I have a hub, with some computers plugged into it, and a cable modem. All of the computers except one have statically-configured addresses. The remaining computer runs a DHCP client, to get an IP address from the cable modem. It routes and NATs for the other boxes.

The router also runs a DHCP server, which I cannot disable. Is there any way that DHCP requests from the internet could reach it, affecting other customers of my ISP?


Not unless your ISP are clueless, DHCP is a L2 protocol that is usually segment-bound, your router would have to be configured as a DHCP-helper to pass client request onto the ISP's DHCP servers for you to be compromised.

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    Actually, DHCP is a L6 protocol. A UDP broadcast is sent, source, destination – Fahad Sadah Nov 13 '10 at 20:37
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    DHCP is L7, just like SMTP or FTP. The RFC even says so. True, the client is using the broadcast IP for much of the protocol, but that's still an IP address. – Jeff Leyser Nov 13 '10 at 22:33
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    DHCP is used for layer 3 configuration but that doesn't have anything to do with the fact that it is an application layer (ie 7) protocol- it just happens to be constrained at layer 2. The OSI model isn't really able to communicate that sort of encapsulation effect. Definitions aside, DHCP operates within layer 2 constraints which is what really matters IMO. – Helvick Nov 13 '10 at 22:48
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    @Helvick DHCP is built on top of UDP/IP. It can be forwarded via routers & DHCP relays. DHCP servers understand, and are configured by, IP subnets. In what way is it constrained to L2? – Jeff Leyser Nov 14 '10 at 2:13
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    @Jeff - I agree completely - it is clearly a high level protocol, built on UDP/IP and by definition it is layer 7. "Constrained" may have been too strong a word choice it is layer 2 constrained in scope in the sense that the request packets are not routable outside of broadcast domains without hardware\services that can infer the additional data that the source is unaware of (the forwarder configs in routers). My point is that the simple structure of the OSI model doesn't really provide a good insight into the multi-layer aspect of a protocol like DHCP, but then again its just a simple model. – Helvick Nov 14 '10 at 9:30

Yes, if your cable modem is plugged directly into your hub it's theoretically possible for other customers' DHCP requests to reach your server and for it to respond, since cable modems typically bridge, making it one big broadcast domain. (It's also possible your cable modem has a built-in router, which would make the answer no).

However, ISPs usually implement filtering so this can't happen, since it would allow one customer to see all of another customer's traffic, which is pretty bad security-wise.

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