Uptime Kuma can't perform DNS monitoring on a DNS server which is on
the same Docker network as itself
reddit / selfhosted

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CincyTriGuy · 2023-11-30 18:20

Apologies if the title is confusing.

I have 2 CoreDNS servers. Both are running on Docker containers, on 2
separate hosts.

I want to stress: NAME RESOLUTION IS WORKING JUST FINE. All of my
clients are pointing at these servers for primary and secondary DNS,
and internal and external resolution is working just fine.

Uptime Kuma and CoreDNS are running on Host 1 (10.118.97.5), on the
same Docker network. The other instance of CoreDNS is running on Host
2 (10.118.97.6).

I have 2 DNS monitors on Uptime Kuma. They are both configured to
resolve an A record for www.google.com. The only difference is one of
the monitors uses 10.118.97.5 as the resolver, and the other uses
10.118.97.6 as the resolver. The one using .6 as the resolver works
just fine, but the one using .5 as the resolver times out and will not
resolve.

Any idea what the issue could be? Uptime Kuma is successfully
monitoring HTTP servers on the same Docker network as itself, as well
as pings, but for some reason port 53 is going into a vacuum.

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rogersa2k · 2023-11-30 20:43

I wish I could be more helpful... I'm wondering if you're using
"macvlan"? I've had to use a "shim" in the past when doing something
similar.

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CincyTriGuy · 2023-11-30 21:04

Good question, I have should clarified in my post. All containers are
on a custom bridge network.

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zoredache · 2023-11-30 23:11

> Any idea what the issue could be?

No idea, but I would start by running netshoot container interactively
on your custom bridge and running some queries with dig against the
two DNS servers.  Maybe try pinging the two IPs.

Then maybe start netshoot and attach it to your uptime_kuma container
namespaces, and run tcpdump and capture dns queries to the failing
server.

https://github.com/nicolaka/netshoot

    # connect to your bridge
    docker run -it --net custom_bridge nicolaka/netshoot

    # monitor your kuma network
    docker run -it --net container:uptime_kuma nicolaka/netshoot

    # tcpdump command
    tcpdump -ni any port 53 and \( host 10.118.97.5 or host
10.118.97.6 \)

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somebodyknows_ · 2023-12-01 12:12

I remember I had a similar issue and the solution was something like
that.