http://www.faqs.org/ftp/internet-drafts/draft-murray-auth-ftp-ssl-12.txt
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
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 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?
Question: Why does
Note that the above happens only if the server requires that TLS be used on data
connections (e.g.
Question: How come
Answer: 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.
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.
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.
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.