## 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.