Script to fix Docker iptables on Synology NAS

docker-iptables-fix.sh

#!/bin/bash
currentAttempt=0
totalAttempts=10
delay=15

while [ $currentAttempt -lt $totalAttempts ]
do
	currentAttempt=$(( $currentAttempt + 1 ))
	
	echo "Attempt $currentAttempt of $totalAttempts..."
	
	result=$(iptables-save)

	if [[ $result =~ "-A DOCKER -i docker0 -j RETURN" ]]; then
		echo "Docker rules found! Modifying..."
		
		iptables -t nat -A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
		iptables -t nat -A PREROUTING -m addrtype --dst-type LOCAL ! --dst 127.0.0.0/8 -j DOCKER
		
		echo "Done!"
		
		break
	fi
	
	echo "Docker rules not found! Sleeping for $delay seconds..."
	
	sleep $delay
done

I've been wasting so much time this week getting NGINX reverse proxy manager to work properly on Synology. If you want IP-whitelisting then being aware of actual outside IP's is essential and I just could not get it to work. Got headaches from macvlan in Docker and still it did not work. Thank you!

Is this problem unique to Synology?
The rule that I applied is the first suggestion by Pedro:
sudo iptables -t nat -A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
sudo iptables -t nat -A PREROUTING -m addrtype --dst-type LOCAL ! --dst 127.0.0.0/8 -j DOCKER

However I have a few thoughts.

The first one is security.
It seems that this is a bit of a brute force method that risks exposing other ports of other containers to the network. An attacker would have to break through the port forwarding of my router which does seem like an unlikely event. Then the solution by @Maypul to only script the ports that you are actually going to use sounds like smart paranoia.

The second thought I have is about complexity.
After a while there are so many tweaks to a system that without proper documentation to self (or others) somebody else or even myself is never going to figure out what or why these tweaks were done in case of a system reset or rebuild. Thus, it might be a smart move to install the docker containers that need to be aware of the outside IP to another system like a Raspberry Pi that runs somewhere in the network in more or less default config and that is dedicated to these tasks. That way it becomes clear what the purpose of that machine is and why it is setup the way that it is.

Anyway, just my thoughts but I'm curious about comments.

This doesn't work when trying to access the container from a reverse proxy

Awesome @Maypul thank you!

The change bypassing the synology firewall is understandable and the default docker behaviour.

Other rules added to the FORWARD chain, either manually, or by another iptables-based firewall, are evaluated after the DOCKER-USER and DOCKER chains. This means that if you publish a port through Docker, this port gets published no matter what rules your firewall has configured

https://docs.docker.com/network/packet-filtering-firewalls/#add-iptables-policies-before-dockers-rules

It should only affect ports that you published with docker.

I have my home network on eth0 and another network on eth1. For this reason I only want to accept connections from eth0.

adding the -i eth0 flag does the trick.

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -m addrtype --dst-type LOCAL -j DOCKER
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -m addrtype --dst-type LOCAL -j DOCKER
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 53 -m addrtype --dst-type LOCAL -j DOCKER
iptables -t nat -A PREROUTING -i eth0 -p udp --dport 53 -m addrtype --dst-type LOCAL -j DOCKER

None of these iptables rules have worked for me :(

I'm using a DS918+ and running DSM 7.2.

When I run the iptables script, the X-Forwarded-For IP address becomes the address of my router for some reason. So I don't get the client IP, but the IP of my router.

Does anyone know a fix? I've also tried disabling userland-proxy in the docker daemon, but that didn't work either. Or maybe I did something wrong.

@ben-ba Not sure if we're talking about the same idea. In my nextcloud container it seems to only see the XFF IP if it's an external/public IP. For example here two request:

Client Proxy Service Request appears to be from
10.0.0.2 172.16.0.2 172.30.1.2 172.16.0.2
42.199.8.17 172.16.0.2 172.30.1.2 42.199.8.17
(My local LAN is 10.0.0.0/24)

What I would like to achieve: In the example above the first request should also appear to be from 10.0.0.2 and not how it currently is 172.16.0.2.

Have you got any fix on this ?