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274 lines
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ReStructuredText
274 lines
11 KiB
ReStructuredText
.. -*- coding: utf-8-with-signature -*-
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=================================
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Tahoe-LAFS SFTP and FTP Frontends
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=================================
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1. `SFTP/FTP Background`_
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2. `Tahoe-LAFS Support`_
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3. `Creating an Account File`_
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4. `Running An Account Server (accounts.url)`_
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5. `Configuring SFTP Access`_
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6. `Configuring FTP Access`_
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7. `Dependencies`_
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8. `Immutable and Mutable Files`_
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9. `Known Issues`_
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SFTP/FTP Background
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===================
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FTP is the venerable internet file-transfer protocol, first developed in
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1971. The FTP server usually listens on port 21. A separate connection is
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used for the actual data transfers, either in the same direction as the
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initial client-to-server connection (for PORT mode), or in the reverse
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direction (for PASV) mode. Connections are unencrypted, so passwords, file
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names, and file contents are visible to eavesdroppers.
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SFTP is the modern replacement, developed as part of the SSH "secure shell"
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protocol, and runs as a subchannel of the regular SSH connection. The SSH
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server usually listens on port 22. All connections are encrypted.
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Both FTP and SFTP were developed assuming a UNIX-like server, with accounts
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and passwords, octal file modes (user/group/other, read/write/execute), and
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ctime/mtime timestamps.
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We recommend SFTP over FTP, because the protocol is better, and the server
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implementation in Tahoe-LAFS is more complete. See `Known Issues`_, below,
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for details.
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Tahoe-LAFS Support
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==================
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All Tahoe-LAFS client nodes can run a frontend SFTP server, allowing regular
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SFTP clients (like ``/usr/bin/sftp``, the ``sshfs`` FUSE plugin, and many
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others) to access the file store. They can also run an FTP server, so FTP
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clients (like ``/usr/bin/ftp``, ``ncftp``, and others) can too. These
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frontends sit at the same level as the web-API interface.
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Since Tahoe-LAFS does not use user accounts or passwords, the SFTP/FTP
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servers must be configured with a way to first authenticate a user (confirm
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that a prospective client has a legitimate claim to whatever authorities we
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might grant a particular user), and second to decide what directory cap
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should be used as the root directory for a log-in by the authenticated user.
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A username and password can be used; as of Tahoe-LAFS v1.11, RSA or DSA
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public key authentication is also supported.
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Tahoe-LAFS provides two mechanisms to perform this user-to-cap mapping.
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The first (recommended) is a simple flat file with one account per line.
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The second is an HTTP-based login mechanism.
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Creating an Account File
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========================
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To use the first form, create a file (for example ``BASEDIR/private/accounts``)
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in which each non-comment/non-blank line is a space-separated line of
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(USERNAME, PASSWORD, ROOTCAP), like so::
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% cat BASEDIR/private/accounts
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# This is a password line: username password cap
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alice password URI:DIR2:ioej8xmzrwilg772gzj4fhdg7a:wtiizszzz2rgmczv4wl6bqvbv33ag4kvbr6prz3u6w3geixa6m6a
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bob sekrit URI:DIR2:6bdmeitystckbl9yqlw7g56f4e:serp5ioqxnh34mlbmzwvkp3odehsyrr7eytt5f64we3k9hhcrcja
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# This is a public key line: username keytype pubkey cap
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# (Tahoe-LAFS v1.11 or later)
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carol ssh-rsa AAAA... URI:DIR2:ovjy4yhylqlfoqg2vcze36dhde:4d4f47qko2xm5g7osgo2yyidi5m4muyo2vjjy53q4vjju2u55mfa
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For public key authentication, the keytype may be either "ssh-rsa" or "ssh-dsa".
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To avoid ambiguity between passwords and public key types, a password cannot
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start with "ssh-".
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Now add an ``accounts.file`` directive to your ``tahoe.cfg`` file, as described in
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the next sections.
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Running An Account Server (accounts.url)
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========================================
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The accounts.url directive allows access requests to be controlled by an
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HTTP-based login service, useful for centralized deployments. This was used
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by AllMyData to provide web-based file access, where the service used a
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simple PHP script and database lookups to map an account email address and
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password to a Tahoe-LAFS directory cap. The service will receive a
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multipart/form-data POST, just like one created with a <form> and <input>
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fields, with three parameters:
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• action: "authenticate" (this is a static string)
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• email: USERNAME (Tahoe-LAFS has no notion of email addresses, but the
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authentication service uses them as account names, so the interface
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presents this argument as "email" rather than "username").
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• passwd: PASSWORD
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It should return a single string that either contains a Tahoe-LAFS directory
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cap (URI:DIR2:...), or "0" to indicate a login failure.
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Tahoe-LAFS recommends the service be secure, preferably localhost-only. This
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makes it harder for attackers to brute force the password or use DNS
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poisoning to cause the Tahoe-LAFS gateway to talk with the wrong server,
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thereby revealing the usernames and passwords.
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Public key authentication is not supported when an account server is used.
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Configuring SFTP Access
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=======================
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The Tahoe-LAFS SFTP server requires a host keypair, just like the regular SSH
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server. It is important to give each server a distinct keypair, to prevent
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one server from masquerading as different one. The first time a client
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program talks to a given server, it will store the host key it receives, and
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will complain if a subsequent connection uses a different key. This reduces
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the opportunity for man-in-the-middle attacks to just the first connection.
