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     accept_filter, accept_filt_add, accept_filt_del, accept_filt_generic_mod_event,
     accept_filt_get — filter incoming connections


     #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

     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(), passing 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 userland.

     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 callback
                    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 socket.

     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 argument 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

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