ACCEPT_FILTER(9)       NetBSD 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-

     #define ACCEPT_FILTER_MOD
     #include <sys/param.h>
     #include <sys/kernel.h>
     #include <sys/sysctl.h>
     #include <sys/signalvar.h>
     #include <sys/socketvar.h>
     #include <netinet/accept_filter.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.  This manual page describes the kernel
     interface for accept filters.  User applications request accept filters
     via the setsockopt(2) system call, passing in an optname of

     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 accept filters currently provided with NetBSD (accf_data(9) and
     accf_http(9)) are implemented as pseudo-devices, but an accept filter may
     use any supported means of initializing and registering itself at system
     startup or later, including the module framework if supported by the run-
     ning kernel.

     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 can be used by accept fil-
     ters which are loadable kernel modules to add and delete themselves.

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

     The accept filter mechanism was introduced in FreeBSD 4.0.  It was ported
     to NetBSD by Coyote Point Systems, Inc. and appeared in NetBSD 5.0.

     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.

NetBSD 9.0                     November 12, 2008                    NetBSD 9.0

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