DHCP Across Wireless Bridge
Anthony Hoppe
anthony.hoppe at gmail.com
Mon Dec 30 22:37:19 UTC 2013
I've done a dhcpdump of my Lubuntu laptop obtaining a DHCP address from
my Cisco 871 router. See the link below:
http://pastebin.com/FE0iuVEB
It looks like Cisco's implementation of DHCP has all communication as
broadcast (with "chaddr" containing the respective client's MAC), as
opposed to isc-dhcp-server responding directly to the client. This
would explain why Cisco's DHCP functions across the wireless bridge.
Is there a way to configure isc-dhcp-server to function similarly?
In most cases I can understand why you wouldn't want to do this, but I
can see it as helpful in other situations (like mine :-D).
~ Anthony
On 12/29/2013 10:33 AM, Anthony Hoppe wrote:
> Hi Glenn,
>
> Thank you for the response! I've done done a few tests using dhcpdump.
> Below are the results.
>
> http://pastebin.com/Ns8jmzSu - This is a server-side dhcpdump showing my
> Dish VIP722 DVR, connected via the wireless bridge, attempting to obtain
> an address.
>
> http://pastebin.com/9yMEUtLG - (Pair 1) Another server-side run of
> dhcpdump showing a Lubuntu laptop, connected via the wireless bridge,
> attempting to obtain an address.
>
> http://pastebin.com/CrzfVJRd - (Pair 1) This is a client-side run of
> dhcpdump showing the Lubuntu laptop, connected to the wireless bridge,
> attempting to obtain an address. It never receives a DHCPOFFER response.
>
> http://pastebin.com/LrRD1wac - (Pair 2) This is a server-side run of
> dhcpdump showing the Lubuntu laptop, hardwired to the network normally
> (NOT using the wireless bridge), successfully obtaining an address.
>
> http://pastebin.com/EjVszsMS - (Pair 2) And lastly, this is a
> client-side run of dhcpdump showing the Lubuntu laptop, hardwired to the
> network normally (NOT using the wireless bridge), successfully obtaining
> an address.
>
> It looks like the wireless bridge is preventing the DHCPOFFER response
> from reaching the client. The DHCPDISCOVER broadcast is received by the
> DHCP server with the MAC address of the wireless bridge as the source.
> The DHCPDISCOVER broadcast has a request to redirect the response to
> the MAC address of the client, which it looks like the DHCP server is
> obeying this request. But, for some reason, the DHCPOFFER response
> never makes it to the client.
>
> Is there a way I can configure isc-dhcp-server and/or the OS it's
> running on to work around this? The Cisco DHCP server seems to do
> things differently that makes this a non-issue. Could it be that the
> DHCPOFFER needs to have a destination of the wireless bridge with a
> redirect request to the MAC address of the client? I don't know...
>
> Thanks for the pointers on my configuration. I fixed the dynamic address
> range so that it does not include the broadcast address (oops!), and I
> modified the fixed address for host dwight so that it's outside the
> dynamic address range. It didn't help this particular problem, but it's
> always good to follow best practice.
>
> ~ Anthony
>
>
>
> On Sun, Dec 29, 2013 at 2:40 AM, Glenn Satchell
> <glenn.satchell at uniq.com.au <mailto:glenn.satchell at uniq.com.au>> wrote:
>
> Hi Anthony
>
> That config looks like it should be ok. As you are seeing the discover
> packets, then traffic is getting through from the clients and the dhcp
> server is replying. However the next thing should be a dhcp request
> packet
> from the client which you're not seeing.
>
> Can you run a packet capture on a client connected to the wireless
> bridge
> and see which parts of the traffic you can see? What you'd be
> looking for
> is whether the client sees the offer from the dhcp server, and if it
> does
> whether it then sends a dhcp request back to the server.
>
> Perhaps also try a packet capture on the dhcp server to see if the
> request
> is coming in, but is somehow not accepted by the dhcp server daemon.
