Vsftpd (Very Secure FTP Daemon) is another of the most popular implementation of FTP (File Transfer Protocol) server for many unix platforms such as: Linux, Solaris and FreeBSD. It is used by FTP servers of many services such as redhat.com, suse.co, debian.com, freebsd.org, gnu.org. As the name suggests, the great pressure developers put on security, nevertheless vsftpd is known as a stable and efficient server also. The source code of the vsftpd is open.

The principle of how does FTP protocol work is simple. First a user is connecting with the server application through the client application (Web browser e.g. FireFox or dedicated client application e.g. LFTP). This means establishing two connections through TCP/IP - one for controlling (exchanging of requests and anwsers by exchanging of commands), one for data transfers.

There are two connection modes through the FTP protocol:
- active mode: for exchanging commands there is used port 21 (a connection established by the client) and for data transmission there is used port 20 (a connection established by the server),
- passive mode: for exchanging commands there is used port 21 and for data transmission there is used port above 1024 (both connections are established by the client).

In migration to IPv6 context it is necessary to remember:

• Even compilation by hand from sources includes IPv6 support. This support apeared in version 1.1.3 (2003), so every application with version equal or above may be configured to work with IPv6.

• In case of Dual Stack vsftpd migration, there is a need of confgiuration of second instance of vsftpd. Here is a part of configuration file that shows this:

  # When "listen" directive is enabled, vsftpd runs in standalone mode and
  # listens on IPv4 sockets. This directive cannot be used in conjunction
  # with the listen_ipv6 directive.
  listen=YES
  #
  # This directive enables listening on IPv6 sockets. To listen on IPv4 and IPv6
  # sockets, you must run two copies of vsftpd whith two configuration files.
  # Make sure, that one of the listen options is commented !!
  #listen_ipv6=YES

• In case of by hand compilation from sources, there is a need of giving an "--enable-ipv6" parameter (the installation with a packet manager system includes this parameter by default). For checking if this parameter is already included in present installation of proftpd server, type: "proftpd -V" from the command line. Then find in the "configure" section the "--enable-ipv6" parameter. In the "Features" section should be "+ IPv6 support".
  

• By default there is an "UseIPv6 on" definition in proftpd.conf. The "UseIPv6 off" deffinition must be changed. In case of no "UseIPv6" definition, application assumes value "on" by default.
  

• By default proftpd listens on every single IP address it finds on the operation system. In this case migration is simplified. However in case of "listen_address" deffinition is found and there is a specific IPv4 address declared, it means, that server listens on that IP address. During the migration from IPv4 to IPv6, there is a need of changing this deffinition to "listen_address6" with a value as IPv6 address.