Re: NT 3.51 dhcp client and server ip address

[Ken Key <[e-mail]>]

[SNIP]
> I agree that the server identifier is used by the client to identify
> the DHCP server which extended the lease, but it shouldn't expect the
> identifier to have any other meaning.  'Fred' should be a valid server
> id as long as its unique among servers that may respond.
[SNIP]

No, it is an IP address of the DHCP server.  From RFC 1533:

# 9.5. Server Identifier
# 
#    This option is used in DHCPOFFER and DHCPREQUEST messages, and may
#    optionally be included in the DHCPACK and DHCPNAK messages.  DHCP
#    servers include this option in the DHCPOFFER in order to allow the
#    client to distinguish between lease offers.  DHCP clients indicate
#    which of several lease offers is being accepted by including this
#    option in a DHCPREQUEST message.
# 
#    The identifier is the IP address of the selected server.
# 
# 
# Alexander & Droms                                              [Page 24]
# 
# RFC 1533        DHCP Options and BOOTP Vendor Extensions    October 1993
# 
# 
#    The code for this option is 54, and its length is 4.
# 
#     Code   Len            Address
#    +-----+-----+-----+-----+-----+-----+
#    |  54 |  4  |  a1 |  a2 |  a3 |  a4 |
#    +-----+-----+-----+-----+-----+-----+

WinNT is not the only DHCP client using the Server Identifier for renewal.
It is in line with the design philosophy of keeping everything the 
protocol needs for operation up at the protocol level.  Requiring the
client to grovel around at the IP layer for the source IP address would be 
like requiring the DHCP server to grovel down in the MAC layer to find 
the from MAC From address the client used rather than specifying an HA
in the protocol packet itself.  You should be using the IP address of the
interface you are going to send the response out of on your multiply homed
host implementation as the Server Identifer.  I know it's a bitch, I had
to implement it myself.

I have always believed this particular use was a tacit design decision of 
the DHCP protocol.  It makes common sense and fits with the rest of the
design of the protocol.  Any of you folks that have been around longer 
than me remember any dicussion on the topic one way or the other?  Any 
other client implementors not use the Server Identifier for the renewal 
IP address?

You have the answer in your own message:  "Otherwise, there needs to exist 
a route between the 138.95.84 and 138.95.105 subnets."  That's absolutely
true.  A properly functioning IP network is a general requirement.

regards,
Ken Key
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