ACCEPT_FILTER(9)

💖 ACCEPT_FILTER(9) — the kernel dev manual, but make it cute 💖

okay so bestie, welcome to ACCEPT_FILTER(9), aka the page where the kernel lets apps pre-chew incoming connections before they even hit userspace. it's giving bouncer-at-the-club energy, fr fr. 🫶

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✨ NAME ✨

accept_filter, accept_filt_add, accept_filt_del, accept_filt_generic_mod_event, accept_filt_get — filter incoming connections 🎀

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🌸 SYNOPSIS 🌸

#include <sys/types.h>
#include <sys/module.h>
#include <sys/socket.h>
#define ACCEPT_FILTER_MOD
#include <sys/socketvar.h>

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);

like yes, that's the whole cast of characters, and yes they all showed up dressed correctly 💅

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🦋 DESCRIPTION 🦋

So the whole vibe here: accept filters let an application ask the kernel to pre-process incoming connections for it. no cap. the app requests one through the setsockopt(2) system call, and it passes in an optname of SO_ACCEPTFILTER. that's the secret handshake. 🤝

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👑 IMPLEMENTATION NOTES 👑

okay so if a module wants to BE an accept filter (main character behavior), it has to hand the system a struct accept_filter:

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 */
};

then the module registers it with the accept_filt_add() function, passing a pointer to a struct accept_filter that was allocated with malloc(9). (yes, you gotta malloc(9) it. the kernel is not your mom, it will not allocate for you. 😭)

and here's the field-by-field breakdown, because every single one matters:

• accf_name — the name of the filter; this is literally how it gets accessed from userland. so pick something cute but, like, legal.
• accf_callback — the callback the kernel does once the connection is established. it's the same thing as a socket upcall, and it gets called when the connection is established and whenever new data arrives on the socket — unless the callback modifies the socket's flags. so, like, don't go changing flags and then act surprised. 🙈
• accf_create — called whenever setsockopt(2) installs the filter onto a listening socket.
• accf_destroy — called whenever the user removes the accept filter on the socket. bye bestie 👋

Now, the accept_filt_del() function: you pass it the same string that you used in accept_filter.accf_name during registration with accept_filt_add(), and then the kernel disallows any further userland use of the filter. periodt. that filter is done. 💀

The accept_filt_get() function is used internally to locate which accept filter to use via the setsockopt(2) system call. it's the matchmaker. iykyk.

And the accept_filt_generic_mod_event() function gives you a simple way to avoid duplicating code for accept filters that don't use the argument field to load and unload themselves. you can use this function in the moduledata_t struct for the DECLARE_MODULE(9) macro. efficiency? slay. ✨

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🔥 SEE ALSO 🔥

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

go read those too, bestie, they're all in the friend group 💜

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🌈 HISTORY 🌈

The accept filter mechanism was introduced in FreeBSD 4.0. an icon, a legend, and the moment is still iconic. ⭐

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🎀 AUTHORS 🎀

This manual page was written by Alfred Perlstein, Sheldon Hearn and Jeroen Ruigrok van der Werven. absolute queens of documentation. 👑

The accept filter concept was pioneered by David Filo at Yahoo! and refined to be a loadable module system by Alfred Perlstein. two icons, one legend of a feature. 🧚

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and that's the whole page!! mwah 💋

this one came to you from the Debian edition of the page, dated June 25, 2000 — so yeah, it's vintage, but it ATE and left no crumbs. 🍓 take care of your accept filters and they'll take care of you. byeeee 🦋✨