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Exercise caution when connecting to the SFTP server remotely. The AES
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implementation used by the SFTP code does not have defenses against timing
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attacks. The code for encrypting the SFTP connection was not written by the
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Tahoe-LAFS team, and we have not reviewed it as carefully as we have reviewed
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the code for encrypting files and directories in Tahoe-LAFS itself. (See
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`Twisted ticket #4633`_ for a possible fix to this issue.)
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.. _Twisted ticket #4633: https://twistedmatrix.com/trac/ticket/4633
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If you can connect to the SFTP server (which is provided by the Tahoe-LAFS
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gateway) only from a client on the same host, then you would be safe from any
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problem with the SFTP connection security. The examples given below enforce
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this policy by including ":interface=127.0.0.1" in the "port" option, which
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causes the server to only accept connections from localhost.
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You will use directives in the tahoe.cfg file to tell the SFTP code where to
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find these keys. To create one, use the ``ssh-keygen`` tool (which comes with
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the standard OpenSSH client distribution)::
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% cd BASEDIR
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% ssh-keygen -f private/ssh_host_rsa_key
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The server private key file must not have a passphrase.
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Then, to enable the SFTP server with an accounts file, add the following
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lines to the BASEDIR/tahoe.cfg file::
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[sftpd]
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enabled = true
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port = tcp:8022:interface=127.0.0.1
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host_pubkey_file = private/ssh_host_rsa_key.pub
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host_privkey_file = private/ssh_host_rsa_key
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accounts.file = private/accounts
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The SFTP server will listen on the given port number and on the loopback
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interface only. The "accounts.file" pathname will be interpreted relative to
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the node's BASEDIR.
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Or, to use an account server instead, do this::
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[sftpd]
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enabled = true
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port = tcp:8022:interface=127.0.0.1
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host_pubkey_file = private/ssh_host_rsa_key.pub
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host_privkey_file = private/ssh_host_rsa_key
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accounts.url = https://example.com/login
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You can provide both accounts.file and accounts.url, although it probably
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isn't very useful except for testing.
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For further information on SFTP compatibility and known issues with various
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clients and with the sshfs filesystem, see wiki:SftpFrontend_
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.. _wiki:SftpFrontend: https://tahoe-lafs.org/trac/tahoe-lafs/wiki/SftpFrontend
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Configuring FTP Access
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======================
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To enable the FTP server with an accounts file, add the following lines to
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the BASEDIR/tahoe.cfg file::
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[ftpd]
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enabled = true
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port = tcp:8021:interface=127.0.0.1
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accounts.file = private/accounts
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The FTP server will listen on the given port number and on the loopback
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interface only. The "accounts.file" pathname will be interpreted relative to
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the node's BASEDIR.
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To enable the FTP server with an account server instead, provide the URL of
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that server in an "accounts.url" directive::
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[ftpd]
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enabled = true
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port = tcp:8021:interface=127.0.0.1
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accounts.url = https://example.com/login
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You can provide both accounts.file and accounts.url, although it probably
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isn't very useful except for testing.
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FTP provides no security, and so your password or caps could be eavesdropped
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if you connect to the FTP server remotely. The examples above include
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":interface=127.0.0.1" in the "port" option, which causes the server to only
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accept connections from localhost.
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Public key authentication is not supported for FTP.
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Dependencies
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============
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The Tahoe-LAFS SFTP server requires the Twisted "Conch" component (a "conch"
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is a twisted shell, get it?). Many Linux distributions package the Conch code
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separately: debian puts it in the "python-twisted-conch" package.
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Immutable and Mutable Files
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===========================
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All files created via SFTP (and FTP) are immutable files. However, files can
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only be created in writeable directories, which allows the directory entry to
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be relinked to a different file. Normally, when the path of an immutable file
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is opened for writing by SFTP, the directory entry is relinked to another
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file with the newly written contents when the file handle is closed. The old
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file is still present on the grid, and any other caps to it will remain
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valid. (See :doc:`../garbage-collection` for how to reclaim the space used by
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files that are no longer needed.)
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The 'no-write' metadata field of a directory entry can override this
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behaviour. If the 'no-write' field holds a true value, then a permission
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error will occur when trying to write to the file, even if it is in a
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writeable directory. This does not prevent the directory entry from being
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unlinked or replaced.
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When using sshfs, the 'no-write' field can be set by clearing the 'w' bits in
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the Unix permissions, for example using the command ``chmod 444 path/to/file``.
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Note that this does not mean that arbitrary combinations of Unix permissions
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are supported. If the 'w' bits are cleared on a link to a mutable file or
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directory, that link will become read-only.
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If SFTP is used to write to an existing mutable file, it will publish a new
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version when the file handle is closed.
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Known Issues
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============
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Known Issues in the SFTP Frontend
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---------------------------------
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Upload errors may not be reported when writing files using SFTP via sshfs
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(`ticket #1059`_).
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Non-ASCII filenames are supported with SFTP only if the client encodes
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filenames as UTF-8 (`ticket #1089`_).
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See also wiki:SftpFrontend_.
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.. _ticket #1059: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1059
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.. _ticket #1089: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1089
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Known Issues in the FTP Frontend
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--------------------------------
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Mutable files are not supported by the FTP frontend (`ticket #680`_).
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Non-ASCII filenames are not supported by FTP (`ticket #682`_).
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The FTP frontend sometimes fails to report errors, for example if an upload
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fails because it does meet the "servers of happiness" threshold (`ticket
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#1081`_).
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.. _ticket #680: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/680
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.. _ticket #682: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/682
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.. _ticket #1081: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1081
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