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How 1500 bytes became the MTU of the internet
Ethernet is everywhere, tens of thousands of hardware vendors speak and implement it. However almost every ethernet link has one number in common, the MTU:
Ben Cox
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Visit blog.benjojo.co.uk → RSS feedOn Hacker News Jun 2015 – Sep 2025
Networking & Internet 25Systems & Performance 15Security & Privacy 11Hardware & Electronics 9Software Engineering 8Society, Policy & Culture 4
2
I may be the only evil (bit) user on the internet (2015)
Almost every year a joke RFC is made on April 1st (these have caught on so well, that it’s now common to see more than one of these every year), or April Fool’s Day, and some of them are pretty great. Here are some of my favourites:
3
Playing battleships over BGP
BGP is the glue of the internet. For a protocol that was produced on two napkins in 1989 it is both amazing and horrifying that it runs almost all of the ISP to ISP interactions and is now a very fundemental part of the internet.
4
BGP handling bug causes widespread internet routing instability
At 7AM (UTC) on Tuesday May 20th 2025 a BGP message was propagated that triggered surprising (to many) behaviours with two major BGP implementations that are often used for carrying internet traffic.
5
Who is squatting IPv4 addresses?
It’s an established fact on the internet that we have ran out of IP(v4) addresses , and we are starting to see the pressure on obtaining IPv4 space. For example if you want to get a /24 block from RIPE NCC when you sign up as a member, then you are currently looking at a 2 month wait for a recycled IPv4 /24 block .
6
Grave flaws in BGP Error handling
Border Gateway Protocol is the de facto protocol that directs routing decisions between different ISP networks, and is generally known as the “glue” that holds the internet together. It’s safe to say that the internet we currently know would not function without working BGP implementations.
7
Building Ultra Long Range Toslink
This post is a textual version of a talk I gave at The 38th Chaos Computer Congress at the end of 2024. You can watch the talk that was recorded by the wonderful C3VOC team below if that’s your preferred medium:
8
The death of a TLD
Another one bites the dust. The gTLD gold rush is now seeing a steady flow of TLD’s that clearly just didnt work out.
9
Sysadmin friendly high speed Ethernet switching
I’ve been on the lookout for a ethernet switch that I don’t hate, the problem with a lot of higher speed (10G and above) ethernet switches is that they are quite expensive new and if you buy the used then they rarely have many years left ( or none at all ) of software patches.
10
Reclaiming IPv4 Class E's 240.0.0.0/4
Ever since the supply of fresh IPv4 address blocks was depleted there has been a number of interesting market changes, mostly around the costs to either acquire or lease IPv4 address blocks. Since the demand for IPv4 addressing has not changed that much, prices have gone up, and more providers like AWS, Hetzner, OVH…
11
DNSFS – Store files in others' DNS resolver caches
A while ago I did a blog post about how long DNS resolvers hold results in cache for, using RIPE Atlas probes testing against their default resolvers (in a lot of cases, the DNS cache on their modem/router).
12
Ghost in the ethernet optic
A few months ago I stumbled on a tweet pointing out a kind of SFP optic that claimed to be smart, made by a Russian company Plumspace . After going through their Smart SFP product listings I was on-board and decided I needed to have some of them.
13
Hunting down the stuck BGP routes
BGP is the glue between all of the thousands of border routers that make up the internet (you can find this post (battleships) and this post (EvE) as a crash course on how BGP works).
14
Mapping the internet with Hilbert curves
The internet is big. Really big . You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think the /22 you got as a LIR was big, but that’s just peanuts to the internet.
15
What would a EvE online Internet look like?
EvE online is a fascinating game. It’s one of the few MMO’s that only have a single “server” to log into, meaning everyone is playing on the same logical world. It’s also had a fascinating set of events that have happened inside it, and also remains a very visually appealing game for what it is:
16
Some interesting stuff I found on IX LANs
These days the internet as a whole is mostly constructed out of point to point ethernet circuits, meaning an ethernet interface (mostly optical) attached directly from one routing device to another routing device.
17
Hacking Ethernet out of Fibre Channel cards
This story, like another in the past , started as an eBay purchase that I would soon regret. I was scrolling through my favorite eBay supplier when I found a listing titled Job Lot Of Roughly 350 Various Network Cards . This of course piqued my interest. Some close photo inspection identified a whole load of cards in…
18
The “simple” 38 step journey to getting an RFC
The Internet is built on the mutual understanding of network protocols and practices, and most of those protocols are defined using Request For Comments (RFC) or Best Common Practices (BCP) documents.
19
How much traffic do I get from North Korea anyway?
While the president of the United States and the leader of North Korea were/are currently beefing on Twitter about who should destroy the world first, North Korea was also causing me some personal frustration on data quality.
20
Building a legacy search engine for a legacy protocol
Most users who use the internet today, are mostly focused on three protocols, HTTP, TLS, and DNS.
21
TOTP SSH port fluxing
Some people change their SSH port on their servers so that it is slightly harder to find for bots or other nasties and while that is generally viewed as an action of security through obscurity it does work very well at killing a lot of the automated logins you always see in /var/log/auth.log .
22
Splitting the Ping
Ping is one of the fundamental pillars of networking. It’s simple, universally supported, and is normally one of the few things that is shipped with all network stacks.
23
Where is all of the Fediverse?
Spurred on by the problems at Twitter, a lot of my social media timeline has “moved out” of Twitter/“X” on to what people mostly describe as mastodon or the “fediverse”. The fediverse being the collection of decentralised “instances” (aka servers) sometimes using different software but speaking a common protocol…
24
Going multipath without Multipath TCP
Gigabit ethernet has been around for a long time, it’s so ubiquitous that there is a very strong chance that if you have a RJ-45 port on your computer, it’s going to be a gigabit ethernet network interface.
25
The speed of BGP network propagation
At the end of March 2019 I did a talk at the INEX’s (Ireland’s biggest internet exchange point) Annual General Meeting. I was supposed to record it, however in a brief panic due to HDMI not working on my laptop I forgot to start a recording of it. Since people found it interesting I figured I would turn it into a blog…