accept_filter man page on FreeBSD

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ACCEPT_FILTER(9)	 BSD Kernel Developer's Manual	      ACCEPT_FILTER(9)

     accept_filter, accept_filt_add, accept_filt_del,
     accept_filt_generic_mod_event, accept_filt_get — filter incoming connec‐

     #include <sys/types.h>
     #include <sys/module.h>
     #include <sys/socket.h>
     #define ACCEPT_FILTER_MOD
     #include <sys/socketvar.h>

     accept_filt_add(struct accept_filter *filt);

     accept_filt_del(char *name);

     accept_filt_generic_mod_event(module_t mod, int event, void *data);

     struct accept_filter *
     accept_filt_get(char *name);

     Accept filters allow an application to request that the kernel pre-
     process incoming connections.  An accept filter is requested via the
     setsockopt(2) system call, passing in an optname of SO_ACCEPTFILTER.

     A module that wants to be an accept filter must provide a struct
     accept_filter to the system:

     struct accept_filter {
	     char    accf_name[16];
	     void    (*accf_callback)(struct socket *so, void *arg, int waitflag);
	     void *  (*accf_create)(struct socket *so, char *arg);
	     void    (*accf_destroy)(struct socket *so);
	     SLIST_ENTRY(accept_filter) accf_next;   /* next on the list */

     The module should register it with the function accept_filt_add(), pass‐
     ing a pointer to a struct accept_filter, allocated with malloc(9).

     The fields of struct accept_filter are as follows:

     accf_name	    Name of the filter; this is how it will be accessed from

     accf_callback  The callback that the kernel will do once the connection
		    is established.  It is the same as a socket upcall and
		    will be called when the connection is established and
		    whenever new data arrives on the socket, unless the call‐
		    back modifies the socket's flags.

     accf_create    Called whenever a setsockopt(2) installs the filter onto a
		    listening socket.

     accf_destroy   Called whenever the user removes the accept filter on the

     The accept_filt_del() function passed the same string used in
     accept_filter.accf_name during registration with accept_filt_add(), the
     kernel will then disallow and further userland use of the filter.

     The accept_filt_get() function is used internally to locate which accept
     filter to use via the setsockopt(2) system call.

     The accept_filt_generic_mod_event() function provides a simple way to
     avoid duplication of code for accept filters which do not use the argu‐
     ment field to load and unload themselves.	This function can be used in
     the moduledata_t struct for the DECLARE_MODULE(9) macro.

     setsockopt(2), accf_data(9), accf_dns(9), accf_http(9), malloc(9)

     The accept filter mechanism was introduced in FreeBSD 4.0.

     This manual page was written by Alfred Perlstein, Sheldon Hearn and
     Jeroen Ruigrok van der Werven.

     The accept filter concept was pioneered by David Filo at Yahoo! and
     refined to be a loadable module system by Alfred Perlstein.

BSD				 June 25, 2000				   BSD

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