ip_mreqn(2type)

💖 ip_mreqn — the manual page, but make it make sense 💖

okay so bestie, welcome to the ✨ ip_mreqn ✨ show. this is a page from section 2type, which is basically the corner of the manual where struct definitions live. we got two structs here — ip_mreqn and its slightly older sibling ip_mreq — and they're both about telling the kernel "hey. i want in on this multicast group. here's where i'm standing and which door i'm using." 📬 no cap.

and it lives in the Standard C library (libc), so you don't even gotta go hunting for it. it's right there. 🫶

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🌸 SYNOPSIS 🌸

grab this header first, she's the one who brings all the IP* constants to the party:

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

then here's the big one, the newer girl, ip_mreqn:

struct ip_mreqn {
    struct in_addr  imr_multiaddr;  /* IP multicast group address */
    struct in_addr  imr_address;    /* IP address of local interface */
    int             imr_ifindex;    /* interface index */
};

and here's the classic, ip_mreq — shorter, no imr_ifindex, still iconic:

struct ip_mreq {
    struct in_addr  imr_multiaddr;  /* IP multicast group address */
    struct in_addr  imr_address;    /* IP address of local interface */
};

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✨ DESCRIPTION ✨

okay so let's go field by field, because each one is doing its own lil job and you gotta know which one is which 💅

• .imr_multiaddr — this is the address of the multicast group your app wants to join or leave. and listen, it must be a valid multicast address. like, that's not optional. the kernel will not be playing games w you on this one. 🚫🎮
• .imr_address — the address of the local interface that the system should use to join the multicast group. and here's the cute part: if it's equal to INADDR_ANY, then the system just picks an appropriate interface for you. it's giving "you choose, i trust you." 😌🌷
• .imr_ifindex — the interface index of the interface that should join/leave the .imr_multiaddr group. orrrr you set it to 0 to indicate any interface. yes bestie, 0 means "any", that's not a typo, that's the vibe. 🎀

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🦋 HISTORY 🦋

• ip_mreqn — Linux 2.2. the newer era. the glow-up. ✨
• ip_mreq — Linux 1.2. the og. respect her. 👑

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🎀 SEE ALSO 🎀

go read these next, they're the whole friend group fr fr:

• IP_ADD_MEMBERSHIP(2const)
• IP_DROP_MEMBERSHIP(2const)
• IP_MULTICAST_IF(2const)
• ip(7)

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quick recap bestie: valid multicast addr in .imr_multiaddr, local interface addr in .imr_address (or INADDR_ANY and let the system cook 🍳), and .imr_ifindex for the interface — or 0 for any. that's the whole assignment. you got this. 🌈

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and that's the page!! 🫶 this lil slice of documentation came from Linux man-pages 6.20, dated 2025-11-25 — so yeah, she's recent, she's fresh, she ate. 💅🌟 now go join a multicast group, main character. 💖🦋🌸