Advanced Tracing
Charly's Column – traceroute
Like every admin, Charly regularly uses the classic traceroute tool. If unfriendly digital natives interfere with an ICMP filter, he simply switches to a clever alternative like LFT.
Practically every admin uses the classic traceroute
tool at more or less regular intervals. This gets me all the more irritated when I find myself in a hotel with a WiFi network where the admin has completely disabled ICMP. Apart from the fact that this causes more trouble than benefits in what is by definition a public network, it can be easily circumvented.
The first version of traceroute
was written in 1988 by a certain Van Jacobsen – Van is his first name, not an honorific. To be able to trace the path of packets through the web, Jacobsen came up with a clever method. He sent test packets through the Internet to a defined destination and increased the time to live (TTL) value for each packet.
The first packet is assigned a TTL of one. Each router that transports the packet further reduces the TTL by one. Once the TTL reaches a value of zero, the router sends it back with an ICMP TTL exceeded
message. By successively increasing the TTL, Jacobsen got the packets back from routers that were further and further away and was able to follow the path of the packet until it finally reached its destination.
This does not work if the remote peer suppresses ICMP messages. However, traceroute
has evolved over the years. It has been able to use an alternative TCP-based method that relies on TCP SYN packets for quite some time. Figure 1 shows two traceroute
s to the same destination, the BBC web server (bbc.co.uk). The first call gets stuck at some point, probably due to an ICMP filter. The second one uses TCP SYN packets – it gets to its destination unhindered.
Alternative traceroute
tools, such as MTR [1], which continuously repeats the trace and thus helps to detect occasional packet losses, take things one step further. Another very interesting tool is Layer Four Traceroute (LFT [2]). It can handle other transport methods and thus makes it through most firewalls. In addition, it can output whose network blocks the packet is passing through, including the number of the autonomous system responsible for it (Figure 2).
It is therefore worthwhile to take a closer look at the different traceroute
variations – if only to keep your blood pressure down during your next hotel stay.
Infos
- "Sys Admin's Daily Grind: Step Counter" by Charly Kühnast, Linux Magazine, issue 119, October 2010, p. 47
- LFT: http://freshmeat.sourceforge.net/projects/LFT
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
New Steam Client Ups the Ante for Linux
The latest release from Steam has some pretty cool tricks up its sleeve.
-
Gnome OS Transitioning Toward a General-Purpose Distro
If you're looking for the perfectly vanilla take on the Gnome desktop, Gnome OS might be for you.
-
Fedora 41 Released with New Features
If you're a Fedora fan or just looking for a Linux distribution to help you migrate from Windows, Fedora 41 might be just the ticket.
-
AlmaLinux OS Kitten 10 Gives Power Users a Sneak Preview
If you're looking to kick the tires of AlmaLinux's upstream version, the developers have a purrfect solution.
-
Gnome 47.1 Released with a Few Fixes
The latest release of the Gnome desktop is all about fixing a few nagging issues and not about bringing new features into the mix.
-
System76 Unveils an Ampere-Powered Thelio Desktop
If you're looking for a new desktop system for developing autonomous driving and software-defined vehicle solutions. System76 has you covered.
-
VirtualBox 7.1.4 Includes Initial Support for Linux kernel 6.12
The latest version of VirtualBox has arrived and it not only adds initial support for kernel 6.12 but another feature that will make using the virtual machine tool much easier.
-
New Slimbook EVO with Raw AMD Ryzen Power
If you're looking for serious power in a 14" ultrabook that is powered by Linux, Slimbook has just the thing for you.
-
The Gnome Foundation Struggling to Stay Afloat
The foundation behind the Gnome desktop environment is having to go through some serious belt-tightening due to continued financial problems.
-
Thousands of Linux Servers Infected with Stealth Malware Since 2021
Perfctl is capable of remaining undetected, which makes it dangerous and hard to mitigate.