FTP and SSL/TLS


RFC 4217 defines FTP over SSL/TLS:

  http://www.faqs.org/rfcs/rfc4217.html
The mod_tls module for proftpd is an implementation of RFC 4217.

RFC 2228 defines FTP Security Extensions, of which mod_tls is one implementation. Another such RFC 2228 implementation is the mod_gss module, available from SourceForge.

Example mod_tls configuration:

  <IfModule mod_dso.c>
    # If mod_tls was built as a shared/DSO module, load it
    LoadModule mod_tls.c
  </IfModule>

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

    # Support both SSLv3 and TLSv1
    TLSProtocol SSLv3 TLSv1

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

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

    # Server's EC certificate
    TLSECCertificateFile /etc/ftpd/server-ec.cert.pem
    TLSECCertificateKeyFile /etc/ftpd/server-ec.key.pem

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

    # Authenticate clients that want to use FTP over TLS?
    TLSVerifyClient off

    # Allow SSL/TLS renegotiations when the client requests them, but
    # do not force the renegotations.  Some clients do not support
    # SSL/TLS renegotiations; when mod_tls forces a renegotiation, these
    # clients will close the data connection, or there will be a timeout
    # on an idle data connection.
    TLSRenegotiate none

  </IfModule>

Debugging
There are a couple of tools available for debugging and analysing FTPS traffic. One of the easiest to use is the s_client application, part of OpenSSL.

The following examples assume that proftpd+mod_tls is listening on 127.0.0.1, port 21:

  # openssl s_client -connect 127.0.0.1:21 -starttls ftp
  CONNECTED(00000003)
  depth=1 /CN=castaglia3-serverCA/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
  verify error:num=20:unable to get local issuer certificate
  verify return:0
  ---
  Certificate chain
   0 s:/CN=castaglia3-server/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
     i:/CN=castaglia3-serverCA/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
   1 s:/CN=castaglia3-serverCA/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
     i:/CN=castaglia3/C=US/ST=Washington/L=Seattle/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/emailAddress=tj@castaglia.org
  ---
  Server certificate
  -----BEGIN CERTIFICATE-----
  ...
  -----END CERTIFICATE-----
  subject=/CN=castaglia3-server/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
  issuer=/CN=castaglia3-serverCA/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
  ---
  Acceptable client certificate CA names
  /CN=castaglia3-clientCA/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
  /CN=castaglia3/C=US/ST=Washington/L=Seattle/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/emailAddress=tj@castaglia.org
  /CN=castaglia3-clientCA/C=US/emailAddress=tj@castaglia.org/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/ST=Washington
  /CN=castaglia3/C=US/ST=Washington/L=Seattle/O=Castaglia/OU=Castaglia Research and Development/OU=TJ Saunders/emailAddress=tj@castaglia.org
  ---
  SSL handshake has read 3731 bytes and written 344 bytes
  ---
  New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA
  Server public key is 1024 bit
  Compression: NONE
  Expansion: NONE
  SSL-Session:
      Protocol  : TLSv1
      Cipher    : DHE-RSA-AES256-SHA
      Session-ID: D1F2E7C001BCFE57D797F34BD1518BDCC6160106D56A238CC0F957337C093435
      Session-ID-ctx: 
      Master-Key: F8EE2812270E8FCD87D1B26F20546EF9BABAE40F36BC6F8DF45B0E2F089571E731EAFAEA5E61BCA7D9D204BE06F28B21
      Key-Arg   : None
      Start Time: 1232997620
      Timeout   : 300 (sec)
      Verify return code: 20 (unable to get local issuer certificate)
  ---
  220 ProFTPD 1.3.2rc4 Server (TJ's FTPS Server) [127.0.0.1]
  quit
  221 Goodbye.
  read:errno=0
Note that the "verify error" message is not of particular concern for us, since we are not using s_client to verify the server's certificate in this example.

The parts of the s_client output which are of particular interest are the highlighted "Certificate chain" and "Acceptable client certificate CA names" sections.

The "Certificate chain" section shows the certificate chain/trust path, from the server's certificate up through the root CA for that certificate. Note that this assumes the certificates in the trust path are available on the server. By default, OpenSSL constructs this certificate chain using the server's certificate (configured using TLSRSACertificateFile or TLSDSACertificateFile) and the trusted CA certificates configured using TLSCACertificateFile and/or TLSCACertificatePath. See the TLSCertificateChainFile FAQ for more details on how to influence the constructed certificate chain.

