IP_ROUTER_ALERT(2const)

💖 IP_ROUTER_ALERT — the "gimme ALL the packets bestie" sockopt ✨

okay so this is a page from the Linux man-pages, and it lives in section 2const (that's the little corner of the man pages where all the constant-ish socket option values hang out). It's IP_ROUTER_ALERT, and honestly? She's niche, she's specific, she's doing her little job. Let's get into it 🫶

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📚 LIBRARY

Standard C library — that's libc, aka -lc. The classic. The blueprint. The one everyone links against 💅

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✨ SYNOPSIS

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

int setsockopt(int sockfd, IPPROTO_IP, IP_ROUTER_ALERT,
               const int *enable, sizeof(int));

So you're calling setsockopt(2), you're passing IPPROTO_IP as the level, IP_ROUTER_ALERT as the option name, and a pointer to an int named enable with sizeof(int) as the length. Clean. Simple. Iconic. 🌟

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🌸 DESCRIPTION

Pass all to-be forwarded packets with the IP Router Alert option set to this socket. Like, all of them. To this socket. She's collecting them 🎀

Valid only for raw sockets. No, fr fr, raw sockets only — don't try to pull this anywhere else, bestie, it's not gonna work out.

This is useful, for instance, for user-space RSVP daemons. Yep, that's the main character use case right there 👑

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😭 ERRORS

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

That's it, that's the whole list — go check those, they've got the receipts 🧾

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🌈 STANDARDS

Linux. Just Linux. That's the vibe. No POSIX, no nothing else — she's a Linux girlie 💜

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🦋 HISTORY

Linux 2.2. Old money. Vintage. Still slaying ✨

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⚠️ CAVEATS

Not me telling you the tapped packets are not forwarded by the kernel 😭 It is the user's responsibility to send them out again. You catch 'em, you throw 'em. That's the deal.

Also, and this is important: socket binding is ignored — such packets are filtered only by protocol. So don't go thinking your bind is doing anything here, because it is not. She's not even looking at it 🙈

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👀 SEE ALSO

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

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💖 okay that's the whole page, you ate that fr fr

Brought to you by Linux man-pages 6.20, dated 2025-11-25 — and yes, the IP_ROUTER_ALERT(2const) page name is real, section and all 💅