[FS#4259] Cannot reach public-facing service in LAN from LAN; only reachable from WAN

[OpenWrt Bugs]

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User who did this - Douglas Silva (harpia) 

Attached to Project - OpenWrt/LEDE Project
Summary - Cannot reach public-facing service in LAN from LAN; only reachable from WAN
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - ====CONTEXT====
I run a publicly accessible web server at home, behind an OpenWrt router. The router has firewall rules allowing incoming connections to ports 80 and 443 (forwarding rules for ipv4; port access rules for ipv6).

====THE PROBLEM====
A recent snapshot upgrade made the web server unreachable from LAN using ipv4, while still being reachable from WAN. Ipv6 is unaffected and works as expected from both LAN and WAN.

====REPRODUCING====
1. Setup a web server inside your LAN network and confirm that it can be reached from outside. I use my phone's 4G connection for that.

2. Download https://downloads.openwrt.org/snapshots/targets/ramips/mt7621/openwrt-ramips-mt7621-tplink_archer-c6-v3-squashfs-sysupgrade.bin and move it to "/tmp" on the router that is already running OpenWrt.

3. Run "sysupgrade openwrt-ramips-mt7621-tplink_archer-c6-v3-squashfs-sysupgrade.bin" and wait for the device to reboot.

4. SSH into it as root, set a root password, apply any needed UCI configuration* and reboot.

5. See if you can reach the web server from inside the LAN using its public ipv4 address.

* I use a shell script to automatically configure everything using UCI commands. It defines DHCP hosts, firewall rules, PPPoE authentication, etc.

====DETAILS====
Device: TP-Link Archer C6 V3 (ramips/mt7621)
OpenWrt version: SNAPSHOT (r18754-f6a01d7f5c)
- Last working snapshot I used was from December 2021, but I don't have its snapshot number.

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=4259

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