matcher: clarify DestsIsTheInternet single-family semantics

DestsIsTheInternet now reports the internet when either family's /0
is contained (0.0.0.0/0 or ::/0), matching what operators expect when
they write the /0 directly. Also documents MatchFromStrings fail-open.

Updates #3157

hscontrol/policy/matcher/matcher.go

@@ -53,6 +53,11 @@ func MatchFromFilterRule(rule tailcfg.FilterRule) Match {
 	return MatchFromStrings(rule.SrcIPs, dests)
 }
 
+// MatchFromStrings builds a Match from raw source and destination
+// strings. Unparseable entries are silently dropped (fail-open): the
+// resulting Match is narrower than the input described, but never
+// wider. Callers that need strict validation should pre-validate
+// their inputs via util.ParseIPSet.
 func MatchFromStrings(sources, destinations []string) Match {
 	srcs := new(netipx.IPSetBuilder)
 	dests := new(netipx.IPSetBuilder)
@@ -96,18 +101,20 @@ func (m *Match) DestsOverlapsPrefixes(prefixes ...netip.Prefix) bool {
 	return slices.ContainsFunc(prefixes, m.dests.OverlapsPrefix)
 }
 
-// DestsIsTheInternet reports if the destination contains "the internet"
-// which is a IPSet that represents "autogroup:internet" and is special
-// cased for exit nodes.
-// This checks if dests is a superset of TheInternet(), which handles
-// merged filter rules where TheInternet is combined with other destinations.
+// DestsIsTheInternet reports whether the destination covers "the
+// internet" — the set represented by autogroup:internet, special-cased
+// for exit nodes. Returns true if either family's /0 is contained
+// (0.0.0.0/0 or ::/0), or if dests is a superset of TheInternet(). A
+// single-family /0 counts because operators may write it directly and
+// it still denotes the whole internet for that family.
 func (m *Match) DestsIsTheInternet() bool {
 	if m.dests.ContainsPrefix(tsaddr.AllIPv4()) ||
 		m.dests.ContainsPrefix(tsaddr.AllIPv6()) {
 		return true
 	}
 
-	// Check if dests contains all prefixes of TheInternet (superset check)
+	// Superset-of-TheInternet check handles merged filter rules
+	// where the internet prefixes are combined with other dests.
 	theInternet := util.TheInternet()
 	for _, prefix := range theInternet.Prefixes() {
 		if !m.dests.ContainsPrefix(prefix) {

hscontrol/policy/matcher/matcher_test.go

@@ -2,6 +2,7 @@ package matcher
 
 import (
 	"net/netip"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -428,4 +429,44 @@ func TestDebugString(t *testing.T) {
 	assert.Contains(t, s, "Destinations:")
 	assert.Contains(t, s, "10.0.0.0/8")
 	assert.Contains(t, s, "192.168.1.0/24")
+
+	// Sources appear before Destinations in the output.
+	assert.Less(
+		t,
+		strings.Index(s, "Sources:"),
+		strings.Index(s, "Destinations:"),
+		"Sources section must precede Destinations",
+	)
+}
+
+func TestDebugString_Empty(t *testing.T) {
+	t.Parallel()
+
+	m := MatchFromStrings(nil, nil)
+
+	s := m.DebugString()
+	assert.Contains(t, s, "Match:")
+	assert.Contains(t, s, "Sources:")
+	assert.Contains(t, s, "Destinations:")
+	assert.NotContains(t, s, "/")
+}
+
+// TestMatchFromStrings_MalformedFailsOpen asserts that unparseable
+// entries are silently dropped and do not crash or widen the Match.
+func TestMatchFromStrings_MalformedFailsOpen(t *testing.T) {
+	t.Parallel()
+
+	m := MatchFromStrings(
+		[]string{"not-a-cidr", "10.0.0.0/8"},
+		[]string{"also-bogus", "192.168.1.0/24"},
+	)
+
+	assert.True(t, m.SrcsContainsIPs(netip.MustParseAddr("10.1.2.3")),
+		"valid src entry must still match")
+	assert.False(t, m.SrcsContainsIPs(netip.MustParseAddr("1.1.1.1")),
+		"malformed src entry must not widen the set")
+	assert.True(t, m.DestsContainsIP(netip.MustParseAddr("192.168.1.10")),
+		"valid dst entry must still match")
+	assert.False(t, m.DestsContainsIP(netip.MustParseAddr("8.8.8.8")),
+		"malformed dst entry must not widen the set")
 }