The first impact

README.md

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+# `docker-zonefiled`
+
+## What's this?
+
+This generates DNS zone files containing DNS hostnames and aliases of running docker containers in all docker networks for a given docker host.
+
+It generates zone files like this:
+
+```
+container_name.network_name.in-docker.internal. A 172.25.1.2
+alias.network_name.in-docker.internal. AAAA 2a02:F00:BA4::1:2
+container_name.backend_network.in-docker.internal. A 192.168.42.2
+```
+
+This is currently implemented as an trivial simple bash script utilizing `docker events` for listening on changes, `docker container inspect` to query the network details and some `jq` magic.
+
+## Why 'tho?
+
+- I wanted sane dns resolution from the docker host to docker containers.
+- nss-docker isn't enough or working.
+- DNS operations are well understood. Automagic name resolution via nss leads to unexpected behavior, if container name vanished and queries drop down to dns.
+- Using DNS subdomains host and aliases are structured per network name.
+- Your networking, routing and firewalling decides if you're able to reach the container interfaces.
+- Passing a different docker socket and alternative domain zone name can support multi-docker-setups.
+
+## Remarks
+
+### Input sanitization of user controlled values
+
+Specially grafted host and alias names of docker containers can inject stuff in the generated zone file, I guess.
+I'm currently open to discuss, how to catch invalid characters in host names.
+
+
+### Currently used `.internal` TLD
+
+This is currently used since it should never be resolved in the internet.
+Since it could be used in a private / enterprise context, it could be conflicting existing setups.
+I'm very open to discuss if a `.in-docker` TLD would be a sane idea.

docker-zonefiled

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+#!/bin/bash
+
+ZONE_FILE="$1"
+SOA="$(hostname -f). $(whoami).$(hostname -f)."
+ZONE="in-docker.internal"
+
+function generate_records() {
+	docker container ls -q \
+		| xargs docker container inspect 2>/dev/null \
+		| jq -r '.[] | select(.State.Running) | .NetworkSettings.Networks | to_entries | map(.key as $net | .value as $conf | (.value.DNSNames[] | (.+"."+$net) as $host | [ ($host + " IN A "+$conf.IPAddress), ($host + " IN AAAA " + $conf.GlobalIPv6Address)])) | flatten | .[]'
+}
+
+function update() {
+	SN="$(date +%y%m%d%H%M%S)"
+	cat <<EOF > $ZONE_FILE
+\$ORIGIN $ZONE.
+\$TTL 10	; default time to live
+
+@ IN SOA $SOA (
+           $SN	; serial number <yymmddHHMM>
+           60	; Refresh
+           60	; Retry
+           60	; Expire
+           10	; Min TTL
+           )
+
+$(generate_records)
+EOF
+}
+update
+
+docker events --filter 'type=network' --filter 'event=connect' --filter 'event=disconnect' --format '{{json .}}' \
+	| while read _event
+	do
+		update
+	done