Tahoe-LAFS SFTP and FTP Frontends

  1. SFTP/FTP Background
  2. Tahoe-LAFS Support
  3. Creating an Account File
  4. Running An Account Server (accounts.url)
  5. Configuring SFTP Access
  6. Configuring FTP Access
  7. Dependencies
  8. Immutable and Mutable Files
  9. Known Issues

SFTP/FTP 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.

We recommend SFTP over FTP, because the protocol is better, and the server implementation in Tahoe-LAFS is more complete. See Known Issues, below, for details.

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. They can also run an FTP server, so FTP clients (like /usr/bin/ftp, ncftp, and others) can too. These frontends sit at the same level as the web-API interface.

Since Tahoe-LAFS does not use user accounts or passwords, the SFTP/FTP 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.

Running An Account Server (accounts.url)

The accounts.url directive allows access requests to be controlled by an HTTP-based login service, useful for centralized deployments. This was used by AllMyData to provide web-based file access, where the service used a simple PHP script and database lookups to map an account email address and password to a Tahoe-LAFS directory cap. The service will receive a multipart/form-data POST, just like one created with a <form> and <input> fields, with three parameters:

  • action: “authenticate” (this is a static string)
  • email: USERNAME (Tahoe-LAFS has no notion of email addresses, but the authentication service uses them as account names, so the interface presents this argument as “email” rather than “username”).
  • passwd: PASSWORD

It should return a single string that either contains a Tahoe-LAFS directory cap (URI:DIR2:...), or “0” to indicate a login failure.

Tahoe-LAFS recommends the service be secure, preferably localhost-only. This makes it harder for attackers to brute force the password or use DNS poisoning to cause the Tahoe-LAFS gateway to talk with the wrong server, thereby revealing the usernames and passwords.

Public key authentication is not supported when an account server is used.

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

Configuring FTP Access

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

[ftpd]
enabled = true
port = tcp:8021:interface=127.0.0.1
accounts.file = private/accounts

The FTP 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.

To enable the FTP server with an account server instead, provide the URL of that server in an “accounts.url” directive:

[ftpd]
enabled = true
port = tcp:8021:interface=127.0.0.1
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.

FTP provides no security, and so your password or caps could be eavesdropped if you connect to the FTP server remotely. The examples above include ”:interface=127.0.0.1” in the “port” option, which causes the server to only accept connections from localhost.

Public key authentication is not supported for FTP.

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. Conch requires the “pycrypto” package, which is a Python+C implementation of many cryptographic functions (the debian package is named “python-crypto”).

Note that “pycrypto” is different than the “pycryptopp” package that Tahoe-LAFS uses (which is a Python wrapper around the C++ -based Crypto++ library, a library that is frequently installed as /usr/lib/libcryptopp.a, to avoid problems with non-alphanumerics in filenames).

Immutable and Mutable Files

All files created via SFTP (and FTP) 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.

Known Issues in the FTP Frontend

Mutable files are not supported by the FTP frontend (ticket #680).

Non-ASCII filenames are not supported by FTP (ticket #682).

The FTP frontend sometimes fails to report errors, for example if an upload fails because it does meet the “servers of happiness” threshold (ticket #1081).