from-header-binding: the sender check, what it costs, its pages; guide, architecture, security

A message's From: line is held against the application that sends it, under
the rule Postfix holds the envelope sender to. OpenDKIM chooses the key by
that line, so until now an application could have mail signed as another
domain of the server or as the server itself.

The check (internal/milter/sender.go)
- A role of the panel on a socket of its own (SENDER_CHECK_SOCKET), first in
  smtpd_milters and strict: default_action=tempfail. The journal milter keeps
  its socket and stays fail-open. postfix-wrapper waits for the new socket.
- It reads no database: what an application may send as comes from Postfix's
  own sender_login_maps, what the server signs for from OpenDKIM's
  SigningTable. Both are read again when they change; one that cannot be
  read defers mail.
- One From: field, one address, allowed by the envelope's lookup; then no
  other name of the SigningTable may appear anywhere in the raw line.
  Refused: 550 5.7.1 naming the line, reduced to printable ASCII.
- A refusal is a send-log row with reason "from" and the header address
  (migration 0006), written by another goroutine and not waited for. The
  journal's refusals record their reason too (limit, client), and the
  notifier takes only a limit's.

What it costs (docs/plans/from-header-binding.md, Statistics)
- The role marks itself alive every 2 s; at start the gap since the mark is
  downtime, counted from Postfix's start if later and not at all when
  supervisord says Postfix is not running.
- The log tailer counts the connections Postfix turned away: a warning that
  names the socket, then milter-reject "Service unavailable" from the same
  smtpd. Seen on the stand: the client is refused from CONNECT on and leaves
  at AUTH (454), so no MAIL line is ever written.
- sender_check_days, a row per UTC day, kept a year.

Pages
- Health: the card "Sender check", the socket's row, and the cards layout -
  boxes numbered by importance in two stacks read in rows; the left stack
  polls, the three readings of the right one are swapped in by id.
- Outbound log: a refused row says why in its tag (wrong From, wrong client,
  rate-limited); the message's page shows the From: line and what the
  application may send as.
- Help: topic "Sender check", word for word the mockup's.

Found by the stand, fixed here: the names for the second look came from the
sender bindings, and a domain with a key and no application yet was not
among them.

e2e: negative_journal_milter_fail_open is gone - it proved that mail passes
with the panel down, which is no longer so. In its place
sender_check_holds_the_from_line (another domain's From:, the hostname's, a
stranger's, an address off the list, another sender in the display name -
refused; own addresses - delivered) and panel_down_defers_mail (4xx with the
panel stopped, the container alive, mail again after, downtime and one
deferred message on Health, the card warning).

Test server, this tree before the last cosmetic edits (a fixture row, a
fact's size, docs): go vet ok, go test 23 packages ok, make e2e ok (337 s).
Locally after them: internal/web/view ok.

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

internal/web/view/pages_test.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"regexp"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -158,47 +159,55 @@ func TestHealthShowsEveryCheckAndBothActions(t *testing.T) {
 		`hx-get="/server/health/fragment"`,
 	)
 	pageLacks(t, "Health", out, `hx-swap-oob`, `/overview/fragment`)
-	if got := strings.Count(out, `class="columns"`); got != 3 {
-		t.Errorf("Health has %d rows of boxes, want 3", got)
+	if got := strings.Count(out, `class="columns sp-cards"`); got != 1 {
+		t.Errorf("Health has %d sets of cards, want 1", got)
 	}
-	// The Configuration box is outside the polled rows: a refresh must not re-render a control.
-	if strings.Index(out, `id="configuration"`) < strings.Index(out, `id="health-checks"`) ||
-		strings.Contains(out[strings.Index(out, `id="health-machine"`):strings.Index(out, `id="configuration"`)], `action="/server/health/reload"`) {
-		t.Error("the reload control is inside the polled rows")
+	// The boxes are numbered by importance and read in rows: the odd ones are
+	// the left stack, the even ones the right.
+	order := []string{`id="processes"`, `id="sender-check"`, `id="hostname"`, `id="machine"`,
+		`id="sockets"`, `id="certificate"`, `id="notification-mail"`, `id="configuration"`}
+	for i := 1; i < len(order); i++ {
+		if strings.Index(out, order[i-1]) < 0 || strings.Index(out, order[i]) < strings.Index(out, order[i-1]) {
+			t.Errorf("%s does not come after %s", order[i], order[i-1])
+		}
+	}
+	for no, title := range []string{"Processes", "Milter sockets", "Sender check", "TLS certificate",
+		"Hostname and reverse DNS", "Notification mail", "Machine", "Configuration"} {
+		if want := `<span class="sp-no">0` + strconv.Itoa(no+1) + `</span><h2>` + title + `</h2>`; !strings.Contains(out, want) {
+			t.Errorf("Health is missing %s", want)
+		}
+	}
+	pageHas(t, "Health", out, `<tr><td>Last 12 months</td><td>41 s</td><td>3</td></tr>`,
+		`Counted since <span class="sp-mono">2026-10-13</span>.`, `href="/help#sender-check"`)
+	// The Configuration box is outside the polled stack: a refresh must not re-render a control.
+	if strings.Contains(out[strings.Index(out, `id="health-stack"`):strings.Index(out, `id="sockets"`)], `action="/server/health/reload"`) {
+		t.Error("the reload control is inside the polled stack")
 	}
 }
 
