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BOOTP Detailed Operation
(Page 2 of 2)
Interpretation of the Client IP Address (CIAddr) Field
A complication can arise when a client
chooses to specify an IP address in the CIAddr field in its request.
The problem is how exactly to interpret this field. Does it mean that
the client is already using this IP address? Or is it just the one it
used last time it was booted? Then there is the related problem of what
to do if the server supplies an address in the YIAddr that is
different from the one the client is using. Should the server's provided
address override the client's address? Or should the client ignore it?
Who makes the decision, the server or the client?
Much confusion occurred due to the
vagueness of the original standard in this regard, and this led to non-uniformity
in how different implementations chose to handle this issue. There were
even some implementations that used the CIAddr to mean the
client requests this IP address, which was never part of BOOTP
functionality. This is an especially bad idea since it could lead directly
to BOOTP replies never reaching the client.
RFC 1542 was written in part to try
to clean up this mess. It suggests that the following is the best way
to handle the meaning of these fields:
- If a client is willing to accept whatever IP
address the server provides, it sets CIAddr to all zeroes, even
if it knows a previous address.
- If the client fills in a value for the field,
it is saying it will use this address, and must be prepared to receive
unicast messages sent to that address.
- If the client specifies an address in CIAddr
and receives a different address in the YIAddr field, the server-provided
address is ignored.
Note that not all hardware devices
may necessarily agree with this interpretation as provided by RFC 1542,
so there are still potential interoperability issues here with older
equipment. RFC 1542 was written in 1993, so this is probably not much
of an issue any more.
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Version 3.0 - Version Date: September 20, 2005
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