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+.\" $NetBSD: accf_http.9,v 1.8 2017/07/03 21:28:48 wiz Exp $
+.\"
+.\" Copyright (c) 2000 Alfred Perlstein
+.\"
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" $FreeBSD: src/share/man/man9/accf_http.9,v 1.11 2005/01/18 20:52:51 ru Exp $
+.\" "
+.Dd September 4, 2008
+.Dt ACCF_HTTP 9
+.Os
+.Sh NAME
+.Nm accf_http
+.Nd "buffer incoming connections until a certain complete HTTP requests arrive"
+.Sh SYNOPSIS
+.Cd "options INET"
+.Cd "pseudo-device accf_http"
+.Sh DESCRIPTION
+This is a filter to be placed on a socket that will be using
+.Fn accept
+to receive incoming HTTP connections.
+.Pp
+Once installed on a listening socket, this filter is activated when a
+connection becomes ready to receive data (at which point
+.Xr accept 2
+would usually return the connected descriptor to the application).
+The filter prevents the descriptor from being returned immediately
+to the application via
+.Xr accept 2 .
+The descriptor is made available to the application via
+.Xr accept 2
+only when one of the following conditions is met:
+.Bl -enum
+.It
+A complete, syntactically valid HTTP/1.0 or HTTP/1.1 HEAD or GET request has
+been buffered by the kernel.
+.It
+The data buffered by the kernel cannot be part of a complete, syntactically
+valid HTTP 1.0 or HTTP/1.1 HEAD or GET request.
+.El
+.Pp
+The utility of
+.Nm
+is that a server will not have to context switch several times
+before performing the initial parsing of the request.
+This effectively reduces the amount of required CPU utilization
+to handle incoming requests by keeping active
+processes in preforking servers such as Apache low
+and reducing the size of the file descriptor set that needs
+to be managed by interfaces such as
+.Fn select ,
+.Fn poll
+or
+.Fn kevent
+based servers.
+.Sh EXAMPLES
+If the accf_data accept filter is present in the kernel configuration,
+this will enable the http accept filter
+on the socket
+.Fa sok .
+.Bd -literal -offset 0i
+	struct accept_filter_arg afa;
+
+	bzero(&afa, sizeof(afa));
+	strcpy(afa.af_name, "httpready");
+	setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa));
+.Ed
+.Sh SEE ALSO
+.Xr setsockopt 2 ,
+.Xr accept_filter 9
+.Sh HISTORY
+The accept filter mechanism and the
+.Nm
+filter were introduced in
+.Fx 4.0 .
+They were ported to
+.Nx
+by Coyote Point Systems and appeared in
+.Nx 5.0 .
+.Sh AUTHORS
+This manual page and the filter were written by
+.An Alfred Perlstein .