The "Acceptable client certificate CA names" section contains the list of CAs that can issue certificates that mod_tls is willing to trust. These CAs always come from the configured TLSCACertificateFile and/or TLSCACertificatePath. This list of acceptable client CAs is presented to the client whenever the server is requesting the client's certificate (which is what mod_tls does by default). If you use:

  TLSOptions NoCertRequest
then the server will not send this list of acceptable client CAs; using s_client in that case, you would see:
  ---
  No client certificate CA names sent
  ---
In case you're wondering, a list of CAs is sent to the client, rather than just a single CA, because this allows clients the ability to determine which certificate (as a client can have many) to use when talking to this particular server.

Another tool, slightly older, is ssldump. This tool is more like ethereal or tcpdump for any SSL/TLS traffic; it is deliberately modeled after tcpdump.

To use ssldump for watching your FTPS traffic, you will need the server key file (for decrypting), and possibly root privileges (for listening on the network interface). Here is an example where ssldump is used to listen on the loopback interface (lo0), port 21:

  # sudo ssldump -d -k server.pem -i lo0 port 21
  New TCP connection #1: localhost(64148) <-> localhost(21)
  0.0423 (0.0423)  S>C
  ---------------------------------------------------------------
  220 ProFTPD 1.3.2rc4 Server (TJ's FTPS Server) [127.0.0.1]
  ---------------------------------------------------------------

  0.0427 (0.0004)  C>S
  ---------------------------------------------------------------
  AUTH TLS
  ---------------------------------------------------------------

  0.0430 (0.0002)  S>C
  ---------------------------------------------------------------
  234 AUTH TLS successful
  ---------------------------------------------------------------

