August 10, 2008

### NAME

accept_filter, accept_filt_add, accept_filt_del, accept_filt_generic_mod_event, accept_filt_get - filter incoming connections

### SYNOPSIS

#define ACCEPT_FILTER_MOD
```int` accept_filt_add(```struct accept_filter *filt`)
```int` accept_filt_del(```char *name`)
```int` accept_filt_generic_mod_event(```module_t mod``, int event``, void *data`)
```struct` `accept_filter``*` accept_filt_get(```char *name`)

### DESCRIPTION

Accept filters allow an application to request that the kernel pre-process incoming connections. This manual page describes the kernel interface for accept filters. User applications request accept filters via the setsockopt(2) system call, passing in an ```optname` of ```SO_ACCEPTFILTER`.

### IMPLEMENTATION NOTES

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

The accept filters currently provided with NetBSD (accf_data(9)and accf_http(9)) are implemented as pseudo-devices, but an accept filter may use any supported means of initializing and registering itself at system startup or later, including the LKM or module frameworks if they are supported by the running kernel.

The fields of are as follows:

accf_name
  Name of the filter; this is how it will be accessed from userland.

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.

accf_create
  Called whenever a setsockopt(2) installs the filter onto a listening socket.

accf_destroy
  Called whenever the user removes the accept filter on the socket.

The accept_filt_del(``) function passed the same string used in accept_filter.accf_name during registration with accept_filt_add(``), the kernel will then disallow and further userland use of the filter.

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

The accept_filt_generic_mod_event(``) function can be used by accept filters which are loadable kernel modules to add and delete themselves.

### SEE ALSO

setsockopt(2), accf_data(9), accf_http(9), malloc(9)

### HISTORY

The accept filter mechanism was introduced in FreeBSD4.0. It was ported to NetBSD by Coyote Point Systems, Inc. and appeared in NetBSD5.0.

### AUTHORS

This manual page was written by

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.