eth_type_trans(9)

💅 eth_type_trans(9) — the Kernel API Manual page, bestie edition 🎀

okay so this is a page from the Kernel API Manual, and it's literally about one little function that figures out what protocol a packet is speaking. short page, iconic function, let's go ✨

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🌸 NAME 🌸

eth_type_trans — determine the packet's protocol ID. that's it. that's the whole job. it looks at a packet and is like "bestie, what even ARE you" and then tells you 💖

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🦋 SYNOPSIS 🦋

__be16 eth_type_trans (struct sk_buff *skb , struct net_device *dev );

it gives you back a __be16 (that's the protocol ID, big-endian, no cap) and it takes two things. we'll get to those 👇

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💜 ARGUMENTS 💜

• skb — received socket data. the actual packet that showed up, in all its struct sk_buff glory 🫶
• dev — receiving network device. the NIC (or whatever) that this packet came in on 🌈

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⭐ DESCRIPTION ⭐

the rule here is lowkey simple and highkey important: we assume 802.3 if the type field is short enough to be a length. like, if that field is smol enough that it could plausibly be a length instead of a type, we go with 802.3. that's the vibe check 🧚

and this is normal practice!! it works for any 'now in use' protocol. so like, no stress, this is the standard move and it slays 💅

heads up: the whole thing hinges on that "short enough to be a length" rule, so don't go assuming it's magic. it's a rule, it's deliberate, that's the tea 🍓

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🔥 SEE ALSO 🔥

all of these live in the kernel file ./net/ethernet/eth.c and honestly they're all giving ethernet main character energy 👑

• eth_header(9)
• eth_get_headlen(9)
• eth_header_cache(9)
• eth_header_cache_update(9)
• eth_header_parse_protocol(9)
• eth_prepare_mac_addr_change(9)
• eth_commit_mac_addr_change(9)
• eth_mac_addr(9)
• ether_setup(9)
• alloc_etherdev_mqs(9)
• platform_get_ethdev_address(9)
• nvmem_get_mac_address(9)
• fwnode_get_mac_address(9)
• device_get_mac_address(9)
• device_get_ethdev_address(9)

like, go read them, they're all part of the same friend group 🫶✨

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and that's the page, bestie!! this one comes straight out of ./net/ethernet, dated October 2026, and it's eth_type_trans(9) — mwah 💋 periodt 🌈👑