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This is a question w.r.t server behavior in the DHCP standard. I've read the RFC and it's still not clear to me.

Situation is this:

There are two DHCP servers on a network. My client initially get IP address from server A. At some stage, server A goes down. My poor client is sending REQUEST in RENEW and then REBIND state, with no response whatsoever.

My questions is: in this situation, should server B response to the REQUEST in REBIND state, e.g. DHCPNAK so that the client has a chance to restart the process?

lang2

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2 Answers

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My understanding is that it could but doesn't have to.

In the RENEWING state, your client should be sending the DHCPREQUEST addressed to server A. Later, in the REBINDING state, your client should be sending the DHCPREQUEST to the broadcast address, so server B could accept it and reply.

However, the last paragraph of section 4.3.2 of RFC 2131 suggests that a server doesn't have to extend a lease that it didn't originally send.

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I wasn't expecting it to renew my lease. But maybe it can send NAK so that the client can restart the whole process. – lang2 Sep 13 '12 at 5:19
The DHCPNAK seems to be used only as a positive indication of bad information from the client. Section 3.2 says, "For example, a server that identifies a request for an expired binding that is owned by another server SHOULD NOT respond with a DHCPNAK unless the servers are using an explicit mechanism to maintain coherency among the servers." The client doesn't need to restart the whole process until its lease expires. At that time it will send a DHCPDISCOVER message which should be answered by server B. – garyjohn Sep 13 '12 at 6:43
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It depends on both the network topology and the actual configuration of each DHCP server. Normally (without DHCP helper on the router) DHCP only works inside an Ethernet segment. If Server B is on a different segment, it will never see requests from the Client, nor will the client see responses from Server B. Also, when the server is configured only with reserved addresses (no dynamic address range), or when the dynamic address range is already exhausted, the server will not respond to requests from clients (because it has nothing to offer).

For a more detailed answer, you'd need to provide more information about your network topology and the configuration of both servers.

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