selfpost/docs/plans/panel-dkim-signing.md

Plan: panel-dkim-signing

Status: draft, 2026-10-15 - the row is agreed (owner, 2026-10-15, the order port25-bare-newline -> base-trixie -> panel-dkim-signing); three questions below go to the owner one at a time; Q1 and Q2 are decided.
Date: 2026-10-15
Version: 2.8.0 MINOR (after base-trixie's 2.7.0). No migration; nothing for an operator to do.

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Goal

The panel signs outgoing mail itself, in its own milter, and OpenDKIM leaves the image. The analysis (own-mta-builds § 3 E) and the roadmap section give the reasons: OpenDKIM is a 2018 beta with three open 2026 CVEs and nobody upstream; every OpenDKIM limit SelfPost works around goes with it (the syslog relay, the From: parser port and its 256-byte refusal, the tables reloaded by a cross-uid signal and CAP_KILL, keys shared through a group).

What stays the same for an operator: the keys, the selectors, the DNS records, the panel's pages (one label on Health excepted, Q3).

What the signer does

• Library: github.com/emersion/go-msgauth/dkim v0.7.0 (MIT; the author of the go-milter v0.4.1 we run; the e2e suite verifies with its v0.6.8 today). Read for this plan, dkim/sign.go: NewSigner hashes the body as it is written (an io.Pipe into a goroutine), so a message is not buffered; HeaderKeys is written to h= in the order given, each name picked last instance first (header.go, newHeaderPicker), and a name listed once more than it occurs is kept in h= with nothing hashed for it - the over-signing RFC 6376 § 5.4.2 describes; RSA-SHA256; the signature is the caller's to insert.
• What is signed: relaxed/simple, as build/opendkim.conf signs today. h=: every header of the message whose name is in OpenDKIM's dkim_should_signhdrs[] (libopendkim/dkim.c:223-246: from, reply-to, subject, date, to, cc, the five resent-, in-reply-to, references, the seven list-), each occurrence, in message order, plus From once more (OversignHeaders From today). Absent names are not listed, as OpenDKIM does not list them.
• Which key: on 465/587 the key of the domain the SASL login belongs to - {auth_authen}, then Bindings.DomainOf(login), the table the sender check already reads (internal/milter/sender.go:262-269). For the panel's own notifications (sendmail(1), non_smtpd_milters) the key of the server's name, as EnsureServerSigned provides it today. The sender check has already held From: to that same domain, so the signature's d= is the From: domain DMARC aligns on.
• What Postfix hands a milter, read in Postfix 3.10.13, src/milter/milter8.c:2329-2400, milter8_header(): Postfix's own Received: is not shown (and an inserted header at index 0 lands above it, "relative to the message as delivered"); a multi-line header comes as text separated by \n; one space after the colon is eaten unless the milter asks for SMFIP_HDR_LEADSPC (OptHeaderLeadingSpace, which the sender check already asks for); body records end in \r\n (milter8_body(), :2470-2500). The signer rebuilds each header as Name: + value with \n made \r\n - go-msgauth's own cmd/dkim-milter writes name + ": " + value + "\r\n" and would sign a folded header wrongly. A unit test signs folded, tab-led and space-free headers and an independent verifier checks them.
• When it cannot sign (no key file, a login with no domain, a read error): temporary failure, default_action=tempfail, as OpenDKIM is attached today. A message is never sent unsigned by accident.
• 8-bit mail: disable_mime_output_conversion=yes. Postfix converts an 8-bit message to 7-bit for a server that does not announce 8BITMIME (default no, src/global/mail_params.h:3088), which breaks any body hash - OpenDKIM's today as well. Not set until now; set with this row.

Decisions settled before this plan

From the roadmap section and the analysis, agreed with the row: the library; relaxed/simple; the header set and From over-signed; the key by the login's domain and by the server's name for local mail; strict; the removals below; keys, selectors and records unchanged; the e2e oracle an independent verifier; a message to a real mailbox before the cut.

Decided here (technical, the owner may overrule):

• The oracle is dkimpy (python3-dkim 1.1.8-2 in trixie, /usr/bin/dkimverify, seen in debian:trixie 2026-10-15) - a separate implementation in another language, maintained, packaged by Debian; not OpenDKIM's opendkim-testmsg (the code this row removes) and not go-msgauth (the signer's own library). It runs in the e2e sink's network against the stand's CoreDNS.
• A cross-check before OpenDKIM goes: on the stand of stage 1 both sign the same messages for a while (OpenDKIM after the panel in the chain) and the two signatures' bh= and h= are compared on every e2e message; they must agree before stage 2 removes OpenDKIM.
• The From: parser port goes. It exists so that the sender check judges the address OpenDKIM chooses the key by. With the key chosen by the login, the check keeps net/mail (one RFC 5322 address at an allowed domain) and the 256-byte refusal goes; the refusal texts that name OpenDKIM are rewritten.

