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An interesting Tailscale + Docker gotcha

2026-01-29T00:00:00Z

As I’ve written about before, I use Tailscale for a lot of things. I thought I had it set up in a reasonably secure manner, but I recently noticed a problem.

I use Tailscale’s ACLs to limit what each node can access, based on the tags I apply to it. So an app node can’t access anything via Tailscale, while an integration or server node can access things tagged with either app or integration. This is expressed pretty simply in the Tailscale ACL JSON:

{
	"tagOwners": {
		// Servers that can be SSH'd into
		"tag:server": [],
		// Applications that are exposed on tailscale but never connect out
		"tag:app": [],
		// Things which talk to other services over tailscale (connecting to apps etc)
		"tag:integration": [],
	},
	"grants": [
		{
			// Users can access everything
			"src": ["autogroup:member"],
			"dst": ["*"],
			"ip":  ["*"],
		},
		{
			// Servers and integrations can access integrations and apps
			"src": ["tag:server", "tag:integration"],
			"dst": ["tag:integration", "tag:app"],
			"ip":  ["*"],
		},
	],
}

Over the past week I’ve been setting up a private Forgejo instance behind Tailscale, complete with an actions runner that runs things using a docker-in-docker container. I didn’t want the runner knowing anything about Tailscale, so I had it configured to speak to Forgejo direct over HTTP (forgejo:3000) instead of using full Tailscale HTTPS URL that I use when accessing it (http://git.example-net.ts.net/)¹.

Everything was going fine, until I forgot to do that translation… and it worked. My Forgejo action runners could access anything on my tailnet. I run several things on Tailscale that just have authentication turned off, on the basis that only authorised devices can access them. Things like the admin interface for this website. I definitely didn’t intend for any workflow I run on my git server to have access to edit my website!

This wasn’t just limited to Forgejo, either. Any docker container I was running could access the tailnet. In hindsight it’s fairly obvious why: the host is running Tailscale, connected as a node tagged with my server tag. That creates a tailscale0 interface, and automagically sets up iptables rules to route Tailscale traffic over the interface. Docker also automagically sets up iptables rules to bridge traffic, and apparently these two sets of rules interact in such a way that traffic from Docker containers is allowed to route via the tailscale0 interface.

I say it’s fairly obvious in hindsight — there’s no reason why Docker would special case any particular host interface after all — but it still feels pretty surprising. Because both bits of software inject their own iptables rules, I never really had a good mental model for how they interact. The host Tailscale node was a completely separate building block to Docker. It would be a pain to use either of them if they didn’t do these rules, but it’s also one of the reasons I don’t really like “magical” things².

There are lots of ways to fix this, but none of them feel particularly great. You can configure both Tailscale and Docker to not automatically fiddle with iptables and handle the rules yourself, but I really hate dealing with iptables³. As a stopgap I did hold my nose and add some iptables rules to drop traffic to the tailscale0 interface if it originated from the IP ranges that Docker was configured to use:

iptables -I DOCKER-USER -s 192.168.0.0/16 -o tailscale0 -j DROP
iptables -I DOCKER-USER -s 172.17.0.0/12 -o tailscale0 -j DROP

The DOCKER-USER chain is a nice little escape hatch; it comes before the main auto-generated DOCKER chain, and Docker leaves the rules in it alone.

So I added these rules, and felt pretty good about myself, and then… everything started breaking in weird ways. After some debugging I realised the problem was DNS⁴. The server’s DNS resolver is 100.100.100.100, a special Tailscale address. This is so that it can resolve tailnet hostnames via MagicDNS (and so I don’t have to configure my custom DNS servers manually on each device; Tailscale does it for me). My new iptables rules inadvertently dropped all the DNS packets coming from docker containers. D’oh.

Obviously the solution here is to double down and add MORE iptables rules:

iptables -I DOCKER-USER -s 192.168.0.0/16 -d 100.100.100.100 -o tailscale0 -j ACCEPT
iptables -I DOCKER-USER -s 172.17.0.0/12 -d 100.100.100.100 -o tailscale0 -j ACCEPT

This got DNS working again, but it all feels a bit gross. Not least because to actually make these persist you need to use iptables-save, and then you also get all the rules that both Docker and Tailscale have inserted. I came across a script to filter out the Docker ones, but… yuck⁵.