  1 1  0.0433 (0.0003)  C>S SSLv2 compatible client hello
    Version 3.1 
    cipher suites
    Unknown value 0x39  
    Unknown value 0x38  
    Unknown value 0x35  
    TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA  
    TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA  
    TLS_RSA_WITH_3DES_EDE_CBC_SHA  
    SSL2_CK_3DES  
    Unknown value 0x33  
    Unknown value 0x32  
    Unknown value 0x2f  
    TLS_RSA_WITH_IDEA_CBC_SHA  
    SSL2_CK_IDEA  
    SSL2_CK_RC2  
    TLS_RSA_WITH_RC4_128_SHA  
    TLS_RSA_WITH_RC4_128_MD5  
    SSL2_CK_RC4  
    TLS_DHE_RSA_WITH_DES_CBC_SHA  
    TLS_DHE_DSS_WITH_DES_CBC_SHA  
    TLS_RSA_WITH_DES_CBC_SHA  
    SSL2_CK_DES  
    TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA  
    TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA  
    TLS_RSA_EXPORT_WITH_DES40_CBC_SHA  
    TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5  
    SSL2_CK_RC2_EXPORT40  
    TLS_RSA_EXPORT_WITH_RC4_40_MD5  
    SSL2_CK_RC4_EXPORT40  
  1 2  0.0658 (0.0225)  S>C  Handshake
        ServerHello
          Version 3.1 
          session_id[32]=
            09 25 df 7a a8 e8 71 a2 9f 56 a6 7b dd 95 ac 67 
            7d 2e 81 b2 1c ca b4 5f 1e 95 13 47 01 28 20 19 
          cipherSuite         Unknown value 0x39
          compressionMethod                   NULL
  1 3  0.0658 (0.0000)  S>C  Handshake
        Certificate
  1 4  0.0658 (0.0000)  S>C  Handshake
        ServerKeyExchange
  1 5  0.0658 (0.0000)  S>C  Handshake
        CertificateRequest
          certificate_types                   rsa_fixed_dh
          certificate_types                   dss_fixed_dh
          certificate_types                   rsa_sign
          certificate_types                   dss_sign
          certificate_authority
            30 81 b8 31 1c 30 1a 06 03 55 04 03 13 13 63 61 
            73 74 61 67 6c 69 61 33 2d 63 6c 69 65 6e 74 43 
            41 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 1f 
            30 1d 06 09 2a 86 48 86 f7 0d 01 09 01 16 10 74 
            6a 40 63 61 73 74 61 67 6c 69 61 2e 6f 72 67 31 
            12 30 10 06 03 55 04 0a 13 09 43 61 73 74 61 67 
            6c 69 61 31 2b 30 29 06 03 55 04 0b 13 22 43 61 
            73 74 61 67 6c 69 61 20 52 65 73 65 61 72 63 68 
            20 61 6e 64 20 44 65 76 65 6c 6f 70 6d 65 6e 74 
            31 14 30 12 06 03 55 04 0b 13 0b 54 4a 20 53 61 
            75 6e 64 65 72 73 31 13 30 11 06 03 55 04 08 13 
            0a 57 61 73 68 69 6e 67 74 6f 6e 
          certificate_authority
            30 81 c1 31 13 30 11 06 03 55 04 03 13 0a 63 61 
            73 74 61 67 6c 69 61 33 31 0b 30 09 06 03 55 04 
            06 13 02 55 53 31 13 30 11 06 03 55 04 08 13 0a 
            57 61 73 68 69 6e 67 74 6f 6e 31 10 30 0e 06 03 
            55 04 07 13 07 53 65 61 74 74 6c 65 31 12 30 10 
            06 03 55 04 0a 13 09 43 61 73 74 61 67 6c 69 61 
            31 2b 30 29 06 03 55 04 0b 13 22 43 61 73 74 61 
            67 6c 69 61 20 52 65 73 65 61 72 63 68 20 61 6e 
            64 20 44 65 76 65 6c 6f 70 6d 65 6e 74 31 14 30 
            12 06 03 55 04 0b 13 0b 54 4a 20 53 61 75 6e 64 
            65 72 73 31 1f 30 1d 06 09 2a 86 48 86 f7 0d 01 
            09 01 16 10 74 6a 40 63 61 73 74 61 67 6c 69 61 
            2e 6f 72 67 
          certificate_authority
            30 81 b8 31 1c 30 1a 06 03 55 04 03 13 13 63 61 
            73 74 61 67 6c 69 61 33 2d 63 6c 69 65 6e 74 43 
            41 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 1f 
            30 1d 06 09 2a 86 48 86 f7 0d 01 09 01 16 10 74 
            6a 40 63 61 73 74 61 67 6c 69 61 2e 6f 72 67 31 
            12 30 10 06 03 55 04 0a 13 09 43 61 73 74 61 67 
            6c 69 61 31 2b 30 29 06 03 55 04 0b 13 22 43 61 
            73 74 61 67 6c 69 61 20 52 65 73 65 61 72 63 68 
            20 61 6e 64 20 44 65 76 65 6c 6f 70 6d 65 6e 74 
            31 14 30 12 06 03 55 04 0b 13 0b 54 4a 20 53 61 
            75 6e 64 65 72 73 31 13 30 11 06 03 55 04 08 13 
            0a 57 61 73 68 69 6e 67 74 6f 6e 
          certificate_authority
            30 81 c1 31 13 30 11 06 03 55 04 03 13 0a 63 61 
            73 74 61 67 6c 69 61 33 31 0b 30 09 06 03 55 04 
            06 13 02 55 53 31 13 30 11 06 03 55 04 08 13 0a 
            57 61 73 68 69 6e 67 74 6f 6e 31 10 30 0e 06 03 
            55 04 07 13 07 53 65 61 74 74 6c 65 31 12 30 10 
            06 03 55 04 0a 13 09 43 61 73 74 61 67 6c 69 61 
            31 2b 30 29 06 03 55 04 0b 13 22 43 61 73 74 61 
            67 6c 69 61 20 52 65 73 65 61 72 63 68 20 61 6e 
            64 20 44 65 76 65 6c 6f 70 6d 65 6e 74 31 14 30 
            12 06 03 55 04 0b 13 0b 54 4a 20 53 61 75 6e 64 
            65 72 73 31 1f 30 1d 06 09 2a 86 48 86 f7 0d 01 
            09 01 16 10 74 6a 40 63 61 73 74 61 67 6c 69 61 
            2e 6f 72 67 
        ServerHelloDone
  1 6  0.1005 (0.0346)  C>S  Handshake
        Certificate
  1 7  0.1005 (0.0000)  C>S  Handshake
        ClientKeyExchange
  1 8  0.1005 (0.0000)  C>S  ChangeCipherSpec
  1 9  0.1005 (0.0000)  C>S  Handshake
  1 10 0.1164 (0.0159)  S>C  ChangeCipherSpec
  1 11 0.1164 (0.0000)  S>C  Handshake
  1 12 1.7856 (1.6692)  C>S  application_data
  1 13 1.7856 (0.0000)  C>S  application_data
  1 14 1.7861 (0.0004)  S>C  application_data
  1    1.7866 (0.0004)  S>C  TCP FIN
  1 15 1.7868 (0.0002)  C>S  Alert

As you can see, the level of granularity used by ssldump is on the level of each SSL/TLS packet; while useful in some cases, I suspect that using openssl s_client will provide most of the information you will want in figuring out your certificate and verification issues.

