traceroute command

Purpose

Prints the route that IP packets take to a network host.

Syntax

traceroute [ -hops Hops ] [ -num ] [ -port Port ] [ -src Address ] Host [ PacketSize ]

Description

The traceroute command attempts to trace the route an IP packet follows to an Internet host by launching UDP probe packets with a small maximum time-to-live (Hops parameter), then listening for an ICMP TIME_EXCEEDED response from gateways along the way. Probes are started with a Hops value of one hop, which is increased one hop at a time until an ICMP PORT_UNREACHABLE message is returned. The ICMP PORT_UNREACHABLE message indicates either that the host has been located or the command has reached the maximum number of hops allowed for the trace.

The traceroute command sends three probes at each Hops setting to record the following:

Hops value
Address of the gateway
Round-trip time of each successful probe

If the probe answers come from different gateways, the command prints the address of each responding system. If there is no response from a probe within a 3-second time-out interval, an * (asterisk) is printed for that probe.

Note: The traceroute command is intended for use in network testing, measurement, and management. It should be used primarily for manual fault isolation. Because of the load it imposes on the network, the traceroute command should not be used during normal operations or from automated scripts.

The traceroute command prints an ! (exclamation mark) after the round-trip time if the Hops value is one hop or less. A maximum time-to-live value of one hop or less generally indicates an incompatibility in the way ICMP replies are handled by different network software. The incompatibility can usually be resolved by doubling the last Hops value used and trying again.

Other possible annotations after the round-trip notation are as follows:

!H: Host unreachable
!N: Network unreachable
!P: Protocol unreachable
!S: Source route failed
!F: Fragmentation needed

If the majority of probes result in an error, the traceroute command exits.

The only mandatory parameter for the traceroute command is the destination host name or IP number. The traceroute command will determine the length of the probe packet based on the Maximum Transmission Unit (MTU) of the outgoing interface. The UDP probe packets are set to an unlikely value so as to prevent processing by the destination host.

Flags

-hops Max_ttl	Sets the maximum time-to-live (maximum number of hops) used in outgoing probe packets. The default is 30 hops (the same default used for TCP connections).
-num	Prints hop addresses numerically rather than symbolically and numerically. This flag saves a name-server address-to-name lookup for each gateway found on the path.
-port Port	Sets the base UDP port number used in probes. The default is 33434. The traceroute command depends on an open UDP port range of base to base + nhops - 1 at the destination host. If a UDP port is not available, this option can be used to pick an unused port range.
-src SRC_Addr	Uses the next IP address in numerical form as the source address in outgoing probe packets. On hosts with more than one IP address, the -src flag can be used to force the source address to be something other than the IP address of the interface on which the probe packet is sent. If the next IP address is not one of the machine's interface addresses, an error is returned and nothing is sent.

Parameters

Host	Specifies the destination host, either by host name or IP number. This parameter is required.
PacketSize	Specifies the probe datagram length. The default packet size is determined by the traceroute command based on the MTU of the outgoing interface.

Exit Status

See Exit status for Virtual I/O Server commands.

Examples

To print the route to host nis.nfs.net, type:

traceroute nis.nsf.net

The output will look similar to the following:

traceroute to rotterdam (35.1.1.48), 30 hops max, 56 byte packet
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

Related Information

The ping command and the optimizenet command.