I realised a cleaner idea might just be to lock down what server tagged nodes could access within Tailscale itself. That way I can avoid touching iptables at all. It’s handy to be able to curl services from the host when debugging, but it’s not really necessary. So I removed that access, and… stuff broke again. I’m using my Forgejo instance as a registry for some of the docker images I run, so the docker daemon needs to be able to reach it. I ended up making a new tag for infrastructure, which can be accessed from server devices. This does still allow all the docker containers to reach Forgejo, but I already have it set up with appropriate access controls and public/private repository splits. Forgejo is a service designed to run publicly, so this seems a reasonable trade-off for convenience. I used the tests feature of Tailscale’s ACL config to make sure I’d got the rules right:

{
	"tests": [
		{
			// Servers can only access infrastructure
			"src":   "tag:server",
			"proto": "tcp",
			"allow": [
				"tag:infrastructure:8080",
			],
			"deny": [
				"100.84.16.43:8080",
				"me@example.com:8080",
				"tag:server:8080",
				"tag:app:8080",
				"tag:integration:8080",
			],
		},
	],
}

The whole situation still feels a bit messy. If I ever get around to switching to nftables I might loop back and manually craft some rules for routing traffic, instead of leaving Tailscale and Docker to do their own thing.

I discovered later on that logging into the container registry at forgejo:3000 actually issued a redirect to https://git.example-net.ts.net/ so this was all basically for naught… ↩︎
I didn’t use Tailscale’s MagicDNS for a long time just because the word “magic” put me off. Only when I eventually got around to learning how it worked, and seeing that it wasn’t really that magical under the hood, did I change my mind. ↩︎
nftables seems far better in lots of ways, but I can’t really be bothered migrating. Maybe next time I reimage the server for whatever reason… ↩︎
It’s always DNS… ↩︎
Again, nftables would almost certainly help here. It actually has (gasp) configuration files. But again, I really didn’t want to spend the time migrating. ↩︎

Blogging and the Imaginary Quality Bar

2025-09-18T00:00:00Z

Recently I realised that I’ve developed a self-imposed quality bar for blog posts. They need to be a certain length, and have a certain substance to them. They need to be generally useful in some way I can’t quite define, to some imagined future audience. They need to have images to break up the page, and opengraph data for when they’re linked to on social media. But… maybe they don’t? Those things all make sense for longer “article” type posts, but not so much for a personal blog.

I’ve done some fiddling so that I can make posts without all that extra stuff. We’ll see how it goes. The posts won’t be distinguished on the site (yet?), but I have set up separate RSS feeds for just short form posts and long form posts in case subscribers are radically opposed to one or the other. Now I can start blogging like it’s 2005 again. But hopefully with a bit less cringe.

I’ve been staring at this post in my editor for about five minutes. The urge to make it longer, more thorough, more article-like is really strong. This entire paragraph is only here as a compromise with myself so I can actually save and commit the post.

The Curse of Knowledge and Blogging

2025-05-16T00:00:00Z

Finding a picture to go with a blog post about blog post ideas is hard, OK?

The single worst part about blogging for me is trying to come up with ideas for what to write about. Not because they’re hard to come up with, but because every idea seems too basic or not worth talking about. Seems.

It’s not just me, either. My friends will occasionally describe some interesting problem they’ve dealt with, and I’ll chime in with “you should blog about that”. Almost invariably the response is something like “it’s not that interesting”.

There’s a cognitive bias called the curse of knowledge¹. Basically, once you know something you can’t accurately reconstruct your previous state of mind from when you didn’t know it. You can struggle to do something for hours, piecing together scraps of information, using all kinds of specialist knowledge, and as soon as you’re finished it all seems so basic why would anyone want to read about that?

Other mental obstacles

Of course, nothing involving people is as simple as having one cause and an obvious solution. Even when I get over the “it’s too basic” road bump, there are other issues. Looking back over some of my posts, I get a horrible cringe-y feeling and the question of “why would anyone be interested in reading that?!” arrives unbidden. Even though I know that I’m interested in reading similar posts written by other people. I’m hesitant to attach labels to everything, but imposter syndrome says hi.

