LO(4)

📦 version 12.2, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 LO(4) — like, the loopback era 💖

Kernel Interfaces Manual ✨ bestie edition ✨

🎀 NAME 🎀

lo — the software loopback network interface. it's giving "talk to yourself but make it networking" 💅

🌸 SYNOPSIS 🌸

device loop

✨ DESCRIPTION ✨

okay so the loop interface is a software loopback mechanism — a whole vibe where packets just... come back to you. bestie it's for:

  • performance analysis 📊
  • software testing 🧪
  • and/or local communication 🏠

fr fr. but here's the thing 👇

Just like every other network interface, the loopback interface must have network addresses assigned for each address family you're gonna use it with. no address, no slay. 😭

You can set those addresses with the appropriate ioctl(2) commands for the corresponding address families. periodt.

🫶 important ordering tea: the loopback interface should be the last interface configured, because protocols may use the order of configuration as an indication of priority. and the loopback should never be configured first — unless no hardware interfaces exist. like, genuinely never. iykyk.

🧚 checksum offload lore 🧚

this is the part where it gets a lil spicy 🌶️

  • If the transmit checksum offload capability flag is enabled on a loopback interface, checksums will not be generated by IP, UDP, or TCP for packets sent on the interface.
  • If the receive checksum offload capability flag is enabled on a loopback interface, checksums will not be validated by IP, UDP, or TCP for packets received on the interface.

By default, both receive and transmit checksum flags will be enabled — because why do the extra work of checksumming for local communication where data corruption is unlikely? literally the whole point of the shortcut. 💅

⚠️ but listen: if transmit checksum generation is disabled, then validation should also be disabled, in order to avoid packets being dropped due to invalid checksums. not me warning you twice 🔥

💀 DIAGNOSTICS 💀

lo%d: can't handle af%d.

The interface was handed a message with addresses formatted in an unsuitable address family; the packet was dropped. rip to that packet 🙈

👑 SEE ALSO 👑

inet(4), intro(4)

🌈 HISTORY 🌈

The lo device appeared in 4.2BSD. and the current checksum generation and validation avoidance policy appeared in FreeBSD 8.0. iconic behavior from start to finish ⭐

okay that's the whole page, bestie — this one came from the Debian crew, dated January 25, 2012. mwah, go configure your interfaces in the right order 🫶💖

📦 where this page comes from