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ACCF_HTTP(9) NetBSD Kernel Developer's Manual ACCF_HTTP(9)
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Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
accf_http -- buffer incoming connections until a certain complete HTTP
This is a filter to be placed on a socket that will be using accept() to
receive incoming HTTP connections.
Once installed on a listening socket, this filter is activated when a
connection becomes ready to receive data (at which point accept(2) would
usually return the connected descriptor to the application). The filter
prevents the descriptor from being returned immediately to the applica-
tion via accept(2). The descriptor is made available to the application
via accept(2) only when one of the following conditions is met:
1. A complete, syntactically valid HTTP/1.0 or HTTP/1.1 HEAD or GET
request has been buffered by the kernel.
2. The data buffered by the kernel cannot be part of a complete, syn-
tactically valid HTTP 1.0 or HTTP/1.1 HEAD or GET request.
The utility of accf_http 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 select(), poll() or kevent() based
If the accf_data accept filter is present in the kernel configuration,
this will enable the http accept filter on the socket sok.
struct accept_filter_arg afa;
setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa));
The accept filter mechanism and the accf_http filter were introduced in
FreeBSD 4.0. They were ported to NetBSD by Coyote Point Systems and
appeared in NetBSD 5.0.
This manual page and the filter were written by Alfred Perlstein.
NetBSD 9.3 September 4, 2008 NetBSD 9.3