I think there’s also a cultural issue at play. British people tend to be somewhat reserved and downplay their accomplishments, especially when compared to people from the US². When I worked for a certain multinational ~~ad conglomerate~~ search engine company we actually had training on how to do performance reviews and speak about ourselves more like our US counterparts.

Put all these things into a cauldron, add a lizard’s leg and howlet’s wing, mix it all up, and you get a brain that despises the idea of blogging about anything. Great.

Dealing

So, now the problem is well-defined, what do we do about it? Clearly I am the best person to be offering this advice, given it’s been four months since my last blog post.

The best counter to the Curse of Knowledge that I’ve found is keeping notes, like a lab notebook. If you finish something and have dozens of links to different resources that you had to piece together, it’s probably something interesting and worth talking about even if it doesn’t seem like it any more. Or maybe you didn’t need a lot of references, but you had to write a lot down to make sense of something, or needed to draw diagrams to solve a problem. Again, probably interesting.

The other issues I’ve mentioned are not so easy to overcome. I’m also not going to try and perform therapy in the form of a blog post! What I personally do is keep a list of topics to write about (that have already passed the “probably interesting even if my brain doesn’t agree” check), and occasionally when the stars align I’ll feel like plucking one off the list and turning thoughts into words.

A call to action

If you’ve got a blog and haven’t written anything recently because you “don’t do anything interesting”, go write something! I promise that someone will be interested. Hell, feel free to send me a link and I’ll be interested for you.

If you don’t have a blog then why not start one? There are lots of great articles out there about why-you-should and how-you-can. And as a bonus, blog posts about how you set up your blog are super interesting. That’s one post sorted already!

As the open internet is slowly subsumed by massive corporate interests, and gummed up with spam and AI slop, the voice of real humans is becoming more and more important. Maybe we can — one blog at a time — get back to the ’90s internet that was full of individuality instead of today’s which is full of ads and spam?

Photo by Mika Baumeister on Unsplash

There’s also something called “Familiarity Blindness” that people like to talk about. It’s a much better name. The only problem is that I can’t find any actual published works that talk about it. No-one authoritative defines or mentions it. It just popped into existence a few years ago. Maybe it’s an LLM hallucination? ↩︎
My favourite commentary on this is Stephen Fry talking about American vs British comedy. ↩︎

Building a travel toolkit

2025-01-02T00:00:00Z

The container for my toolkit, a UGREEN “travel accessories” bag

For a while now, whenever I’ve traveled anywhere I’ve dumped a selection of tools into my backpack just in case I need them. Originally this was mostly focused around being able to open and fix my laptop if anything went wrong, but over time has slowly expanded to include other useful tools.

Repeatedly packing these tools and then returning them to where they belonged was tedious, though. I couldn’t just leave them in the backpack as I often needed them for non-travelling purposes, and I didn’t want to just buy duplicate tools that spent most of their life a few meters away from each other, so I ignored the problem and just dealt with the tedium.

Eventually I acquired enough tools that — while not quite duplicates — I wouldn’t miss often if they were kept in the backpack. Building on these I’ve created a little travel toolkit that contains everything I might want (and more) while away.

The base

The biggest challenge I had before was finding a screwdriver set. I have an iFixit Mako set that did the in-and-out-of-the-bag routine for several years. Nothing else covered the range of drivers I wanted for generic electronic tinkering and more chunky work. I finally found one in the form of a Wera Tool-Check PLUS TR-Edition. “TR” stands for “Tool Rebel” and is Wera’s “fan club” brand. Its signature features over the normal Tool-Check PLUS are a fancy fabric case and a 152mm bit extender; but it also comes with a much different set of bits, which is why I picked it up.

