+ DNS: TLS handshake: terminate handshake on bad SNI

dnsforward/dnsforward.go

@@ -2,10 +2,12 @@ package dnsforward
 
 import (
 	"crypto/tls"
+	"crypto/x509"
 	"fmt"
 	"net"
 	"net/http"
 	"runtime"
+	"sort"
 	"strings"
 	"sync"
 	"time"
@@ -16,6 +18,7 @@ import (
 	"github.com/AdguardTeam/dnsproxy/proxy"
 	"github.com/AdguardTeam/dnsproxy/upstream"
 	"github.com/AdguardTeam/golibs/log"
+	"github.com/AdguardTeam/golibs/utils"
 	"github.com/joomcode/errorx"
 	"github.com/miekg/dns"
 )
@@ -151,6 +154,7 @@ type FilteringConfig struct {
 // TLSConfig is the TLS configuration for HTTPS, DNS-over-HTTPS, and DNS-over-TLS
 type TLSConfig struct {
 	TLSListenAddr    *net.TCPAddr `yaml:"-" json:"-"`
+	StrictSNICheck   bool         `yaml:"strict_sni_check" json:"-"`                  // Reject connection if the client uses server name (in SNI) that doesn't match the certificate
 	CertificateChain string       `yaml:"certificate_chain" json:"certificate_chain"` // PEM-encoded certificates chain
 	PrivateKey       string       `yaml:"private_key" json:"private_key"`             // PEM-encoded private key
 
@@ -159,6 +163,9 @@ type TLSConfig struct {
 
 	CertificateChainData []byte `yaml:"-" json:"-"`
 	PrivateKeyData       []byte `yaml:"-" json:"-"`
+
+	cert     tls.Certificate // nolint(structcheck) - linter thinks that this field is unused, while TLSConfig is directly included into ServerConfig
+	dnsNames []string        // nolint(structcheck) // DNS names from certificate (SAN) or CN value from Subject
 }
 
 // ServerConfig represents server configuration.
@@ -234,6 +241,7 @@ func (s *Server) startInternal() error {
 }
 
 // Prepare the object
+// nolint(gocyclo)
 func (s *Server) Prepare(config *ServerConfig) error {
 	if config != nil {
 		s.conf = *config
@@ -305,12 +313,28 @@ func (s *Server) Prepare(config *ServerConfig) error {
 
 	if s.conf.TLSListenAddr != nil && len(s.conf.CertificateChainData) != 0 && len(s.conf.PrivateKeyData) != 0 {
 		proxyConfig.TLSListenAddr = s.conf.TLSListenAddr
-		keypair, err := tls.X509KeyPair(s.conf.CertificateChainData, s.conf.PrivateKeyData)
+		s.conf.cert, err = tls.X509KeyPair(s.conf.CertificateChainData, s.conf.PrivateKeyData)
 		if err != nil {
 			return errorx.Decorate(err, "Failed to parse TLS keypair")
 		}
+
+		if s.conf.StrictSNICheck {
+			x, err := x509.ParseCertificate(s.conf.cert.Certificate[0])
+			if err != nil {
+				return errorx.Decorate(err, "x509.ParseCertificate(): %s", err)
+			}
+			if len(x.DNSNames) != 0 {
+				s.conf.dnsNames = x.DNSNames
+				log.Debug("DNS: using DNS names from certificate's SAN: %v", x.DNSNames)
+				sort.Strings(s.conf.dnsNames)
+			} else {
+				s.conf.dnsNames = append(s.conf.dnsNames, x.Subject.CommonName)
+				log.Debug("DNS: using DNS name from certificate's CN: %s", x.Subject.CommonName)
+			}
+		}
+
 		proxyConfig.TLSConfig = &tls.Config{
-			Certificates: []tls.Certificate{keypair},
+			GetCertificate: s.onGetCertificate,
 			MinVersion:     tls.VersionTLS12,
 		}
 	}
@@ -329,6 +353,53 @@ func (s *Server) Prepare(config *ServerConfig) error {
 	return nil
 }
 
+// Find value in a sorted array
+func findSorted(ar []string, val string) int {
+	i := sort.SearchStrings(ar, val)
+	if i == len(ar) || ar[i] != val {
+		return -1
+	}
+	return i
+}
+
+func isWildcard(host string) bool {
+	return len(host) >= 2 &&
+		host[0] == '*' && host[1] == '.'
+}
+
+// Return TRUE if host name matches a wildcard pattern
+func matchDomainWildcard(host, wildcard string) bool {
+	return isWildcard(wildcard) &&
+		strings.HasSuffix(host, wildcard[1:])
+}
+
+// Return TRUE if client's SNI value matches DNS names from certificate
+func matchDNSName(dnsNames []string, sni string) bool {
+	if utils.IsValidHostname(sni) != nil {
+		return false
+	}
+	if findSorted(dnsNames, sni) != -1 {
+		return true
+	}
+
+	for _, dn := range dnsNames {
+		if matchDomainWildcard(sni, dn) {
+			return true
+		}
+	}
+	return false
+}
+
+// Called by 'tls' package when Client Hello is received
+// If the server name (from SNI) supplied by client is incorrect - we terminate the ongoing TLS handshake.
+func (s *Server) onGetCertificate(ch *tls.ClientHelloInfo) (*tls.Certificate, error) {
+	if s.conf.StrictSNICheck && !matchDNSName(s.conf.dnsNames, ch.ServerName) {
+		log.Info("DNS: TLS: unknown SNI in Client Hello: %s", ch.ServerName)
+		return nil, fmt.Errorf("Invalid SNI")
+	}
+	return &s.conf.cert, nil
+}
+
 // Stop stops the DNS server
 func (s *Server) Stop() error {
 	s.Lock()

dnsforward/dnsforward_test.go

@@ -10,6 +10,7 @@ import (
 	"encoding/pem"
 	"math/big"
 	"net"
+	"sort"
 	"sync"
 	"testing"
 	"time"
@@ -887,3 +888,15 @@ func TestIpFromAddr(t *testing.T) {
 	a = ipFromAddr(nil)
 	assert.True(t, a == "")
 }
+
+func TestMatchDNSName(t *testing.T) {
+	dnsNames := []string{"host1", "*.host2", "1.2.3.4"}
+	sort.Strings(dnsNames)
+	assert.True(t, matchDNSName(dnsNames, "host1"))
+	assert.True(t, matchDNSName(dnsNames, "a.host2"))
+	assert.True(t, matchDNSName(dnsNames, "b.a.host2"))
+	assert.True(t, matchDNSName(dnsNames, "1.2.3.4"))
+	assert.True(t, !matchDNSName(dnsNames, "host2"))
+	assert.True(t, !matchDNSName(dnsNames, ""))
+	assert.True(t, !matchDNSName(dnsNames, "*.host2"))
+}