From b2f8eb68e8f431f803531a87821a3238faf821dc Mon Sep 17 00:00:00 2001
From: Mark Peek <mark@peek.org>
Date: Sat, 13 Jun 2015 17:15:49 -0700
Subject: [PATCH] Enable ssh agent forwarding #1066

---
 CHANGELOG.md                                  |  2 +
 communicator/ssh/communicator.go              | 50 +++++++++++++++++++
 .../docs/provisioners/shell.html.markdown     | 27 ++++++++++
 3 files changed, 79 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4391393d0..9e26c7818 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,8 @@ FEATURES:
       being built. This should be used for template-relative paths. [GH-54]
   * **Disable SSH:** Set `communicator` to "none" in any builder to disable SSH
       connections. Note that provisioners won't work if this is done. [GH-1591]
+  * **SSH Agent Forwarding:** SSH Agent Forwarding will now be enabled
+      to allow access to remote servers such as private git repos. [GH-1066]
 
 IMPROVEMENTS:
 
diff --git a/communicator/ssh/communicator.go b/communicator/ssh/communicator.go
index 217afc940..193ecae42 100644
--- a/communicator/ssh/communicator.go
+++ b/communicator/ssh/communicator.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"github.com/mitchellh/packer/packer"
 	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
 	"io"
 	"io/ioutil"
 	"log"
@@ -226,10 +227,59 @@ func (c *comm) reconnect() (err error) {
 	if sshConn != nil {
 		c.client = ssh.NewClient(sshConn, sshChan, req)
 	}
+	c.connectToAgent()
 
 	return
 }
 
+func (c *comm) connectToAgent() {
+	if c.client == nil {
+		return
+	}
+
+	// open connection to the local agent
+	socketLocation := os.Getenv("SSH_AUTH_SOCK")
+	if socketLocation == "" {
+		log.Printf("no local agent socket")
+		return
+	}
+	agentConn, err := net.Dial("unix", socketLocation)
+	if err != nil {
+		log.Printf("could not connect to local agent socket: %s", socketLocation)
+		return
+	}
+
+	// create agent and add in auth
+	forwardingAgent := agent.NewClient(agentConn)
+	if forwardingAgent == nil {
+		log.Printf("could not create agent client")
+		agentConn.Close()
+		return
+	}
+
+	// add callback for forwarding agent to SSH config
+	// XXX - might want to handle reconnects appending multiple callbacks
+	auth := ssh.PublicKeysCallback(forwardingAgent.Signers)
+	c.config.SSHConfig.Auth = append(c.config.SSHConfig.Auth, auth)
+	agent.ForwardToAgent(c.client, forwardingAgent)
+
+	// Setup a session to request agent forwarding
+	session, err := c.newSession()
+	if err != nil {
+		return
+	}
+	defer session.Close()
+
+	err = agent.RequestAgentForwarding(session)
+	if err != nil {
+		log.Printf("RequestAgentForwarding:", err)
+		return
+	}
+
+	log.Printf("agent forwarding enabled")
+	return
+}
+
 func (c *comm) scpSession(scpCommand string, f func(io.Writer, *bufio.Reader) error) error {
 	session, err := c.newSession()
 	if err != nil {
diff --git a/website/source/docs/provisioners/shell.html.markdown b/website/source/docs/provisioners/shell.html.markdown
index e57910cb0..89a442a83 100644
--- a/website/source/docs/provisioners/shell.html.markdown
+++ b/website/source/docs/provisioners/shell.html.markdown
@@ -146,6 +146,33 @@ on reboot or in your shell script. For example, on Gentoo:
 /etc/init.d/net.eth0 stop
 ```
 
+## SSH Agent Forwarding
+
+Some provisioning requires connecting to remote SSH servers from within the
+packer instance. The below example is for pulling code from a private git
+repository utilizing openssh on the client. Make sure you are running
+`ssh-agent` and add your git repo ssh keys into it using `ssh-add /path/to/key`.
+When the packer instance needs access to the ssh keys the agent will forward
+the request back to your `ssh-agent`.
+
+Note: when provisioning via git you should add the git server keys into
+the `~/.ssh/known_hosts` file otherwise the git command could hang awaiting
+input. This can be done by copying the file in via the
+[file provisioner](/docs/provisioners/file.html) (more secure)
+or using `ssh-keyscan` to populate the file (less secure). An example of the
+latter accessing github would be:
+
+```
+{
+  "type": "shell",
+  "inline": [
+    "sudo apt-get install -y git",
+    "ssh-keyscan github.com >> ~/.ssh/known_hosts",
+    "git clone git@github.com:exampleorg/myprivaterepo.git"
+  ]
+}
+```
+
 ## Troubleshooting
 
 *My shell script doesn't work correctly on Ubuntu*