docs: add mail runbook page (mailbox and domain add procedures)

docs/src/mail.md

@@ -0,0 +1,90 @@
+# Mail
+
+A single mail server, `mx1`, runs the [Simple NixOS Mailserver](https://nixos-mailserver.readthedocs.io/)
+stack (Postfix + Dovecot + Rspamd + OpenDKIM) for the `cnx.email` domain. All of
+it is declared in `modules/mail.nix`, imported by `machines/mx1/configuration.nix`.
+
+Mailboxes are **virtual** (not system users): each address is a login account
+whose password is a four-word passphrase auto-generated by a clan vars generator
+(e.g. `otter-lantern-cobalt-driftwood-42`). The generator stores both the
+passphrase (handed to the user) and its sha-512 hash (consumed by SNM); plaintext
+never lands in the repo.
+
+The mail cert is obtained by `mx1` itself via ACME DNS-01 against `ns1` (key
+`acme_mx1`), and carries `mx1.cnx.email` plus the `mta-sts.cnx.email` and
+`mail.cnx.email` SANs. See [DNS → ACME DNS-01](./dns.md#acme-dns-01).
+
+## Add a mailbox
+
+1. Append the address to `accounts` in `modules/mail.nix`:
+   ```nix
+   accounts = [
+     "postmaster@cnx.email"
+     "alice@cnx.email"      # new
+   ];
+   ```
+2. Mint its generated password (user-run, YubiKey touch):
+   ```
+   clan vars generate mx1
+   ```
+3. Redeploy: `clan machines update mx1`.
+4. Hand the passphrase to the user. The generator name replaces `@`→`-at-` and
+   `.`→`-`:
+   ```
+   clan vars get mx1 mail-passwd-alice-at-cnx-email/passphrase
+   ```
+
+The DMARC report inbox (`dmarc@cnx.email`) is special: its password comes from
+the **shared** `mail-dmarc-cred` generator (not the per-machine set) so parsedmarc
+on `control` can read the same passphrase over the mesh.
+
+## Add a domain
+
+Adding a second mail domain is more than a config line — it needs its own DNS
+records and DKIM key. Steps:
+
+1. **Serve the zone** (if not already): add it in `modules/dns/domains.nix` and
+   drop a `<domain>.zone` file — see [DNS](./dns.md).
+2. **Declare the domain** in `modules/mail.nix`:
+   ```nix
+   mailserver.domains = [ "cnx.email" "newdomain.tld" ];
+   ```
+   Add at least a `postmaster@newdomain.tld` account (RFC 5321 requires it) the
+   same way as above.
+3. **Publish the mail DNS records** in the new zone, mirroring `cnx.email`'s
+   `; ---- Mail ----` block:
+   - `MX 10 mx1.cnx.email.` — point at the existing MX; do **not** stand up a new
+     hostname unless you really want a separate server.
+   - SPF: `@ TXT "v=spf1 mx -all"`.
+   - DKIM: SNM generates a key per domain on first start under
+     `mailserver.dkimKeyDirectory` (selector `mail`). After deploy, read the
+     public key off `mx1` and paste it as `mail._domainkey TXT`. The private keys
+     are declared as clan state and backed up.
+   - DMARC: `_dmarc TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@cnx.email; ..."`
+     — reports can keep flowing to the existing `dmarc@cnx.email` inbox.
+4. **DANE / MTA-STS are tied to `mx1.cnx.email`**, the MX hostname — they do **not**
+   need duplicating per sending-domain. A new domain that uses `mx1.cnx.email` as
+   its MX inherits the existing TLSA and MTA-STS policy. Only add new TLSA/MTA-STS
+   records if you introduce a new MX hostname.
+5. `clan vars generate mx1` (if you added accounts), then
+   `clan machines update mx1`.
+
+## DKIM, DANE, MTA-STS reminders
+
+These three are easy to get subtly wrong; the live records for `cnx.email` and
+the exact commands to regenerate them are in `modules/dns/zones/cnx.email.zone`:
+
+- **DKIM** key rotation = regenerate under `dkimKeyDirectory`, then republish the
+  `mail._domainkey TXT`.
+- **DANE TLSA** (`_25._tcp.mx1 TLSA 3 1 1 …`) is the SHA-256 of `mx1`'s cert
+  **public key**. It stays valid across renewals because lego runs with
+  `--reuse-key`; only recompute it if the key changes. The `openssl` one-liner is
+  in the zone file.
+- **MTA-STS**: bump the `_mta-sts TXT` id **every time** the policy in
+  `modules/mail.nix` changes, or senders keep the cached policy.
+
+## DMARC reporting
+
+`mx1` delivers aggregate/forensic DMARC reports to `dmarc@cnx.email`; parsedmarc
+on `control` polls that mailbox over the mesh and feeds the results into Grafana.
+See [Monitoring](./monitoring.md).