crypto/tls: add server side SNI support.

With this in place, a TLS server is capable of selecting the correct
certificate based on the client's ServerNameIndication extension.

The need to call Config.BuildNameToCertificate is unfortunate, but
adding a sync.Once to the Config structure made it uncopyable and I
felt that was too high a price to pay. Parsing the leaf certificates
in each handshake was too inefficient to consider.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5151048

src/pkg/crypto/tls/common.go

@@ -10,6 +10,7 @@ import (
 	"crypto/x509"
 	"io"
 	"io/ioutil"
+	"strings"
 	"sync"
 	"time"
 )
@@ -101,6 +102,10 @@ type ConnectionState struct {
 	NegotiatedProtocol         string
 	NegotiatedProtocolIsMutual bool
 
+	// ServerName contains the server name indicated by the client, if any.
+	// (Only valid for server connections.)
+	ServerName string
+
 	// the certificate chain that was presented by the other side
 	PeerCertificates []*x509.Certificate
 	// the verified certificate chains built from PeerCertificates.
@@ -124,6 +129,14 @@ type Config struct {
 	// Server configurations must include at least one certificate.
 	Certificates []Certificate
 
+	// NameToCertificate maps from a certificate name to an element of
+	// Certificates. Note that a certificate name can be of the form
+	// '*.example.com' and so doesn't have to be a domain name as such.
+	// See Config.BuildNameToCertificate
+	// The nil value causes the first element of Certificates to be used
+	// for all connections.
+	NameToCertificate map[string]*Certificate
+
 	// RootCAs defines the set of root certificate authorities
 	// that clients use when verifying server certificates.
 	// If RootCAs is nil, TLS uses the host's root CA set.
@@ -179,6 +192,59 @@ func (c *Config) cipherSuites() []uint16 {
 	return s
 }
 
+// getCertificateForName returns the best certificate for the given name,
+// defaulting to the first element of c.Certificates if there are no good
+// options.
+func (c *Config) getCertificateForName(name string) *Certificate {
+	if len(c.Certificates) == 1 || c.NameToCertificate == nil {
+		// There's only one choice, so no point doing any work.
+		return &c.Certificates[0]
+	}
+
+	name = strings.ToLower(name)
+	for len(name) > 0 && name[len(name)-1] == '.' {
+		name = name[:len(name)-1]
+	}
+
+	if cert, ok := c.NameToCertificate[name]; ok {
+		return cert
+	}
+
+	// try replacing labels in the name with wildcards until we get a
+	// match.
+	labels := strings.Split(name, ".")
+	for i := range labels {
+		labels[i] = "*"
+		candidate := strings.Join(labels, ".")
+		if cert, ok := c.NameToCertificate[candidate]; ok {
+			return cert
+		}
+	}
+
+	// If nothing matches, return the first certificate.
+	return &c.Certificates[0]
+}
+
+// BuildNameToCertificate parses c.Certificates and builds c.NameToCertificate
+// from the CommonName and SubjectAlternateName fields of each of the leaf
+// certificates.
+func (c *Config) BuildNameToCertificate() {
+	c.NameToCertificate = make(map[string]*Certificate)
+	for i := range c.Certificates {
+		cert := &c.Certificates[i]
+		x509Cert, err := x509.ParseCertificate(cert.Certificate[0])
+		if err != nil {
+			continue
+		}
+		if len(x509Cert.Subject.CommonName) > 0 {
+			c.NameToCertificate[x509Cert.Subject.CommonName] = cert
+		}
+		for _, san := range x509Cert.DNSNames {
+			c.NameToCertificate[san] = cert
+		}
+	}
+}
+
 // A Certificate is a chain of one or more certificates, leaf first.
 type Certificate struct {
 	Certificate [][]byte

src/pkg/crypto/tls/conn_test.go

@@ -50,3 +50,57 @@ func TestRemovePadding(t *testing.T) {
 		}
 	}
 }
+
+var certExampleCom = `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`
+
+var certWildcardExampleCom = `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`
+
+var certFooExampleCom = `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`
+
+var certDoubleWildcardExampleCom = `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`
+
+func TestCertificateSelection(t *testing.T) {
+	config := Config{
+		Certificates: []Certificate{
+			{
+				Certificate: [][]byte{fromHex(certExampleCom)},
+			},
+			{
+				Certificate: [][]byte{fromHex(certWildcardExampleCom)},
+			},
+			{
+				Certificate: [][]byte{fromHex(certFooExampleCom)},
+			},
+			{
+				Certificate: [][]byte{fromHex(certDoubleWildcardExampleCom)},
+			},
+		},
+	}
+
+	config.BuildNameToCertificate()
+
+	pointerToIndex := func(c *Certificate) int {
+		for i := range config.Certificates {
+			if c == &config.Certificates[i] {
+				return i
+			}
+		}
+		return -1
+	}
+
+	if n := pointerToIndex(config.getCertificateForName("example.com")); n != 0 {
+		t.Errorf("example.com returned certificate %d, not 0", n)
+	}
+	if n := pointerToIndex(config.getCertificateForName("bar.example.com")); n != 1 {
+		t.Errorf("bar.example.com returned certificate %d, not 1", n)
+	}
+	if n := pointerToIndex(config.getCertificateForName("foo.example.com")); n != 2 {
+		t.Errorf("foo.example.com returned certificate %d, not 2", n)
+	}
+	if n := pointerToIndex(config.getCertificateForName("foo.bar.example.com")); n != 3 {
+		t.Errorf("foo.bar.example.com returned certificate %d, not 3", n)
+	}
+	if n := pointerToIndex(config.getCertificateForName("foo.bar.baz.example.com")); n != 0 {
+		t.Errorf("foo.bar.baz.example.com returned certificate %d, not 0", n)
+	}
+}

src/pkg/crypto/tls/handshake_server.go

@@ -115,7 +115,12 @@ FindCipherSuite:
 	}
 
 	certMsg := new(certificateMsg)
+	if len(clientHello.serverName) > 0 {
+		c.serverName = clientHello.serverName
+		certMsg.certificates = config.getCertificateForName(clientHello.serverName).Certificate
+	} else {
 		certMsg.certificates = config.Certificates[0].Certificate
+	}
 	finishedHash.Write(certMsg.marshal())
 	c.writeRecord(recordTypeHandshake, certMsg.marshal())