panel-dkim-signing stage 1: the panel's DKIM signer, beside OpenDKIM

internal/milter/sign.go: a milter on two sockets (cmd/panel/signer.go):
sign.sock for 465/587, the key of the domain the SASL login belongs to
(Bindings.DomainOf, the sender_login_maps Postfix reads), and
sign-local.sock for the panel's own sendmail(1) mail, the server's name.
go-msgauth/dkim v0.7.0, relaxed/simple, OpenDKIM's dkim_should_signhdrs[]
(libopendkim/dkim.c:223-246) in message order plus From once more; keys
and selectors from the KeyTable the panel writes, a key path outside the
key directory refused; strict - whatever cannot be signed is deferred.

From the sources: Postfix 3.10.13 milter8.c:2329-2400 hands a folded
header \n-separated, so the signer joins its lines with CRLF; inserts are
1-based in cleanup_ins_header() (cleanup_milter.c:1193), so index 0 is the
top, where OpenDKIM puts its own (opendkim.c:3673). go-milter v0.4.1 calls
no Abort when a connection ends, so a watchdog closes go-msgauth's
streaming signer after an hour and a late end of message is deferred.

Postfix: smtpd_milters = sender check, signer, OpenDKIM, journal;
non_smtpd_milters = the signer's local socket, OpenDKIM - both sign every
message until stage 2. disable_mime_output_conversion=yes (default off,
mail_params.h:3088): an 8-bit message is no longer re-encoded after it is
signed.

Tests: unit tests sign and verify folded, tab-led and space-free headers,
an added From:, a changed body, two messages on a connection, odd bodies,
and five cases deferred. e2e: the sink image has python3-dkim (dkimpy
1.1.8-2) and the stand's DNS; on an application's message and on a
notification the two signatures must agree on h=, bh=, c=, s=, a= and each
must pass dkimverify. On the server: make e2e exit 0 (714 s), go vet + go
test 27 packages ok.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build/postfix-wrapper.sh

@@ -2,8 +2,9 @@
 # Postfix start wrapper: solves the cold-start race where Postfix would try to
 # reach the milter sockets before they are listening.
 #
-# It blocks until the milter sockets — OpenDKIM, the panel's sender check and
-# journal-milter and, with the inbound relay on, its inbound journal milter —
+# It blocks until the milter sockets — OpenDKIM, the panel's sender check,
+# DKIM signer (two) and journal-milter and, with the inbound relay on, its
+# inbound journal milter —
 # are present, then runs `postfix start-fg` and stays its parent. If they are
 # not ready within the timeout it exits non-zero WITHOUT starting Postfix.
 # supervisord counts that exit as one that came too quickly - the program's
@@ -22,10 +23,12 @@ set -eu
 OPENDKIM_SOCK="${OPENDKIM_SOCKET:-/run/opendkim/opendkim.sock}"
 JOURNAL_SOCK="${JOURNAL_MILTER_SOCKET:-/run/selfpost/journal.sock}"
 SENDER_SOCK="${SENDER_CHECK_SOCKET:-/run/selfpost/sender.sock}"
+SIGN_SOCK="${DKIM_SIGN_SOCKET:-/run/selfpost/sign.sock}"
+SIGN_LOCAL_SOCK="${DKIM_SIGN_LOCAL_SOCKET:-/run/selfpost/sign-local.sock}"
 TIMEOUT="${MILTER_WAIT_TIMEOUT:-30}"
 INTERVAL=1
 
-SOCKS="$OPENDKIM_SOCK $JOURNAL_SOCK $SENDER_SOCK"
+SOCKS="$OPENDKIM_SOCK $JOURNAL_SOCK $SENDER_SOCK $SIGN_SOCK $SIGN_LOCAL_SOCK"
 if [ "${INBOUND_RELAY_ENABLE:-false}" = "true" ]; then
 	SOCKS="$SOCKS ${INBOUND_JOURNAL_MILTER_SOCKET:-/run/selfpost/inbound-journal.sock}"
 fi