add session level keep-alives for ssh communicator

communicator/ssh/communicator.go

@@ -55,6 +55,10 @@ type Config struct {
 
 	// UseSftp, if true, sftp will be used instead of scp for file transfers
 	UseSftp bool
+
+	// KeepAliveInterval sets how often we send a channel request to the
+	// server. A value < 0 disables.
+	KeepAliveInterval time.Duration
 }
 
 // Creates a new packer.Communicator implementation over SSH. This takes
@@ -104,6 +108,20 @@ func (c *comm) Start(cmd *packer.RemoteCmd) (err error) {
 		return
 	}
 
+	go func() {
+		if c.config.KeepAliveInterval < 0 {
+			return
+		}
+		c := time.NewTicker(c.config.KeepAliveInterval)
+		defer c.Stop()
+		for range c.C {
+			_, err := session.SendRequest("keepalive@packer.io", true, nil)
+			if err != nil {
+				return
+			}
+		}
+	}()
+
 	// Start a goroutine to wait for the session to end and set the
 	// exit boolean and status.
 	go func() {

helper/communicator/config.go

@@ -37,6 +37,7 @@ type Config struct {
 	SSHProxyPort              int           `mapstructure:"ssh_proxy_port"`
 	SSHProxyUsername          string        `mapstructure:"ssh_proxy_username"`
 	SSHProxyPassword          string        `mapstructure:"ssh_proxy_password"`
+	SSHKeepAliveInterval      time.Duration `mapstructure:"ssh_keep_alive_interval"`
 
 	// WinRM
 	WinRMUser               string        `mapstructure:"winrm_username"`
@@ -131,6 +132,10 @@ func (c *Config) prepareSSH(ctx *interpolate.Context) []error {
 		c.SSHTimeout = 5 * time.Minute
 	}
 
+	if c.SSHKeepAliveInterval == 0 {
+		c.SSHKeepAliveInterval = 5 * time.Second
+	}
+
 	if c.SSHHandshakeAttempts == 0 {
 		c.SSHHandshakeAttempts = 10
 	}

helper/communicator/step_connect_ssh.go

@@ -182,6 +182,7 @@ func (s *StepConnectSSH) waitForSSH(state multistep.StateBag, cancel <-chan stru
 			Pty:        s.Config.SSHPty,
 			DisableAgentForwarding: s.Config.SSHDisableAgentForwarding,
 			UseSftp:                s.Config.SSHFileTransferMethod == "sftp",
+			KeepAliveInterval:      s.Config.SSHKeepAliveInterval,
 		}
 
 		log.Println("[INFO] Attempting SSH connection...")

website/source/docs/templates/communicator.html.md

@@ -91,6 +91,10 @@ The SSH communicator has the following options:
 -   `ssh_host` (string) - The address to SSH to. This usually is automatically
     configured by the builder.
 
+*   `ssh_keep_alive_interval` (string) - How often to send "keep alive"
+    messages to the server. Set to a negative value (`-1`) to disable. Example value:
+    "10s". Defaults to "5s".
+
 -   `ssh_password` (string) - A plaintext password to use to authenticate
     with SSH.