RFC 4217 defines FTP over SSL/TLS:

  http://www.faqs.org/rfcs/rfc4217.html

Example mod_tls configuration:

  <IfModule mod_tls.c>
    TLSEngine on
    TLSLog /var/ftpd/tls.log
    TLSProtocol TLSv1

    # Are clients required to use FTP over TLS when talking to this server?
    TLSRequired off

    # Server's certificate
    TLSRSACertificateFile /etc/ftpd/server.cert.pem
    TLSRSACertificateKeyFile /etc/ftpd/server.key.pem

    # CA the server trusts
    TLSCACertificateFile /etc/ftpd/root.cert.pem

    # Authenticate clients that want to use FTP over TLS?
    TLSVerifyClient off
  </IfModule>

Debugging
ssldump

Frequently Asked Questions

Question: How do I generate the certificate files used by mod_tls?
Answer: The mod_tls module uses the same certificate files as Apache's mod_ssl. The mod_ssl documentation explains what the files are, how they are used, and how to generate your own:

  http://www.modssl.org/docs/2.7/ssl_faq.html

Question: Where can I find a list of clients that support FTPS?
Answer: This page is a good FTPS resource:

  http://www.ford-hutchinson.com/~fh-1-pfh/ftps-ext.html

Note, though, that there are known issues with some FTPS clients:

  http://www.runestig.com/osp.html#NOTE1

Question: How come mod_tls does not support "implicit" FTPS (i.e. automatically encrypting sessions on port 990)?
Answer: The short answer is because the Draft no longer specifies support for such a mode. Here is a description of why the alternatives to the current mode (client-requested encryption using standard control channel) are "bad".

The long answer is covered in Eric Rescorla's excellent book, "SSL and TLS". There tend to be two different strategies used when adding new features to a protocol: separate ports for protocol variants, or upward negotiation. Port 443 for HTTPS is an example of the separate ports strategy. The drawback to the separate ports approach is that there is a finite number of ports available, and so this approach does not scale well. The benefit is that use of separate ports tends to require smaller changes to client and server code. Upward negotiation is more flexible, but requires that the protocol support some sort of feature negotiation or extension discovery, allowing clients and servers to easily agree to negotiate "upward" into a secure channel. The authors of the FTPS Draft felt that upward negotiation was the more appropriate of these two approaches for encrypting FTP channels.

Question: Can I require TLS on a per-user basis?
Answer: Prior to ProFTPD 1.2.10rc2, no. The IETF Draft specifying FTP over TLS requires that the TLS handshake occur before the client sends the USER command. This means that the server does not know the name of the user that the client will be using when the TLS session is established. It is possible that the client's certificate, if one is even presented, may contain information the server may use to map that certificate to a user, but such mapping is not currently supported by mod_tls. Note that this is also the reason the TLSRequired directive cannot appear in the <Anonymous> context: anonymous logins are based on the USER command.

However, in 1.2.10rc2, the mod_tls module was modified to allow such per-user TLS requirements. To do this, the AllowPerUser parameter of the TLSOptions directive is used. For example, the following example mod_tls configuration allows non-SSL anonymous sessions, but requires SSL/TLS for all other sessions:

  <IfModule mod_tls.c>
    TLSEngine on
    TLSRSACertificateFile ...
    TLSCACertificateFile ...
    TLSOptions AllowPerUser
    TLSRequired on

    <Anonymous ~ftp>
      User ftp
      Group ftp
      UserAlias anonymous ftp

      RequireValidShell off

      # Note how TLSRequired is set to off here in the <Anonymous> context
      TLSRequired off
    </Anonymous>
  </IfModule>

  <IfGroup trusted>
    TLSRequired off
  </IfGroup>

Question: Why does mod_tls break FXP transfers?
Answer: The Draft specifying FTP over SSL explicitly omits site-to-site transfers. A TLS session is established between the client and the server on the control channel and, to save on the expensive overhead of TLS handshake, that session is reused for encrypting traffic on the data channel. In a site-to-site transfer, the client opens two control channels, one with each server, and then arranges for those servers to open a data channel between themselves. However, since the servers have not established a TLS session between themselves, that opening of the data channel fails.

Note that the above happens only if the server requires that TLS be used on data connections (e.g. TLSRequired is either on or data), of if the client tells the server that the client will be using TLS on the data connections (e.g. when it sends the AUTH command with an argument of TLS-P). Without these conditions, site-to-site transfers can occur normally, albeit unencrypted. Encrypted site-to-site transfers are not supported.

