The Fritzbox and Devices With Multiple IP Addresses

For years I have wondered why accessing my Fritzbox DSL router at home via my LTE backup connectivity doesn’t work reliably. While web pages load at first, things quickly get stuck. After 4 years I finally found the answer and a fix for it.

As I run a number of services at home I need reliable Internet connectivity and thus have an LTE backup in place via a separate router in case the DSL line fails. Over the years it has helped me out on quite a number of occasions.

When my DSL line fails I can reconfigure my cloud at home to use the LTE router as gateway for all services. To access the Fritzbox GUI via the backup link I have to use SSH port forwarding on another device in the local network because the Fritzbox, by design, doesn’t use the LTE router as default gateway and thus all TCP connections coming from there can’t be answered. And this is what I had trouble with over the years. In most cases I could see the first few pages of the Fritzbox GUI in my browser but then the connection got stuck. I tried putting switches in between, I tried different physical setups but nothing helped.

And then one day I noticed that the Raspberry Pi I used as an SSH port forwarding gateway with a fixed IP address didn’t have DHCP deactivated for the interface so it had 2 IP addresses on the same network interface with a single MAC address. And it is this combination of single MAC address and multiple IP addresses that the Fritzbox can’t handle. While binding several IP addresses to a single interface and single MAC seems to be a totally valid configuration, I’d still classify it as an unintended misconfiguration with effects that really mystified me.

Note that the Fritzbox has no problem to handle Ethernet interfaces with several MAC addresses each bound to a separate IP address which is the configuration of my cloud setup at home, i.e. a dozen virtual machines in Ethernet bridged mode, each with its own MAC and IP addresses mapped to the single physical Ethernet interface of the host!

Glad I figured that one out!