Tahoe-LAFS SFTP Frontend

  1. SFTP Background
  2. Tahoe-LAFS Support
  3. Creating an Account File
  4. Configuring SFTP Access
  5. Dependencies
  6. Immutable and Mutable Files
  7. Known Issues

SFTP Background

FTP is the venerable internet file-transfer protocol, first developed in 1971. The FTP server usually listens on port 21. A separate connection is used for the actual data transfers, either in the same direction as the initial client-to-server connection (for PORT mode), or in the reverse direction (for PASV) mode. Connections are unencrypted, so passwords, file names, and file contents are visible to eavesdroppers.

SFTP is the modern replacement, developed as part of the SSH “secure shell” protocol, and runs as a subchannel of the regular SSH connection. The SSH server usually listens on port 22. All connections are encrypted.

Both FTP and SFTP were developed assuming a UNIX-like server, with accounts and passwords, octal file modes (user/group/other, read/write/execute), and ctime/mtime timestamps.

Previous versions of Tahoe-LAFS supported FTP, but now only the superior SFTP frontend is supported. See Known Issues, below, for details on the limitations of SFTP.

Tahoe-LAFS Support

All Tahoe-LAFS client nodes can run a frontend SFTP server, allowing regular SFTP clients (like /usr/bin/sftp, the sshfs FUSE plugin, and many others) to access the file store.

Since Tahoe-LAFS does not use user accounts or passwords, the SFTP servers must be configured with a way to first authenticate a user (confirm that a prospective client has a legitimate claim to whatever authorities we might grant a particular user), and second to decide what directory cap should be used as the root directory for a log-in by the authenticated user. A username and password can be used; as of Tahoe-LAFS v1.11, RSA or DSA public key authentication is also supported.

Tahoe-LAFS provides two mechanisms to perform this user-to-cap mapping. The first (recommended) is a simple flat file with one account per line. The second is an HTTP-based login mechanism.

Creating an Account File

To use the first form, create a file (for example BASEDIR/private/accounts) in which each non-comment/non-blank line is a space-separated line of (USERNAME, PASSWORD, ROOTCAP), like so:

% cat BASEDIR/private/accounts
# This is a password line: username password cap
alice password URI:DIR2:ioej8xmzrwilg772gzj4fhdg7a:wtiizszzz2rgmczv4wl6bqvbv33ag4kvbr6prz3u6w3geixa6m6a
bob sekrit URI:DIR2:6bdmeitystckbl9yqlw7g56f4e:serp5ioqxnh34mlbmzwvkp3odehsyrr7eytt5f64we3k9hhcrcja

# This is a public key line: username keytype pubkey cap
# (Tahoe-LAFS v1.11 or later)
carol ssh-rsa AAAA... URI:DIR2:ovjy4yhylqlfoqg2vcze36dhde:4d4f47qko2xm5g7osgo2yyidi5m4muyo2vjjy53q4vjju2u55mfa

For public key authentication, the keytype may be either “ssh-rsa” or “ssh-dsa”. To avoid ambiguity between passwords and public key types, a password cannot start with “ssh-“.

Now add an accounts.file directive to your tahoe.cfg file, as described in the next sections.

Configuring SFTP Access

The Tahoe-LAFS SFTP server requires a host keypair, just like the regular SSH server. It is important to give each server a distinct keypair, to prevent one server from masquerading as different one. The first time a client program talks to a given server, it will store the host key it receives, and will complain if a subsequent connection uses a different key. This reduces the opportunity for man-in-the-middle attacks to just the first connection.

Exercise caution when connecting to the SFTP server remotely. The AES implementation used by the SFTP code does not have defenses against timing attacks. The code for encrypting the SFTP connection was not written by the Tahoe-LAFS team, and we have not reviewed it as carefully as we have reviewed the code for encrypting files and directories in Tahoe-LAFS itself. (See Twisted ticket #4633 for a possible fix to this issue.)

If you can connect to the SFTP server (which is provided by the Tahoe-LAFS gateway) only from a client on the same host, then you would be safe from any problem with the SFTP connection security. The examples given below enforce this policy by including “:interface=127.0.0.1” in the “port” option, which causes the server to only accept connections from localhost.

You will use directives in the tahoe.cfg file to tell the SFTP code where to find these keys. To create one, use the ssh-keygen tool (which comes with the standard OpenSSH client distribution):

% cd BASEDIR
% ssh-keygen -f private/ssh_host_rsa_key

The server private key file must not have a passphrase.

Then, to enable the SFTP server with an accounts file, add the following lines to the BASEDIR/tahoe.cfg file:

[sftpd]
enabled = true
port = tcp:8022:interface=127.0.0.1
host_pubkey_file = private/ssh_host_rsa_key.pub
host_privkey_file = private/ssh_host_rsa_key
accounts.file = private/accounts

The SFTP server will listen on the given port number and on the loopback interface only. The “accounts.file” pathname will be interpreted relative to the node’s BASEDIR.

Or, to use an account server instead, do this:

[sftpd]
enabled = true
port = tcp:8022:interface=127.0.0.1
host_pubkey_file = private/ssh_host_rsa_key.pub
host_privkey_file = private/ssh_host_rsa_key
accounts.url = https://example.com/login

You can provide both accounts.file and accounts.url, although it probably isn’t very useful except for testing.

For further information on SFTP compatibility and known issues with various clients and with the sshfs filesystem, see wiki:SftpFrontend

Dependencies

The Tahoe-LAFS SFTP server requires the Twisted “Conch” component (a “conch” is a twisted shell, get it?). Many Linux distributions package the Conch code separately: debian puts it in the “python-twisted-conch” package.

Immutable and Mutable Files

All files created via SFTP are immutable files. However, files can only be created in writeable directories, which allows the directory entry to be relinked to a different file. Normally, when the path of an immutable file is opened for writing by SFTP, the directory entry is relinked to another file with the newly written contents when the file handle is closed. The old file is still present on the grid, and any other caps to it will remain valid. (See Garbage Collection in Tahoe for how to reclaim the space used by files that are no longer needed.)

The ‘no-write’ metadata field of a directory entry can override this behaviour. If the ‘no-write’ field holds a true value, then a permission error will occur when trying to write to the file, even if it is in a writeable directory. This does not prevent the directory entry from being unlinked or replaced.

When using sshfs, the ‘no-write’ field can be set by clearing the ‘w’ bits in the Unix permissions, for example using the command chmod 444 path/to/file. Note that this does not mean that arbitrary combinations of Unix permissions are supported. If the ‘w’ bits are cleared on a link to a mutable file or directory, that link will become read-only.

If SFTP is used to write to an existing mutable file, it will publish a new version when the file handle is closed.

Known Issues

Known Issues in the SFTP Frontend

Upload errors may not be reported when writing files using SFTP via sshfs (ticket #1059).

Non-ASCII filenames are supported with SFTP only if the client encodes filenames as UTF-8 (ticket #1089).

See also wiki:SftpFrontend.