» Template Communicators

Communicators are the mechanism Packer uses to upload files, execute scripts, etc. with the machine being created.

Communicators are configured within the builder section. Packer currently supports three kinds of communicators:

  • none - No communicator will be used. If this is set, most provisioners also can't be used.

  • ssh - An SSH connection will be established to the machine. This is usually the default.

  • winrm - A WinRM connection will be established.

In addition to the above, some builders have custom communicators they can use. For example, the Docker builder has a "docker" communicator that uses docker exec and docker cp to execute scripts and copy files.

» Using a Communicator

By default, the SSH communicator is usually used. Additional configuration may not even be necessary, since some builders such as Amazon automatically configure everything.

However, to specify a communicator, you set the communicator key within a build. Multiple builds can have different communicators. Example:

  "builders": [
      "type": "amazon-ebs",
      "communicator": "ssh"

After specifying the communicator, you can specify a number of other configuration parameters for that communicator. These are documented below.

» SSH Communicator

The SSH communicator connects to the host via SSH. If you have an SSH agent configured on the host running Packer, and SSH agent authentication is enabled in the communicator config, Packer will automatically forward the SSH agent to the remote host.

The SSH communicator has the following options:

  • ssh_agent_auth (boolean) - If true, the local SSH agent will be used to authenticate connections to the remote host. Defaults to false.

  • ssh_bastion_agent_auth (boolean) - If true, the local SSH agent will be used to authenticate with the bastion host. Defaults to false.

  • ssh_bastion_host (string) - A bastion host to use for the actual SSH connection.

  • ssh_bastion_password (string) - The password to use to authenticate with the bastion host.

  • ssh_bastion_port (number) - The port of the bastion host. Defaults to 22.

  • ssh_bastion_private_key_file (string) - Path to a PEM encoded private key file to use to authenticate with the bastion host. The ~ can be used in path and will be expanded to the home directory of current user.

  • ssh_bastion_username (string) - The username to connect to the bastion host.

  • ssh_clear_authorized_keys (boolean) - If true, Packer will attempt to remove its temporary key from ~/.ssh/authorized_keys and /root/.ssh/authorized_keys. This is a mostly cosmetic option, since Packer will delete the temporary private key from the host system regardless of whether this is set to true (unless the user has set the -debug flag). Defaults to "false"; currently only works on guests with sed installed.

  • ssh_disable_agent_forwarding (boolean) - If true, SSH agent forwarding will be disabled. Defaults to false.

  • ssh_file_transfer_method (scp or sftp) - How to transfer files, Secure copy (default) or SSH File Transfer Protocol.

  • ssh_handshake_attempts (number) - The number of handshakes to attempt with SSH once it can connect. This defaults to 10.

  • 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 (-1s) to disable. Example value: 10s. Defaults to 5s.

  • ssh_password (string) - A plaintext password to use to authenticate with SSH.

  • ssh_port (number) - The port to connect to SSH. This defaults to 22.

  • ssh_private_key_file (string) - Path to a PEM encoded private key file to use to authenticate with SSH. The ~ can be used in path and will be expanded to the home directory of current user.

  • ssh_proxy_host (string) - A SOCKS proxy host to use for SSH connection

  • ssh_proxy_password (string) - The password to use to authenticate with the proxy server. Optional.

  • ssh_proxy_port (number) - A port of the SOCKS proxy. Defaults to 1080.

  • ssh_proxy_username (string) - The username to authenticate with the proxy server. Optional.

  • ssh_pty (boolean) - If true, a PTY will be requested for the SSH connection. This defaults to false.

  • ssh_read_write_timeout (string) - The amount of time to wait for a remote command to end. This might be useful if, for example, packer hangs on a connection after a reboot. Example: 5m. Disabled by default.

  • ssh_timeout (string) - The time to wait for SSH to become available. Packer uses this to determine when the machine has booted so this is usually quite long. Example value: 10m.

  • ssh_username (string) - The username to connect to SSH with. Required if using SSH.

» SSH Communicator Details

Packer will only use one authentication method, either publickey or if ssh_password is used packer will offer password and keyboard-interactive both sending the password. In other words Packer will not work with sshd configured with more than one configured authentication method using AuthenticationMethods.

Packer supports the following ciphers:

  • aes128-ctr
  • aes192-ctr
  • aes256-ctr
  • arcfour128
  • arcfour256
  • arcfour
  • es128-gcm@openssh.com
  • acha20-poly1305@openssh.com

And the following MACs:

  • hmac-sha1
  • hmac-sha1-96
  • hmac-sha2-256
  • hmac-sha2-256-etm@openssh.com

» WinRM Communicator

The WinRM communicator has the following options.

  • winrm_host (string) - The address for WinRM to connect to.

    NOTE: If using an Amazon EBS builder, you can specify the interface WinRM connects to via ssh_interface

  • winrm_insecure (boolean) - If true, do not check server certificate chain and host name.

  • winrm_password (string) - The password to use to connect to WinRM.

  • winrm_port (number) - The WinRM port to connect to. This defaults to 5985 for plain unencrypted connection and 5986 for SSL when winrm_use_ssl is set to true.

  • winrm_timeout (string) - The amount of time to wait for WinRM to become available. This defaults to 30m since setting up a Windows machine generally takes a long time.

  • winrm_use_ntlm (boolean) - If true, NTLMv2 authentication (with session security) will be used for WinRM, rather than default (basic authentication), removing the requirement for basic authentication to be enabled within the target guest. Further reading for remote connection authentication can be found here.

  • winrm_use_ssl (boolean) - If true, use HTTPS for WinRM.

  • winrm_username (string) - The username to use to connect to WinRM.

» Pausing Before Connecting

We recommend that you enable SSH or WinRM as the very last step in your guest's bootstrap script, but sometimes you may have a race condition where you need Packer to wait before attempting to connect to your guest.

If you end up in this situation, you can use the template option pause_before_connecting. By default, there is no pause. For example:

  "communicator": "ssh",
  "ssh_username": "myuser",
  "pause_before_connecting": "10m"

In this example, Packer will check whether it can connect, as normal. But once a connection attempt is successful, it will disconnect and then wait 10 minutes before connecting to the guest and beginning provisioning.