init

README.md

@@ -0,0 +1,86 @@
+## DNSMasq divergence testbed
+
+This project spins up one `dnsmasq` in front of four backend DNS servers (CoreDNS). Each backend returns a different A record for the same name so you can observe how `dnsmasq` behaves when upstreams disagree.
+
+### Topology
+
+- **dnsmasq**: listens on host UDP/TCP `5353`, forwards to backends at `192.168.243.11-14`
+- **backend1..backend4 (CoreDNS)**: each serves a different answer for `test.local`:
+  - backend1 → `10.0.0.1`
+  - backend2 → `10.0.0.2`
+  - backend3 → `10.0.0.3`
+  - backend4 → `10.0.0.4`
+
+Custom bridge network: `192.168.243.0/24` with static IPs for reproducibility.
+
+### Files
+
+- `docker-compose.yaml`: services and fixed IP networking
+- `dnsmasq/dnsmasq.conf`: forwards to all four backends, logging enabled, caching enabled
+- `backends/backend*/Corefile`: CoreDNS configs returning distinct answers
+
+### Run
+
+```bash
+docker compose -f /home/akos/docker/dnsmasq/docker-compose.yaml up -d
+```
+
+Wait a few seconds until all containers are healthy.
+
+### Test from the host
+
+Query via `dnsmasq` on port 5353:
+
+```bash
+dig @127.0.0.1 -p 5353 test.local A +short
+```
+
+Run several times to observe responses and `dnsmasq` caching behavior. You should see one of: `10.0.0.1`, `10.0.0.2`, `10.0.0.3`, `10.0.0.4`.
+
+### Test from within the dnsmasq container (optional)
+
+The `andyshinn/dnsmasq` image is Alpine-based; install `dig` temporarily:
+
+```bash
+docker exec -it dnsmasq sh -c "apk add --no-cache bind-tools >/dev/null && dig @127.0.0.1 test.local A +short"
+```
+
+### Inspect logs
+
+`dnsmasq` query logging is enabled:
+
+```bash
+docker logs -f dnsmasq
+```
+
+### Adjusting behavior
+
+To explore how `dnsmasq` handles disagreement:
+
+- **Disable cache** (no stored answers): set `cache-size=0` in `dnsmasq/dnsmasq.conf`, then recreate the service.
+- **Force first-server order**: add `strict-order` to `dnsmasq/dnsmasq.conf` so servers are queried in listed order.
+- **Query all upstreams**: add `all-servers` so `dnsmasq` queries every upstream in parallel and picks the first reply.
+
+Apply changes by recreating the service:
+
+```bash
+docker compose -f /home/akos/docker/dnsmasq/docker-compose.yaml up -d --force-recreate dnsmasq
+```
+
+### Resetting the cache
+
+```bash
+docker restart dnsmasq
+```
+
+### Clean up
+
+```bash
+docker compose -f /home/akos/docker/dnsmasq/docker-compose.yaml down -v
+```
+
+### Notes
+
+- The backends are simple CoreDNS instances using the `hosts` plugin for `test.local`; unknown names forward to public resolvers.
+- The compose file exposes `53/udp` and `53/tcp` on host port `5353` to avoid clashing with any local resolver.
+