In computing, traceroute is a computer network diagnostic tool for displaying the route (path) and measuring transit delays of packets across an Internet Protocol (IP) network. The history of the route is recorded as the round-trip times of the packets received from each successive host (remote node) in the route (path); the sum of the mean times in each hop is a measure of the total time spent to establish the connection. Traceroute proceeds unless all (three) sent packets are lost more than twice, then the connection is lost and the route cannot be evaluated. Ping, on the other hand, only computes the final round-trip times from the destination point.
The command traceroute is available on many modern operating systems. On Apple macOS, it is available by opening the menu Network Utilities and selecting Traceroute, as well as from the command line interface in a terminal. On other Unix systems, such as FreeBSD or Linux, it is also available as a command line tool. On Microsoft Windows, it is named tracert. Windows NT-based operating systems also provide PathPing, with similar functionality. For Internet Protocol Version 6 (IPv6) the tool sometimes has the name traceroute6 or tracert6.
In Linux, traceroute by default sends a sequence of User Datagram Protocol (UDP) packets addressed to a destination host; ICMP Echo Request or TCP SYN packets can also be used. In Windows, traceroute sends ICMP echo requests instead of UDP packets. The time-to-live (TTL) value, also known as hop limit, is used in determining the intermediate routers being traversed towards the destination. Routers decrement TTL values of packets by one when routing and discard packets whose TTL value has reached zero, returning the ICMP error message ICMP Time Exceeded. Common default values for the initial TTL are 128 (Windows OS) and 64 (Unix-based OS).
Traceroute works by sending packets with gradually increasing TTL value, starting with TTL value of one. The first router receives the packet, decrements the TTL value and drops the packet because it then has TTL value zero. The router sends an ICMP Time Exceeded message back to the source. The next set of packets are given a TTL value of two, so the first router forwards the packets, but the second router drops them and replies with ICMP Time Exceeded. Proceeding in this way, traceroute uses the returned ICMP Time Exceeded messages to build a list of routers that packets traverse, until the destination is reached and returns an ICMP Echo Reply message.
The timestamp values returned for each router along the path are the delay (latency) values, typically measured in milliseconds for each packet.
The sender expects a reply within a specified number of seconds. If a packet is not acknowledged within the expected interval, an asterisk is displayed. The Internet Protocol does not require packets to take the same route towards a particular destination, thus hosts listed might be hosts that other packets have traversed. If the host at hop #N does not reply, the hop is skipped in the output.
On Unix-like operating systems, traceroute employs User Datagram Protocol (UDP) datagrams by default, with destination port numbers ranging from 33434 to 33534. The traceroute utility usually has an option to instead use ICMP Echo Request (type 8) packets, like the Windows utility tracert does, or to use TCP SYN packets. If a network has a firewall and operates both Windows and Unix-like systems, more than one protocol must be enabled inbound through the firewall for traceroute to work and receive replies.
Some traceroute implementations use TCP packets, such as tcptraceroute and layer four traceroute. PathPing is a utility introduced with Windows NT that combines ping and traceroute functionality. MTR is an enhanced version of ICMP traceroute available for Unix-like and Windows systems. The various implementations of traceroute all rely on ICMP Time Exceeded (type 11) packets being sent to the source.
The implementations of traceroute shipped with Linux, FreeBSD, NetBSD, OpenBSD, DragonFly BSD, and macOS include an option to use ICMP Echo packets (-I), or any arbitrary protocol (-P) such as UDP, TCP or ICMP. On Linux, tracepath is a utility similar to traceroute, with the primary difference of not requiring superuser privileges.
Cisco's implementation of traceroute also uses a sequence of UDP datagrams, each with incrementing TTL values, to an invalid port number at the remote host; by default, UDP port 33434 is used. Extended version of this command (known as the extended traceroute command) can change the destination port number used by the UDP probe messages.
Most implementations include at least options to specify the number of queries to send per hop, time to wait for a response, the hop limit and port to use. Invoking traceroute with no specified options displays the list of available options, while man traceroute presents more details, including the displayed error flags. Simple example on Linux:
$ traceroute -w 3 -q 1 -m 16 example.com
In the example above, selected options are to wait for three seconds (instead of five), send out only one query to each hop (instead of three), limit the maximum number of hops to 16 before giving up (instead of 30), with example.com as the final host.
This can help identify incorrect routing table definitions or firewalls that may be blocking ICMP traffic, or high port UDP in Unix ping, to a site. Note that a firewall may permit ICMP packets but not permit packets of other protocols.
Traceroute is also used by penetration testers to gather information about network infrastructure and IP ranges around a given host.
It can also be used when downloading data, and if there are multiple mirrors available for the same piece of data, one can trace each mirror to get a good idea of which mirror would be the fastest to use.
The traceroute manual page states that the original traceroute program was written by Van Jacobson in 1987 from a suggestion by Steve Deering, with particularly cogent suggestions or fixes from C. Philip Wood, Tim Seaver and Ken Adelman. The author of the ping program, Mike Muuss, states on his website that traceroute was written using kernel ICMP support that he had earlier coded to enable raw ICMP sockets when he first wrote the ping program.
- Looking Glass server
- MTR (software) – computer software which combines the functionality of the traceroute and ping programs in a single network diagnostic tool
- netsniff-ng – a Linux networking toolkit with an autonomous system traceroute utility
- PathPing – a Windows NT network utility that combines the functionality of ping with that of traceroute (or tracert)
- "traceroute(8) - Linux man page". linux.die.net. Retrieved 2014-02-26.
- "Tracert". Retrieved April 17, 2016.
- Comer, Douglas (2004). Computer Network and Internets with Internet Applications. Pearson Education, Inc. pp. 360–362. ISBN 0131433512.
- "tracepath(8) – Linux man page". linux.die.net. Retrieved 2015-06-21.
- "Understanding the Ping and Traceroute Commands". Cisco IOS Software Releases 12.1 Mainline. cisco.com. 2006-11-29. Retrieved 2013-12-08.
- The Story of the PING Program