GRE tunnel through NAT

[Bob George]

I'm trying to set up a GRE tunnel through a NAT gateway as follows:

1. External GRE traffic arriving on the external interface of gateway A 
should be redirected to an inside interface on B. The GRE tunnel 
endpoints are the remote system C and eth1 on B.

2. Traffic exiting machine A's external interface (eth1) should be NAT'd 
to the external IP of that interface.

Hopefully this drawing will make everything clear:

           eth1 +---+ eth2      eth1 +---+
                |   | .254      .253 |   |
C --- INTERNET -+ A +----- DMZ ------+ B |
  <external_ip> |   | 172.16.23.0/24 |   |
                +---+                +---+
                ^                    ^
                NAT                  tunnel endpoint
 <----- GRE TUNNEL ---------------->

I HAVE been able to:

* Create GRE tunnels between C-A, and A-B with no problems.
* Redirect other non-GRE inbound traffic on A eth1 to B eth1.
* Connect from B to external systems with TCP, UDP & ICMP.

I have tried various combinations of iptables configurations with no 
luck so far with GRE. Here are the relevant parts of the configuration on A:

--- cut here --- cut here ---

iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to
iptables -N eth1-in
iptables -A INPUT -i eth1 -j eth1-in

# NAT B traffic - THESE WORK
iptables -A eth1-in --protocol tcp -i eth1 --destination-port 8200:8202 
-j ACCEPT
iptables -t nat -A PREROUTING -i eth1 --protocol tcp --destination-port 
8200:8202 -j DNAT --to 172.16.23.253

# NAT inbound GRE traffic - THIS DOESN'T WORK
iptables -A eth1-in --protocol 47 -i eth1 -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 --protocol 47 -j DNAT --to 
172.16.23.253

--- cut here --- cut here ---

I have also configured A with basic rules NAT'ing all outbound traffic 
of eth1, and setting ALLOW policies for FORWARD, INPUT and OUTPUT. I've 
also tried removing the DNAT and SNAT rules for GRE in various 
combinations. All other traffic worked well, but still no GRE.

I did some captures using tcpdump on both systems. I could see GRE 
traffic coming in to eth1 on A, and ICMP responses from A to C 
indicating "protocol unreachable." I never saw the outbound GRE traffic 
going through NAT, though other outbound traffic from B was fine.

I've searched various howtos, and found numerous articles that make me 
think it should work easily, yet I'm stumped. I'm curious if anyone has 
successfully done GRE via NAT. Am I (hopefully) misssing something obvious?

Thanks,

- Bob