TLS Client Auth/Mutual Auth
Like most web servers, when mod_tls is used, it does not require that the connecting client present a certificate for verification by default. That is, mod_tls does not require "client auth" or "mutual auth" by default. To require that clients present a valid certificate, you would use the TLSVerifyClient directive like so:

  <IfModule mod_tls.c>
    TLSEngine on
    ...
    # Verify clients that want to use FTP over TLS
    TLSVerifyClient on
  </IfModule>

With this directive enabled in your configuration, if a client connects and performs the SSL/TLS handshake but does not present a valid certificate, then the TLSLog would contain error messages like this:

  mod_tls/2.4.3[12065]: TLS/TLS-C requested, starting TLS handshake
  mod_tls/2.4.3[12065]: unable to accept TLS connection: protocol error: 
    (1) error:140890C7:SSL routines:SSL3_GET_CLIENT_CERTIFICATE:peer did not return a certificate
  mod_tls/2.4.3[12065]: TLS/TLS-C negotiation failed on control channel
The client failed to provide a valid certificate, and so the connection was rejected.

Frequently Asked Questions

Question: When I enable mod_tls, I can't upload or download files, or list directories. I see the following error in my client:

  425 Unable to build data connection: Operation not permitted.
Why?
Answer: By default, mod_tls requires that the same SSL session be used data transfers (see
TLSOptions), which includes uploads, downloads, and directory listings. If your data transfers are failing, you might look in your TLSLog, and see a log message like this:
  client did not reuse SSL session, rejecting data connection (see the NoSessionReuseRequired TLSOptions parameter)
The workaround, then, is to add the TLSOption mentioned in the log message to your configuration, i.e.:
  # Relax the requirement that the SSL session be reused for data transfers
  TLSOptions NoSessionReuseRequired
Then restart proftpd, and your data transfers should work.

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: Does FTPS protect both the control connection and the data connections?
Question: Short answer: yes.

The long answer is, of course, that it depends. In the case of mod_tls, it depends on your TLSRequired setting. If you use:

  TLSRequired on
then you are configuring mod_tls to require SSL/TLS protection for both control connections (e.g. protecting the username and password used to log in) and data connections. If you have:
  TLSRequired off
then it is up to the FTPS client whether both control and data connections will be protected via SSL/TLS. Other TLSRequired settings can be used to specify specific combinations: data connections only, control connections only, authentication plus data data connections only, etc. The TLSRequired
documentation has the details.

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
including the list of FTPS clients. On a related note, there have been some reports that Debian's ftp-ssl client has a few bugs; using Peter Runestig's ftp-tls is known to work.

Note, though, that there are known issues with some older versions of FTPS clients, most notably SmartFTP and CuteFTP Pro. These clients had the following behavior: RFC 2246 states that, if a client receives a CertificateRequest, and the client has no certificate, it "should send a certificate message containing no certificates" (Section 7.4.6). Instead, after negotiating to use TLS version 1, these clients choke and break the connection (which is actually SSL version 2 behavior). The NoCertRequest TLSOption is designed for use for such clients.

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.

All that said, in ProFTPD 1.3.3rc2, the mod_tls module was enhanced to support implicit FTPS via the UseImplicitSSL TLSOption.

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>
The modification also allows mod_ifsession-based conditions, so that one can have settings like:
  <IfGroup trusted>
    TLSRequired off
  </IfGroup>
However, there is a risk involved in using the AllowPerUser option: it causes mod_tls not to enforce TLSRequired until after the potentially sensitive USER and PASS commands have been sent by the client. This allows clients, even when TLSRequired on or TLSRequired ctrl are in effect, to send the USER and PASS commands unencrypted. Depending on your site's security needs, the ability to require SSL/TLS on a per-user basis may or may not be worth the ability to require SSL/TLS for the USER and PASS commands.

Question: When I use the following in my proftpd.conf:

  TLSRequired on # or "TLSRequired auth", or "TLSRequired auth+data"
  TLSOptions AllowPerUser
I get the following error when I try to start proftpd:
  cannot enforce both 'TLSRequired auth' and 'TLSOptions AllowPerUser' at the same time
Answer: The reason this error occurs is because you have a configuration which is impossible to support. The TLSRequired setting (e.g. "on", "auth", or "auth+data") requires that SSL/TLS be in use during authentication. But in order to support the TLSOption AllowPerUser setting, the mod_tls cannot require that SSL/TLS be in use during authentication, since it does not know the user until after authentication has completed. The requested configuration cannot be supported, and thus the server will refuse to start.

