diff --git a/ssh/certs.go b/ssh/certs.go
index 67600e2..2fc8af1 100644
--- a/ssh/certs.go
+++ b/ssh/certs.go
@@ -251,10 +251,18 @@
 	// for user certificates.
 	SupportedCriticalOptions []string
 
-	// IsAuthority should return true if the key is recognized as
-	// an authority. This allows for certificates to be signed by other
-	// certificates.
-	IsAuthority func(auth PublicKey) bool
+	// IsUserAuthority should return true if the key is recognized as an
+	// authority for the given user certificate. This allows for
+	// certificates to be signed by other certificates. This must be set
+	// if this CertChecker will be checking user certificates.
+	IsUserAuthority func(auth PublicKey) bool
+
+	// IsHostAuthority should report whether the key is recognized as
+	// an authority for this host. This allows for certificates to be
+	// signed by other keys, and for those other keys to only be valid
+	// signers for particular hostnames. This must be set if this
+	// CertChecker will be checking host certificates.
+	IsHostAuthority func(auth PublicKey, address string) bool
 
 	// Clock is used for verifying time stamps. If nil, time.Now
 	// is used.
@@ -356,7 +364,13 @@
 		}
 	}
 
-	if !c.IsAuthority(cert.SignatureKey) {
+	// if this is a host cert, principal is the remote hostname as passed
+	// to CheckHostCert.
+	if cert.CertType == HostCert && !c.IsHostAuthority(cert.SignatureKey, principal) {
+		return fmt.Errorf("ssh: no authorities for hostname: %v", principal)
+	}
+
+	if cert.CertType == UserCert && !c.IsUserAuthority(cert.SignatureKey) {
 		return fmt.Errorf("ssh: certificate signed by unrecognized authority")
 	}
 
diff --git a/ssh/certs_test.go b/ssh/certs_test.go
index c5f2e53..fba6310 100644
--- a/ssh/certs_test.go
+++ b/ssh/certs_test.go
@@ -104,7 +104,7 @@
 		t.Fatalf("got %v (%T), want *Certificate", key, key)
 	}
 	checker := CertChecker{}
-	checker.IsAuthority = func(k PublicKey) bool {
+	checker.IsUserAuthority = func(k PublicKey) bool {
 		return bytes.Equal(k.Marshal(), validCert.SignatureKey.Marshal())
 	}
 
@@ -142,7 +142,7 @@
 		checker := CertChecker{
 			Clock: func() time.Time { return time.Unix(ts, 0) },
 		}
-		checker.IsAuthority = func(k PublicKey) bool {
+		checker.IsUserAuthority = func(k PublicKey) bool {
 			return bytes.Equal(k.Marshal(),
 				testPublicKeys["ecdsa"].Marshal())
 		}
@@ -160,7 +160,7 @@
 
 func TestHostKeyCert(t *testing.T) {
 	cert := &Certificate{
-		ValidPrincipals: []string{"hostname", "hostname.domain"},
+		ValidPrincipals: []string{"hostname", "hostname.domain", "otherhost"},
 		Key:             testPublicKeys["rsa"],
 		ValidBefore:     CertTimeInfinity,
 		CertType:        HostCert,
@@ -168,8 +168,8 @@
 	cert.SignCert(rand.Reader, testSigners["ecdsa"])
 
 	checker := &CertChecker{
-		IsAuthority: func(p PublicKey) bool {
-			return bytes.Equal(testPublicKeys["ecdsa"].Marshal(), p.Marshal())
+		IsHostAuthority: func(p PublicKey, h string) bool {
+			return h == "hostname" && bytes.Equal(testPublicKeys["ecdsa"].Marshal(), p.Marshal())
 		},
 	}
 
@@ -178,7 +178,7 @@
 		t.Errorf("NewCertSigner: %v", err)
 	}
 
-	for _, name := range []string{"hostname", "otherhost"} {
+	for _, name := range []string{"hostname", "otherhost", "lasthost"} {
 		c1, c2, err := netPipe()
 		if err != nil {
 			t.Fatalf("netPipe: %v", err)
diff --git a/ssh/client_auth_test.go b/ssh/client_auth_test.go
index dd83a3c..bd9f8a1 100644
--- a/ssh/client_auth_test.go
+++ b/ssh/client_auth_test.go
@@ -38,7 +38,7 @@
 	defer c2.Close()
 
 	certChecker := CertChecker{
-		IsAuthority: func(k PublicKey) bool {
+		IsUserAuthority: func(k PublicKey) bool {
 			return bytes.Equal(k.Marshal(), testPublicKeys["ecdsa"].Marshal())
 		},
 		UserKeyFallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
diff --git a/ssh/knownhosts/knownhosts.go b/ssh/knownhosts/knownhosts.go
index d1f3718..ea92b29 100644
--- a/ssh/knownhosts/knownhosts.go
+++ b/ssh/knownhosts/knownhosts.go
@@ -144,11 +144,16 @@
 	return bytes.Equal(a.Marshal(), b.Marshal())
 }
 
-// IsAuthority can be used as a callback in ssh.CertChecker
-func (db *hostKeyDB) IsAuthority(remote ssh.PublicKey) bool {
+// IsAuthorityForHost can be used as a callback in ssh.CertChecker
+func (db *hostKeyDB) IsHostAuthority(remote ssh.PublicKey, address string) bool {
+	h, p, err := net.SplitHostPort(address)
+	if err != nil {
+		return false
+	}
+	a := addr{host: h, port: p}
+
 	for _, l := range db.lines {
-		// TODO(hanwen): should we check the hostname against host pattern?
-		if l.cert && keyEq(l.knownKey.Key, remote) {
+		if l.cert && keyEq(l.knownKey.Key, remote) && l.match([]addr{a}) {
 			return true
 		}
 	}
@@ -409,9 +414,7 @@
 
 // New creates a host key callback from the given OpenSSH host key
 // files. The returned callback is for use in
-// ssh.ClientConfig.HostKeyCallback. Hostnames are ignored for
-// certificates, ie. any certificate authority is assumed to be valid
-// for all remote hosts.  Hashed hostnames are not supported.
+// ssh.ClientConfig.HostKeyCallback. Hashed hostnames are not supported.
 func New(files ...string) (ssh.HostKeyCallback, error) {
 	db := newHostKeyDB()
 	for _, fn := range files {
@@ -425,12 +428,8 @@
 		}
 	}
 
-	// TODO(hanwen): properly supporting certificates requires an
-	// API change in the SSH library: IsAuthority should provide
-	// the address too?
-
 	var certChecker ssh.CertChecker
-	certChecker.IsAuthority = db.IsAuthority
+	certChecker.IsHostAuthority = db.IsHostAuthority
 	certChecker.IsRevoked = db.IsRevoked
 	certChecker.HostKeyFallback = db.check