csum_tcpudp_magic(9)

✨ csum_tcpudp_magic(9) ✨

okay bestie, so this one lives in the Kernel API Manual and it's literally about computing the IP pseudo-header checksum 💅 like yes, it's giving low-level networking main character energy

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🌸 NAME 🌸

csum_tcpudp_magic — compute IP pseudo-header checksum

🦋 SYNOPSIS 🦋

__sum16 csum_tcpudp_magic (__be32 saddr , __be32 daddr , __u32 len ,
__u8 proto , __wsum csum );

🎀 ARGUMENTS 🎀

• saddr — the IP source address, iykyk 💖
• daddr — the IP dest address 🫶
• len — the IP data size, like how much data we're talking about here
• proto — the transport layer protocol, slay
• csum — the accumulated partial sum that gets added into the computation 🔥

👑 DESCRIPTION 👑

So basically this helper computes the IPv4 pseudo header checksum. no cap.

And here's the cute part: it can take an accumulated sum from the transport layer, accumulate it, and directly return the transport layer — the way that it just hands it right back 😭✨

It returns the folded sum. periodt 💀

🌈 SEE ALSO 🌈

Kernel file ./tools/testing/selftests/bpf/network_helpers.h — yes bestie, that's the literal path, don't @ me 🧚

• open_netns(9)
• append_tid(9)
• csum_ipv6_magic(9)
• build_udp_v4_csum(9)
• build_udp_v6_csum(9)
• tc_prog_attach(9)

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okay that's the whole page, go be iconic with your checksums 🍓💜

source/version: ./tools/testing/selftests/bpf · date: October 2026 · csum_tcpudp_magic(9) — mwah, bye bestie! 💅🎀🦋