Layer Four Traceroute (LFT) and WhoB

The alternative traceroute and whois tools for network (reverse) engineers

04/20  LFT 3.91 released! Several improvements.

LFT, short for Layer Four Traceroute, is a sort of 'traceroute' that often works much faster (than the commonly-used Van Jacobson method) and goes through many configurations of packet-filters (firewalls). More importantly, LFT implements numerous other features including AS number lookups through several reliable sources, loose source routing, netblock name lookups, et al. What makes LFT unique? LFT is the all-in-one traceroute tool because it can launch a variety of different probes using ICMP, UDP, and TCP protocols, or the RFC1393 trace method. For example, rather than only launching UDP probes in an attempt to elicit ICMP "TTL exceeded" from hosts in the path, LFT can send TCP SYN or FIN probes to target arbitrary services. Then, LFT listens for "TTL exceeded" messages, TCP RST (reset), and various other interesting heuristics from firewalls or other gateways in the path. LFT also distinguishes between TCP-based protocols (source and destination), which make its statistics slightly more realistic, and gives a savvy user the ability to trace protocol routes, not just layer-3 (IP) hops. With LFT's verbose output, much can be discovered about a target network.

WhoB is a likable whois client (see whois(1)) designed to provide everything a network engineer needs to know about a routed IP address by typing one line and reading one line. But even so, it's worth typing a few more lines because WhoB can do lots of other cool things for you! It can display the origin-ASN based on the global routing table at that time (according to Prefix WhoIs, RIPE NCC, or Cymru), the 'origin' ASN registered in the RADB (IRR), the netname and orgname, etc. By querying pWhoIs, WhoB can even show you all prefixes being announced by a specific Origin-ASN. WhoB performs the lookups quickly, the output is easily parsed by automated programs, and it's included as part of the Layer Four Traceroute (LFT) software package. LFT uses WhoB as a framework (and you can too, quite easily--see whois.h). Recent LFT releases (as of version 2.5) include WhoB functionality through a standalone "whob" client/command placed in the LFT binary directory.

LFT and WhoB continue to evolve and provide more and more useful data to network engineers and to anyone else that cares how IP datagrams are being routed. With the advent of smarter firewalls, traffic engineering, QoS, and per-protocol packet forwarding, LFT and WhoB have become invaluable tools for many network managers worldwide.

LFT and WhoB are released under our open source license.

Download LFT

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Latest Stable Source (3.91)

We're no longer linking to binaries or providing builds. Please use your package manager (or ports tree) or build LFT from source.

LFT Links


Many people have complained about various problems on the Windows platform. Both LFT and the WhoB client compile and run well under Cygwin environments on Windows. Unfortunately, Microsoft's changes to the Windows IP stack (as of XP Service Pack 2) reduced their raw socket functionality significantly as part of their security bolstering process. These changes have effectively stopped LFT from working properly while using TCP. LFT's UDP tracing and other advanced features still work properly. For more information on Windows raw sockets, consult


LFT builds and tests successfully on most Linux systems, most BSD-based systems (OpenBSD, FreeBSD), Darwin (MacOS X), and Solaris. The autoconf (./configure) script should appropriately identify and configure makefiles for these and various other target platforms.


Note that before you can compile LFT on UNIX-like systems, you need to install libpcap. It is available from the tcpdump/libpcap website.

LFT supports autoconf! To build LFT for your system, use the ./configure script.

The easy way: This installs LFT under /usr/local/bin/lft after compilation:

# ./configure ; make install

IF YOU DO NOT USE "make install" - Don't forget to SETUID-root the executable.

# chown root lft ; chmod 4755 lft


Please send bug reports to lft AT

When reporting bugs, please include level-2 verbose output (e.g., "lft -VV <target>") in your e-mail / bug report.


We thank the following contributors that either coded or helped test and debug problems with LFT (not ordered intentionally):

  • Victor Oppleman - project maintainer, contributor
  • Eugene Antsilevitch - contributor
  • Sergey Kondryukov - contributor
  • Zachary Kanner - contributor
  • Lane Davis - contributor
  • Robb Ballard - package maintainer
  • Florin Andrei - packaging contributor
  • Jim McKim - solaris port contributor
  • Nils McCarthy - FFT's original author


To those who would ask the question "who did that first?" with regard to utilizing TCP for traceroute, the answer is "We don't know." However, LFT was first released to the public in 1998 under the name FFT, and that was the first we'd heard of.

Currently Nov 26th 2020, 3:55am GMT