Question: How come mod_tls does not support SSLv2?
Answer: Various defects have been found in the SSLv2 protocol. Some legacy sites need to support SSLv2 for their HTTP traffic, in spite of its flaws. Use of FTP over TLS is fairly new, however, and there is not much "legacy" in that regard; it was felt that, as mod_tls aims to provide strong cryptographic security, supporting a known bad protocol is a Bad Idea.

Question: Using mod_tls, FTP sessions through my firewall now no longer work. What's going on?
Answer: The short answer is that FTPS and firewalls (and devices performing NAT) do not interact well. The control connection happens on a well-known port, and has no issues; it is the data connection that poses problems for FTP-aware firewalls. In a non-FTPS session, the firewall can inspect the FTP server's responses on the control connection to a client's PASV or PORT command, and thus know which on which ports/addresses the data connection will be established. In an FTPS session, though, those control connection messages are encrypted (that is the point of using FTPS, right?), and so the FTP-aware firewall cannot peek. Hence, it cannot know which on which ports the data connection will be established. For firewalls that are configured to always allow a certain range of ports (such as might be configured using the PassivePorts directive), FTPS should function without issue.

Unfortunately, this is a rather intractable--and known--issue. Earlier versions of the Draft defining FTPS used to allow something known as "implicit" FTPS, by which a client could contact a well-known port (akin to port 443 for HTTPS; FTPS used port 990) and the server, simply because the client contacted that certain port, would automatically encrypt the session. This approach has several drawbacks (the reason why it was removed from later versions of the Draft), but it did allow for simple TCP proxying.

To attempt to deal with the above issue, the RFC for FTP over SSL/TLS suggests using the CCC FTP command (Clear Command Channel). The CCC command makes an encrypted control channel revert back to an unencrypted channel. This helps to solve data connection problems in situations where network equipment (such as firewalls, routers, NAT) peek at the control channel in order to open ports. By sending the CCC command and unecrypting the control channel, the network equipment can once again peek at the commands (i.e. PORT and EPRT) in the control channel. Since the CCC command must come after the client has logged in, the USER and PASS commands on the control channel will still be protected by SSL/TLS.

Note that in order to configure the mod_tls module to allow use of the CCC command by clients, the following must appear in your proftpd.conf:

  TLSRequired auth+data

Question: Sometimes my encrypted transfers are slow. Is there a way to speed them up?
Answer: There have been reports that increasing the tunable buffer size (using the --enable-buffer-size option of the configure script) to 8192 increases transfer speeds, most notably on very high speed networks. Increasing the buffer size does not appear to affect normal FTP transfers (in fact, it may benefit them as well, depending on the client).

Question: I can login using FTPS, but I cannot see any directories. Why not?
Answer: You have most likely configured mod_tls to require SSL/TLS protection for data transfers as well as control commands, by using:

  TLSRequired on

  PROT: unwilling to accept security parameter (C), declining

Question: Why would I see the following errors while attempting to build proftpd with mod_tls?

  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x35): In function `dlfcn_load': : undefined reference to `dlopen'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x95): In function `dlfcn_load': : undefined reference to `dlclose'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0xbc): In function `dlfcn_load': : undefined reference to `dlerror'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x147): In function `dlfcn_bind_var': : undefined reference to `dlsym'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x172): In function `dlfcn_bind_var': : undefined reference to `dlerror'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x237): In function `dlfcn_bind_func': : undefined reference to `dlsym'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x262): In function `dlfcn_bind_func': : undefined reference to `dlerror'
  /usr/local/ssl/lib/libcrypto.a(dso_dlfcn.o)(.text+0x50b): In function `dlfcn_unload': : undefined reference to `dlclose'
  collect2: ld returned 1 exit status
  make: *** [proftpd] Error 1

  make clean
  ./configure LDFLAGS=-ldl ...
  make

Question: Why would I see a "no shared cipher" error in the TLSLog when attempting to connect with my FTPS client?

  Jan 10 17:15:18 mod_tls/2.1.1[2212]: TLS/TLS-C requested, starting TLS handshake
  Jan 10 17:15:18 mod_tls/2.1.1[2212]: unable to accept TLS connection:
    (1) error:1408A0C1:SSL routines:SSL3_GET_CLIENT_HELLO:no shared cipher
  Jan 10 17:15:18 mod_tls/2.1.1[2212]: TLS/TLS-C negotiation failed on control channel

Another possibility is a misconfiguration. If your TLSCACertificateFile and TLSRSACertificateFile directives are in the "server config" context of the proftpd.conf file, and you have >VirtualHost< sections in your proftpd.conf, then you might see the "no shared cipher" error. The easiest thing is to move those TLSCACertificateFile and TLSRSACertificateFile directives into a >Global< section in your proftpd.conf, so that they apply to all vhosts configured. The virtual hosting howto describes this in more detail.