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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 indicates 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.
On the Apple Mac OS, the traceroute tool is available through opening 'Network Utilities' then selecting 'Traceroute' tab, or typing the "traceroute" command in the terminal. On Microsoft Windows operating systems it is named
tracert. Windows NT-based operating systems also provide PathPing, with similar functionality. Variants with similar functionality are also available, such as
tracepath on Linux installations. For Internet Protocol Version 6 (IPv6) the tool sometimes has the name
Traceroute sends a sequence of three Internet Control Message Protocol (ICMP) echo request packets addressed to a destination host. 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 packets' TTL value by 1 when routing and discard packets whose TTL value has reached zero, returning the ICMP error message ICMP Time Exceeded. Common default values for TTL are 128 (Windows OS) and 64 (Linux-based OS).
Traceroute works by sending packets with gradually increasing TTL value, starting with TTL value = 1. 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 2, 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.
Hop 192.168.1.2 Depth 1 Probe status: unsuccessful Parent: () Return code: Label-switched at stack-depth 1 Sender timestamp: 2008-04-17 09:35:27 EDT 400.88 msec Receiver timestamp: 2008-04-17 09:35:27 EDT 427.87 msec Response time: 26.92 msec MTU: Unknown Multipath type: IP Address Range 1: 127.0.0.64 ~ 127.0.0.127 Label Stack: Label 1 Value 299792 Protocol RSVP-TE
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, the traceroute utility uses 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), like the Windows tracert utility does. If a network has a firewall and operates both Windows and Unix-like systems, both protocols must be enabled inbound through the firewall for traceroute to work and receive replies.
Some traceroute implementations use TCP packets, such as tcptraceroute or 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 Mac OS X include an option to use ICMP Echo packets (-I) or any arbitrary protocol (-P) such as UDP, TCP, ICMP.
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.
traceroute will display the options if invoked without any,
man traceroute will display details including error flags displayed. Simple example on Linux:
traceroute -w 3 -q 1 -m 16 example.com
Only wait 3 seconds (instead of 5), only send out 1 query to each hop (instead of 3), limit the maximum number of hops to 16 before giving up (instead of 30) with 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. Also, the inventor 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.
See also 
- Hop (networking)
- Hop (telecommunications)
- Hop count
- Time to live
- Looking Glass servers
- MTR (software) – computer software which combines the functionality of the traceroute and ping programs in a single network diagnostic tool.
- PathPing – a Windows NT network utility that combines the functionality of ping with that of traceroute (or tracert).
- netsniff-ng, a Linux networking toolkit with an autonomous system traceroute utility
- Layer four traceroute
- traceroute(8) – Linux man page
- Tracert – Windows XP Command-line reference
- RFC 1393: Traceroute using an IP Option Internet RFC
- RFC 792: Internet Control Message Protocol (ICMP)
- How traceroute works – InetDaemon
Online traceroute services:
- Dazzlepod: Multi-source visual traceroute on Google Map
- traceroute.org: monthly updated list of online traceroutes
- IPv4 traceroute sites, listed by ASN. (An IPv6 Traceroute page is also available.) (BGP4.net)
- Misk.com Traceroute, Supports IPv6
- Online Traceroute with GeoIP mapping
- IPv6 Tools (Euro6IX)
- TRACEROUTE6.NET: Located in Japan, Asia. Online IPv6 and IPv4 Ping and Traceroute tools
- Traceroute from multiple locations: Locations in Netherlands, Hong Kong, USA, Croatia, UK, Ukraine, Malaysia, Russia, India, Turkey.