Routing problem with udp, and a multihomed host in 2.4.20

[Neil Brown]

I have three subnets.  A, B and C.

I have a Linux 2.4.20 server, named bartok, with three interfaces, one
on each subnet (so it can listen to broadcast requests everywhere).

This server has a default route pointing at A.

I have a router that routes between all these subnets, and others.

I have a client that sits on subnet B.

When my client tries to talk to bartok, it might try any of 3 IP
addresses (Due to round-robining in the DNS).

All IP addresses work find when establishing a TCP connection
e.g. telnet or ssh.
They don't for UDP. 
  e.g.   rpcinfo -u bartok mountd

If I use the address on B, it works fine (as you would expect - same
subnet).
If I use the address on A it works fine (that is the default route
interface).

But if I use the address on C, it doesn't.

What happens is:

    - request goes from client to router
    - request goes from router to bartok interface on C
    - bartok issues an ARP for client on C interface which is WRONG
    - nobody replies to the ARP because client is on B, not C.

If I turn on proxy-arp on the router I can get the reply back, but I
would rather not do that.

So why does a reply to a UDP request arriving on subnet C from some
other subnet try to ARP out on subnet C instead of being routed
normally, while replies to TCP requests get routed properly and work
fine?

Is this a bug, or is there some configuration I can change?

I have double checked the subnet masks and broadcast addresses and
they work fine.
We have rp_filter set to 0 on all interfaces.
forwarding is set to 0, but setting it to 1 makes no difference.

Thanks,
NeilBrown