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 (FXP) 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 (FXP) transfers can occur normally, albeit unencrypted.

Note that as of proftpd-1.3.5rc4, encrypted site-to-site (FXP) transfers are supported via the SSCN FTP command. The TLSVerifyServer directive is also needed for secure FXP transfers.

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
See the TLSRequired description for more details.

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
However, if your FTPS client does not expect to handle encrypted data transfers (and directory listings count as data transfers, as they are sent over a data channel), you may see your client appear to hang, waiting for data it can read. Make sure your client is prepared to handle the security requirements you have configured on the server.

The following may also appear in the TLS for any data transfers (which include directory listings):

  client did not reuse SSL session, rejecting data connection (see the NoSessionReuseRequired TLSOptions parameter
This message appears because an additional security restriction that was added in ProFTPD 1.3.3rc1. The
TLSOptions documentation for this "NoSessionReuseRequired" option describes the situation in more detail.

You may also see the following appear in the TLSLog on occasion:

  PROT: unwilling to accept security parameter (C), declining
The PROT FTP command is used to set the protection level to be used for data transfers. Some clients send a PROT command with a security parameter of C, meaning "Clear", which effectively tells the server not to protect data transfers. The mod_tls module will refuse the C security parameter if, like above, there is "TLSRequired on" in your proftpd.conf. This case also indicates a disagreement between the client's security expectations and the security policy you have configured on the server.

In order to accept a "PROT C" FTP command, your mod_tls configuration would need to use a TLSRequired value other than required, e.g. something like:

  # We only require SSL/TLS protection during authentication
  TLSRequired auth

  # We will accept SSL/TLS protection for the control channel if the
  # client wants to use it, but NOT for data transfers
  TLSRequired !data

Question: Using FTPS, after uploading a very large file, my next directory listing fails:

  425 Unable to build data connection: Operation not permitted
The TLSLog contains:
  client did not reuse SSL session, rejecting data connection (see the NoSessionReuseRequired TLSOptions parameter)
but I do not want to use that option, and would like to rely on the additional security protection provided by requring SSL session reuse. And my FTPS client is correctly reusing SSL session IDs (as earlier data transfers were working properly). So why is my data transfer failing after the upload of a very large file?
Answer: The answer involves SSL session caching on the server side (i.e. mod_tls), cache timeouts, and session renegotiations.

By default, mod_tls uses OpenSSL's "internal" session cache, which is an in-memory caching of SSL session IDs. And by default, OpenSSL's internal session cache has a cache timeout of 5 minutes; after that amount of time in the internal session cache, a cached SSL session ID is considered stale and is available for reuse.

This means that 5 minutes or more into an FTPS session, even if your FTPS client reused an SSL session ID, the OpenSSL internal session cache will time out that SSL session ID. The next time your FTPS client goes to reuse that session ID for a data transfer, mod_tls won't find it in the OpenSSL internal session cache, and will think that your FTPS client is not reusing the SSL session ID as is required, and fail the transfer.

Fixing this situation requires two parts: a) the ability to change the cache timeout used for the OpenSSL internal session cache, and b) renegotiating the SSL session ID with the FTPS client periodically, to keep the SSL session ID up-to-date in the session cache.

The first part, configuring the session cache timeout for the OpenSSL internal session cache, is only possible in ProFTPD 1.3.4rc2 and later (see Bug#3580). The TLSSessionCache directive was modified to allow a configuration such as:

  TLSSessionCache internal: 1800
(Unfortunately, the ':' after "internal" is necessary.) This configures mod_tls such that the OpenSSL internal session cache uses a cache timeout of 1800 seconds (30 minutes), rather than the default of 300 seconds (5 minutes).

No matter how long you configure the cache timeout, eventually you will have a session which lasts longer than that timeout. Which brings us to the second part of the solution: renegotiating a new SSL session ID periodically, which keeps it fresh in the session cache. The TLSRenegotiate directive is needed for this. For example, the following configuration should address the issue of failed data transfers after very large uploads:

  TLSRenegotiate ctrl 1500 timeout 300
  TLSSessionCache internal: 1800
This tells mod_tls to request a renegotiation of the SSL session on the control channel every 1500 seconds (25 minutes), and to allow 300 seconds (5 minutes) for the client to perform the renegotiation. It also tells mod_tls to cache the SSL session data for 1800 seconds (30 minutes), i.e. longer than the renegotiation time of 1500 seconds.

This way, as long as your client supports renegotiations and is updating the SSL session ID properly for data transfers, when a data transfer is requested, the SSL session ID presented by the client should always be fresh and in the session cache.

