TRACEROUTE(8) FreeBSD System Manager's Manual TRACEROUTE(8)
NAME
traceroute - print the route packets take to network host
SYNOPSIS
traceroute [-adDeEFISnrvx] [-A as_server] [-f first_ttl] [-g gateway]
[-i iface] [-m max_ttl] [-M first_ttl] [-p port] [-P proto]
[-q nprobes] [-s src_addr] [-t tos] [-w waittime]
[-z pausemsecs] host [packetlen]
DESCRIPTION
The Internet is a large and complex aggregation of network hardware,
connected together by gateways. Tracking the route one's packets follow
(or finding the miscreant gateway that's discarding your packets) can be
difficult. traceroute utilizes the IP protocol `time to live' field and
attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along
the path to some host.
The only mandatory parameter is the destination host name or IP number.
The default probe datagram length is 40 bytes, but this may be increased
by specifying a packet length (in bytes) after the destination host name.
Other options are:
-a Turn on AS# lookups for each hop encountered.
-A as_server
Turn on AS# lookups and use the given server instead of the
default.
-d Enable socket level debugging.
-D When an ICMP response to our probe datagram is received, print
the differences between the transmitted packet and the packet
quoted by the ICMP response. A key showing the location of
fields within the transmitted packet is printed, followed by the
original packet in hex, followed by the quoted packet in hex.
Bytes that are unchanged in the quoted packet are shown as
underscores. Note, the IP checksum and the TTL of the quoted
packet are not expected to match. By default, only one probe per
hop is sent with this option.
-e Firewall evasion mode. Use fixed destination ports for UDP, UDP-
Lite, TCP and SCTP probes. The destination port does NOT
increment with each packet sent.
-E Detect ECN bleaching. Set the IPTOS_ECN_ECT1 Explicit Congestion
Notification (ECN) bits (01), and report if the hop has bleached
(00) or mangled (10) them, or if it is experiencing congestion
(11). Otherwise, report that it passed the bits appropriately.
If -t is also specified, the corresponding ECN bits will be
replaced.
-f first_ttl
Set the initial time-to-live used in the first outgoing probe
packet.
-F Set the "don't fragment" bit.
-g gateway
Specify a loose source route gateway (8 maximum).
-i iface
Specify a network interface to obtain the source IP address for
outgoing probe packets. This is normally only useful on a multi-
homed host. (See the -s flag for another way to do this).
-I Use ICMP ECHO instead of UDP datagrams. (A synonym for "-P
icmp").
-m max_ttl
Set the max time-to-live (max number of hops) used in outgoing
probe packets. The default is the value of the net.inet.ip.ttl
sysctl(8) (the same default used for TCP connections).
-M first_ttl
Set the initial time-to-live value used in outgoing probe
packets. The default is 1, i.e., start with the first hop.
-n Print hop addresses numerically rather than symbolically and
numerically (saves a nameserver address-to-name lookup for each
gateway found on the path).
-p port
Protocol specific. For UDP, UDP-Lite, TCP and SCTP, sets the
base port number used in probes (default is 33434). Traceroute
hopes that nothing is listening on UDP ports (or UDP-Lite ports
if used by traceroute and supported by the peer) port + 1 to port
+ (max_ttl - first_ttl + 1) * nprobes at the destination host (so
an ICMP PORT_UNREACHABLE message will be returned to terminate
the route tracing). If something is listening on a port in the
default range, this option can be used to pick an unused port
range.
-P proto
Use packets of specified IP protocol when sending probes. The
proto argument may be one of the following:
udp Use udp(4) packets. This is the default.
icmp Use icmp(4) "echo request" packets.
udplite Use udplite(4) packets.
tcp Use tcp(4) "SYN" packets. This will cause a
successful traceroute to end with no response
(i.e., a "*" response) since traceroute does not
know how to detect the RST or SYN+ACK response from
the destination host.
sctp Use sctp(4) packets. The packetlen argument must
be a multiple of 4. SCTP probes will be
constructed as SCTP "INIT" chunks, unless the
packet length is too small, in which case the
probes will be SCTP "SHUTDOWN-ACK" chunks followed
by zero or one "PAD" chunks.
gre Use gre(4) packets. The GRE packets will be
constructed as if they contain a PPTP (Point-to-
Point Tunneling Protocol) payload.
