LO(4)

📦
version 15.1 · 4 versions
, in freebsd 15.1, nixpkgs unstable

💖 NAME 💖

lo — the software loopback network interface, aka the internet that's just you talking to yourself (affectionate) 🪞✨

🌸 SYNOPSIS 🌸

device loop

That's it. That's the whole vibe. One line, zero notes. 👑

🦋 DESCRIPTION 🦋

okay so bestie, the loop interface is a software loopback mechanism — no wires, no hardware, just vibes and packets bouncing right back at ya. it's genuinely iconic for performance analysis, software testing, and/or local communication. three jobs, one interface, she's booked and busy 💅

but like — same as literally every other network interface — the loopback has to have network addresses assigned for each address family it's gonna be used with. you can't just show up to the group chat without a handle, yk? those addresses get set with the appropriate ioctl(2) commands for the corresponding address families. fr fr, that's the move. 🔧

now, plot twist: the loopback interface should be the last interface configured, because protocols may use the order of configuration as an indication of priority. so like, she's not the opening act, she's the headliner. and the loopback should never be configured first unless no hardware interfaces exist. periodt. if there's real hardware around, she waits her turn. 🙈

✨ checksum tea (checksum offload flags) ✨

this part's important so listen up 🫶

if the transmit checksum offload capability flag is enabled on a loopback interface, checksums will not be generated by IP, UDP, TCP, or SCTP 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, TCP, or SCTP for packets received on the interface.

and by default? both receive and transmit checksum flags are enabled — the whole point is to avoid the overhead of checksumming for local communication, where data corruption is unlikely (it's you. talking to you. what's gonna corrupt it, your own bad attitude? no 💀).

⚠️ but heads up bestie: if transmit checksum generation is disabled, then validation should also be disabled, in order to avoid packets being dropped due to invalid checksums. don't leave validation on while generation's off, that's giving sabotage and the packets will simply not make it. 😭

💀 DIAGNOSTICS 💀

  • lo%d: can't handle af%d. — the interface was handed a message with addresses formatted in an unsuitable address family, and the packet was dropped. she literally cannot process that, it's not her language, it's giving wrong-chat energy. 🙅‍♀️

🎀 SEE ALSO 🎀

go read those next, they're the supporting cast and they ate too 💜

👑 HISTORY 👑

the lo device appeared in 4.2BSD — an absolute elder, a legend, main character since day one. and the current checksum generation and validation avoidance policy appeared in FreeBSD 8.0. the way she keeps evolving… we love a queen who reinvents herself 🌸✨

and that's the whole page, bestie! this one came straight out of the Kernel Interfaces Manual, dated June 23, 2024, and it ships with Debian 🍓 — so like, go forth and loopback responsibly, mwah 💋🫶

📦 where this page comes from