udp_csum_outgoing(9)

✨ udp_csum_outgoing(9) ✨

okay so this is a page straight out of the Kernel API Manual, bestie — section 9, kernel internals, the good stuff 💅 and today's main character is udp_csum_outgoing, which is literally the function that computes the UDPv4/v6 checksum over fragments. no cap.

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🎀 NAME 🎀

udp_csum_outgoing — compute UDPv4/v6 checksum over fragments 🔥

She does v4 and v6, and she does it across fragments, which is honestly iconic of her.

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🦋 SYNOPSIS 🦋

__wsum udp_csum_outgoing (struct sock *sk , struct sk_buff *skb );

Returns a __wsum — that's the checksum vibe. Takes two args, both pointers, both important, don't skip either 🫶

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🌸 ARGUMENTS 🌸

• sk — the socket we are writing to 💌 like, that's the socket, she's the destination of all this outgoing energy.
• skb — the sk_buff containing the filled-in UDP header. the checksum field must be zeroed out — not "kind of zeroed", not "mostly zeroed", literally zeroed out before you call this. do not forget. i'm begging 🙈

fr fr, that zeroed-checksum thing is the whole bit — the function is computing over that header, so a leftover checksum in there would be a whole situation 😭

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👑 SEE ALSO 👑

• Kernel file ./include/net/udp.h — the source of truth, go read her 💜
• struct udp_skb_cb(9)
• struct udp_hslot(9)
• struct udp_hslot_main(9)
• struct udp_table(9)

like, the whole udp header family reunion, honestly. ate. 🍓

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that's the whole page, bestie — no filler, no skips, every single detail accounted for ✨

signing off from ./include/net, dated October 2026, per the Kernel API Manual — mwah 💋