Redirect incoming connection on a given interface to the loopback interface

Hello,

I am using Shorewall 5.2.8 on a server that has three interfaces, one 
internal, one DMZ and one connected to the Internet.
On that server, there is a service that binds itself only on localhost 
and that cannot be configured otherwise.
As I would like to access it from the "loc" zone, I have tried writing 
this REDIRECT rule:
REDIRECT       loc             lo:1883 tcp     1883    - &loc

But when I check the configuration, I get this error:
  ERROR: Unknown destination zone (lo) /etc/shorewall/rules

Fair enough, I thus went at defining "lo" in zones despite my initial 
feeling it exists implicitly and did it like this in the zones file:
lo    locahost

This changes the error message to this:
ERROR: Rules may not override a NONE policy /etc/shorewall/rules

As this mentions policies, I thus went into the policy file and added 
this line:
loc             lo              ACCEPT

But this in turn gives me another error message:
ERROR: Policy "loc lo ACCEPT" duplicates earlier policy "loc lo NONE"

But there is no such policy in my policy file.

Thinking that there might be something special with the loopback 
interface, I also tried the following rule:
REDIRECT        loc             net:1883        tcp     1883 -       &loc

And this gives me yet another error:
  ERROR: A server IP address (1883) may not be specified in a REDIRECT 
rule /etc/shorewall/rules

So it seems that the syntax I'm trying to use is bogus, but I can't 
figure out how to express what I want in shorewall, being:

On the server itself, redirect connection from port PPPP on interface of 
zone ZZZ to port MMMM on interface of zone YYYY

I tried looking in the documentation but I could find an example for 
REDIRECT rules

Thanks for your help

Olivier