egress: tighten DNS answer filtering and TTL clamping

net/egress/dns.go

@@ -6,10 +6,21 @@ package egress
 import (
 	"fmt"
 	"net"
+	"strings"
 
 	"github.com/miekg/dns"
 )
 
+// normalizeDNSName returns s lower-cased with a single trailing dot stripped,
+// so that names from DNS wire format (often FQDN with trailing dot) compare
+// consistently against caller-supplied policy names (typically without a
+// trailing dot). DNS names are case-insensitive by RFC 1035.
+func normalizeDNSName(s string) string {
+	s = strings.ToLower(s)
+	s = strings.TrimSuffix(s, ".")
+	return s
+}
+
 // ParseDNSQuery extracts the transaction ID, question name, and query type
 // from a DNS query payload (UDP payload, not including Ethernet/IP/UDP headers).
 func ParseDNSQuery(payload []byte) (txnID uint16, qname string, qtype uint16, err error) {
@@ -52,13 +63,48 @@ func BuildNXDOMAIN(txnID uint16, qname string, qtype uint16) ([]byte, error) {
 
 // ParseDNSResponse extracts the question name, A-record IPs, and minimum
 // TTL from a DNS response payload.
+//
+// Only A records whose owner name is reachable from the question via the
+// response's own CNAME chain are returned. An answer with an unrelated
+// owner name — for example, an injected out-of-bailiwick record — is
+// discarded. Without this filter, a compromised allowed zone could
+// smuggle arbitrary IPs into dynamic egress rules by returning them in
+// the Answer section under any name.
+//
+// The returned qname is normalized (lower-cased, trailing dot stripped).
 func ParseDNSResponse(payload []byte) (qname string, ips []net.IP, ttl uint32, err error) {
 	var msg dns.Msg
 	if err := msg.Unpack(payload); err != nil {
 		return "", nil, 0, fmt.Errorf("unpack DNS response: %w", err)
 	}
 	if len(msg.Question) > 0 {
-		qname = msg.Question[0].Name
+		qname = normalizeDNSName(msg.Question[0].Name)
+	}
+
+	// Build the set of owner names reachable from qname via CNAMEs in this
+	// response. Multi-pass until no new names are added; bounded by the
+	// Answer count so CNAME loops cannot cause infinite iteration.
+	validNames := map[string]struct{}{qname: {}}
+	for i := 0; i < len(msg.Answer); i++ {
+		progress := false
+		for _, rr := range msg.Answer {
+			cn, ok := rr.(*dns.CNAME)
+			if !ok {
+				continue
+			}
+			owner := normalizeDNSName(cn.Hdr.Name)
+			if _, have := validNames[owner]; !have {
+				continue
+			}
+			target := normalizeDNSName(cn.Target)
+			if _, already := validNames[target]; !already {
+				validNames[target] = struct{}{}
+				progress = true
+			}
+		}
+		if !progress {
+			break
+		}
 	}
 
 	var minTTL uint32
@@ -68,6 +114,9 @@ func ParseDNSResponse(payload []byte) (qname string, ips []net.IP, ttl uint32, e
 		if !ok {
 			continue
 		}
+		if _, ok := validNames[normalizeDNSName(a.Hdr.Name)]; !ok {
+			continue
+		}
 		ips = append(ips, a.A)
 		if first || a.Hdr.Ttl < minTTL {
 			minTTL = a.Hdr.Ttl

net/egress/dns_test.go

@@ -12,6 +12,31 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestNormalizeDNSName(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		in, want string
+	}{
+		{"example.com", "example.com"},
+		{"example.com.", "example.com"},
+		{"Example.COM.", "example.com"},
+		{"API.GitHub.com", "api.github.com"},
+		{"", ""},
+		{".", ""},
+		// Only a single trailing dot is stripped; double-trailing is a
+		// malformed name and should not be coerced.
+		{"foo..", "foo."},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, tt.want, normalizeDNSName(tt.in))
+		})
+	}
+}
+
 func TestParseDNSQuery(t *testing.T) {
 	t.Parallel()
 
@@ -121,13 +146,114 @@ func TestParseDNSResponse(t *testing.T) {
 
 	qname, ips, ttl, err := ParseDNSResponse(payload)
 	require.NoError(t, err)
-	assert.Equal(t, "example.com.", qname)
+	assert.Equal(t, "example.com", qname)
 	assert.Len(t, ips, 2)
 	assert.Equal(t, "93.184.216.34", ips[0].String())
 	assert.Equal(t, "93.184.216.35", ips[1].String())
 	assert.Equal(t, uint32(60), ttl) // minimum TTL
 }
 
