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   6 .TH "TRACEPATH" "8" "07 May 2014" "iputils-121221" "System Manager's Manual: iputils"
   7 .SH NAME
   8 tracepath, tracepath6 \- traces path to a network host discovering MTU along this path
   9 .SH SYNOPSIS
  10
  11 \fBtracepath\fR [\fB-n\fR] [\fB-b\fR] [\fB-l \fIpktlen\fB\fR] [\fB-m \fImax_hops\fB\fR] [\fB-p \fIport\fB\fR] \fB\fIdestination\fB\fR
  12
  13 .SH "DESCRIPTION"
  14 .PP
  15 It traces path to \fIdestination\fR discovering MTU along this path.
  16 It uses UDP port \fIport\fR or some random port.
  17 It is similar to \fBtraceroute\fR, only does not require superuser
  18 privileges and has no fancy options.
  19 .PP
  20 \fBtracepath6\fR is good replacement for \fBtraceroute6\fR
  21 and classic example of application of Linux error queues.
  22 The situation with IPv4 is worse, because commercial
  23 IP routers do not return enough information in ICMP error messages.
  24 Probably, it will change, when they will be updated.
  25 For now it uses Van Jacobson's trick, sweeping a range
  26 of UDP ports to maintain trace history.
  27 .SH "OPTIONS"
  28 .TP
  29 \fB-n\fR
  30 Print primarily IP addresses numerically.
  31 .TP
  32 \fB-b\fR
  33 Print both of host names and IP addresses.
  34 .TP
  35 \fB-l\fR
  36 Sets the initial packet length to \fIpktlen\fR instead of
  37 65535 for \fBtracepath\fR or 128000 for \fBtracepath6\fR.
  38 .TP
  39 \fB-m/term> \fR
  40 Set maximum hops (or maximum TTLs) to \fImax_hops\fR
  41 instead of 30.
  42 .TP
  43 \fB-p\fR
  44 Sets the initial destination port to use.
  45 .SH "OUTPUT"
  46 .PP
  47
  48 .nf
  49 root@mops:~ # tracepath6 3ffe:2400:0:109::2
  50  1?: [LOCALHOST]                              pmtu 1500
  51  1:  dust.inr.ac.ru                   0.411ms
  52  2:  dust.inr.ac.ru        asymm  1   0.390ms pmtu 1480
  53  2:  3ffe:2400:0:109::2               463.514ms reached
  54      Resume: pmtu 1480 hops 2 back 2
  55 .fi
  56 .PP
  57 The first column shows TTL of the probe, followed by colon.
  58 Usually value of TTL is obtained from reply from network,
  59 but sometimes reply does not contain necessary information and
  60 we have to guess it. In this case the number is followed by ?.
  61 .PP
  62 The second column shows the network hop, which replied to the probe.
  63 It is either address of router or word [LOCALHOST], if
  64 the probe was not sent to the network.
  65 .PP
  66 The rest of line shows miscellaneous information about path to
  67 the correspinding network hop. As rule it contains value of RTT.
  68 Additionally, it can show Path MTU, when it changes.
  69 If the path is asymmetric
  70 or the probe finishes before it reach prescribed hop, difference
  71 between number of hops in forward and backward direction is shown
  72 following keyword async. This information is not reliable.
  73 F.e. the third line shows asymmetry of 1, it is because the first probe
  74 with TTL of 2 was rejected at the first hop due to Path MTU Discovery.
  75 .PP
  76 The last line summarizes information about all the path to the destination,
  77 it shows detected Path MTU, amount of hops to the destination and our
  78 guess about amount of hops from the destination to us, which can be
  79 different when the path is asymmetric.
  80 .SH "SEE ALSO"
  81 .PP
  82 \fBtraceroute\fR(8),
  83 \fBtraceroute6\fR(8),
  84 \fBping\fR(8).
  85 .SH "AUTHOR"
  86 .PP
  87 \fBtracepath\fR was written by
  88 Alexey Kuznetsov
  89 <kuznet@ms2.inr.ac.ru>.
  90 .SH "SECURITY"
  91 .PP
  92 No security issues.
  93 .PP
  94 This lapidary deserves to be elaborated.
  95 \fBtracepath\fR is not a privileged program, unlike
  96 \fBtraceroute\fR, \fBping\fR and other beasts of this kind.
  97 \fBtracepath\fR may be executed by everyone who has some access
  98 to network, enough to send UDP datagrams to investigated destination
  99 using given port.
 100 .SH "AVAILABILITY"
 101 .PP
 102 \fBtracepath\fR is part of \fIiputils\fR package
 103 and the latest versions are  available in source form at
 104 http://www.skbuff.net/iputils/iputils-current.tar.bz2.