[Bug 6614] Questionable implementation of RFC2132 - udhcpc

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=6614

--- Comment #2 from David Freedman <lcreg-busybox at convergence.cx> 2013-10-28 23:21:20 UTC ---
Hi Denys,

""'
2. Protocol Summary
...
   ...  A DHCP server always returns its own address in the 'server
   identifier' option. ..."

No RFC2119 language here, my server doesn't have an address, therefore it won't
respond with one.

This is quite deliberate, as it needs to impersonate a server on many VLANs
that may belong to customers, it can't be seen to be stealing an address from
their subnets. 

"3.1 Client-server interaction - allocating a network address
...
     ... The client broadcasts a DHCPREQUEST message
     that MUST include the 'server identifier' option to indicate which
     server it has selected ...
""""

That's fine, we just use 0.0.0.0 as the server identifier, the option MUST be
included, but it doesn't say anything about the option value. 

"""
4.3.2 DHCPREQUEST message

   A DHCPREQUEST message may come from a client responding to a
   DHCPOFFER message from a server, from a client verifying a previously
   allocated IP address or from a client extending the lease on a
   network address.  If the DHCPREQUEST message contains a 'server
   identifier' option, the message is in response to a DHCPOFFER
   message.  Otherwise, the message is a request to verify or extend an
   existing lease.
"""

yes, the key bit of this sentence is "If the DHCPREQUEST message contains a
'server identifier' option" , 
we do have one present, it is just blank. 

"If server ID is not provided, client can't send it back
in its DHCPREQUEST answer to DHCPOFFER."

We can send back Server-ID : 0.0.0.0, we're still including the option. 

"it can't specify to which of potentially many DHCP
servers it is replying to."

It's replying to the server at 0.0.0.0

>A server-id-less DHCP server looks like a corner case

Not sure I would call it a 'corner case', there is good support in ISC dhcp
client and Microsoft Windows for this. 

Dave.

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