isc-dhcp-server not receiving DHCPDISCOVER

["Russell L. Carter" <rcarter@pinyon.org>]

Hi,

After wearing out the google I'm going to foist my problem onto you
good folk.

I've got an internal net with several dual-homed jessie boxen.  I
would like to get isc-dhcp-server up so I can configure my new UniFi
toy.  The server has two statically configured interfaces, eth0 and
eth1.

The server's dhcpd.conf contains:

ddns-update-style none;
log-facility local7;
option domain-name "xx.pinyon.org";
option domain-name-servers 10.0.10.5, 10.0.10.7;
option routers 10.0.10.5;
default-lease-time 600;
max-lease-time 7200;
authoritative;
subnet 10.0.11.0 netmask 255.255.255.0 {}

subnet 10.0.10.0 netmask 255.255.255.0 {
       pool { range 10.0.10.50 10.0.10.100; }
}

and I've set INTERFACES="eth0 eth1" in /etc/default/isc-dhcp-server.

isc-dhcp-server starts fine.  I then listen for DHCP packets on the
10.0.10.0 net with

$ dhcpdump -i eth0

On the test (jessie) client I have ifdown'd eth0 and execute

$ dhclient -v eth0

(I can bring up static eth0 on the client and ping the server fine)

The problem is that no DHCP packet is received by the eth0 interface
on the server.  As a first test I've ifdown'd the eth1 interface on
the server and tried running a dhclient and I DO see the DHCPDISCOVER
packets.

Neither the server nor the client is running a firewall.  iptables is
not even installed.  For kicks I enabled /proc/sys/net/ipv4/ip_forward
but this had no effect.  I've got a smart switch in the middle but I
checked and everything looks fine.

It looks to me that the client UDP broadcast is blocked from reaching
the server, but what could cause that?

Any ideas much appreciated.

Russell