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Use a DHCP client identifier
Authored by: iljitsch on Aug 27, '04 03:43:46AM

I've been doing this for a while, and I always inteded to write up a hint after some more testing.

Anyway, what I do to get the same IP address on both interfaces, is give them both the same client identifier. AFAIK, most DHCP servers will look at the client identifier rather than the MAC address so this should give you the same address for both interfaces. However, I've only tried this with a Cisco router as a DHCP server, so I can't be sure that more generic stuff works the same way.

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Use a DHCP client identifier
Authored by: jtrott on Aug 27, '04 04:22:48AM

Actually, what seems to happen is that MacOS X doesn't use the MAC address as a DHCP identifier. It generates another unique string from the DHCP Client ID and other information. The side effect of this is that if all interfaces have the same DHCP Client ID entered then all interfaces get the same IP address if they are talking to the same DHCP server. You can see this if you look at the DHCP server leases, for windows boxes they will show the MAC address. For MacOS X boxes it will show a much longer string.

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Use a DHCP client identifier
Authored by: snarke on Oct 07, '04 07:57:31AM

My, that's very cool. I've had two IP addresses for my laptop, because I couldn't figure out how to give both the Ethernet and Airport the same one. It hasn't really been a big deal, since the connection would switch back and forth even though the IP was different, even for a chat client (amazing!), but it did make my DNS entries annoying.

At first, setting the Client ID didn't seem to work. As it turns out, that's because my D-Link router would assign a dynamic IP based on the full Client ID (the word changed to hex), but when I tried to associate it with a fixed, reserved IP, it would only remember the first six characters. Changing the ID to a six character word means it now assigns the same IP address to either interface, despite the different MAC addresses. Nifty!

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