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traceroute outputs the list of traversed routers in simple text format, together with timing information

traceroute is a computer network tool used to determine the route taken by packets across an IP network. An IPv6 variant, traceroute6, is also widely available.

The traceroute tool is available on practically all Unix-like operating systems. Variants with similar functionality are also available, such as tracepath on modern Linux installations and tracert on Microsoft Windows operating systems. Windows NT-based operating systems also provide pathping, which provides similar functionality.

Traceroute works by increasing the "time-to-live" value of each successive batch of packets sent. The first three packets have a time-to-live (TTL) value of one (implying that they make a single hop). The next three packets have a TTL value of 2, and so on. When a packet passes through a host, normally the host decrements the TTL value by one, and forwards the packet to the next host. When a packet with a TTL of one reaches a host, the host discards the packet and sends an ICMP time exceeded (type 11) packet to the sender. The traceroute utility uses these returning packets to produce a list of hosts that the packets have traversed en route to the destination. The three timestamp values returned for each host along the path are the delay (aka latency) values typically in milliseconds (ms) for each packet in the batch. If a packet does not return within the expected timeout window, a star (asterisk) is traditionally printed. traceroute may not list the real hosts. It indicates that the first host is at one hop, the second host at two hops, etc. IP does not guarantee that all the packets take the same route.

On modern Unix and Linux-based operating systems, the traceroute utility by default uses UDP datagrams with a destination port number starting at 33434. The traceroute utility usually has an option to specify use of ICMP echo request (type 8) instead, as used by the Windows tracert utility.

There are also traceroute implementations that use TCP packets, such as tcptraceroute or Layer Four Trace. pathping is a utility introduced with Windows NT that combines ping and traceroute functionality. mtr (My traceroute) is an enhanced version of ICMP traceroute which is available for Unix and Windows systems. All implementations of traceroute rely on ICMP (type 11) packets being sent to the originator.

Estonia to the United States. 195.80.96.219 (kauge.aso.ee) to 130.94.122.199 (larousse.wikipedia.org).

Windows command : tracert 130.94.122.199

Linux or Mac OS X command : traceroute 130.94.122.199

AS/400 command : QSYS/TRCTCPRTE RMTSYS('130.94.122.199')

1   et-gw.aso.ee
2   kjj-bb2-fe-0-1-4.ee.estpak.ee
3   noe-bb2-ge-0-0-0-1.ee.estpak.ee
4   s-b3-pos0-3.telia.net
5   s-bb1-pos1-2-0.telia.net
6   adm-bb1-pos1-1-0.telia.net
7   adm-b1-pos2-0.telia.net
8   p4-1-2-0.r00.amstnl02.nl.bb.verio.net
9   p4-0-3-0.r01.amstnl02.nl.bb.verio.net
10  p4-0-1-0.r80.nwrknj01.us.bb.verio.net
11  p4-0-3-0.r00.nwrknj01.us.bb.verio.net
12  p16-0-1-1.r20.mlpsca01.us.bb.verio.net
13  xe-1-2-0.r21.mlpsca01.us.bb.verio.net
14  xe-0-2-0.r21.snjsca04.us.bb.verio.net
15  p64-0-0-0.r21.lsanca01.us.bb.verio.net
16  p16-3-0-0.r01.sndgca01.us.bb.verio.net
17  ge-1-2.a03.sndgca01.us.da.verio.net
18  larousse.wikipedia.org

Traceroute is often used for network troubleshooting. By showing a list of routers traversed, it allows the user to identify the path taken to reach a particular destination on the network. This can help identify routing problems or firewalls that may be blocking access to a site. 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, as 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.

Supplying such detailed information about the pathways taken was considered acceptable and convenient in the early days of the Internet, but later was considered questionable for privacy and security reasons. Traceroute information has been frequently used by hackers as a way to acquire sensitive information about a company's network architecture. By using the traceroute command, a hacker or several hackers can quickly map out what nodes are available on a company's network architecture and exploit vulnerable or compromised nodes/computers.

For these reasons, while traceroute was widely used during the early days of Internet, by the 1990s many Internet sites have blocked traceroute requests, however, in general one may still perform a traceroute up to the borders of such blocked sites. This allows one to see the path taken to a given site, but in many cases not the network within that site.

The traceroute man 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.

• Layer Four Trace, a more modern traceroute (IP network tracing) implementation that supports a multitude of layer-4 protocols

• RFC 1393 - Traceroute using an IP Option Internet RFC
• RFC 792 - Internet Control Message Protocol (ICMP)
• traceroute man page
• traceroute.org — Monthly updated list of online traceroutes
• BGP4.net Traceroute Wiki — Hundreds of traceroute links, sorted by Autonomous System Number.
• mapulator.com Combines traceroute with Google Maps
• VisualRoute, online traceroute, shows connection path and IP locations on global map
• Paris traceroute, allows routes to be traced correctly in the presence of routers that balance traffic across multiple links
• Layer Four Traceroute (LFT),a traceroute program with TCP and UDP tracing support

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.