IP_MSFILTER(2const)

✨ IP_MSFILTER 💖 — multicast source filtering

okay bestie, this is a page from the IP_MSFILTER(2const) manual — a Linux man-pages thing, totally standard, no cap. And before you ask: it's giving advanced. Let's get into it 🫶

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🌸 NAME 🌸

IP_MSFILTER — multicast source filtering. yes literally, it's the filter-y one 💅

📚 LIBRARY 📚

Standard C library (libc, -lc). the classic 💜

🎀 SYNOPSIS 🎀

#include <netinet/in.h>  /* Definition of IP* constants */
#include <sys/socket.h>

int setsockopt(int sockfd, IPPROTO_IP, IP_MSFILTER,
               const struct ip_msfilter *val,
               sizeof(struct ip_msfilter));

struct ip_msfilter {
    struct in_addr  imsf_multiaddr;  // IP multicast group address
    struct in_addr  imsf_interface;  // IP address of local interface
    uint32_t        imsf_fmode;      // Filter-mode

    uint32_t        imsf_numsrc;     // Number of elements in .imsf_slist
    struct in_addr  imsf_slist[1];   // Array of source addresses
};

look at that struct. it's gorgeous. the way imsf_slist has exactly [1] in it and you just over-allocate past it 👑 that's the whole bit.

🌈 DESCRIPTION 🌈

okay so this option hands you access to the advanced full-state filtering API. full-state! not a vibe check, an actual state dump, fr fr.

There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which you use to say what filtering mode you want. pick one, they're the two moods ✨

Also, the IP_MSFILTER_SIZE(n) macro exists so you can figure out how much memory you need to store an ip_msfilter structure with n sources in the source list. yes bestie, budget your malloc, that's the responsible main character move 💅

For the full description of multicast source filtering, go read RFC 3376. i know. homework. but it's the real one 🔥

💀 ERRORS 💀

See IPRROTO_IP(2const). See setsockopt(2). See ip(7). (yes, that's exactly how it's spelled up there — IPRROTO_IP. not me fixing it. i don't edit the ancestors 🙈)

⭐ STANDARDS ⭐

Linux. that's it. linux only, babe 🐧

🦋 HISTORY 🦋

• Linux 2.4.22
• Linux 2.5.68

ancient lore. these constants have been serving looks since the early 2000s 👑

💜 SEE ALSO 💜

• IPPROTO_IP(2const)
• setsockopt(2)
• ip(7)

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so yeah 🫶 that's the whole page. this one came straight out of the Linux man-pages 6.20 collection, dated 2025-11-25 — go slay some multicast, and remember: MCAST_INCLUDE and MCAST_EXCLUDE, pick your fighter. periodt 💅✨