Here’s a comparison

Type	Standard version	Tool Rebel edition
Phillips	2x PH1 3x PH2 1x PH3	1x PH00 1x PH0 1x PH1 1x PH2 1x PH3
Pozidrive	None	1x PZ1 3x PZ2
Slotted	1x 5.5mm	1x 4mm 1x 4.5mm 2x 5.5mm 1x 6.5mm
Torx	1x T10 1x T15 1x T20 1x T25 1x T30 1x T40	1x T10 1x T15 1x T20 1x T25 1x T30 1x T40
Security Torx	1x T10 1x T15 1x T20 1x T25 1x T30	1x T25
Hex	1x H3 1x H4 1x H5 1x H6 1x H8	1x H1.5 1x H2 1x H2.5 1x H3 1x H4 1x H6 1x H8

There are only five bits present in the standard version that are missing in the TR edition: 5mm hex (pretty uncommon) and four sizes of security torx (rare). In exchange for those and the removal of a bunch of duplicates, you get two extra phillips sizes (0 and 00, both very useful for electronics and spectacles), some pozidrive bits (extremely common on random hardware/furniture/etc), smaller hex bits (for electronics, 3D printers, and so on), and a few other bits and pieces.

Overall it’s a vastly superior selection for my needs. And while it doesn’t have the full range of bits provided by the iFixit kit, it does cover almost everything I’m ever actually likely to need while travelling, and it’s probably around a third of the size.

The other core part of the toolkit is the bag. I had a UGREEN “travel accessory” bag hanging around unused, and noticed it was almost the exact right size for the Tool-Check. Now, how many other tools could I stuff in there?

All the tools

The array of tools that make up the travel toolkit

The kit can be broken down into a few sections:

Screwdriving

The aforementioned Wera Tool-Check
Its 152mm bit extender

I’ve talked enough about this already, I think, so I’ll spare you any additional commentary on screwdrivers and bits.

Splitting, gripping and cutting

Engineer tweezers
Engineer mini snips
Engineer mini screw removal pliers
Knipex mini waterpump pliers
Two iFixit spudgers (one metal, one plastic)
iFixit jimmy

I have bigger or slightly different versions of these that stay in my normal toolkit. The tweezers and snips are useful in a variety of situations, and the spudgers are handy for getting inside laptops and other devices. The pliers are less useful, but they’re one of those things where when you do need them there aren’t any good alternatives.

Sewing

Two needles
Some black thread
Set of mini clips

You never know when you’re going to rip a seam in something. I find the clips a lot less fiddly than using pins for keeping material in place when I’m trying to sew.

Computering

Retractable ethernet cable
RJ45 coupler
USB-C to RJ45 adapter
Dual USB-A and USB-C thumb drive, with an Arch ISO on it

Sometimes Wi-Fi just doesn’t cut it, or you need to fiddle with a router that doesn’t expose its admin UI over Wi-Fi. The USB-C adapter is for use with my phone, as it’s often easier to move that get the laptop where it needs to be.

I can boot from the thumb drive into an Arch environment, and from there reinstall my laptop from scratch if needed. There’s enough space left over so it can be used to transfer files as well.

Miscellaneousing

Fabric tape measure
Spirit level
Sharpie
Superglue
Spare batteries: 2 CR2032s, 2 AA, 2 AAA

I’m almost certain I won’t use the spirit level, but it’s tiny and I had a spare so it went in. Everything else is just in the “generally handy to have” category.

Putting it all together

The nicely-organised side of the toolkit

The Tool-Check PLUS fits almost perfectly into one side of the UGREEN bag, leaving a little wiggle room along the side for the longer parts like spudgers. The middle pockets can’t fit much in, but keep the batteries and sewing bits out of the way. Everything else is in a big zip-up compartment in the other side, which is slightly messy but still easy enough to dig through to find whatever I need.

The complete kit weighs in at around 1.2KG, so it’s not exactly lightweight, but it goes in a bag with a heavy laptop and all of its peripherals so it’s not as noticeable as it might otherwise be.

It remains to be seen how much use I’ll actually get out of any of this. Maybe I’ll put a bit of paper in so that I can track how often I use each tool…

Docker reverse proxying, redux

2022-12-06T00:00:00Z

Six years ago, I described my system for configuring a reverse proxy for docker containers. It involved six containers including a key-value store and a webserver. Nothing in that system has persisted to this day. Don’t get me wrong – it worked – but there were a lot of rough edges and areas for improvement.