+func TestParseDNSResponse_DropsOutOfBailiwickAnswers(t *testing.T) {
+	t.Parallel()
+
+	// Question for example.com; attacker-controlled response slips an A
+	// record with a different owner name (typical out-of-bailiwick injection
+	// attempt to smuggle an internal IP into dynamic rules).
+	msg := &dns.Msg{
+		MsgHdr: dns.MsgHdr{Id: 0x1234, Response: true},
+		Question: []dns.Question{
+			{Name: "example.com.", Qtype: dns.TypeA, Qclass: dns.ClassINET},
+		},
+		Answer: []dns.RR{
+			&dns.A{
+				Hdr: dns.RR_Header{Name: "example.com.", Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
+				A:   net.ParseIP("93.184.216.34"),
+			},
+			&dns.A{
+				Hdr: dns.RR_Header{Name: "internal.local.", Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
+				A:   net.ParseIP("169.254.169.254"),
+			},
+		},
+	}
+	payload, err := msg.Pack()
+	require.NoError(t, err)
+
+	qname, ips, _, err := ParseDNSResponse(payload)
+	require.NoError(t, err)
+	assert.Equal(t, "example.com", qname)
+	require.Len(t, ips, 1)
+	assert.Equal(t, "93.184.216.34", ips[0].String())
+}
+
+func TestParseDNSResponse_FollowsCNAMEChain(t *testing.T) {
+	t.Parallel()
+
+	// Legitimate CNAME chain: example.com -> cdn.example.net -> 1.2.3.4.
+	// The A record's owner matches the CNAME target, which is reachable
+	// from the question via the chain, so the IP is accepted.
+	msg := &dns.Msg{
+		MsgHdr: dns.MsgHdr{Id: 0x2345, Response: true},
+		Question: []dns.Question{
+			{Name: "example.com.", Qtype: dns.TypeA, Qclass: dns.ClassINET},
+		},
+		Answer: []dns.RR{
+			&dns.CNAME{
+				Hdr:    dns.RR_Header{Name: "example.com.", Rrtype: dns.TypeCNAME, Class: dns.ClassINET, Ttl: 300},
+				Target: "cdn.example.net.",
+			},
+			&dns.A{
+				Hdr: dns.RR_Header{Name: "cdn.example.net.", Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
+				A:   net.ParseIP("1.2.3.4"),
+			},
+		},
+	}
+	payload, err := msg.Pack()
+	require.NoError(t, err)
+
+	qname, ips, _, err := ParseDNSResponse(payload)
+	require.NoError(t, err)
+	assert.Equal(t, "example.com", qname)
+	require.Len(t, ips, 1)
+	assert.Equal(t, "1.2.3.4", ips[0].String())
+}
+
+func TestParseDNSResponse_DropsUnreachableCNAMEA(t *testing.T) {
+	t.Parallel()
+
+	// An A record whose owner is NOT reachable via any CNAME chain from
+	// the question must be dropped even if another A record from the same
+	// name-chain is valid.
+	msg := &dns.Msg{
+		MsgHdr: dns.MsgHdr{Id: 0x3456, Response: true},
+		Question: []dns.Question{
+			{Name: "example.com.", Qtype: dns.TypeA, Qclass: dns.ClassINET},
+		},
+		Answer: []dns.RR{
+			&dns.CNAME{
+				Hdr:    dns.RR_Header{Name: "example.com.", Rrtype: dns.TypeCNAME, Class: dns.ClassINET, Ttl: 300},
+				Target: "cdn.example.net.",
+			},
+			// A record for an unrelated name slipped into the Answer section.
+			&dns.A{
+				Hdr: dns.RR_Header{Name: "attacker.example.net.", Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
+				A:   net.ParseIP("10.0.0.1"),
+			},
+			// Legitimate A record at the CNAME target.
+			&dns.A{
+				Hdr: dns.RR_Header{Name: "cdn.example.net.", Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
+				A:   net.ParseIP("1.2.3.4"),
+			},
+		},
+	}
+	payload, err := msg.Pack()
+	require.NoError(t, err)
+
+	_, ips, _, err := ParseDNSResponse(payload)
+	require.NoError(t, err)
+	require.Len(t, ips, 1)
+	assert.Equal(t, "1.2.3.4", ips[0].String())
+}
+
 func TestParseDNSResponse_NoARecords(t *testing.T) {
 	t.Parallel()
 

net/egress/interceptor.go

@@ -16,6 +16,13 @@ const (
 	// if the DNS response has a shorter TTL. This prevents excessive
 	// rule churn from very short TTLs.
 	defaultMinTTL = 60 * time.Second
+
+	// defaultMaxTTL is the maximum TTL applied to dynamic rules, even
+	// if the DNS response advertises a longer TTL. This bounds how long
+	// a resolved IP remains allowed — useful if an upstream server
+	// returns very long TTLs, and as belt-and-suspenders against a
+	// compromised zone returning long-lived rogue answers.
+	defaultMaxTTL = 5 * time.Minute
 )
 
