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This image allows you to create a container that exposes a single volume (from another container or a host directory) over SFTP.

The image supports a single SFTP user authenticated via any number of private keys (the main pitfall of this approach is that there is no way to tie an action to a specific person or computer when multiple people or computers are authenticated via their own private keys; this approach works well when only a single person or computer is expected to access this share).

##Configuration

Server Keys

Expose a volume at /volumes/ssh_keys which contains the following files:

  • /volumes/ssh_keys/ssh_host_rsa_key
  • /volumes/ssh_keys/ssh_host_dsa_key
  • /volumes/ssh_keys/ssh_host_ecdsa_key
  • /volumes/ssh_keys/ssh_host_ed25519_key

If you do not expose this volume, you must set SFTP_CONTAINER_KEYS='CREATE' (see below).

Automatically Create Keys

To have the container automaticall create its keys, set the environment variable SFTP_CONTAINER_KEYS='CREATE'.

WARNING: If you expose a volume with keys and set this variable, your keys will be overwritten!

Using this option repeatedly will confuse SFTP clients when they see new host keys each time they connect; however, it can be a good way to create initial keys.

Manually Create Keys

In your key directory on the host, run the following commands:

cd /my/key/directory

ssh-keygen -f ./ssh_host_rsa_key -N '' -t rsa -b 4096
ssh-keygen -f ./ssh_host_dsa_key -N '' -t dsa
ssh-keygen -f ./ssh_host_ecdsa_key -N '' -t ecdsa -b 521
ssh-keygen -f ./ssh_host_ed25519_key -N '' -t ed25519

User ID

This image supports a single user authenticated by one or more private keys. Specify the username for this user via the SFTP_CONTAINER_USER environment variable. Specify the group name for this user via the SFTP_CONTAINER_GROUP environment variable.

This user and group will be created if they do not exist in the container (the container has stock ubuntu users already). Pass in a user and group ID to control which IDs they should be created with as SFTP_CONTAINER_USER_ID and SFTP_CONTAINER_GROUP_ID.

To manage permissions, the user and user id as well as the group and group id should map to the the same values on the system or container hosting the volumes.

User Keys

Expose a volume at /volumes/user/ which contains an authorized_keys file (so /volumes/user/authorized_keys).

This file should mimic a standard ~/.ssh/authorized_key file and list the public keys that should be allowed access.

Data

Any directories that should be available for SFTP should be expose as volumes of a subdirectory under /volumes/sftp_root.
For example, to share the directory "www", add it as a volume named "/volumes/sftp_root/www".

The permissions for these directories will transfer, so they should be readable and writable by SFTP_CONTAINER_USER or SFTP_CONTAINER_GROUP.
Unfortunately, this mechanism works extremely poorly (or, not at all) with docker-machine or boot2docker and host-mounted volumes. When using these tools, data-only containers should be considered.

Debug

To see debug output directly from the SSH daemon, set the SFTP_CONTAINER_DEBUG environment variable to "DEBUG". Note that this puts the SSH daemon in a debug mode which is not suitable for production use.

Example

In this example, the host is serving static files from /www via nginx. The files are owned by www-data. To make these files accessible over SFTP, the following steps could be taken.

Create a directory to hold the host keys for the SFTP server and create the keys:

mkdir /sftp_keys
ssh-keygen -f /sftp_keys/ssh_host_rsa_key -N '' -t rsa -b 4096
ssh-keygen -f /sftp_keys/ssh_host_dsa_key -N '' -t dsa
ssh-keygen -f /sftp_keys/ssh_host_ecdsa_key -N '' -t ecdsa -b 521
ssh-keygen -f /sftp_keys/ssh_host_ed25519_key -N '' -t ed25519

Create the authorized_keys file for your user(s) who should have SFTP access:

mkdir /sftp_user
cp $AUTHORIZED_KEYS /sftp_user/

Get the user ID and group ID of the www-data users and group:

id www-data
    uid=33(www-data) gid=33(www-data) groups=33(www-data)

Ensure the permissions are correct on the host system:

chown www-data:www-data /www

Run the container; pick an SFTP port to represent this directory and map it to port 22:

docker run -d \
-e SFTP_CONTAINER_USER="www-data" \
-e SFTP_CONTAINER_USER_ID="33" \
-e SFTP_CONTAINER_GROUP="www-data" \
-e SFTP_CONTAINER_GROUP_ID="33" \
-v "/sftp_keys:/volumes/ssh_keys" \
-v "/www:/volumes/sftp_root/www" \
-v "/sftp_user:/volumes/user" \
-p 4577:22 willia4/sftp_volume

From a client, connect an SFTP client to port 4577 using the appropriate private key.

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