods for MTU probing.  One
uses the Linux-specific
.B IP_MTU
and
.B IP_MTU_DISCOVER
socket options; this works fine even as an unprivileged user.  The other
uses raw sockets, so it's fairly portable, but
.B pathmtu
must be installed setuid-root to work.  (It attempts to create its raw
sockets as its first action \(en before processing the command line \(en
and drops privileges immediately afterwards, so the attack surface is
very tiny.)  The raw sockets method is very slightly more robust:
specifically, it's much less likely to get confused by delayed 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 "\-g, \-\-growth=" factor
Sets the retransmit interval growth factor.  Each time a packet is
retransmitted,
.B pathmtu
increases the amount of time it waits before retransmitting again by
this
.IR factor .
The default growth factor is 3.
.TP
.BI "\-m, \-\-method=" name
Select the MTU probing method.  The available methods are shown by
.BR \-\-help .
The
.B linux
method is Linux-specific and might be confused by delayed errors under
some circumstances, but it's usable by unprivileged users; the
.B raw
method is portable but requires
.B pathmtu
to be installed setuid-root.
.TP
.BI "\-r, \-\-retransmit=" interval
Sets the initial retransmit interval, in seconds.  If no reply is
received to a probe within the interval, then a second packet is sent,
and the retransmit interval increased by the growth factor (see
.BR \-g ).
The default initial retransmit interval is 0.333 seconds.
.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 no reply will be
forthcoming.  If we've ever received a reply from the remote host in the
past, then
.B pathmtu
assumes that a timeout indicates that the packet was too large, but the
ICMP fragmentation-required error was suppressed as a result of
administrative incompetence by someone responsible for an intermediate
router.  Otherwise,
.B pathmtu
sends a small packet to settle the question of where packets are being
dropped: if it doesn't receive a response to this packet either, then it
assumes that the timeout means that the remote host
.I did
receive the packet.  The default timeout is 8 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.  A sequence number
(in order to disambiguate replies) and some pseudorandom data are
appended to the header.  The default header is empty.
.
.\"--------------------------------------------------------------------------
.SH "BUGS"
.
The whole business of probing path MTUs is rather unpleasant.
.
.\"--------------------------------------------------------------------------
.SH "AUTHOR"
.
Mark Wooding, <mdw@distorted.org.uk>
.
.\"----- That's all, folks --------------------------------------------------