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
Answer: Add -ldl to your configure command, for example:
  make clean
  ./configure LDFLAGS=-ldl ...
  make
This tells the proftpd build system to add libdl, an OS-specific library for handling dynamically loaded code, to the list of libraries used when linking proftpd. On some systems, the functions in the libdl library are part of libc, and are not in a separate library.

I suspect that the reason this build option may be necessary is that OpenSSL 0.9.8 contained changes regarding how OpenSSL loads "engines", software modules that talk to hardware devices that can do specialized cryptographic operations. These changes involve being able to dynamically load the "engine" software modules.

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
Answer: There are two reasons this might happen.

The first is that there is indeed no ciphersuite in common between mod_tls and your FTPS client. The SSL/TLS ciphersuites used by mod_tls are configured using the TLSCipherSuite directive. If you have a complex or restrictive TLSCipherSuite configured, that could be the culprit. Try relaxing (or removing outright) your TLSCipherSuite, or try to configure your FTPS client to use the same SSL/TLS ciphersuites.

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.

Question: My FTPS client sometimes times out after uploading/downloading more than 1 GB of data. When I turn off SSL/TLS, the upload/download works. Why?
Answer: The culprit behind this is most likely SSL/TLS renegotiations. By default, mod_tls uses SSL/TLS renegotiations to periodically update the session key which protects the data being transferred; see the
TLSRenegotiate documentation for more details, particularly the time-based and bytes-based limits at which renegotations are forced.

Some FTPS clients, however, do not support server-initiated SSL/TLS renegotations. When the server does try to force a renegotiation, the client fails that new handshake, cannot upload/download any more data over the protected channel, and the transfer will eventually time out. Alternatively, the transfer could terminate strangely in the middle of the upload/download. Note, however, that not all transfer issues will be caused by SSL/TLS renegotiations. Bugs in firewalls and routers can also cause these symptoms.

Should you suspect that you are having issues with your FTPS client because of SSL/TLS renegotiations, you can configure mod_tls to accept renegotiations if the client requests one, but not to otherwise force them:

  TLSRenegotiate required off

Question: My FTPS client has trouble connecting to proftpd using SSL/TLS, with the following error appearing in the TLSLog:

  Sep 17 11:03:46 mod_tls/2.1.2[9628]: TLS/TLS-C requested, starting TLS handshake
  Sep 17 11:03:46 mod_tls/2.1.2[9628]: unable to accept TLS connection: received EOF that violates protocol
  Sep 17 11:03:46 mod_tls/2.1.2[9628]: TLS/TLS-C negotiation failed on control channel
Is this a bug in mod_tls, in the client, or something else?

Answer: There might be several different causes for this error. It could be a bug in the OpenSSL library, in mod_tls, in the FTPS client, or it could be a transient network issue.

Now, one possible thing to try is to use the following in your proftpd.conf file:

  TLSOptions NoCertRequest
This option tells the OpenSSL library to not include a message requesting the client's certificate in the SSL/TLS handshake messages. Some older SSL implementations seem to have trouble with this certificate request message, and react badly. This includes some Windows FTP clients, as well as some FTP clients for the Mac.

One user tried using both the Transmit and the Fetch applications for the Mac; both state that they can handle FTP over SSL/TLS. Using both of these applications, the user saw the above error. The user, when using Transmit, saw the following error message appear from the client:

  Server said:
  AUTH TLS successful
  error:00000000:lib(0):func(0):reason(0)
And when using Fetch, the error message displayed by that client was:
 
  SSL Error -9844
  Server responded: "AUTH TLS successful"
In both causes, using "TLSOptions NoCertRequest" appeared to make those clients happy.

Question: When proftpd starts up, I am prompted to enter the passphrases for my certificates. How can I get the daemon to start without having to enter passphrases?
Answer: You can either remove the passphrase from the certificate key file (as mentioned in the Apache mod_ssl
FAQ), or you can use the TLSPassPhraseProvider directive to configure a program will which provide the passphrases to proftpd automatically.

Question: How can I configure mod_tls to use OpenSSL in FIPS mode?
Answer: Using OpenSSL in FIPS mode requires quite a few steps. First, you would configure proftpd to use the mod_tls module as normal, assuming your OpenSSL installation has been compiled with FIPS support:

  ./configure --with-modules=mod_tls ...

