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UPC vs port forwarding

Estimated read time: 2 minutes

UPC traditionally had a setup consisting of a cable modem providing internet access to a single computer, and then it was up to the users if they use that access to really connect to a computer or to a router, providing wireless access and so on. It seems, these days they are more after actually encouraging people to use their subscription on multiple devices — possibly that way it’s easier to sell larger packages (like 60 MBit/s download rate instead of 30 MBit/s, etc). One fallout from this move is that they started to replace modems with a combination of modems and routers, in this case this is an Ubee EVW3226, with the brand removed. I wanted to try out if this new device could replace my previous router or not — so far it seems to be good enough, though there was one pitfall, hence this post.

It’s possible to define a range of IP addresses to be used for DHCP purposes, though you can’t serve fixed IP addresses based on the MAC address of the clients. Given that my home network isn’t that large, I can tolerate that: as long as there is a range that can be safely used for fixed addresses, I can configure that manually. It’s also possible to do port forwarding, e.g. redirecting the incoming ssh traffic to a given address — except you can’t do both at the same time: you can’t redirect traffic to an address that’s not known (served via DHCP) to the router. Which is a shame, the #1 use case for port forwarding is to redirect traffic to a home-server that will then also have a fixed IP internally…

So here is a hack that allowed me to still do this: set the start of the range of the DHCP served IP’s exactly to the address of the (to be used in future as) fixed address, e.g. Connect with one client, so that the address will be known to the router. Then add the port-forwarding rule, finally set the DHCP range back to its original value (in my case I use for fixed addresses and 100+ for dynamic purposes). It’s a stupid trick, but it works… ;-)

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