Bind 0.0.0.0 to IPv4 only (closes #528)

Go listens on any unspecified address with one socket that answers
IPv4 and IPv6, so BIND_ADDRESS=0.0.0.0 also bound every IPv6 address,
which the README says it does not. The listener now uses tcp4 for an
IPv4 address; :: still binds both.

The image's health check asked for localhost, which wget reached over
::1 only, so it now asks for 127.0.0.1. The README says what that
means for a loopback bind in a container, drops the bindv6only
condition from :: (Go sets that socket option itself), and names an
ss check.

Model: opus-5-5

internal/server/bind_address_test.go

@@ -5,6 +5,7 @@ import (
 	"net"
 	"net/http"
 	"strconv"
+	"syscall"
 	"testing"
 	"time"
 
@@ -25,6 +26,11 @@ const (
 	// its network namespace even with a published port.
 	wildcardV4 = "0.0.0.0"
 
+	// loopbackV6 and wildcardV6 are the IPv6 counterparts of the two
+	// above.
+	loopbackV6 = "::1"
+	wildcardV6 = "::"
+
 	// unavailableAddr is a TEST-NET-1 address (RFC 5737). It is a
 	// well-formed literal that no host is assigned, so binding it
 	// fails with EADDRNOTAVAIL rather than succeeding somewhere
@@ -116,6 +122,24 @@ func otherLocalAddr(t *testing.T) string {
 	return ""
 }
 
+// skipWithoutIPv6 skips the test when this host cannot bind
+// loopbackV6. Without IPv6 there is no second family for a bind to
+// claim or leave alone.
+func skipWithoutIPv6(t *testing.T) {
+	t.Helper()
+
+	var listenCfg net.ListenConfig
+
+	l, err := listenCfg.Listen(
+		t.Context(), "tcp6", net.JoinHostPort(loopbackV6, "0"),
+	)
+	if err != nil {
+		t.Skip("host has no IPv6 loopback to bind")
+	}
+
+	require.NoError(t, l.Close())
+}
+
 // startBoundServer starts the wired app with the given bind address
 // on a free port and returns that port. The app is stopped on
 // cleanup.
@@ -292,6 +316,50 @@ func TestBindAddress_WildcardReachesOtherAddresses(t *testing.T) {
 	requireDialable(t, net.JoinHostPort(other, strconv.Itoa(port)))
 }
 
+// TestBindAddress_WildcardV4IsIPv4Only pins that 0.0.0.0 answers on
+// IPv4 alone, as the README says. Go listens on 0.0.0.0 with a socket
+// that answers IPv6 too unless told otherwise, which put the admin UI
+// and the receiver on every IPv6 address of a host whose operator had
+// chosen an IPv4 bind.
+func TestBindAddress_WildcardV4IsIPv4Only(t *testing.T) {
+	t.Parallel()
+
+	skipWithoutIPv6(t)
+
+	port := startBoundServer(t, wildcardV4)
+
+	// Positive control: the service really is up, on IPv4.
+	requireDialable(t, net.JoinHostPort(loopbackV4, strconv.Itoa(port)))
+
+	dialer := net.Dialer{Timeout: dialTimeout}
+
+	conn, err := dialer.DialContext(
+		t.Context(), "tcp",
+		net.JoinHostPort(loopbackV6, strconv.Itoa(port)),
+	)
+	if err == nil {
+		_ = conn.Close()
+	}
+
+	require.ErrorIs(
+		t, err, syscall.ECONNREFUSED,
+		"a %s bind answered on %s", wildcardV4, loopbackV6,
+	)
+}
+
+// TestBindAddress_WildcardV6ReachesBothFamilies is the counterpart:
+// :: is the value that binds IPv6, and it answers IPv4 as well.
+func TestBindAddress_WildcardV6ReachesBothFamilies(t *testing.T) {
+	t.Parallel()
+
+	skipWithoutIPv6(t)
+
+	port := startBoundServer(t, wildcardV6)
+
+	requireDialable(t, net.JoinHostPort(loopbackV6, strconv.Itoa(port)))
+	requireDialable(t, net.JoinHostPort(loopbackV4, strconv.Itoa(port)))
+}
+
 // TestBindAddress_ServesRequestsOnConfiguredAddress proves the bound
 // listener serves the application rather than merely accepting TCP,
 // so a bind address that is honoured cannot be mistaken for one that