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.\" Copyright (c) 2000 Alfred Perlstein
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.\" $FreeBSD: src/share/man/man9/accept_filter.9,v 1.2.2.6 2002/12/29 16:35:39 schweikh Exp $
.\" $DragonFly: src/share/man/man9/accept_filter.9,v 1.3 2003/11/14 03:54:32 dillon Exp $
.\" "
.Dd June 25, 2000
.Os
.Dt ACCEPT_FILTER 9
.Sh NAME
.Nm accept_filter ,
.Nm accept_filt_add ,
.Nm accept_filt_del ,
.Nm accept_filt_generic_mod_event ,
.Nm accept_filt_get
.Nd filter incoming connections
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In sys/socketvar.h
.Ft int
.Fn accept_filt_add "struct accept_filter *filt"
.Ft int
.Fn accept_filt_del "char *name"
.Ft int
.Fn accept_filt_generic_mod_event "module_t mod" "int event" "void *data"
.Ft struct accept_filter *
.Fn accept_filt_get "char *name"
.Sh DESCRIPTION
Accept filters allow an application to request
that the kernel pre-process incoming connections.
An accept filter is requested via the
.Xr setsockopt 2
system call, passing in an
.Fa optname
of
.Dv SO_ACCEPTFILTER .
.Sh IMPLEMENTATION NOTES
A module that wants to be an accept filter
must provide a struct accept_filter to the system:
.Bd -literal
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 */
};
.Ed
.Pp
The module should register it with the function
.Fn accept_filt_add ,
passing a pointer to a struct accept_filter, allocated with
.Xr MALLOC 9
.Pp
The fields of
.Fa struct accept_filter
are as follows:
.Bl -tag -width accf_callbackXXX
.It accf_name
Name of the filter;
this is how it will be accessed from userland.
.It 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.
.It accf_create
Called whenever a
.Xr setsockopt 2
installs the filter onto
a listening socket.
.It accf_destroy
Called whenever the user removes the accept filter on the socket.
.El
.Pp
.Fn accept_filt_del
passed the same string used in accept_filter.accf_name during registration
with
.Fn accept_filt_add ,
the kernel will then disallow and further userland use of the filter.
.Pp
.Fn accept_filt_get
is used internally to locate which accept filter to use via the
.Fn setsockopt
syscall.
.Pp
.Fn accept_filt_generic_mod_event
provides a simple way to avoid duplicate
code for accept filters which don't use
argument field to load and unload
themselves.  It is a function that can be
put in the load/unload struct
for the
.Fn DECLARE_MODULE
macro.
.Sh SEE ALSO
.Xr setsockopt 2 ,
.Xr accf_data 9 ,
.Xr accf_http 9 ,
.Xr malloc 9
.Sh HISTORY
The accept filter mechanism was introduced in
.Fx 4.0 .
.Sh AUTHORS
This manual page was written by
.An 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.