>
> Two minor things with the config: the top end of the range is the
> broadcast address (10.7.17.255), perhaps reduce that to 10.7.17.254. The
> host dwight has a fixed ip address that is inside the dynamic range.
> While
> it may never be a problem, it's better to either break the range around
> that address (have two ranges: 100-199 and 201-254) or change it to have
> an ip outside the dynamic range. Neither of these two problems would
> stop
> dhcp working though.
>
> regards,
> -glenn
>
> On Sun, December 29, 2013 6:36 pm, Anthony Hoppe wrote:
> > Hello All,
> >
> > I've been experimenting with isc-dhcp-server on my home network
> and have
> > run into a snag. I have three devices (Xbox 360, Samsung Blu-Ray
> Player,
> > and a Dish VIP 722 HD-DVR) connected to a switch which connects to a
> > wireless bridge. In my previous setup (using my Cisco 871 router
> running
> > C870-ADVIPSERVICESK9-M ver 12.4(24)T as a DHCP server) these
> devices were
> > able to receive DHCP addresses without any problems. However, with
> > isc-dhcp-server, they are unable to receive addresses. Reviewing
> > /var/log/syslog shows many DHCPDISCOVER/DHCPOFFER pairings as
> seen below
> > (an example of one device):
> >
> > -----
> >
> > Dec 28 23:28:13 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:13 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:15 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:15 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:17 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:17 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:31 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:31 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:33 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:33 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:35 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:35 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:49 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:49 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:51 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:51 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> > Dec 28 23:28:53 dns dhcpd: DHCPDISCOVER from 00:08:89:e4:dd:df
> via eth0
> > Dec 28 23:28:53 dns dhcpd: DHCPOFFER on 10.7.17.101 to
> 00:08:89:e4:dd:df
> > via eth0
> >
> > -----
> >
> > Connecting a known working computer to the switch behind the wireless
> > bridge also results the same...it is not able to obtain a DHCP
> address.
> > DHCP works anywhere else on the network without a hitch.
> >
> > Some Googling around leads me to believe the culprit is likely the
> > wireless
> > bridge. I am using an EnGenius WAP (I can't find and/or recall
> the model
> > at the moment) in client bridge mode. However, like I said
> earlier, DHCP
> > worked fine in my previous setup. Is there a way to configure
> > isc-dhcp-server so that it will work, too?
> >
> > Here is my dhcpd.conf:
> >
> > -----
> >
> > authoritative;
> > option domain-name "hhsn.net <http://hhsn.net>";
> > option domain-name-servers 10.7.17.24;
> >
> > ddns-updates on;
> > ddns-update-style interim;
> > ignore client-updates;
> > update-static-leases on;
> >
> > default-lease-time 86400;
> > max-lease-time 86400;
> > log-facility local7;
> >
> >
> > include "/etc/dhcp/ddns.key";
> >
> > zone hhsn.net <http://hhsn.net>. {
> > primary 10.7.17.24;
> > key DDNS_UPDATE;
> > }
> >
> > zone 17.7.10.in-addr.arpa. {
> > primary 10.7.17.24;
> > key DDNS_UPDATE;
> > }
> >
> >
> > subnet 10.7.17.0 netmask 255.255.255.0 {
> > range 10.7.17.100 10.7.17.255;
> > option routers 10.7.17.1;
> > option broadcast-address 10.7.17.255;
> > option subnet-mask 255.255.255.0;
> > }
> >
> > host dwight {
> > hardware ethernet 00:23:DF:7F:28:04;
> > fixed-address 10.7.17.200;
> > }
> >
> > host nettalk {
> > hardware ethernet 00:25:F6:00:3A:B4;
> > fixed-address 10.7.17.20;
> > }
> >
> > -----
> >
> > Any assistance would be greatly appreciated. I am using
> isc-dhcp-server
> > version 4.2.2 on Debian 7.
> >
> > Thanks!
> >
> > ~ Anthony
>
>
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