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 theclamd.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).
- ALLMATCHSCAN file/directory
 ALLMATCHSCAN works just like SCAN except that it sets a mode
	    where, after finding a virus within a file, continues scanning for
            additional viruses.
- 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.
Cisco 2015-10-07