Manual browser: accept_filter(9)

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


accept_filter, accept_filt_add, accept_filt_del, accept_filt_generic_mod_event, accept_filt_getfilter incoming connections



#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 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(), passing 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 running kernel.

The fields of struct accept_filter are as follows:

Name of the filter; this is how it will be accessed from userland.
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.
Called whenever a setsockopt(2) installs the filter onto a listening socket.
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 can be used by accept filters which are loadable kernel modules to add and delete themselves.


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.

November 12, 2008 NetBSD 7.0