Open questions (to the owner, one at a time)

• Q1. Where the signer runs. (a) Its own role in the panel process on two sockets of its own: sign.sock for 465/587 (the key by the login) and sign-local.sock for local submission (the server's key only), in the chain where OpenDKIM is now: sender check, signer, journal. (b) Inside the sender check's milter: one socket fewer, but the check would receive every body, and local mail - which has no login - would need the same milter to tell "no login" from "local", deciding a key by the absence of something. Recommended: (a) - the socket a message comes through, not a missing login, decides that the server's key may sign it; the sender check stays a header-only milter; and moving the signer into a process of its own later is a change of where it is started. Decided (owner, 2026-10-15): (a).
• Q2. The key directory's name. /data/opendkim/keys/<domain>/<sel>.private stays as it is, or becomes /data/dkim/... (the entrypoint moves the tree once; KeyTable and SigningTable are deleted). A backup restores only into the image of its own version, so no older archive meets the new name. Recommended: rename - the cost is the entrypoint's move, the paths in the code and the backup tests; a directory named after a program the image no longer has would mislead whoever reads /data. Decided (owner, 2026-10-15): rename to /data/dkim.
• Q3. Health. The Overview's socket check and the process list name OpenDKIM and the syslog relay. Recommended: the socket row becomes "DKIM signer" (the panel's sign.sock, checked the same way), the two programs leave the process list. A mockup first, as for any page change.

Stages

Stage 1 - the signer, beside OpenDKIM (no release)

• internal/milter/sign.go: the session (Connect, MailFrom with {auth_authen}, Header, Headers, BodyChunk, Body), OptHeaderLeadingSpace, InsertHeader(0, "DKIM-Signature", ...) (the server's Actions gain add-header), the key read per message from the key file (a small cache keyed by path and mtime), the selector from the store as Record reads it.
• Unit tests: sign and verify with go-msgauth; folded and tab-led headers; a header absent and present twice; an empty body; a body without a final newline; a login with no domain and a missing key file answer tempfail; the server's socket never signs with a domain's key.
• cmd/panel: the role, its two sockets (listenMilter, group selfpost, 0660), the wrapper waits for them.
• build/postfix-config.sh: smtpd_milters = sender, sign, OpenDKIM, journal (OpenDKIM kept for the cross-check); non_smtpd_milters = sign-local, OpenDKIM; disable_mime_output_conversion=yes.
• e2e: dkimpy in the stand; every positive send and the notification verified by dkimpy against the panel's signature; the panel's and OpenDKIM's bh=/h= compared.

Stage 2 - OpenDKIM removed (no release)

• Chain: OpenDKIM out of both milter lists.
• Go: internal/domain/opendkim.go reduced to key storage (tables, Rebuild, assertConfigSafe, the USR1 reload go), internal/supervisor (its only caller is the reload), internal/syslogrelay, internal/milter/mailparse.go and its tests and testdata, the sender-check texts; Resync's "OpenDKIM's tables" step; Health's liveness set and socket row (Q3); OPENDKIM_SOCKET/OPENDKIM_DIR.
• Image: the opendkim package, opendkim.conf, the two supervisord programs, the /dev/log link and /run/opendkim, usermod -aG selfpost opendkim, crashexit's spared list; deploy/docker-compose.yml's KILL capability (its only use was the reload).
• Keys: /data/opendkim/keys -> /data/dkim/keys (Q2): the entrypoint copies the tree once and leaves the old one for a rollback to 2.7.x until the next release; KeyTable and SigningTable go with the new layout.
• Tests: test/upstream/*.sh, e2e's OpenDKIM log and process checks, the 256-byte test rewritten for the new rule.

Stage 3 - documents, the canary, the cut

• architecture § OpenDKIM -> § DKIM signing; security.md (the mail-path bullets, the accepted risks that were OpenDKIM's); guide; development's cited-package table (opendkim and libopendkim11 leave); the assumptions register's header notes what left; help (design commit first) and Health's label (Q3, mockup first).
• Reviews: technical and security.
• On the test server: a day of the owner's mail; one message from an application through 465 and one notification to a real mailbox, the headers read (dkim=pass with the panel's d=).
• Cut 2.8.0.

Risks

• A signing defect fails DMARC for every tenant at once. The answer is the oracle (another implementation), the cross-check against OpenDKIM on the same messages before it goes, and the canary before the cut.
• go-msgauth is small and released slowly. It can be vendored if it ever matters; the signing path is about 300 lines of it.
• Rollback: until stage 2 ships, the chain change is one line; after the cut, the previous image (2.7.x) still has OpenDKIM and reads the same keys - under Q2's rename, from the old path, which the move leaves as a copy until the next release.