udp4_lib_lookup1(9)

✨ udp4_lib_lookup1 ✨

okay bestie, so this one's from the Kernel API Manual (yes, the kernel kernel, we're literally in the source code rn 💅) and it's about the little lookup that could 🫶

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🌸 NAME 🌸

udp4_lib_lookup1 — a simplified lookup using the primary hash, and the hash key is the destination port. that's the whole vibe. 🎀

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🦋 SYNOPSIS 🦋

struct sock * udp4_lib_lookup1 (const struct net *net , __be32 saddr ,
       __be16 sport , __be32 daddr , unsigned int hnum , int dif , int sdif ,
       const struct udp_table *udptable );

like, yes, that's a lot of arguments. it ate and left no crumbs, but also please count your parameters before calling it 😭

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💖 ARGUMENTS 💖

• net — the network namespace. your lil world. 🌍
• saddr — source address, in network order (do not reorder it, i'm serious 🙈)
• sport — source port, also in network order
• daddr — destination address, network order again, consistency is iconic
• hnum — destination port, but in host order this time. yes, this one's different. i don't make the rules 💀
• dif — destination interface index
• sdif — destination bridge port index, if relevant. if it's not relevant, don't come for me ✋
• udptable — the set of UDP hash tables. the whole menu, basically 🍓

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🌈 DESCRIPTION 🌈

so here's the tea ☕ — this is a simplified lookup meant to be used as a fallback if no sockets are found, because of a potential race between an (receive) address change and a lookup happening before the rehash operation. like the socket's still moving boxes and you're already knocking on the door 😭

the way this function does it: it ignores SO_REUSEPORT groups while scoring result sockets. why? because if we already have one, we literally don't need the fallback at all. no cap. 🧚

⚠️ important bestie: this gets called under rcu_read_lock. don't forget the lock, i'm begging 🫶

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⭐ RETURN ⭐

the socket with the highest matching score if there is one, or NULL if there isn't. that's it, that's the result 🔥

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👑 SEE ALSO 👑

• Kernel file ./net/ipv4/udp.c
• udp_lib_get_port(9)
• udp4_hwcsum(9)
• first_packet_length(9)
• udp_poll(9)

go read those next, i promise they're worth it 💜

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kisses from ./net/ipv4 mwah 💋 this page was dated October 2026, source and version and everything, so if it's stale that's a future problem, not mine 🌸 bye bestie!! 🦋💅🎀