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I have quite a complicated home setup and I have just ordered a new phone line for a separate ADSL connection for my business. I will be routing this through my CentOS Linux firewall/router and wondered if it would be better to go for a straight pass-through ADSL modem or for an ADSL router (bearing in mind CentOS will do all my routing).

I was thinking of getting something like the Draytek Vigor 120 which is a simple modem. Is it easy to configure this in CentOS?

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Voted down because? – Mark Allison Sep 19 '13 at 10:09
up vote 4 down vote accepted

You don't have a choice. I'm pretty sure you can't even buy a plain bridging-only DSL modem today. Almost all DSL modems will support PPPoA or PPPoE, and they never support bridging over PPP*, so if you had any of the PPP* modes enabled, it would be, at the very least, doing one-address NAT gatewaying, not plain bridging.

The standard for plain bridging over DSL is RFC 2684 (which obsoleted RFC 1483), but I doubt you can find a modem that just supports that without also supporting some flavor of PPP endpoint and NAT at the same time. Even that Draytek Vigor 120 you pointed at supports PPPoA and PPPoE and NAT gatewaying, just like any "DSL router" product would.

Whatever DSL modem solution you buy, try to configure it to be as simple of a bridging device (no NAT, no DHCP server) as you can, so it doesn't get in the way of what you're trying to do with your CentOS router box.

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According to the manual… the external IP is passed through to the host (i.e. without NAT), which is different to how most ADSL routers work. I'm wondering if there's any advantage with this type of product over the more common ADSL routers. – Mark Allison Sep 17 '13 at 21:10

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