Tag - Queries

DHCP Questions and Answers

27. Can a client have a home address and still float?
There is nothing in the protocol to keep a client that already has a leased or permanent IP number from getting a(nother) lease on a temporary basis on another subnet (i.e., for that laptop which is almost always in one office, but occasionally is plugged in in a conference room or class room). Thus it is left to the server implementation to support such a feature. I’ve heard that Microsoft’s NT-based server can do it.

28. How can I relay DHCP if my router does not support it?
A server on a net(subnet) can relay DHCP or BOOTP for that net. Microsoft has software to make Windows NT do this.

29. How do I migrate my site from BOOTP to DHCP?
I don’t have an answer for this, but will offer a little discussion. The answer depends a lot on what BOOTP server you are using and how you are maintaining it. If you depend heavily on BOOTP server software to support your existing clients, then the demand to support clients that support DHCP but not BOOTP presents you with problems. In general, you are faced with the choice:

Find a server that is administered like your BOOTP server only that also serves DHCP. For example, one popular BOOTP server, the CMU server, has been patched so that it will answer DHCP queries.
Run both a DHCP and a BOOTP server. It would be good if I could find out the gotcha’s of such a setup.
Adapt your site’s administration to one of the available DHCP/BOOTP servers.
Handle the non-BOOTP clients specially, e.g. turn off DHCP and configure them statically: not a good solution, but certainly one that can be done to handle the first few non-BOOTP clients at your site.
30. Can you limit which MAC addresses are allowed to roam?
Sites may choose to require central pre-configuration for all computers that will be able to acquire a dynamic address. A DHCP server could be designed to implement such a requirement, presumably as an option to the server administrator.

31. Is there an SNMP MIB for DHCP?
There is no standard MIB; creating one is on the list of possible activities of the DHCP working group. It is possible that some servers implement private MIBs.

32. What is DHCP Spoofing?
Ascend Pipeline ISDN routers (which attach Ethernets to ISDN lines) incorporate a feature that Ascend calls “DHCP spoofing” which is essentially a tiny server implementation that hands an IP address to a connecting Windows 95 computer, with the intention of giving it an IP number during its connection process.


BOOTP client boot from a DHCP and BOOTPserver

15. Can a BOOTP client boot from a DHCP server?
Only if the DHCP server is specifically written to also handle BOOTP queries.

16. Can a DHCP client boot from a BOOTP server?
Only if the DHCP client were specifically written to make use of the answer from a BOOTP server. It would presumably treat a BOOTP reply as an unending lease on the IP address.

In particular, the TCP/IP stack included with Windows 95 does not have this capability.

17. Is a DHCP server “supposed to” be able to support a BOOTP client?
The RFC on such interoperability (1534) is clear: “In summary, a DHCP server: … MAY support BOOTP clients,” (section 2). The word “MAY” indicates such support, however useful, is left as an option.

A source of confusion on this point is the following statement in section 1.5 of RFC 1541: “DHCP must provide service to existing BOOTP clients.” However, this statement is one in a list of “general design goals for DHCP”, i.e. what the designers of the DHCP protocol set as their own goals. It is not in a list of requirements for DHCP servers.

18. Is a DHCP client “supposed to” be able to use a BOOTP server?
The RFC on such interoperability (1534) is clear: “A DHCP client MAY use a reply from a BOOTP server if the configuration returned from the BOOTP server is acceptable to the DHCP client.” (section 3). The word “MAY” indicates such support, however useful, is left as an option.

19. Can a DHCP client or server make a DNS server update the client’s DNS entry to match the client’s dynamically assigned address?
RFCs 2136 and 2137 indicate a way in which DNS entries can be updated dynamically. Using this requires a DNS server that supports this feature and a DHCP server that makes use of it. The RFCs are very recent (as of 5/97) and implementations are few. In the mean time, there are DNS and DHCP servers that accomplish this through proprietary means.


Copyright ©2010 - 2022 Ciscoforall.com | Privacy Policy | Terms & Conditions