DHCP Across Wireless Bridge

Anthony Hoppe anthony.hoppe at gmail.com
Tue Dec 31 19:20:56 UTC 2013

Hi Ted,

Thanks for letting me know that reaching out to the dhcp-users list was 
not completely bone-head of me! :-)

The wireless access point I'm using as a bridge simply has one setting, 
"client bridge mode."  Whether or not this turns it into a relay agent 
or not I haven't a clue.  However, it definitely affects DHCP traffic. 
See my example below:


The above shows two DHCPDISCOVER requests as seen by the DHCP server. 
The first is with the client (a laptop) behind the wireless bridge.  The 
second is with the client connected to the network normally.  Take note 
of the IP line.  In sample 1, the sending MAC address is 
0:2:6f:9b:fe:52, which is the wireless bridge.  In sample 2, the sending 
MAC address is 0:14:22:ad:8b:b7, which is the client needing the IP.  In 
both cases, the MAC specified as the CHADDR bit is the client's MAC.

So the wireless bridge is definitely interfering.  My hope is to be able 
to construct a class/subclass combo that will match based on the MAC in 
the IP line so that the DHCP server always broadcasts only for clients 
behind the wireless bridge...if even possible.

~ Anthony

On 12/31/2013 05:12 AM, Ted Lemon wrote:
> On Dec 31, 2013, at 12:35 AM, Anthony Hoppe <anthony.hoppe at gmail.com> wrote:
>> This seems to have done the trick!  Thanks!  I think this is definitely a case of "RTFM."  I am going to become a lot more familiar with the dhcpd.conf man page.
> You did the right thing—the man page wouldn't have helped until you figured out what was being done differently by the Cisco server.   The wireless relay is broken.
>> class "WirelessBridge" {
>> 	match hardware;
>> }
>> subclass "WirelessBridge" 1:00:02:6f:9b:fe:52;
>> Even if the above is remotely correct, I'm not sure how to write "always-broadcast for clients in class WirelessBridge".
> Unless the bridge is configured to act as a layer-two relay agent (doubtful) there should be no sign that packets have traversed it.   IOW, no way to detect that the client needs broadcast.
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