Document the kernel interface for accept filters.
Thanks to sheldonh for cleaning up the rough initial manpage.
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share/man/man9/accept_filter.9
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share/man/man9/accept_filter.9
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.\"
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.\" Copyright (c) 2000 Alfred Perlstein
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.\"
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\" "
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.Dd June 25, 2000
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.Os
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.Dt ACCEPT_FILTER 9
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.Sh NAME
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.Nm accept_filter ,
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.Nm accept_filt_add ,
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.Nm accept_filt_del ,
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.Nm accept_filt_generic_mod_event ,
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.Nm accept_filt_get
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.Nd filter incoming connections
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <sys/socket.h>
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.Fd #include <sys/socketvar.h>
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.Ft int
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.Fn accept_filt_add "struct accept_filter *filt"
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.Ft int
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.Fn accept_filt_del "char *name"
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.Ft int
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.Fn accept_filt_generic_mod_event "module_t mod" "int event" "void *data"
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.Ft struct accept_filter *
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.Fn accept_filt_get "char *name"
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.Sh DESCRIPTION
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Accept filters allow an application to request
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that the kernel pre-process incoming connections.
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An accept filter is requested via the
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.Xr setsockopt 2
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system call, passing in an
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.Fa optname
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of
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.Dv SO_ACCEPTFILTER .
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.Sh IMPLEMENTATION NOTES
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A module that wants to be an accept filter
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must provide a struct accept_filter to the system:
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.Bd -literal
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struct accept_filter {
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char accf_name[16];
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void (*accf_callback)
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__P((struct socket *so, void *arg, int waitflag));
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void * (*accf_create)
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__P((struct socket *so, char *arg));
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void (*accf_destroy)
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__P((struct socket *so));
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SLIST_ENTRY(accept_filter) accf_next; /* next on the list */
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};
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.Ed
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.Pp
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The module should register it with the function
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.Fn accept_filt_add ,
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passing a pointer to a struct accept_filter, allocated with
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.Xr MALLOC 9
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.Pp
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The fields of
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.Fa struct accept_filter
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are as follows:
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.Bl -tag -width accf_callbackXXX
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.It accf_name
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Name of the filter;
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this is how it will be accessed from userland.
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.It accf_callback
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The callback that the kernel will do
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once the connection is established.
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It is the same as a socket upcall
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and will be called when the connection is established
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and whenever new data arrives on the socket,
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unless the callback modifies the socket's flags.
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.It accf_create
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Called whenever a
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.Xr setsockopt 2
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installs the filter onto
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a listening socket.
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.It accf_destroy
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Called whenever the user removes the accept filter on the socket.
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.El
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.Pp
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.Fn accept_filt_del
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passed the same string used in accept_filter.accf_name during registration
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with
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.Fn accept_filt_add ,
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the kernel will then disallow and further userland use of the filter.
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.Pp
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.Fn accept_filt_get
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is used internally to locate which accept filter to use via the
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.Fn setsocketopt
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syscall.
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.Pp
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.Fn accept_filt_generic_mod_event
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provides a simple way to avoid duplicate
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code for accept filters which don't use
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argument field to load and unload
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themselves. It is a function that can be
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put in the load/unload struct
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for the
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.Fn DECLARE_MODULE
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macro.
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.Sh SEE ALSO
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.Xr setsockopt 2 ,
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.Xr malloc 9
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.Sh HISTORY
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The accept filter mechanism was introduced in
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.Fx 4.0 .
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.Sh AUTHORS
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This manual page has been written by
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.An Alfred Perlstein .
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The accept filter concept was pioneered by engineers at Yahoo.com
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and refined to be a loadable module system by Alfred Perlstein.
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