Networking → Why are most router firmware Linux 2.4 or 2.6?

I am evaluating alternatives for router / firewall, and see most router firmware seems to be based on Linux 2.4 or Linux 2.6. These seem to be relatively older versions of Linux. Is there a reason these are used or preferred to more modern versions?

Thanks for helping.

There may be a more technical reason, but I'd guess that it's due to code-bloat in newer versions. The typical consumer router has a limited amount of flash memory available, so the smallest OS install that can do the job will be utilized.

Remember when Windows 3.1 came on a few floppy disks? Windows 8 comes on a DVD and clocks in at over 3GB of compressed data. It takes up over 12GB when installed. Linux has also grown over the years.

Stability, as well..

2.4 and 2.6 have been well reviewed, and proven stable - it's not likely that an unexpected 'undocumented feature' will pop up in older kernel releases...

It takes time to fine tune kernel build options and then verify proper operations of all components—so sometimes people just stick with what works. That said, there’s nothing stopping you from building your own router/firewall with the latest kernel code.

I am working on modifying a router to permit access to jtag in the likely event it's bricked. I'm trying to understand the reasons for the various build decisions before attempting to merely compile the current standard distribution of DD-WRT for the router. I note some builds use 2.4 some 2.6, but there isn't much other than that on existing Atheros or Broadcom devices that I've found. I was wondering why that should be.

Except proprietary SoCs with little to no public documentation and binary-only drivers. If you just wanted to use the thing as a Linux box via the serial port without Ethernet or WiFi connectivity then sure you can use the latest kernel code. Otherwise you're stuck using the kernel the chip maker provides.

/M

Proven stability could be the big one. I know some corporate systems still use 2.4 or 2.6 due to it being a known variable. NASA does the same thing too on spacecraft systems, They tend to use what we on this site would see as obsolete because its proven rock solid.