bongo/docs/tls.md

# TLS policy

Bongo offers TLS 1.3 and TLS 1.2 by default. Older protocol versions are only
available on listeners whose explicit legacy-TLS option is enabled. Keep those
listeners restricted to trusted legacy networks; do not expose them directly
to the Internet.

The SMTP delivery agent uses opportunistic STARTTLS. When a peer offers
STARTTLS, it encrypts the connection. `outbound_tls_verify` additionally
verifies the certificate chain against the system trust store and verifies the
connected MX or configured LMTP host name. It defaults to `false`, because
public Internet MX servers cannot universally be assumed to have a usable
public-PKI certificate. Enable it for a closed set of known destinations.

`outbound_tls_required` defaults to `false`, so delivery remains possible to
SMTP servers which do not offer STARTTLS. Enabling it defers mail instead of
sending plaintext. It is a global policy and should only be enabled when every
configured destination is known to support TLS.

`outbound_dane_enabled` defaults to `true`. RFC 7672 DANE is activated only
when both the MX and the address used for the connection were authenticated by
DNSSEC and the TLSA lookup is independently DNSSEC-validated by GnuTLS/libdane.
A secure usable DANE-EE(3) or DANE-TA(2) record makes STARTTLS and DANE
authentication mandatory; DANE-TA also checks the MX and original next-hop
names. PKIX-TA(0) and PKIX-EE(1) records do not become SMTP DANE trust anchors.
A secure TLSA RRset containing no usable SMTP records still makes TLS
mandatory. Bogus or indeterminate DNSSEC results defer delivery. Insecure DNS
or securely proven TLSA absence leaves the existing opportunistic STARTTLS
policy in place.

`outbound_mta_sts_enabled` also defaults to `true`. When DANE is inactive,
Bongo discovers the RFC 8461 policy at `_mta-sts.<domain>`, downloads it only
over certificate-validated HTTPS, and keeps the validated policy in
`outbound_mta_sts_cache_directory`. An `enforce` policy permits only listed
MX hosts and requires authenticated TLS 1.2 or newer; policy, certificate,
STARTTLS, and handshake failures defer delivery without a plaintext retry. A
`testing` policy records the same failures without changing delivery. A valid
cached policy remains authoritative through its `max_age` when DNS discovery
or HTTPS refresh temporarily fails. DANE always takes precedence over
MTA-STS.

When outbound TLS reporting is enabled, the SMTP delivery agent records one
final TLS policy result for each attempted destination. The worker aggregates
completed UTC days into RFC 8460 JSON reports without losing late events or
double-counting retries. It preserves the exact DNS TLSRPT policy that was
used for delivery, waits a randomized interval of up to four hours, and sends
one durable delivery to every `mailto:` and `https:` reporting URI. Reports
are gzip-compressed; mail reports use `multipart/report` with an
`application/tlsrpt+gzip` attachment, while HTTPS reports use a validated
HTTPS POST. Temporary failures are retried with persisted exponential backoff
for 24 hours after the initial attempt.

TLS report messages carry `TLS-Required: No` and bypass outbound DANE,
MTA-STS, and TLSRPT accounting so a receiver's TLS failure cannot create a
report loop. They still prefer STARTTLS. RFC 8460 requires email reports to be
DKIM-signed, so `mailto:` delivery remains pending unless outbound DKIM is
enabled and the configured key for the reporting domain is readable. The
submitter defaults to the DKIM signing domain, or otherwise to the configured
Bongo host name. HTTPS report delivery does not depend on DKIM.

The resolver named in `resolv.conf` must therefore be a trusted validating
resolver reached over a trusted local path; Bongo must not trust an AD bit
received from an untrusted network resolver. GnuTLS/libdane also needs its
DNSSEC root trust anchor installed by the operating system.

External-account collection and submission use libcurl and independently
verify certificates and host names by default. See `external-accounts.md` for
the credential and provider policy.

Initial setup stores the default certificate and key as
`/etc/bongo/ssl.d/server.crt` and `/etc/bongo/ssl.d/server.key`. The key is
root-owned, group-readable by Bongo, and never world-readable. Setup creates a
3072-bit RSA self-signed certificate only when no pair exists. A complete
existing pair is parsed and checked for a matching key before reuse; unsafe,
invalid, or incomplete material fails closed.

The `bongoworker` ACME job implements RFC 8555 directly. It honors CA
`Retry-After` limits and renewal information, writes candidate material first,
validates the complete certificate/key pair, then deploys it atomically and
signals the manager to reload TLS listeners. HTTP-01 is the default. DNS-01
publishes `base64url(SHA-256(keyAuthorization))`, waits for the exact TXT value
through ldns, and removes only the value created for that authorization.

For DNS-01, `/etc/bongo/acme.d/providers` selects the longest matching zone.
Use `type: "nsupdate"` for the native RFC 2136 adapter; `rfc2136` is an alias.
No external `nsupdate` process is used. The adapter supports SOA-primary
discovery, explicit IPv4/IPv6 or host-name servers, non-default ports, and
TSIG HMAC algorithms. Delegated CNAME challenges use `match_zone`,
`challenge_alias`, and the provider's target `zone`. Installed examples are
in `/usr/share/bongo/examples/acme`.

`bongo-admin migrate` moves the historical `osslcert.pem` and `osslpriv.pem`
pair from the state database directory into `ssl.d` only when both files are
present and the destination is empty. ACME deployments should atomically
install a complete pair and reload or restart the affected Bongo agents.