Compiling proftpd requires the following, for FIPS support to work properly:

  make CC=/path/to/openssl/bin/fipsld FIPSLD_CC=gcc
The FIPSLD_CC variable should point to your normal C compiler, e.g. gcc. The use of this fipsld program is mandatory. The FIPS standard requires that the linking process happen a very specific way, involving verification of calculated and expected checksums of compiled code, etc. The OpenSSL packages with FIPS support supply this fipsld program which will link the compiled code according to the FIPS specifications. If you do not use fipsld, then attempts to use OpenSSL in FIPS mode will fail. For example, you would see the following if starting a proftpd daemon which has not been linked using fipsld while requesting use of FIPS:
  - mod_tls/2.1.2: unable to use FIPS mode: (unknown)
  - Fatal: unable to load module 'mod_tls.c': Operation not permitted

Now, assuming you have compiled and installed your proftpd properly, e.g.:

  make CC=/path/to/openssl/bin/fipsld FIPSLD_CC=gcc
  make CC=/path/to/openssl/bin/fipsld FIPSLD_CC=gcc install
you will now be ready to start proftpd.

In order for FIPS mode to be effective, OpenSSL must be told to run in FIPS mode from the very beginning. The mod_tls module initializes the OpenSSL library when the mod_tls module is loaded, before the proftpd.conf file is parsed. Thus the requesting of FIPS mode cannot be done via a setting in proftpd.conf. (Annoying, I know.)

Instead, you must use the -D command-line parameter when starting proftpd (see the docs for the <IfDefine> and Define directives) to define a specific variable, which the mod_tls module will look for. Specifically, you will need to start proftpd like thus:

  /path/to/proftpd -DTLS_USE_FIPS ...
This will define the TLS_USE_FIPS variable; this tells mod_tls to initialize OpenSSL using FIPS mode. When this works, you will see the following when proftpd starts up:
  - mod_tls/2.1.2: FIPS mode enabled

For additional reading on OpenSSL and FIPS, see:

  http://www.openssl.org/docs/fips/fipsnotes.html

Question: Why do I see the following in my logs when I start proftpd using mod_tls?

 - mod_tls/2.2: compiled using OpenSSL version 'OpenSSL 0.9.7i 14 Oct 2005'
headers, but linked to OpenSSL version 'OpenSSL 0.9.7l 28 Sep 2006' library
What does this mean?
Answer: That is an informational/warning message.

Some systems are badly maintained by their admins (and/or by the packages installed on the systems), such that the OpenSSL headers can become quite badly out of sync with the OpenSSL libraries. If this discrepancy becomes bad enough, you can see strange behavior from OpenSSL, ranging from random behavior to segfaults. So mod_tls tries to let the admin know about the system's mismatched OpenSSL header/library versions.

Usually a minor OpenSSL version difference like the example above is OK, but it really depends on exactly what changed in OpenSSL, and how.

If you see the above message, it is not a requirement that you recompile proftpd against the OpenSSL headers of the same version as the OpenSSL libraries. However, the version discrepancy is a possible source of trouble.

This header/library version check was added recently, hence why older proftpd releases do not log the warning.

Question: When should the TLSCertificateChainFile directive be used?
Answer: Short answer: only in very specific arrangements of CA hierarchies. Most of the time, you probably do not need it.

The long explanation requires an illustration. Let's say we have a CA hierarchy that looks something like this:

       MyRootCA                      TheirRootCA
           |                             |
      MyServerCA                    TheirClientCA
           |                             |
    +------+------+               +------+------+
    |      |      |               |      |      |
  certA  certB  certC           cert1  cert2  cert3

Another way of asking the question thus is: "How to I send MyServerCA and MyRootCA to the client without having them in my trusted CA locations"?

We might configure mod_tls to have "certA" as the server's certificate, via TLSRSACertificateFile. The "certA" certificate is issued by MyServerCA. And let's assume that we do not want to verify any client certificates issued by MyServerCA.

We do want to be able to verify client certs issued by a different CA, say, TheirClientCA. We configure TheirClientCA in mod_tls using TLSCACertificateFile or TLSCACertificatePath.

The client connects to mod_tls, and starts the SSL/TLS handshake. mod_tls sends its "certA" certificate, along with any certs that may be needed on the client for verifying the server's certificate. (This is what appears in the "Certificate chain" output from s_client; see SSL/TLS debugging above.) Perhaps the client does not have MyServerCA present in the client certificate store. So we need to tell mod_tls to send the MyServerCA and MyRootCA certs, along with "certA". We could place the MyServerCA and MyRootCA certs in TLSCACertificatePath, but then any client certs issued by MyServerCA would be trusted as well (and that's not what we want for this example).

