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SSH Communicator

Communicators are the mechanism Packer uses to upload files, execute scripts, etc. on the machine being created, and are configured within the builder section.

The SSH communicator does this by using the SSH protocol. It is the default communicator for a majority of builders.

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.

Getting Ready to Use the SSH Communicator

The SSH communicator is the default communicator for a majority of builders, but depending on your builder it may not work "out of the box".

If you are building from a cloud image (for example, building on Amazon), there is a good chance that your cloud provider has already preconfigured SSH on the image for you, meaning that all you have to do is configure the communicator in the Packer template.

However, if you are building from a brand-new and unconfigured operating system image, you will almost always have to perform some extra work to configure SSH on the guest machine. For most operating system distributions, this work will be performed by a boot command that references a file which provides answers to the normally-interactive questions you get asked when installing an operating system. The name of this file varies by operating system; some common examples are the "preseed" file required by Debian, the "kickstart" file required by CentOS or the "answer file", also known as the Autounattend.xml file, required by Windows. For simplicity's sake, we'll refer to this file as the "preseed" file in the rest of the documentation.

If you are unfamiliar with how to use a preseed file for automatic bootstrapping of an image, please either take a look at our quick guides to image bootstrapping, or research automatic configuration for your specific guest operating system. Knowing how to automatically initalize your operating system is critical for being able to successfully use Packer.

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_host (string) - The address to SSH to. This usually is automatically configured by the builder.

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

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

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

  • ssh_ciphers ([]string) - This overrides the value of ciphers supported by default by Golang. The default value is [ "aes128-gcm@openssh.com", "chacha20-poly1305@openssh.com", "aes128-ctr", "aes192-ctr", "aes256-ctr", ]

    Valid options for ciphers include: "aes128-ctr", "aes192-ctr", "aes256-ctr", "aes128-gcm@openssh.com", "chacha20-poly1305@openssh.com", "arcfour256", "arcfour128", "arcfour", "aes128-cbc", "3des-cbc",

  • ssh_clear_authorized_keys (bool) - 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_key_exchange_algorithms ([]string) - If set, Packer will override the value of key exchange (kex) algorithms supported by default by Golang. Acceptable values include: "curve25519-sha256@libssh.org", "ecdh-sha2-nistp256", "ecdh-sha2-nistp384", "ecdh-sha2-nistp521", "diffie-hellman-group14-sha1", and "diffie-hellman-group1-sha1".

  • ssh_certificate_file (string) - Path to user certificate used to authenticate with SSH. The ~ can be used in path and will be expanded to the home directory of current user.

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

  • ssh_timeout (duration string | ex: "1h5m2s") - 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. This defaults to 5m, unless ssh_handshake_attempts is set.

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

  • ssh_handshake_attempts (int) - The number of handshakes to attempt with SSH once it can connect. This defaults to 10, unless a ssh_timeout is set.

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

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

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

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

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

  • ssh_bastion_interactive (bool) - If true, the keyboard-interactive used to authenticate with bastion host.

  • 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_certificate_file (string) - Path to user certificate used to authenticate with bastion host. The ~ can be used in path and will be expanded to the home directory of current user.

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

    NOTE: Guests using Windows with Win32-OpenSSH v9.1.0.0p1-Beta, scp (the default protocol for copying data) returns a non-zero error code since the MOTW cannot be set, which cause any file transfer to fail. As a workaround you can override the transfer protocol with SFTP instead ssh_file_transfer_protocol = "sftp".

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

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

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

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

  • ssh_keep_alive_interval (duration string | ex: "1h5m2s") - 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_read_write_timeout (duration string | ex: "1h5m2s") - 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_remote_tunnels ([]string) -

  • ssh_local_tunnels ([]string) -

  • ssh_agent_auth (bool) - If true, the local SSH agent will be used to authenticate connections to the source instance. No temporary keypair will be created, and the values of ssh_password and ssh_private_key_file will be ignored. The environment variable SSH_AUTH_SOCK must be set for this option to work properly.
  • temporary_key_pair_name (string) - The name of the temporary key pair to generate. By default, Packer generates a name that looks like packer_<UUID>, where <UUID> is a 36 character unique identifier.
  • 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.

Note: The options ssh_keypair_name, ssh_agent_auth, temporary_key_pair_name, and ssh_private_key_file are supported by the communicator; however, they may not be supported for every builder. Please refer to the builder documentation for supported options.

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 MACs:

For more information on the ciphers that Packer supports, check the docs for the ssh_ciphers template option.

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