-// The fragment is the polled part of the page and nothing else: the same rows,
-// the second one marked to be swapped in by its id, and the Notification mail
-// card, a column of the last row, swapped in the same way. No shell and no
-// actions.
-func TestHealthFragmentIsThePolledRows(t *testing.T) {
+// The fragment is the polled part of the page and nothing else: the left
+// stack as it is, and the three readings of the right stack, each marked to be
+// swapped in by its id. No shell and no actions.
+func TestHealthFragmentIsThePolledBoxes(t *testing.T) {
 	page := renderSignedIn(t, "health", healthFixture(false))
 	frag := renderFragment(t, "health_body", healthFixture(true))
 
-	const card = `<div class="column is-6" id="health-mail"`
-	// The part of s before the card, and the card itself up to what follows it
-	// on the page (the Configuration box).
-	rows := func(s string) string {
-		return s[strings.Index(s, `<div class="columns" id="health-machine"`):strings.Index(s, card)]
-	}
-	mail := func(s string) string {
-		a := strings.Index(s, card)
-		if b := strings.Index(s, `id="configuration"`); b > a {
-			return s[a:b]
+	// The polled boxes of s: from the left stack to the Configuration box, or
+	// to the end of a fragment. On the page the right stack's column opens
+	// between them; the fragment has no such line.
+	polled := func(s string) string {
+		s = s[strings.Index(s, `<div class="column is-6" id="health-stack"`):]
+		if b := strings.Index(s, `<div class="box" id="configuration"`); b > 0 {
+			s = s[:b]
 		}
-		return s[a:]
+		s = strings.ReplaceAll(s, ` hx-swap-oob="true"`, "")
+		return strings.Join(strings.Fields(strings.Replace(s, `<div class="column is-6">`, "", 1)), " ")
 	}
-	// Same markup, apart from the out-of-band marks and what follows the card on the page.
-	oob := func(s string) string { return strings.ReplaceAll(strings.TrimSpace(s), ` hx-swap-oob="true"`, "") }
-	if got, want := oob(rows(frag)), oob(rows(page)); !strings.HasPrefix(want, got) {
-		t.Errorf("the fragment is not the page's polled rows\nfragment: %.400s\npage:     %.400s", got, want)
+	if got, want := polled(frag), polled(page); got != want {
+		t.Errorf("the fragment is not the page's polled boxes\nfragment: %.600s\npage:     %.600s", got, want)
 	}
-	if got, want := oob(mail(frag)), oob(mail(page)); !strings.HasPrefix(want, got) {
-		t.Errorf("the fragment's Notification mail card is not the page's\nfragment: %.400s\npage:     %.400s", got, want)
-	}
-	pageHas(t, "fragment", frag, `id="health-machine" data-poll`, `id="health-checks" hx-swap-oob="true"`,
-		`id="health-mail" hx-swap-oob="true"`, `id="notification-mail"`)
+	pageHas(t, "fragment", frag, `id="health-stack" data-poll`, `id="sockets" hx-swap-oob="true"`,
+		`id="certificate" hx-swap-oob="true"`, `id="notification-mail" hx-swap-oob="true"`, `id="sender-check"`)
 	pageLacks(t, "fragment", frag, `<h1`, `navbar`, `action="/server/health/reload"`, `action="/server/health/recheck"`)
 }
 
@@ -257,7 +266,7 @@ func TestHealthStatesTheFixtureDoesNotDraw(t *testing.T) {
 		`<div class="notification is-success is-light">`, `DNS re-checked.`,
 	)
 	// the flash sits between the head and the first row, as the kit puts it
-	if strings.Index(out, `class="notification`) > strings.Index(out, `id="health-machine"`) ||
+	if strings.Index(out, `class="notification`) > strings.Index(out, `id="health-stack"`) ||
 		strings.Index(out, `class="notification`) < strings.Index(out, `class="sp-head"`) {
 		t.Error("the flash is not between page_head and the first box")
 	}