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Clam daemon

`clamd` is a multi-threaded daemon that uses libclamav to scan files for viruses. It may work in one or both modes listening on:

- Unix (local) socket
- TCP socket

The daemon is fully configurable via the `clamd.conf` file 9. `clamd` recognizes the following commands:

- PING
  Check the daemon's state (should reply with "PONG").
- VERSION
  Print program and database versions.
- RELOAD
  Reload the databases.
- SHUTDOWN
  Perform a clean exit.
- SCAN file/directory
  Scan file or directory (recursively) with archive support enabled (a full path is required).
- RAWSCAN file/directory
  Scan file or directory (recursively) with archive and special file support disabled (a full path is required).
- CONTSCAN file/directory
  Scan file or directory (recursively) with archive support enabled and don't stop the scanning when a virus is found.
- MULTISCAN file/directory
  Scan file in a standard way or scan directory (recursively) using multiple threads (to make the scanning faster on SMP machines).
- INSTREAM
  It is mandatory to prefix this command with n or z.
  Scan a stream of data. The stream is sent to clamd in chunks, after INSTREAM, on the same socket on which the command was sent. This avoids the overhead of establishing new TCP connections and problems with NAT. The format of the chunk is: `<length><data>` where `<length>` is the size of the following data in bytes expressed as a 4 byte unsigned integer in network byte order and `<data>` is the actual chunk. Streaming is terminated by sending a zero-length chunk. Note: do not exceed StreamMaxLength as defined in clamd.conf, otherwise clamd will reply with INSTREAM size limit exceeded and close the connection.
- FILDES
  It is mandatory to newline terminate this command, or prefix with n or z. This command only works on UNIX domain sockets.
  Scan a file descriptor. After issuing a FILDES command a subsequent rfc2292/bsd4.4 style packet (with at least one dummy character) is sent to clamd carrying the file descriptor to be scanned inside the ancillary data. Alternatively the file descriptor may be sent in the same packet, including the extra character.
- STATS
  It is mandatory to newline terminate this command, or prefix with n or z, it is recommended to only use the z prefix.
  On this command clamd provides statistics about the scan queue, contents of scan queue, and memory usage. The exact reply format is subject to changes in future releases.
- IDSESSION, END
  It is mandatory to prefix this command with n or z, also all commands inside IDSESSION must be prefixed.
  Start/end a clamd session. Within a session multiple SCAN, INSTREAM, FILDES, VERSION, STATS commands can be sent on the same socket without opening new connections. Replies from clamd will be in the form `<id>: <response>` where `<id>` is the request number (in ASCII, starting from 1) and `<response>` is the usual clamd reply. The reply lines have the same delimiter as the corresponding command had. Clamd will process the commands asynchronously, and reply as soon as it has finished processing. Clamd requires clients to read all the replies it sent, before sending more commands to prevent send() deadlocks. The recommended way to implement a client that uses IDSESSION is with non-blocking sockets, and a select()/poll() loop: whenever send would block, sleep in select/poll until either you can write more data, or read more replies. Note that using non-blocking sockets without the select/poll loop and alternating recv()/send() doesn't comply with clamd's requirements. If clamd detects that a client has deadlocked, it will close the connection. Note that clamd may close an IDSESSION connection too if the client doesn't follow the protocol's requirements.
- STREAM (deprecated, use INSTREAM instead)
  Scan stream: clamd will return a new port number you should connect to and send data to scan.

It's recommended to prefix clamd commands with the letter z (eg. zSCAN) to indicate that the command will be delimited by a NULL character and that clamd should continue reading command data until a NULL character is read. The null delimiter assures that the complete command and its entire argument will be processed as a single command. Alternatively commands may be prefixed with the letter n (e.g. nSCAN) to use a newline character as the delimiter. Clamd replies will honour the requested terminator in turn. If clamd doesn't recognize the command, or the command doesn't follow the requirements specified below, it will reply with an error message, and close the connection. Clamd can handle the following signals:

- SIGTERM - perform a clean exit
- SIGHUP - reopen the log file
- SIGUSR2 - reload the database

Clamd should not be started in the background using the shell operator `&` or external tools. Instead, you should run and wait for clamd to load the database and daemonize itself. After that, clamd is instantly ready to accept connections and perform file scanning.