traceroute

In computing, traceroute and tracert are computer network diagnostic commands for displaying possible routes (paths) 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 (usually 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.

This article is about the computer network diagnostic tool. For other uses, see Traceroute (disambiguation).

traceroute
The traceroute command
Original author(s)Van Jacobson
Initial release1987 (1987)
PlatformUnix-like systems
TypeCommand
tracert
Developer(s)Microsoft, ReactOS Contributors
PlatformWindows, ReactOS
TypeCommand
LicenseMicrosoft Windows: Proprietary commercial software
ReactOS: GNU General Public License

For Internet Protocol Version 6 (IPv6) the tool sometimes has the name traceroute6 or tracert6.

Implementations

The command traceroute is available on many modern operating systems. On Unix-like systems such as FreeBSD, macOS, and Linux it is available as a command line tool. Traceroute is also graphically accessible in macOS within the Network Utilities suite.

Microsoft Windows and ReactOS provide a program named tracert that performs the same route-tracing function. Windows NT-based operating systems also provide PathPing, with similar functionality. The ReactOS version was developed by Ged Murphy and is licensed under the GPL.[1]

On Unix-like operating systems, traceroute sends, by default, a sequence of User Datagram Protocol (UDP) packets, with destination port numbers ranging from 33434 to 33534; the implementations of traceroute shipped with Linux,[2] FreeBSD,[3] NetBSD,[4] OpenBSD,[5] DragonFly BSD,[6] and macOS include an option to use ICMP Echo Request packets (-I), or any arbitrary protocol (-P) such as UDP, TCP using TCP SYN packets, or ICMP.[7]

On Windows, tracert sends ICMP Echo Request packets, rather than the UDP packets traceroute sends by default.[8]

The time-to-live (TTL) value, also known as hop limit, is used in determining the intermediate routers being traversed towards the destination. Traceroute sends packets with TTL values that gradually increase from packet to packet, starting with TTL value of one. 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.[7] For the first set of packets, 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 Destination Unreachable message if UDP packets are being used or an ICMP Echo Reply message if ICMP Echo messages are being used.[7]

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.

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 (lft). 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.

On Linux, tracepath is a utility similar to traceroute, with the primary difference of not requiring superuser privileges.[9]

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. An extended version of this command (known as the extended traceroute command) can change the destination port number used by the UDP probe messages.[10]

Usage
Router addresses can be superimposed upon maps of their physical locations. This example shows a request from New Zealand to an IP address in Massachusetts which takes a route that passes through Europe.

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. An 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. 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, each mirror can be traced to get an idea of which mirror would be the fastest to use.

Origins

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.[11]

Limitations

Traceroute limitations[12] are well known and should be taken into account when using the tool. For example, traceroute does not discover paths at the router level, but at the interface level. Another limitation appears when routers do not respond to probes or when routers have a limit for ICMP responses.[13] In the presence of traffic load balancing, traceroute may indicate a path that does not actually exist; to minimize this problem there is a traceroute modification called Paris-traceroute,[14] which maintains the flow identifier of the probes to avoid load balancing.

gollark: Ah, I see.
gollark: Wait, how come it's `[8]` and not passing 8 as an actual parameter?
gollark: It's in-place.
gollark: Maybe it should be `res[8..<^1]` but who knows.
gollark: Perhaps its random byte generation is isomorphic to cryoapioids. Or the timestamp is wrong. Or ???.

See also
  • 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)

References
  1. https://github.com/reactos/reactos/blob/master/base/applications/network/tracert/tracert.cpp
  2. traceroute(8) – Linux Programmer's Manual – Administration and Privileged Commands
  3. traceroute(8) – FreeBSD System Manager's Manual
  4. traceroute(8) – NetBSD System Manager's Manual
  5. traceroute(8) – OpenBSD System Manager's Manual
  6. traceroute(8) – DragonFly BSD System Maintenance and Operation Commands Manual
  7. Comer, Douglas (2004). Computer Network and Internets with Internet Applications. Pearson Education, Inc. pp. 360–362. ISBN 978-0131433519.
  8. "Tracert". Retrieved April 17, 2016.
  9. "tracepath(8) – Linux man page". linux.die.net. Retrieved 2015-06-21.
  10. "Understanding the Ping and Traceroute Commands". Cisco IOS Software Releases 12.1 Mainline. cisco.com. 2006-11-29. Retrieved 2013-12-08.
  11. The Story of the PING Program
  12. Ray Belleville (March 3, 2017). "Limitations of Traceroute Explained".
  13. Marchetta, P.; Montieri, A.; Persico, V.; Pescapé, A.; Cunha, Í; Katz-Bassett, E. (June 2016). "How and how much traceroute confuses our understanding of network paths". 2016 IEEE International Symposium on Local and Metropolitan Area Networks (LANMAN): 1–7. doi:10.1109/lanman.2016.7548847. ISBN 978-1-4673-9882-4.
  14. "Paris Traceroute". paris-traceroute.net. Retrieved 2017-06-10.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

Further reading
  • Frisch, Æleen (2001). Windows 2000 Commands Pocket Reference. O'Reilly. ISBN 978-0-596-00148-3.
  • Stanek, William R. (2008). Windows Command-Line Administrator's Pocket Consultant, 2nd Edition. Microsoft Press. ISBN 978-0735622623.
  • Barrett, Daniel J. (2012). Macintosh Terminal Pocket Guide: Take Command of Your Mac. O'Reilly. ISBN 978-1449328986.
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