accept_filt_add(9) accept_filt_generic_mod_event

Other Alias

accept_filter, accept_filt_del


In sys/types.h In sys/module.h In sys/socket.h Fd #define ACCEPT_FILTER_MOD 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


Accept filters allow an application to request that the kernel pre-process incoming connections. An accept filter is requested via the setsockopt(2) system call, passing in an Fa optname of SO_ACCEPTFILTER


A module that wants to be an accept filter must provide a Vt 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 Fn accept_filt_add , passing a pointer to a Vt struct accept_filter , allocated with malloc(9).

The fields of Vt 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 Fn accept_filt_del function 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.

The Fn accept_filt_get function is used internally to locate which accept filter to use via the setsockopt(2) system call.

The Fn accept_filt_generic_mod_event function provides a simple way to avoid duplication of code for accept filters which do not use the argument field to load and unload themselves. This function can be used in the Vt moduledata_t struct for the DECLARE_MODULE9 macro.


The accept filter mechanism was introduced in Fx 4.0 .


This manual page was written by An -nosplit An Alfred Perlstein , An Sheldon Hearn and An Jeroen Ruigrok van der Werven.

The accept filter concept was pioneered by An David Filo at Yahoo! and refined to be a loadable module system by An Alfred Perlstein .