IPV6_MULTICAST_IF(2const)

✨ IPV6_MULTICAST_IF — bestie edition ✨

okay so you're here because you want to pick which network device your multicast packets go OUT of 💅 and babes, you came to the right manpage. this is IPV6_MULTICAST_IF, a 2const page, and it's lowkey the whole personality of one socket option. let's get into it 🦋

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🌸 NAME 🌸

IPV6_MULTICAST_IF — the local device for a multicast socket 🎀

📚 LIBRARY 📚

Standard C library — that's libc, -lc ✨

🩷 SYNOPSIS 🩷

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

int setsockopt(int sockfd, IPPROTO_IPV6, IPV6_MULTICAST_IF,
               const int *ifindex, sizeof(int));
int getsockopt(int sockfd, IPPROTO_IPV6, IPV6_MULTICAST_IF,
               int *ifindex, sizeof(int));

not me dropping both the setter AND the getter like it's nothing 😌 slay.

💖 DESCRIPTION 💖

So the way this works is: it sets the device for outgoing multicast packets on the socket. that's it, that's the job. you're telling the socket "hey bestie, send your multicast vibes out through this network interface specifically" 🌈

⚠️ heads up though — this is allowed only for SOCK_DGRAM and SOCK_RAW socket. if your socket isn't one of those two, this is not your moment, iykyk 💀

The argument is a pointer to an interface index (peep netdevice(7) for the whole lore) in an integer. so like, an int, but it's pointing at it. pointer energy 👑

😭 ERRORS 😭

ngl this part just points you elsewhere, and honestly? valid of it:

• See IPPROTO_IPV6(2const).
• See setsockopt(2).
• See ipv6(7).

📜 HISTORY 📜

• IPV6_MULTICAST_IF — POSIX.1-2001. yes she's old, and yes she still ate 💅

👀 SEE ALSO 👀

• IPPROTO_IPV6(2const)
• setsockopt(2)
• ipv6(7)

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and that's the whole page bestie, go forth and multicast responsibly 🫶✨

Linux man-pages 6.20 — dated 2026-01-05 — IPV6_MULTICAST_IF(2const), signing off, no cap 💜