Other protocols may also be specified, either by number or by
name (see protocols(5)), though traceroute does not implement any
special knowledge of their packet formats. This option is useful
for determining which router along a path may be blocking packets
based on IP protocol number. But see BUGS below.
-q nprobes
Set the number of probes per hop (default is 3, unless -D is
specified, when it is 1).
-r Bypass the normal routing tables and send directly to a host on
an attached network. If the host is not on a directly-attached
network, an error is returned. This option can be used to ping a
local host through an interface that has no route through it
(e.g., after the interface was dropped by routed(8).
-s src_addr
Use the following IP address (which usually is given as an IP
number, not a hostname) as the source address in outgoing probe
packets. On multi-homed hosts (those with more than one IP
address), this option can be used to force the source address to
be something other than the IP address of the interface the probe
packet is sent on. If the IP address is not one of this
machine's interface addresses, an error is returned and nothing
is sent. (See the -i flag for another way to do this).
-S Print a summary of how many probes were not answered for each
hop.
-t tos Set the type-of-service in probe packets to the following value
(default zero). The value must be a decimal integer in the range
0 to 255. This option can be used to see if different types-of-
service result in different paths. The upper six bits are the
Differentiated Services Codepoint (RFC4594). The lower two bits
are the Explicit Congestion Notification field (RFC3168).
-v Verbose output. Received ICMP packets other than TIME_EXCEEDED
and UNREACHABLEs are listed.
-w waittime
Set the time (in seconds) to wait for a response to a probe
(default 5 sec.).
-x Toggle ip checksums. Normally, this prevents traceroute from
calculating ip checksums. In some cases, the operating system
can overwrite parts of the outgoing packet but not recalculate
the checksum (so in some cases the default is to not calculate
checksums and using -x causes them to be calculated). Note that
checksums are usually required for the last hop when using ICMP
ECHO probes (-I). So they are always calculated when using ICMP.
-z pausemsecs
Set the time (in milliseconds) to pause between probes (default
0). Some systems such as Solaris and routers such as Ciscos rate
limit ICMP messages. A good value to use with this is 500 (e.g.,
1/2 second).
This program attempts to trace the route an IP packet would follow to
some internet host by launching UDP probe packets with a small TTL (time
to live) then listening for an ICMP "time exceeded" reply from a gateway.
We start our probes with a TTL of one and increase by one until we get an
ICMP "port unreachable" (which means we got to "host") or hit a max
(which defaults to the amount of hops specified by the net.inet.ip.ttl
sysctl(8) and can be changed with the -m flag). Three probes (change
with -q flag) are sent at each TTL setting and a line is printed showing
the TTL, address of the gateway and round trip time of each probe. If
the probe answers come from different gateways, the address of each
responding system will be printed. If there is no response within a 5
sec. timeout interval (changed with the -w flag), a "*" is printed for
that probe.
We don't want the destination host to process the UDP probe packets so
the destination port is set to an unlikely value (if some clod on the
destination is using that value, it can be changed with the -p flag).
A sample use and output might be:
% traceroute nis.nsf.net.
traceroute to nis.nsf.net (35.1.1.48), 64 hops max, 40 byte packets
1 helios.ee.lbl.gov (128.3.112.1) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 39 ms 39 ms 39 ms
6 128.32.197.4 (128.32.197.4) 40 ms 59 ms 59 ms
7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 59 ms
8 129.140.70.13 (129.140.70.13) 99 ms 99 ms 80 ms
9 129.140.71.6 (129.140.71.6) 139 ms 239 ms 319 ms
10 129.140.81.7 (129.140.81.7) 220 ms 199 ms 199 ms
11 nic.merit.edu (35.1.1.48) 239 ms 239 ms 239 ms
Note that lines 2 & 3 are the same. This is due to a buggy kernel on the
2nd hop system - lilac-dmc.Berkeley.EDU - that forwards packets with a
zero TTL (a bug in the distributed version of 4.3BSD). Note that you
have to guess what path the packets are taking cross-country since the
NSFNet (129.140) doesn't supply address-to-name translations for its
NSSes.
