IPV6_V6ONLY(2const)

💖 IPV6_V6ONLY(2const) 💖

okay bestie, so this one lives in the IPV6_V6ONLY(2const) corner of the manual pages, and it's literally the little socket flag that decides whether your v6 socket is a v6-only girlie or a dual-stack queen. library-wise it's just the standard C library, libc, -lc, nothing extra to install, we love a low-effort icon ✨

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🌸 SYNOPSIS 🌸

so you're gonna pull in these headers, right:

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

and then you hit setsockopt() like this:

int setsockopt(int sockfd, IPPROTO_IPV6, IPV6_V6ONLY,
               const int *enable, sizeof(int));

that's it. that's the whole call. IPPROTO_IPV6 level, IPV6_V6ONLY option, and a pointer to an int named enable that's sizeof(int) big. slay 💅

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✨ DESCRIPTION ✨

okay so here's the tea 🍓 — it all depends on what you put in enable.

• if this flag is set to true (that means nonzero), your socket is restricted to sending and receiving IPv6 packets only. that's the v6-only era. and the cute part? an IPv4 application and an IPv6 application can bind to a single port at the same time. no fighting, no EADDRINUSE drama, both get their lil port. iconic 🎀
• if this flag is set to false (zero), then the socket can be used to send and receive packets to and from an IPv6 address or an IPv4-mapped IPv6 address. dual-stack mode. main character energy 🌈

now, what's the default? bestie, it's not vibes-based, it's file-based 😭 — the default value for this flag is defined by the contents of the file

/proc/sys/net/ipv6/bindv6only

and the default value for that file is 0 (false). so out of the box, dual-stack. noted. 💜

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💀 ERRORS 💀

ngl this page doesn't spell them out, it just points you elsewhere:

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

that's where the error lore lives, go read those 🫶

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👑 HISTORY 👑

• POSIX.1-2001
• Linux 2.4.21
• Linux 2.6

so she's been around a minute. vintage. legendary. periodt. ⭐

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🦋 SEE ALSO 🦋

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

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reminder, bestie: IPV6_V6ONLY only does its thing at the IPPROTO_IPV6 level, and the value you hand it is a pointer to an int — not the int itself. don't be out here passing integers raw, iykyk 🙈

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and that's the whole page, no cap 💖 this lil writeup came from Linux man-pages 6.20, dated 2026-06-21 — go forth and set your socket options, you absolute IPV6_V6ONLY icon 🌟