Microservices and their limitations

My goal was to follow the UNIX philosophy of “do one thing and do it well”. Unfortunately, that doesn’t really work when applied to network services that have to interact with one and other. UNIX tools are built upon a common file system and simple data passed over STDIN. Microservices don’t have that shared foundation. You could make one: companies that use microservices in anger often have a team that deals with the “developer experience” of creating and using microservices. But as a solo developer that’s not something I wanted to spend my time doing.

This became very apparent when trying to debug issues. In the UNIX world, if your series of commands piped together does something strange you can simply echo the data at various stages. Not so much when that data is flying around on a network, going into and out of things like etcd. Trying to figure out why a certificate hadn’t been acquired was a case of searching through logs from four containers, none of which had particularly good logging. There are many ways to get insight into what’s happening with microservices but, again, that’s not something I wanted to do myself.

Over time, and with experience in other projects, I came to realise that microservices only really make sense if you’re unable to deploy a monolith. For tech companies this naturally happens when different teams contribute to the same service: splitting it out into smaller services that are wholly owned by individual teams makes sense. For solo developers, that never happens. You can still gain the other benefits of microservices – such as code separation and having clearly defined APIs – by sticking to certain coding standards.

Proxy inconveniences

As well as being unhappy with the microservice nature of the solution, I wasn’t pleased with nginx. If you requested an unknown domain, nginx would use the first server block in its config to serve a response, instead of sending an “unrecognised name” alert as I wanted. It was a minor issue, but it irked me.

So from nginx I switched to haproxy. It has a strict-sni option when configuring TLS connections which makes it behave properly. It also performs a lot better for this type of workload than nginx. All was well for a while, but then I started getting alerts that requests were occasionally failing. I couldn’t reproduce the issue, but my nightly jobs to build and push containers managed to hit it nearly every night, causing them to fail.

After some investigation, I found that the haproxy developers had refactored the header parsing code, and neglected to properly reset flags when multiple requests were sent over the same connection. There was a patch, but it wasn’t released. No problem, I thought, I’ll just cherry-pick it onto the last release… Except that haproxy use Git in the most convoluted manner I’ve ever seen – they have one repository per release. This makes it harder to patch, but it also made me question whether I trusted them to ship stable software: there were no tests for the header parsing code (which is both fundamental and finicky, the perfect target for tests), the source code management was weird, and they didn’t seem in any rush to patch this bug.

Not long after that issue, Greg managed to encounter another bug where haproxy returned a 500 error whenever the upstream server replied with a particular, perfectly valid, header. The die was cast – it was time to move to something else.

Not Invented Here syndrome

Looking for a new solution, there were many more options than back in 2016. I’m still convinced, however, that anything exposed to the Internet should not have access to run docker containers. It’s the modern equivalent of running a CGI script as root. That single requirement eliminates most off-the-shelf solutions. What do you do when nothing quite meets your specific requirements? You make something yourself! My new solution has two components: Dotege and Centauri.

Dotege is a replacement for the microservices that monitored containers and obtained certificates. It’s fundamentally a templating engine - whenever the containers change, it evaluates a template and saves the result to disk. The template has access to details about the containers, their labels, ports, and so on. Dotege can also obtain certificates from Let’s Encrypt, and raise a signal against another process whenever the template or certificates change. I used this to generate the configuration and certificates used by haproxy for a while, and more recently changed the template so that it works for Centauri.

Centauri is my own reverse proxy. It’s configured using a simple text file and can also obtain certificates from an ACME provider. It doesn’t serve static content, has no knowledge about docker, and avoids the other bells and whistles that adorn most reverse proxies. It also has good test coverage to ensure that I don’t, say, accidentally break header parsing.

As a software engineer I enjoy writing software, but I also enjoy running simple, easy to understand software. That’s what I’ve achieved here: it’s very easy to identify where the problem is if anything goes wrong, both are small Go programs rather than vast sprawling C monstrosities, and their interaction is primarily through a file written to disk that can be inspected or edited as needed.