A more interesting example is:
% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (18.26.0.115), 64 hops max, 40 byte packets
1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 20 ms 39 ms 39 ms
6 128.32.197.4 (128.32.197.4) 59 ms 119 ms 39 ms
7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 39 ms
8 129.140.70.13 (129.140.70.13) 80 ms 79 ms 99 ms
9 129.140.71.6 (129.140.71.6) 139 ms 139 ms 159 ms
10 129.140.81.7 (129.140.81.7) 199 ms 180 ms 300 ms
11 129.140.72.17 (129.140.72.17) 300 ms 239 ms 239 ms
12 * * *
13 128.121.54.72 (128.121.54.72) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (18.26.0.115) 339 ms 279 ms 279 ms
Note that the gateways 12, 14, 15, 16 & 17 hops away either don't send
ICMP "time exceeded" messages or send them with a TTL too small to reach
us. 14 - 17 are running the MIT C Gateway code that doesn't send "time
exceeded"s. God only knows what's going on with 12.
The silent gateway 12 in the above may be the result of a bug in the
4.[23]BSD network code (and its derivatives): 4.x (x <= 3) sends an
unreachable message using whatever TTL remains in the original datagram.
Since, for gateways, the remaining TTL is zero, the ICMP "time exceeded"
is guaranteed to not make it back to us. The behavior of this bug is
slightly more interesting when it appears on the destination system:
1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (128.32.168.35) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (128.32.133.254) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (128.32.131.22) 59 ms ! 39 ms ! 39 ms !
Notice that there are 12 "gateways" (13 is the final destination) and
exactly the last half of them are "missing". What's really happening is
that rip (a Sun-3 running Sun OS3.5) is using the TTL from our arriving
datagram as the TTL in its ICMP reply. So, the reply will time out on
the return path (with no notice sent to anyone since ICMP's aren't sent
for ICMP's) until we probe with a TTL that's at least twice the path
length. I.e., rip is really only 7 hops away. A reply that returns with
a TTL of 1 is a clue this problem exists. traceroute prints a "!" after
the time if the TTL is <= 1. Since vendors ship a lot of obsolete (DEC's
Ultrix, Sun 3.x) or non-standard (HP-UX) software, expect to see this
problem frequently and/or take care picking the target host of your
probes.
Other possible annotations after the time are:
!H Host unreachable.
!N Network unreachable.
!P Protocol unreachable.
!S Source route failed.
!F-<pmtu> Fragmentation needed. The RFC1191 Path MTU Discovery
value is displayed.
!U Destination network unknown.
!W Destination host unknown.
!I Source host is isolated.
!A Communication with destination network
administratively prohibited.
!Z Communication with destination host administratively
prohibited.
!Q For this ToS the destination network is unreachable.
!T For this ToS the destination host is unreachable.
!X Communication administratively prohibited.
!V Host precedence violation.
!C Precedence cutoff in effect.
!<num> ICMP unreachable code <num>.
These are defined by RFC1812 (which supersedes RFC1716). If almost all
the probes result in some kind of unreachable, traceroute will give up
and exit.
This program is intended for use in network testing, measurement and
management. It should be used primarily for manual fault isolation.
Because of the load it could impose on the network, it is unwise to use
traceroute during normal operations or from automated scripts.
SEE ALSO
netstat(1), ping(8), traceroute6(8)
AUTHORS
Implemented by Van Jacobson from a suggestion by Steve Deering. Debugged
by a cast of thousands with particularly cogent suggestions or fixes from
C. Philip Wood, Tim Seaver and Ken Adelman.
BUGS
When using protocols other than UDP, functionality is reduced. In
particular, the last packet will often appear to be lost, because even
though it reaches the destination host, there's no way to know that
because no ICMP message is sent back. In the TCP case, traceroute should
listen for a RST from the destination host (or an intermediate router
that's filtering packets), but this is not implemented yet.
The AS number capability reports information that may sometimes be
inaccurate due to discrepancies between the contents of the routing
database server and the current state of the Internet.
FreeBSD 15.1-STABLE-HBSD May 14, 2025 TRACEROUTE(8)