[Bug] Local domain resolution suddenly stopped working

[WildeBeast2521]

This is a: BUG

Details

I'm using both DHCP and DNS from Pi-Hole on my Ubuntu Server machine. When using Pi-Hole as DHCP, it automatically fills in the DNS servers for the clients as itself. So for example, if my server is on 192.168.100.10 and fe80::abcd:efg:hijk:12l3, these are the DNS servers for each client.
Today, suddenly all my local domain resolutions are failing (except on the DNS server itself) from my client devices. When I look up a local device with nslookup, e.g. mobile.home.arpa (home.arpa is the domain name I have set in Pi-Hole), it returns:

nslookup mobile.home.arpa
Server:  UnKnown
Address:  fe80::1

*** UnKnown can't find mobile.home.arpa: Non-existent domain

I noticed that the address fe80::1 looked odd so I pinged the address and it actually is the local-link address of the DNS server:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute        <======
       valid_lft forever preferred_lft forever

But it seems like it's the loopback address? So I tried adding the local-link address that I mentioned above (fe80::abcd:efg:hijk:12l3) into my router, and surely enough, domain resolutions started to work again:

nslookup mobile.home.arpa
Server:  dns
Address:  fe80::abcd:efg:hijk:12l3

Name:    mobile.home.arpa
Address:  192.168.100.12

When I removed the local-link address from the router, domain resolutions stopped working again.
I thought it must be a temporary issue so I restarted my entire network, from the router to the DNS server itself, but I couldn't fix it the address problem; only the local-link address fe80::abcd:efg:hijk:12l3 seems to work.
I'm not sure if this is the way it's configured from the start but I'm noticing this now. There hasn't been any change in Pi-Hole's settings recently.
Is this the default behaviour for filling up DNS servers? If so, should it suddenly be causing issues?

Related Issues

• I have searched this repository/Pi-hole forums for existing issues and pull requests that look similar

How to reproduce the issue

1. Environment data

• Operating System: Ubuntu
• Hardware: PC
• Kernel Architecture: amd64
• Docker Install Info and version:
  • Software source: official docker-ce
  • Supplimentary Software: none
• Hardware architecture: x86

2. compose.yml

services:
  pihole:
    container_name: pihole
    image: pihole/pihole:latest
    ports:
      - "53:53/tcp"
      - "53:53/udp"
      - "80:80/tcp"
      - "443:443/tcp"
      - "67:67/udp"
    environment:
      TZ: 'Asia/Karachi'
    volumes:
      - './pihole:/etc/pihole'
    cap_add:
      - NET_ADMIN
      - SYS_NICE
    restart: unless-stopped
    network_mode: "host"

These common fixes didn't work for my issue

• I have tried removing/destroying my container, and re-creating a new container
• I have tried fresh volume data by backing up and moving/removing the old volume data
• I have tried running the stock docker run example(s) in the readme (removing any customizations I added)
• I have tried a newer or older version of Docker Pi-hole (depending what version the issue started in for me)
• I have tried running without my volume data mounts to eliminate volumes as the cause

[Tamas92]

I had similar issue in the past. I put the host IP (Pihole's IP) before the DNS ports and it solved my issue. Give it a try, maybe it helps.
...

 ports:
      - "xxx.xxx.xxx.xxx:53:53/tcp"
      - "xxx.xxx.xxx.xxx:53:53/udp"

....

[yubiuser]

So basically your issue is that Pi-hole adds its loopback address as DNS server to the DHCP offering instead of its link-local address?

[WildeBeast2521]

@Tamas92
I don't think you quite understand the issue.

I made the original issue at pi-hole/pi-hole and pi-hole/docker-pi-hole. I don't know how the issue is in pi-hole/FTL. I sent this comment in pi-hole/pi-hole:

Found out that fe80::1 is my gateway, which means for IPv6, Pi-hole is making the router my IPv6 DNS. 🤔

Edit: It was transferred by yubiuser, I see.

When acting as a DHCP server, Pi-hole should automatically fill in the right IP addresses for the clients, which should be the server's link-local address for IPv6. Here, it's giving the clients the router's link-local address, which I found by checking my IPv6 default gateway.

@yubiuser
Situation explained above.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Please comment or update this issue or it will be closed in 5 days.

[WildeBeast2521]

Hey! Anyone available to review and figure this bug out, please?

@yubiuser
Any updates?

[github-actions]

This issue is stale because it has been open 30 days with no activity. Please comment or update this issue or it will be closed in 5 days.

[WildeBeast2521]

Please acknowledge.