Objective: When an external (ie, wlan) connection is attempted to port 26 on the radio, forward that traffic ("destination NAT") to a computer on my internal LAN.

Firewall rules in the radio (rules #3 & #7 in the filter chain, and rule #1 in the NAT chain, have been inserted by me):

/ip firewall filter print
Flags: X - disabled, I - invalid, D - dynamic
 0   ;;; default configuration
     chain=input action=accept protocol=icmp src-address=44.0.0.0/8

 1   ;;; default configuration
     chain=input action=accept connection-state=established

 2   ;;; default configuration
     chain=input action=accept connection-state=related

 3   chain=input action=accept protocol=tcp in-interface=wlan1-gateway dst-port=26

 4   ;;; default configuration
     chain=input action=drop in-interface=wlan1-gateway

 5   ;;; default configuration
     chain=forward action=accept connection-state=established

 6   ;;; default configuration
     chain=forward action=accept connection-state=related

 7   chain=forward action=accept protocol=tcp in-interface=wlan1-gateway dst-port=26

 8   ;;; default configuration
     chain=forward action=drop connection-state=invalid

/ip firewall nat print
Flags: X - disabled, I - invalid, D - dynamic
 0   ;;; default configuration
     chain=srcnat action=masquerade to-addresses=0.0.0.0 out-interface=wlan1-gateway

 1   chain=dstnat action=dst-nat to-addresses=192.168.0.250 protocol=tcp in-interface=wlan1-gateway dst-port=26

I use the same technique on my Linux boxes, and it works fine (albeit iptables is slightly different).  However, when accessing my radio from an external IP address, no connection is made (times out).  If I change the dstnat rule action to "accept", the connection is refused.  The logs for port 26 on the target device (192.168.0.250) show no connection attempt.  In the (default) srcnat chain, "action=masquerade" implies NATting on the return trip (into the LAN).  The same thing needs to happen in a dstnat chain, but I don't see a way to do that;  I'm "assuming" that the OS automatically does that.  When doing DNAT on Linux, I have to do that manually, with the same rule in the "PREROUTING" and "OUTPUT" NAT chains, but those chains don't exist in my radio.

Ideas welcome (note that "action=masquerade" is not valid in a dstnat chain).

-- Dean