delicate DHCP problem with bridged eth devices
From: KaiRo - Robert Kaiser (kairo_at_kairo.at)
Date: 10/30/04
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Date: Sat, 30 Oct 2004 16:01:41 +0200
Hi all,
In our student home, we got a new connection that offered us 128 public
IPs, for a maximum of 63 people living in here, that's a quite nice
thing to have.
It gave me some headaches though, I solved most cases, but I still have
a DHCP problem. First I gotta tell you the delicate situation:
[For better reading, I'll substitute the constant xx.xx.xx part of the
addresses to X (so xx.xx.xx.0 becomes X.0) in the following writing.]
Our provider has a gateway installed at the X.1 address of our subnet,
which is set to the full netmask of this subnet and we can't reconfigure
that.
We want all of our packets to run through our Linux server to be able to
filter as well as monitor the traffic of our clients, eventually even
shape their bandwidth or similar things.
The clients themselves are wired to three 24-port switches, that's why
we have 4 network cards in the server, one leading to the provider, the
other three to our switches / clients.
I've set up transparent bridging, and went through some loops with
ebtables and routing so that all does work now (at least it's supposed
to, it's tested to be working but the clients don't get their IPs
assigned like that yet):
subnet xx.xx.xx.0/25 [X.0/25] (128 addresses, netmask 255.255.255.128)
|---------| X.1 X.2 |------------------|
---| gateway |------------| eth0 --- eth1 |--- clients (X.34-X.56)
|---------| | | | |
| br0 ---+-- eth2 |--- clients (X.66-X.88)
| X.3 | |
| --- eth3 |--- clients (X.98-X.120)
|------------------|
Linux server
Don't be disturbed by br0 and eth0 having two IPs on the same subnet, I
was able to solve the routing problems with that...
We want to serve the clients' IP addresses via DHCP, as we do now, but
here's where I'm running into problems.
Wait a minute, I'm telling "as we do now" but I didn't tell what we do
now...
Well, eth1-eth3 do have 192.168.xx.xx addresses with seperated subnets
assigned, which do get NATed to X.2, and DHCP assigns IPs to the clients
on those subnets. That does work well, and has done so for a while.
If I manually assign a public IP to a client, it does work as well.
But here's the problem:
How do I tell DHCP to assign an IP on the bridged subnet?
I have 4 subnet declarations in dhcpd.conf:
subnet xx.xx.xx.0 netmask 255.255.255.128 {
server-identifier xx.xx.xx.2;
option routers xx.xx.xx.1;
}
subnet 192.168.23.32 netmask 255.255.255.224 {
server-identifier 192.168.23.33;
option routers 192.168.23.33;
}
subnet 192.168.23.64 netmask 255.255.255.224 {
server-identifier 192.168.23.65;
option routers 192.168.23.65;
}
subnet 192.168.23.96 netmask 255.255.255.224 {
server-identifier 192.168.23.97;
option routers 192.168.23.97 ;
}
Those are followed by a bunch of host blocks like:
host test {
hardware ethernet zz:zz:zz:zz:zz:zz;
fixed-address 192.168.23.85;
}
[same for addresses on the bridged public subnet]
The first problem is seen in syslog when starting dhcpd:
dhcpd: Multiple interfaces match the same subnet: eth0 br0
dhcpd: Multiple interfaces match the same shared network: eth0 br0
dhcpd: Listening on Socket/eth3/192.168.23.96/27
dhcpd: Sending on Socket/eth3/192.168.23.96/27
dhcpd: Listening on Socket/eth2/192.168.23.64/27
dhcpd: Sending on Socket/eth2/192.168.23.64/27
dhcpd: Listening on Socket/eth1/192.168.23.32/27
dhcpd: Sending on Socket/eth1/192.168.23.32/27
dhcpd: Listening on Socket/br0/xx.xx.xx.0/25
dhcpd: Sending on Socket/br0/xx.xx.xx.0/25
Then, for 192.168.23.xx addresses, everything works, see e.g.
dhcpd: DHCPDISCOVER from zz:zz:zz:zz:zz:zz via eth2
dhcpd: DHCPOFFER on 192.168.23.85 to zz:zz:zz:zz:zz:zz via eth2
dhcpd: DHCPREQUEST for 192.168.23.85 (192.168.23.65) from
zz:zz:zz:zz:zz:zz via eth2
dhcpd: DHCPACK on 192.168.23.85 to zz:zz:zz:zz:zz:zz via eth2
OTOH, dhcpd thinks it shouldn't hand out the public addresses because it
fails to know the bridged subnet does apply to the eth1-eth3 subnets:
dhcpd: DHCPDISCOVER from 00:50:da:74:7f:79 via eth1: network
192.168.23.32/27: no free leases
last message repeated 2 times
How can I get dhcpd to offer the public addresses on eth1-eth3?
[The Linux system is SuSE 9.1 with a self-compiled Linux-2.6.8.1, btw.]
Thanks in advance for your help,
Robert Kaiser
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