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       ipl - IP packet log device


       The  ipl  pseudo  device's  purpose  is to provide an easy way to gather packet headers of
       packets you wish to log.  If a packet header is to be logged, the entire header is  logged
       (including  any  IP  options  - TCP/UDP options are not included when it calculates header
       size) or not at all.  The packet contents are also logged after the header.   If  the  log
       reader  is  busy  or  otherwise  unable to read log records, up to IPLLOGSIZE (8192 is the
       default) bytes of data are stored.

       Prepending every packet header logged is a structure containing  information  relevant  to
       the packet following and why it was logged.  The structure's format is as follows:

        * Log structure.  Each packet header logged is prepended by one of these.
        * Following this in the log records read from the device will be an ipflog
        * structure which is then followed by any packet data.
       typedef struct iplog    {
               u_long  ipl_sec;
               u_long  ipl_usec;
               u_int   ipl_len;
               u_int   ipl_count;
               size_t  ipl_dsize;
               struct  iplog   *ipl_next;
       } iplog_t;

       typedef struct  ipflog  {
       #if (defined(NetBSD) && (NetBSD <= 1991011) && (NetBSD >= 199603))
               u_char  fl_ifname[IFNAMSIZ];
               u_int   fl_unit;
               u_char  fl_ifname[4];
               u_char  fl_plen;        /* extra data after hlen */
               u_char  fl_hlen;        /* length of IP headers saved */
               u_short fl_rule;        /* assume never more than 64k rules, total */
               u_32_t  fl_flags;
       } ipflog_t;

       When reading from the ipl device, it is necessary to call read(2) with a buffer big enough
       to hold at least 1 complete log record - reading of partial log records is not supported.

       If the packet contents are more than 128 bytes when log body is used, then only 128  bytes
       of the packet contents are logged.

       Although  it  is  only  possible  to  read  from the ipl device, opening it for writing is
       required when using an ioctl which changes any kernel data.

       The ioctls which are loaded with this device can be found under ipf(4).  The ioctls  which
       are for use with logging and don't affect the filter are:

               ioctl(fd, SIOCIPFFB, int *)
               ioctl(fd, FIONREAD, int *)

       The  SIOCIPFFB  ioctl  flushes  the  log  buffer  and returns the number of bytes flushed.
       FIONREAD returns the number of bytes currently used for storing log data.  If IPFILTER_LOG
       is not defined when compiling, SIOCIPFFB is not available and FIONREAD will return but not
       do anything.

       There is currently no support for non-blocking IO  with  this  device,  meaning  all  read
       operations  should  be considered blocking in nature (if there is no data to read, it will
       sleep until some is made available).




       Packet headers are dropped when the internal buffer (static size) fills.