Pi-hole with OpenWRT not resolving CNAME subdomains

Help
1

Details

I'm a networking newbie and I'm trying to set up a home server so I can run various applications including applications in docker containers. I've been trying to use subdomain names in CNAME entries on pihole, but I can't get them to resolve to the home server.

I've been poring over openwrt and pihole docs, forums, and guides trying to find a solution to this, but I haven't been able to crack it so I'm looking for help.

My setup includes:

  • Router: Netgear R6220
    • Running OpenWRT configured with pihole as the DNS server from the instructions (method 2).
    • IP: 10.0.0.1
  • PiHole: Pi Zero W
    • IP: 10.0.0.2
    • CNAME entry mapping api.iron.lan to iron.lan
  • Home Server: ASUS PN51
    • named iron.lan
    • no static ip
  • Docker Containers running on iron.lan:
    • envoy front proxy routing api.iron.lan to a container running a fastapi server
    • fastapi server

Expected Behaviour:

  • Subdomains should resolve to the home server.
  • An HTTP request from any machine on my lan to api.iron.lan should make it to the docker container running a fastapi server

Actual Behaviour:

  • Domains that map to actual machines on my lan resolve correctly (e.g. iron.lan, pihole.lan)
  • Subdomains mapped in CNAME to an actual machine do not resolve in browser or nslookup unless explicitly pointed to pihole as the DNS
  • When originating from the home server, I can make HTTP requests to "api.iron.lan" without issue.

From WSL on my laptop

pdoran@sodium:~$ nslookup iron.lan
Server:         172.22.208.1
Address:        172.22.208.1#53

Non-authoritative answer:
Name:   iron.lan
Address: 10.0.0.203
Name:   iron.lan
Address: fd12:193c:86bb::cfe

pdoran@sodium:~$ nslookup api.iron.lan
Server:         172.22.208.1
Address:        172.22.208.1#53

** server can't find api.iron.lan: NXDOMAIN

pdoran@sodium:~$ nslookup api.iron.lan 10.0.0.2
Server:         10.0.0.2
Address:        10.0.0.2#53

api.iron.lan    canonical name = iron.lan.
Name:   iron.lan
Address: fd12:193c:86bb::cfe

From the home server iron.lan

pdoran@iron:~$ curl http://api.iron.lan:10000/api/rest/time -w "\n"
{"time":"2023-03-30T06:01:36.956975"}

Debug Token:

https://tricorder.pi-hole.net/RkWK6OO2/

2

Note that Pi-hole does not see any HTTP requests.
A client will try to resolve a given domain from a URL prior to sending the actual HTTP request.
If indeed the HTTP request would get sent and would not make it to its destination, then Pi-hole would not be involved.

Pi-hole only ever sees DNS requests - including those for your CNAME.

CNAME-target
CNAME-target962×363 11.3 KB

With your current configuration as apparent from your debug log, a lookup for your CNAME would only work if if Pi-hole's cache is populated accordingly, e.g. after a previous nslookup iron.lan.

To make Pi-hole authoritative for iron.lan, just create the respective Local DNS Record.

Alternatively, create the respective subdomains like your api.iron.lan directly as Local DNS records.

EDIT:

That machine is using 172.22.208.1 for DNS, not Pi-hole.

3

Thanks for the advice. I did this and it worked immediately. However, a day later, I'm again unable to locate iron.lan and api.iron.lan. I don't understand why this would happen.

DNS:

Screenshot 2023-04-01 194005
Screenshot 2023-04-01 194005986×228 6.57 KB

CNAME:

Screenshot 2023-04-01 194052
Screenshot 2023-04-01 194052983×229 6.72 KB

OpenWRT
Screenshot 2023-04-01 194154

nslookup from pihole

pdoran@pihole:~ $ nslookup api.iron.lan
Server:         fd12:193c:86bb::1
Address:        fd12:193c:86bb::1#53

** server can't find api.iron.lan: NXDOMAIN

pdoran@pihole:~ $ nslookup api.iron.lan 10.0.0.2
Server:         10.0.0.2
Address:        10.0.0.2#53

api.iron.lan    canonical name = iron.lan.
Name:   iron.lan
Address: 10.0.0.194
Name:   iron.lan
Address: fd12:193c:86bb::cfe

nslookup from wsl on my laptop

pdoran@sodium:~$ nslookup api.iron.lan
Server:         172.22.208.1
Address:        172.22.208.1#53

** server can't find api.iron.lan: NXDOMAIN

pdoran@sodium:~$ nslookup api.iron.lan 10.0.0.2
Server:         10.0.0.2
Address:        10.0.0.2#53

api.iron.lan    canonical name = iron.lan.
Name:   iron.lan
Address: 10.0.0.194
Name:   iron.lan
Address: fd12:193c:86bb::cfe

nslookup from a Windows command prompt:

C:\Users\pdoran>nslookup iron.lan
Server:  OpenWrt.lan
Address:  fd12:193c:86bb::1

Name:    iron.lan
Addresses:  fd12:193c:86bb::cfe
          10.0.0.194


C:\Users\pdoran>nslookup api.iron.lan
Server:  OpenWrt.lan
Address:  fd12:193c:86bb::1

*** OpenWrt.lan can't find api.iron.lan: Non-existent domain

I suspect there are issues with wsl and windows, but looking at the network connection my OpenWRT settings are definitely broadcasting that pihole is the DNS server:

Screenshot 2023-04-01 194750

Are there additional settings with OpenWRT or pihole that I must do in order to get this to work?

4

That's a separate issue:
The failing nslookups are not using Pi-hole, but resolvers at 172.22.208.1 and fd12:193c:86bb::1, e.g.:

You should probably point the machines that ran those nsloookups to your Pi-hole for DNS as well.

But if those alternates are used by intention, you should make sure that those 172.22.208.1 and fd12:193c:86bb::1: use your Pi-hole as their only(!) upstream.

You may verify that your Pi-hole is in the upstream resolution chain by running:

nslookup pi.hole

By default, only Pi-hole knows how to resolve pi.hole.

5

This last bit helped tremendously; pi.hole was not resolving because it was pointing to the IPv6 server. Turns out that OpenWRT router was broadcasting itself as the IPv6 DNS server; I believe this was a default setting.

It needs to be disabled in OpenWRT under the lan interface settings:

Interfaces > Lan > DHCP Server > IPv6 Settings

Screenshot 2023-04-03 205708

Disabled the setting and rebooted the router (just in case) and everything seems to work as expected.

Thank you for your help!

6

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