Mario Fetka
57 lines
2.7 KiB
Markdown
57 lines
2.7 KiB
Markdown
# Mail authentication
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Bongo 0.7 uses separate, established libraries for each mail-authentication
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job:
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* libspf2 checks the SMTP client and envelope identity on incoming mail.
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* OpenDKIM verifies incoming signatures and signs outgoing mail.
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* OpenDMARC evaluates SPF and all DKIM results against the single RFC5322
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`From` domain.
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* GNU Mailutils parses the RFC5322 `From` field; it is not an SPF, DKIM or
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DMARC engine.
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* libsrs2 rewrites the envelope sender when mail is forwarded and reverses the
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resulting SRS recipient when a bounce returns.
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Incoming checks run on unauthenticated external SMTP sessions. Authenticated
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submission and the restricted internal-relay listener are not rejected by
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DMARC. Bongo writes the local SPF, DKIM and DMARC decisions into one
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`Authentication-Results` field before handing the message to the queue.
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At that trust boundary, all incoming `Authentication-Results` fields are
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removed because an unauthenticated sender can forge them. Bongo then adds one
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fresh local result field after verification.
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The relevant `smtp` configuration keys are:
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* `spf_verify`, `dkim_verify`, and `dmarc_verify` enable the incoming checks.
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* `dmarc_enforce` defers temporary lookup failures and rejects messages when a
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sampled `p=reject` policy applies. A `p=quarantine` result remains accepted
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and is recorded for later filtering; it is never treated as `reject`.
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* `dkim_sign_outgoing`, `dkim_key_directory`, `dkim_selector`, and
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`dkim_signing_domain` control outgoing DKIM signatures.
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* `srs_forward`, `srs_reverse`, `srs_domain`, and `srs_secret_file` control
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SRS forwarding and bounce reversal.
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The SRS secret and DKIM private keys must not be readable by other users. SRS
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addresses are envelope addresses such as `SRS0=...@example.org`; SRS is not a
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message-header format.
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SPF and DMARC are DNS policies rather than per-message signatures. The setup
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wizard validates their DNS records, while the SMTP agents only evaluate them
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at delivery time. It writes public-key hints below
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`/etc/bongo/dkim.d/DOMAIN` and suffixless per-domain check state below
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`/etc/bongo/dmarc.d`.
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The generated starting policy is deliberately conservative:
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```text
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DOMAIN TXT v=spf1 ip4:PUBLIC_IPV4 -all
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SELECTOR._domainkey.DOMAIN TXT v=DKIM1; k=rsa; p=PUBLIC_KEY
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_dmarc.DOMAIN TXT v=DMARC1; p=none; rua=mailto:dmarc-reports@DOMAIN
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```
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If no public outbound IPv4 address is supplied, the SPF suggestion uses `mx`
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and is marked for review. Bongo follows SPF RFC 7208, DKIM RFC 6376, current
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DMARC RFC 9989, and aggregate-report RFC 9990. RFC 9989 removed the historical
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`pct` tag; use `t=y` while testing a stricter policy. Start with `p=none`,
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review reports, then deliberately progress to `quarantine` or `reject`.
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