 // DNSInterceptor intercepts DNS traffic at the relay level to enforce
@@ -25,21 +32,47 @@ type DNSInterceptor struct {
 	policy       *Policy
 	dynamicRules *firewall.DynamicRules
 	minTTL       time.Duration
+	maxTTL       time.Duration
 	gatewayIP    [4]byte
 }
 
+// DNSInterceptorOption customizes a DNSInterceptor.
+type DNSInterceptorOption func(*DNSInterceptor)
+
+// WithMinTTL sets the minimum TTL applied to dynamic rules. A zero or
+// negative value leaves the default in place.
+func WithMinTTL(d time.Duration) DNSInterceptorOption {
+	return func(i *DNSInterceptor) {
+		if d > 0 {
+			i.minTTL = d
+		}
+	}
+}
+
+// WithMaxTTL sets the maximum TTL applied to dynamic rules. A zero or
+// negative value disables the cap (any TTL is accepted).
+func WithMaxTTL(d time.Duration) DNSInterceptorOption {
+	return func(i *DNSInterceptor) { i.maxTTL = d }
+}
+
 // NewDNSInterceptor creates an interceptor with the given policy, dynamic
 // rule set, and gateway IP. Only DNS responses from the gateway are
 // snooped to prevent spoofed responses from creating dynamic rules.
-// Dynamic rules created from DNS responses will have at least minTTL
-// duration (use 0 for the default of 60 seconds).
-func NewDNSInterceptor(policy *Policy, dr *firewall.DynamicRules, gatewayIP [4]byte) *DNSInterceptor {
-	return &DNSInterceptor{
+//
+// By default, dynamic rules are clamped to minTTL=60s and maxTTL=5m;
+// override via WithMinTTL / WithMaxTTL.
+func NewDNSInterceptor(policy *Policy, dr *firewall.DynamicRules, gatewayIP [4]byte, opts ...DNSInterceptorOption) *DNSInterceptor {
+	i := &DNSInterceptor{
 		policy:       policy,
 		dynamicRules: dr,
 		minTTL:       defaultMinTTL,
+		maxTTL:       defaultMaxTTL,
 		gatewayIP:    gatewayIP,
 	}
+	for _, o := range opts {
+		o(i)
+	}
+	return i
 }
 
 // HandleEgress processes an outbound DNS query frame. If the queried
@@ -118,6 +151,9 @@ func (d *DNSInterceptor) HandleIngress(frame []byte, hdr *firewall.PacketHeader)
 	if ttl < d.minTTL {
 		ttl = d.minTTL
 	}
+	if d.maxTTL > 0 && ttl > d.maxTTL {
+		ttl = d.maxTTL
+	}
 
 	ports, proto := d.policy.HostPorts(qname)
 

net/egress/interceptor_test.go

@@ -7,6 +7,7 @@ import (
 	"encoding/binary"
 	"net"
 	"testing"
+	"time"
 
 	mdns "github.com/miekg/dns"
 	"github.com/stretchr/testify/assert"
@@ -304,6 +305,37 @@ func TestDNSInterceptor_ResponseFromNonGateway_Ignored(t *testing.T) {
 		"DNS responses from non-gateway sources must not create dynamic rules")
 }
 
+func TestDNSInterceptor_ClampsTTLAtMaximum(t *testing.T) {
+	t.Parallel()
+
+	policy := NewPolicy([]HostSpec{{Name: "example.com"}})
+	dr := firewall.NewDynamicRules()
+	interceptor := NewDNSInterceptor(policy, dr, testDstIP,
+		WithMinTTL(1*time.Microsecond),
+		WithMaxTTL(5*time.Millisecond),
+	)
+
+	// Response advertises TTL = 1 hour; should be clamped to 5 ms.
+	ips := []net.IP{net.ParseIP("1.2.3.4")}
+	frame := buildDNSResponseFrame(testDstMAC, testSrcMAC, testDstIP, testSrcIP, 12345, "example.com", ips, 3600)
+	hdr := firewall.ParseHeaders(frame)
+
+	interceptor.HandleIngress(frame, hdr)
+
+	probe := &firewall.PacketHeader{
+		DstIP:    [4]byte{1, 2, 3, 4},
+		Protocol: 6,
+		DstPort:  443,
+	}
+	_, ok := dr.Match(firewall.Egress, probe)
+	require.True(t, ok, "rule should be live immediately after ingress")
+
+	// Give the clamp time to elapse; the rule must no longer match.
+	time.Sleep(25 * time.Millisecond)
+	_, ok = dr.Match(firewall.Egress, probe)
+	assert.False(t, ok, "rule should have expired past maxTTL clamp")
+}
+
 func TestDNSInterceptor_ExplicitProtocol_SingleRule(t *testing.T) {
 	t.Parallel()