# BIND TSIG configuration for go-dns

This guide describes how to configure **BIND 9** so a **TSIG shared secret** can perform:

- **Dynamic updates** (RFC 2136) — the same operations as `nsupdate`
- **Full zone transfer (AXFR)** — read the entire zone over TCP

The **go-dns** library (`godns`) signs requests with TSIG; BIND verifies the signature and applies ACLs for that key. **Key generation is not part of go-dns** — operators create keys with BIND tooling or copy secrets from existing secret management.

## Prerequisites

- BIND 9.x authoritative server for the target zone
- Shell access to edit `named.conf` (or included snippets) and reload the server
- A TSIG key name, algorithm, and Base64 secret (see [Define the TSIG key](#define-the-tsig-key))

## Define the TSIG key

Generate a key on the server (or any host with BIND utilities):

```bash
tsig-keygen -a hmac-sha256 go-dns-key
```

Or:

```bash
rndc tsig-keygen -a hmac-sha256 go-dns-key
```

Output is a `key { ... };` block suitable for `named.conf`. Example:

```bind
key "go-dns-key" {
    algorithm hmac-sha256;
    secret "q4f6j2+8vK9mN3pR7sT1uW5xY0zA2bC4dE6fG8hI0jK=";
};
```

Place the key in `named.conf` or in an included file (e.g. `/etc/bind/keys.conf`) referenced with:

```bind
include "/etc/bind/keys.conf";
```

**Important:** Treat the secret like a password. Do not commit it to version control. go-dns callers pass the same key name, algorithm, and secret at runtime.

## Configure the zone

Reference the key in the zone block for the origin you want to update and transfer. Both ACLs are required for go-dns clients that perform updates and AXFR with the same key:

```bind
zone "example.com" {
    type master;
    file "/var/lib/bind/db.example.com";
    allow-update { key "go-dns-key"; };
    allow-transfer { key "go-dns-key"; };
};
```

| Directive | Purpose |
|-----------|---------|
| `allow-update` | Permits RFC 2136 UPDATE messages signed with this TSIG key |
| `allow-transfer` | Permits AXFR (full zone transfer) to clients signing with this TSIG key |

If you only need updates or only transfers, grant the corresponding directive alone. go-dns supports separate keys per client instance when policy requires different read and write credentials.

## Optional: stricter update policy

Broad `allow-update { key "..."; };` grants update access to the entire zone. For finer control, use **`update-policy`** instead of or in addition to `allow-update`:

```bind
zone "example.com" {
    type master;
    file "/var/lib/bind/db.example.com";
    allow-transfer { key "go-dns-key"; };
    update-policy {
        grant go-dns-key subdomain example.com. A AAAA TXT;
    };
};
```

Policy syntax and grant types are documented in the BIND 9 Administrator Reference Manual. Choose `allow-update` or `update-policy` according to your security model; do not leave zones open to unauthenticated updates.

## Reload and validate configuration

1. Check syntax:

   ```bash
   named-checkconf
   ```

2. Reload BIND (adjust command for your installation):

   ```bash
   rndc reload
   ```

3. Verify **dynamic update** (replace host, zone, and key values):

   ```bash
   nsupdate -y hmac-sha256:go-dns-key:BASE64SECRET <<EOF
   server ns.example.com
   zone example.com
   update add test.example.com 300 IN A 192.0.2.10
   send
   EOF
   ```

4. Verify **AXFR** with TSIG:

   ```bash
   dig @ns.example.com example.com AXFR -y hmac-sha256:go-dns-key:BASE64SECRET +tcp
   ```

   A successful transfer lists all zone records and terminates with the zone's SOA RR.

## Security notes

- **Restrict scope** — Grant the key only on zones that automation must manage. Avoid `allow-update { key ...; };` on parent zones unless intentional.
- **Do not expose AXFR publicly** — Zone transfers reveal full zone contents. Restrict `allow-transfer` to trusted networks or TSIG keys; never use `allow-transfer { any; };` on the Internet.
- **Separate read and write keys** — Use one TSIG key with `allow-transfer` only and another with `allow-update` only when policy requires split privileges. go-dns clients configure one key per session.
- **Rotate secrets** — When rotating a TSIG secret, update BIND and all go-dns callers together; mismatched secrets produce `BADSIG` errors.
- **Algorithm choice** — Prefer **hmac-sha256** (or stronger) over legacy hmac-md5 for new keys.

## Complete minimal example

The following fragment is a self-contained illustration (adjust paths and zone data file):

```bind
key "go-dns-key" {
    algorithm hmac-sha256;
    secret "q4f6j2+8vK9mN3pR7sT1uW5xY0zA2bC4dE6fG8hI0jK=";
};

zone "example.com" {
    type master;
    file "/var/lib/bind/db.example.com";
    allow-update { key "go-dns-key"; };
    allow-transfer { key "go-dns-key"; };
};
```

Ensure `db.example.com` exists with a valid SOA and NS records before loading the zone.

## Related documentation

- [Project specification](project-specification.md) — library scope, TSIG model, AXFR-only transfers
- [Change 00001 spec](changes/00001-setup-project/spec.md) — full DNS operation catalog and BIND guide requirements
- [Product backlog](backlog.md) — future work (e.g. IXFR)