The solution here is to use TLSCertificateChainFile to supply the MyServerCA and MyRootCA certs, as part of the server cert chain. The configured TLSCertificateChainFile would contain the PEM-formatted MyServerCA and MyRootCA public certs. And although the TLSCertificateChainFile states that the certs contained in the file should be in certificate chain order, this is not strictly necessary; the OpenSSL library will adjust the ordering as appropriate.

Question: I am having trouble connecting to my SSL/TLS-enabled proftpd; my FTPS client shows this error:

  error:14094410:SSL routines:SSL3_READ_BYTES:sslv3 alert handshake failure
What is wrong?
Answer: It depends; the first thing is to check your TLSLog to see what errors, if any, are logged by the mod_tls module. For example, you might see:
  Dec 14 10:47:58 mod_tls/2.4.1[13393]: unable to accept TLS connection: protocol error: 
    (1) error:1408A0C1:SSL routines:SSL3_GET_CLIENT_HELLO:no shared cipher
The most common causes of these problems are: a) overly restrictive TLSCipherSuite configuration, or b) missing server certificate (i.e. TLSRSACertificateFile, TLSDSACertificateFile, or TLSPKCS12File). The file configured for the server certificate might also be badly formatted, which would result in the same error.

Question: Is there a way to require TLS (FTPS) for remote clients only, and allow simple FTP (without TLS) for local clients (i.e. for clients in networks which we will be able to define as "local")?
Answer: Yes.

To do this, you would use a combination of <Class> sections and mod_ifsession's <IfClass>, e.g.:

  <Class local>
    From ...
  </Class>

  <IfModule mod_tls.c>
    # Normal mod_tls configuration here

    <IfClass local>
      # Don't require FTPS from local clients
      TLSRequired off
    </IfClass>

    <IfClass !local>
      # Require FTPS from remote/non-local clients
      TLSRequired on
    </IfClass>

  </IfModule>

Question: I have configured my proftpd server for FTPS. When I use FileZilla to try to connect to it, though, I see this error in the FileZilla logs:

  GnuTLS error -9: A TLS packet with unexpected length was received
Is there a ProFTPD directive to fix this error?
Answer: In most cases, the above client error indicates an error unrelated to SSL/TLS on your server. Check the proftpd debug logging, the SQLLogFile if you are using the mod_sql module, etc.

Question: What is the difference between the mod_tls_shmcache and mod_tls_memcache modules?
Answer: Both of these modules are used to support session caching/resumption in mod_tls. The difference between these modules is in where the SSL session data is cached/stored.

The mod_tls_shmcache module stores SSL session data in a SysV shared memory ("shm") segment, which can be accessed by the different proftpd processes on the same machine. The mod_tls_memcache module stores SSL session data in a memcached server (configured using the mod_memcache module); this allows different proftpd processes on different machines to access/reuse the cached session data; this can be quite useful when operating a pool of proftpd servers e.g. behind a load balancer.

Question: When I try to connect using FTPS to my proftpd server, the SSL/TLS connection fails, and I see the following errors in the TLSLog:

  Jul 23 01:15:46 mod_tls/2.4.3[10481]: using default OpenSSL verification locations (see $SSL_CERT_DIR environment variable)
  Jul 23 01:15:46 mod_tls/2.4.3[10481]: error checking key from 
    TLSRSACertificateKeyFile '/usr/local/etc/proftpd/ssl/proftpd.key.pem':
    (1) error:140A80B1:SSL routines:SSL_CTX_check_private_key:no certificate assigned
  Jul 23 01:15:46 mod_tls/2.4.3[10481]: error initializing OpenSSL context for this session
  Jul 23 01:15:46 mod_tls/2.4.3[10481]: TLS/TLS-C requested, starting TLS handshake
  Jul 23 01:15:47 mod_tls/2.4.3[10481]: unable to accept TLS connection: protocol error:
    (1) error:1408A0C1:SSL routines:SSL3_GET_CLIENT_HELLO:no shared cipher
  Jul 23 01:15:47 mod_tls/2.4.3[10481]: TLS/TLS-C negotiation failed on control channel
What does this mean?
Answer: This error means that, somehow, you have configured a key for a certificate, but don't have the matching certificate configured. For example, if you configured mod_tls like so:
  #TLSRSACertificateFile /usr/local/etc/proftpd/ssl/server.cert.pem
  TLSRSACertificateKeyFile /usr/local/etc/proftpd/ssl/proftpd.key.pem
then you would see the above error.

This could also happen if, for some reason, the certificate and key in the configured files were not properly matched up.


Last Updated: $Date: 2014/03/26 18:22:32 $