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Authored by: dahjelle on Apr 02, '04 03:52:35PM

Out of curiosity, what is the DHCP Client ID field for, anyway? All I've ever seen is instructions like the above: "Type anything into the DHCP Client ID field." Surely there is a better purpose for it than that?



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Authored by: nat5an on Apr 02, '04 05:28:00PM

Some DHCP servers will require you to use a special ID to get an IP address, Excite @home used to do this. It's a way to prevent random people from connecting to your service.



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Authored by: billabOng on Apr 02, '04 06:31:51PM

I believe it's so you can figure out who's who in your DHCP logs.



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The purpose of the DHCP Client Identifier
Authored by: src-kc8rmb on Apr 03, '04 12:50:49PM

The DHCP Client Identifier is just that, a unique (per subnet) identifier (such as a hostname or serial number) allowing the server to identify the client and select the appropriate response. In the absence of a Client Identifier, the "chaddr" field (hardware address) is used.

Since every Ethernet/WiFi device has a unique hardware address w/o any configuration, that's what's used most of the time. Unless the server has specific per client configuration it barely matters which you use.

@HOME and others use this because it's easier for them to restrict who can get an IP address by using a fixed per client id than by forcing customers to accurately provide them a 12 hex digit Ethernet address and change it everytime they change computers or interfaces cards.

The use in logging is secondary, and primarily for network troubleshooting. A client specified field isn't a very good choice for auditing and security purposes.

Changing the client id or entering junk willy-nilly is bad for two reasons:

(1) The protocol basically requires the server to see each different client id (plus the original chaddr) as a different client, forcing the server to keep issuing new leases without invalidating the old (chewing up the IP space).

(2) It greatly increases the likelyhood that you will enter a client id which someone else on your subnet has used. This can result in duplicate IP addresses, which is a major headache (for you too), defeating one of the biggest benefits of DHCP.

For more information, please see RFC 2131: Dynamic Host Configuration Protocol, available whereever your favorite RFC's are given away, not sold (ftp://ftp.rfc-editor.org/in-notes/rfc2131.txt).



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