3 files changed, 93 insertions, 2 deletions
diff --git a/plumbing/transport/ssh/auth_method.go b/plumbing/transport/ssh/auth_method.go
index f53e510..82e3453 100644
--- a/plumbing/transport/ssh/auth_method.go
+++ b/plumbing/transport/ssh/auth_method.go
@@ -5,7 +5,10 @@ import (
 	"fmt"
 	"net"
 	"os"
+	"os/user"
+	"path/filepath"
 
+	"github.com/src-d/crypto/ssh/knownhosts"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/agent"
 )
@@ -17,6 +20,7 @@ var ErrEmptySSHAgentAddr = errors.New("SSH_AUTH_SOCK env variable is required")
 // configuration needed to establish an ssh connection.
 type AuthMethod interface {
 	clientConfig() *ssh.ClientConfig
+	hostKeyCallback() (ssh.HostKeyCallback, error)
 }
 
 // The names of the AuthMethod implementations. To be returned by the
@@ -35,6 +39,7 @@ const (
 type KeyboardInteractive struct {
 	User      string
 	Challenge ssh.KeyboardInteractiveChallenge
+	baseAuthMethod
 }
 
 func (a *KeyboardInteractive) Name() string {
@@ -56,6 +61,7 @@ func (a *KeyboardInteractive) clientConfig() *ssh.ClientConfig {
 type Password struct {
 	User string
 	Pass string
+	baseAuthMethod
 }
 
 func (a *Password) Name() string {
@@ -78,6 +84,7 @@ func (a *Password) clientConfig() *ssh.ClientConfig {
 type PasswordCallback struct {
 	User     string
 	Callback func() (pass string, err error)
+	baseAuthMethod
 }
 
 func (a *PasswordCallback) Name() string {
@@ -100,6 +107,7 @@ func (a *PasswordCallback) clientConfig() *ssh.ClientConfig {
 type PublicKeys struct {
 	User   string
 	Signer ssh.Signer
+	baseAuthMethod
 }
 
 func (a *PublicKeys) Name() string {
@@ -122,6 +130,7 @@ func (a *PublicKeys) clientConfig() *ssh.ClientConfig {
 type PublicKeysCallback struct {
 	User     string
 	Callback func() (signers []ssh.Signer, err error)
+	baseAuthMethod
 }
 
 func (a *PublicKeysCallback) Name() string {
@@ -163,3 +172,80 @@ func NewSSHAgentAuth(user string) (*PublicKeysCallback, error) {
 		Callback: agent.NewClient(pipe).Signers,
 	}, nil
 }
+
+// NewKnownHostsCallback returns ssh.HostKeyCallback based on a file based on a
+// know_hosts file. http://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT
+//
+// If files is empty, the list of files will be read from the SSH_KNOWN_HOSTS
+// environment variable, example:
+//   /home/foo/custom_known_hosts_file:/etc/custom_known/hosts_file
+//
+// If SSH_KNOWN_HOSTS is not set the following file locations will be used:
+//   ~/.ssh/known_hosts
+//   /etc/ssh/ssh_known_hosts
+func NewKnownHostsCallback(files ...string) (ssh.HostKeyCallback, error) {
+	files, err := getDefaultKnownHostsFiles()
+	if err != nil {
+		return nil, err
+	}
+
+	files, err = filterKnownHostsFiles(files...)
+	if err != nil {
+		return nil, err
+	}
+
+	return knownhosts.New(files...)
+}
+
+func getDefaultKnownHostsFiles() ([]string, error) {
+	files := filepath.SplitList(os.Getenv("SSH_KNOWN_HOSTS"))
+	if len(files) != 0 {
+		return files, nil
+	}
+
+	user, err := user.Current()
+	if err != nil {
+		return nil, err
+	}
+
+	return []string{
+		filepath.Join(user.HomeDir, "/.ssh/known_hosts"),
+		"/etc/ssh/ssh_known_hosts",
+	}, nil
+}
+
+func filterKnownHostsFiles(files ...string) ([]string, error) {
+	var out []string
+	for _, file := range files {
+		_, err := os.Stat(file)
+		if err == nil {
+			out = append(out, file)
+			continue
+		}
+
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+	}
+
+	if len(out) == 0 {
+		return nil, fmt.Errorf("unable to find any valid know_hosts file, set SSH_KNOWN_HOSTS env variable")
+	}
+
+	return out, nil
+}
+
+type baseAuthMethod struct {
+	// HostKeyCallback is the function type used for verifying server keys.
+	// If nil default callback will be create using NewKnownHostsHostKeyCallback
+	// without argument.
+	HostKeyCallback ssh.HostKeyCallback
+}
+
+func (m *baseAuthMethod) hostKeyCallback() (ssh.HostKeyCallback, error) {
+	if m.HostKeyCallback == nil {
+		return NewKnownHostsCallback()
+	}
+
+	return m.HostKeyCallback, nil
+}
diff --git a/plumbing/transport/ssh/common.go b/plumbing/transport/ssh/common.go
index 5ed64d4..9b484f9 100644
--- a/plumbing/transport/ssh/common.go
+++ b/plumbing/transport/ssh/common.go
@@ -82,7 +82,13 @@ func (c *command) connect() error {
 	}
 
 	var err error
-	c.client, err = ssh.Dial("tcp", c.getHostWithPort(), c.auth.clientConfig())
+	config := c.auth.clientConfig()
+	config.HostKeyCallback, err = c.auth.hostKeyCallback()
+	if err != nil {
+		return err
+	}
+
+	c.client, err = ssh.Dial("tcp", c.getHostWithPort(), config)
 	if err != nil {
 		return err
 	}
diff --git a/plumbing/transport/ssh/upload_pack_test.go b/plumbing/transport/ssh/upload_pack_test.go
index 8194770..54d523a 100644
--- a/plumbing/transport/ssh/upload_pack_test.go
+++ b/plumbing/transport/ssh/upload_pack_test.go
@@ -33,5 +33,4 @@ func (s *UploadPackSuite) SetUpSuite(c *C) {
 	ep, err = transport.NewEndpoint("git@github.com:git-fixtures/non-existent.git")
 	c.Assert(err, IsNil)
 	s.UploadPackSuite.NonExistentEndpoint = ep
-
 }