in_pktinfo(2type)

✨ in_pktinfo(2type) ✨

okay bestie, pull up a chair, because this page from the in_pktinfo(2type) manual is literally about knowing exactly where your packets came from and where they're headed. it's giving "packet detective" energy and we're obsessed 💖

🌸 NAME 🌸

in_pktinfo — all the tea about the incoming packet 🫖✨

💅 LIBRARY 💅

Standard C library (libc, -lc). yes, the classic. the blueprint. no cap.

🦋 SYNOPSIS 🦋

#include <netinet/in.h>

struct in_pktinfo {
    unsigned int    ipi_ifindex;   /* Interface index */
    struct in_addr  ipi_spec_dst;  /* Local address */
    struct in_addr  ipi_addr;      /* Header Destination address */
};

look at her. three fields. main character energy in a struct. 👑

---

🔥 DESCRIPTION 🔥

🎀 recvmsg(2)

when recvmsg(2) hands this struct back to you, here's what you're lookin at:

• .ipi_ifindex is the unique index of the interface the packet was received on. like, that specific one. no imposters. 💅
• .ipi_spec_dst is the preferred source address for replies to the given packet — aka "hey, if you're gonna clap back, reply from this address." 🫶
• .ipi_addr is the destination address in the packet header.

and here's the plot twist 😭: these addresses are usually the same, but they can differ for broadcast or multicast packets. and depending on the configured routes, .ipi_spec_dst might lowkey belong to a different interface from the one that actually received the packet. the way that routes can just switch things up on you… iconic but also unhinged. 🌀

🌈 sendmsg(2)

so when you're sending, it's a whole different vibe:

• if IP_PKTINFO is passed to sendmsg(2) and .ipi_spec_dst is not zero, then it gets used as the local source address, for the routing table lookup, and for setting up IP source route options. triple threat fr fr. 👑
• when .ipi_ifindex is not zero, the primary local address of the interface specified by the index overwrites .ipi_spec_dst for the routing table lookup. yeah — it just overrides it like that. rude but that's the rules. 💀
• .ipi_addr? ignored. 💀 not even invited to the function call. sorry girl, you're benched.

⚠️ heads up bestie: the overwrite thing in that second bullet is real and it's coming for your .ipi_spec_dst. set .ipi_ifindex with intention, iykyk.

---

⭐ STANDARDS ⭐

Linux. that's it. that's the standard. 🐧✨

🦄 HISTORY 🦄

Linux 2.2 — so she's been around a minute, but she still ate. 💖

👑 SEE ALSO 👑

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

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and that's the page!! 🌟 this one came from Linux man-pages 6.20, dated 2025-11-26 — so you know it's fresh 🍓. now go forth and read your packet headers like the networking baddie you are. periodt. 💅🌸🦋