builder/null: Support SSH Agent Auth

builder/null/builder.go

@@ -32,6 +32,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 			Config: &b.config.CommConfig,
 			Host:   CommHost(b.config.CommConfig.Host()),
 			SSHConfig: SSHConfig(
+				b.config.CommConfig.SSHAgentAuth,
 				b.config.CommConfig.SSHUsername,
 				b.config.CommConfig.SSHPassword,
 				b.config.CommConfig.SSHPrivateKey),

builder/null/config.go

@@ -41,14 +41,16 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 			fmt.Errorf("a Username must be specified, please reference your communicator documentation"))
 	}
 
-	if c.CommConfig.Password() == "" && c.CommConfig.SSHPrivateKey == "" {
+	if !c.CommConfig.SSHAgentAuth && c.CommConfig.Password() == "" && c.CommConfig.SSHPrivateKey == "" {
 		errs = packer.MultiErrorAppend(errs,
 			fmt.Errorf("one authentication method must be specified, please reference your communicator documentation"))
 	}
 
-	if c.CommConfig.SSHPassword != "" && c.CommConfig.SSHPrivateKey != "" {
+	if (c.CommConfig.SSHAgentAuth &&
+		(c.CommConfig.SSHPassword != "" || c.CommConfig.SSHPrivateKey != "")) ||
+		(c.CommConfig.SSHPassword != "" && c.CommConfig.SSHPrivateKey != "") {
 		errs = packer.MultiErrorAppend(errs,
-			fmt.Errorf("only one of ssh_password and ssh_private_key_file must be specified"))
+			fmt.Errorf("only one of ssh_agent_auth, ssh_password, and ssh_private_key_file must be specified"))
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {

builder/null/ssh.go

@@ -2,10 +2,14 @@ package null
 
 import (
 	"fmt"
+	"io/ioutil"
+	"net"
+	"os"
+
 	"github.com/hashicorp/packer/communicator/ssh"
 	"github.com/mitchellh/multistep"
 	gossh "golang.org/x/crypto/ssh"
-	"io/ioutil"
+	"golang.org/x/crypto/ssh/agent"
 )
 
 func CommHost(host string) func(multistep.StateBag) (string, error) {
@@ -17,8 +21,26 @@ func CommHost(host string) func(multistep.StateBag) (string, error) {
 // SSHConfig returns a function that can be used for the SSH communicator
 // config for connecting to the specified host via SSH
 // private_key_file has precedence over password!
-func SSHConfig(username string, password string, privateKeyFile string) func(multistep.StateBag) (*gossh.ClientConfig, error) {
+func SSHConfig(useAgent bool, username string, password string, privateKeyFile string) func(multistep.StateBag) (*gossh.ClientConfig, error) {
 	return func(state multistep.StateBag) (*gossh.ClientConfig, error) {
+		if useAgent {
+			authSock := os.Getenv("SSH_AUTH_SOCK")
+			if authSock == "" {
+				return nil, fmt.Errorf("SSH_AUTH_SOCK is not set")
+			}
+
+			sshAgent, err := net.Dial("unix", authSock)
+			if err != nil {
+				return nil, fmt.Errorf("Cannot connect to SSH Agent socket %q: %s", authSock, err)
+			}
+
+			return &gossh.ClientConfig{
+				User: username,
+				Auth: []gossh.AuthMethod{
+					gossh.PublicKeysCallback(agent.NewClient(sshAgent).Signers),
+				},
+			}, nil
+		}
 
 		if privateKeyFile != "" {
 			// key based auth