Source code Files
~~~~~~~~~~~~~~~~~
Always compiled:
  main.c      - General setup. Listen for and fork new sessions.
  data.c      - Data connection forwarding.
  control.c   - Session startup. Control session parsing commands.
  ftp-cmds.c  - List of ftp commands and functions to act on them.
  misc.c      - Miscellaneous functions, mainly socket stuff.
  configs.c   - Config file parsing, and carrying out some config file options.
  str.c       - Safe (?) buffer handling code.

Conditionally compiled:
  ntp.c       - Non transparent proxy support. (default on)
 (linux|bsd).c - Data connection NAT stuff.
  cache.c     - Generic cache code.
  httpcache.c - Cache module to route requests through an HTTP proxy.
 localcache.c - Local caching module.
  cachemgr.c  - Cache manager code for local cache.
  vscan.c     - Virus scanning for local cache.
  transdata.c - Client/server stuff for transparent proxied data connections.
  ssl.c       - ssl connections between frox and the ftp server

Important Functions
~~~~~~~~~~~~~~~~~~~
The main loop is run_daemon() in main.c - it forks a new process for
each new connection and only terminates when sent a SIGKILL/SIGSTOP.

Each new connection starts in init_session() which gets a USER command
for ntp if necessary, makes the connection to the remote host, and
initialises the cache if this is an anonymous session.

run_loop() is the main loop for each session. It just loops calling
get_control_line() and {client,data}_control_forward().

get_control_line() selects on the necessary fds. It deals with
accepting and forwarding data connections as necessary, and only
returns once it has the control line it was asked for. If it was asked
for a control line from one of the client/server and data arrives on
the other it is buffered.

extract_{server,client}_cmd() are the functions which do the initial
reading and parsing of the raw control streams.

parse_client_cmd() decides what to do with the ftp commands. There are
a number of arrays that match commands to functions to process them.
The main one is in ftp_cmds.c, but there are also some in the caching
code if it is compiled in. All these arrays are held in
info->cmd_arrays, and parse_client_cmd() works through them until it
finds a command that matches.

We set a SIGALRM each time we receive some activity on a connection.
If a write blocks or the session is quiet for too long it doesn't get
reset and we die.

Caching
~~~~~~~

Caching code gets called at session initialisation and with a number
of ftp commands so it can follow what is going on. It gets called for
a file retrieve and can either forward it on or return a file
descriptor the file can be read from. During file retrieval each read
buffer is sent to cache_inc_data() which has the chance to read or
edit it.

Local caching is almost all implemented in the cachemgr.c file which
runs in a separate process. It sends back file descriptors to the
other processes - for cached files a file descriptor to read the file
from, and for uncached ones one to write to.

Transparent proxying of data Connections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Transparently proxying data connections requires doing NAT both on
connections we make and to intercept other connections. This requires
root priveliges which are retained by a separate process forked off at
startup. This process is communicated with over a socket - requests
are sent, and file descriptors passed back.