Chameth.com - posts like debugging-beyond-the-debugger, exposing-game-servers-over-tailscale, finding-an-awkward-bug-with-claude-code, why-you-should-be-using-https but not break-everything-fuzz-testing, simple-backups-restic-hetzner

Lego, ACME DNS-01, and negatively-cached surprises

2026-09-09T00:00:00Z

Recently a friend was deploying Centauri, my reverse proxy service that can obtain TLS certificates using ACME DNS-01 challenges. He asked a seemingly innocent question: “When Centauri is ‘waiting for record propagation’ is there an option to increase the time?” But that’s a question with a hidden, concerning XY problem. We both use the same DNS provider, and it definitely shouldn’t take more than a minute to see a DNS record after it’s been created. Plus, I don’t have issues with it timing out!

Centauri uses the excellent lego library to do the actual ACME dance, and I’ve dug into it before pretty extensively to debug other DNS issues. When I last looked, it used the local recursive DNS resolver to check for CNAMEs and find the authoritative server, then checked for record propagation there. So, naturally, I blamed Simon’s local resolver doing something weird. Turns out that was surprisingly close to correct, but not for the reasons I thought.

Unfortunately for my snap judgements, Simon had a perfect rejoinder: he’d deployed the exact same VM image before and Centauri worked fine. And also while we’d been speaking it had managed to get a certificate after waiting 58 seconds for DNS propagation. Simon suggested that maybe it was systemd-resolved caching something stale, but I dismissed that based on what I knew of how Lego worked.

Diving back into Lego

Of course, after I dismissed the idea, I immediately started doubting myself. Last time I was looking at DNS issues I traced the code all the way across the Lego codebase. This time I just had a coding agent explore and summarise for me. Turns out it DOES use the recursive resolver to check for the TXT challenge record.

The DNS functionality looks like this:

CNAME query on the _acme-challenge record using the recursive resolver.
Maybe an SOA query on the same, depending on the DNS provider, using the recursive resolver.
Actually create the challenge record using the DNS provider.
Query records recursively to discover CNAMEs using the recursive resolver. It doesn’t matter if this fails.
Query the TXT record on every configured recursive resolver, blocking until they all work.
Query the TXT record on the authoritative nameserver.

So yes, a stale response from the recursive resolver can indeed ruin the whole thing. Simon’s domain had a minimum TTL of 60 seconds, so that links in nicely with it working at 58 seconds and failing most of the time: the 1 minute timeout is racing the 1 minute TTL, somehow. But my domains use a much longer TTL and I can see renewals happening in my logs basically immediately. What gives?

NXDOMAIN caching and RFC fun

So I had a theory as to what was happening, but it still had some loose ends that I didn’t understand. I did get a pretty big hint when I started doing lookups from various locations, though. On my desktop if I do a CNAME query then a TXT query for a record that doesn’t exist, I get this:

$ drill CNAME nonexistent.chameth.com
;; ->>HEADER<<- opcode: QUERY, rcode: NXDOMAIN, id: 12630
chameth.com.	3600	IN	SOA	chameth.com. root.chameth.com. 2026339162 7200 3600 86400 3600

$ drill TXT nonexistent.chameth.com
;; ->>HEADER<<- opcode: QUERY, rcode: NXDOMAIN, id: 56402
chameth.com.	3590	IN	SOA	chameth.com. root.chameth.com. 2026339162 7200 3600 86400 3600

And from my server:

$ drill CNAME nonexistent.chameth.com
;; ->>HEADER<<- opcode: QUERY, rcode: NXDOMAIN, id: 7807
chameth.com.	3600	IN	SOA	chameth.com. root.chameth.com. 2026339162 7200 3600 86400 3600

$ drill TXT nonexistent.chameth.com 
;; ->>HEADER<<- opcode: QUERY, rcode: NXDOMAIN, id: 5069
chameth.com.	3600	IN	SOA	chameth.com. root.chameth.com. 2026339162 7200 3600 86400 3600

Notice how the TTL (the first number) decreases locally, but not on the server? That implies my local resolver is reusing the same result when I do the second query, and the resolver on the server isn’t. It turns out that on my desktop I use systemd-resolved, which sends requests to Tailscale, which passes them off to NextDNS. On the server they go straight to Tailscale and then onwards to NextDNS. So systemd-resolved is what’s causing the behaviour. Like Simon guessed right at the start.

But why is it actually doing that? It’s kind of annoying. Well, it’s doing what it’s meant to. RFC2308 says:

A negative answer that resulted from a name error (NXDOMAIN) should be cached such that it can be retrieved and returned in response to another query for the same <QNAME, QCLASS> that resulted in the cached negative response.

A negative answer that resulted from a no data error (NODATA) should be cached such that it can be retrieved and returned in response to another query for the same <QNAME, QTYPE, QCLASS> that resulted in the cached negative response.

Because the record doesn’t exist at all (there’s no other records for _acme-challenge: why would there be?) we get an NXDOMAIN response, and that “should” be cached for any query type. So when Lego queries for the CNAME record, we’re priming the cache with “_acme-challenge doesn’t exist” with a potentially huge TTL on that cache entry. When we then try and query the TXT record we created we get the cached NXDOMAIN back.

If the TXT record was created first then the CNAME query would get a NODATA response instead, which wouldn’t affect us as that is understandably cached per query type. But the point in Lego’s CNAME check is to decide where to put the TXT record, so it can’t just flip the ordering.

I apparently dodged this problem entirely on my servers, because nothing in that resolution path implements this particular caching behaviour.

But why now?

But Simon had this working before. It’s possible that his existing deployments are having this issue now, and are just retrying enough to win the race, but he would’ve noticed if they failed this much when he set them up.

So I did some archaeology. Here’s the timeline:

February 2016: CNAME check added to deal with, well, CNAMEs.
February 2016: Immediate follow-up making a failed CNAME check non-blocking.
September 2024: Option added to check propagation using custom recursive resolvers.
February 2026: That option now defaults to on.

And the mystery is solved: Lego v5 enabled the recursive check by default. That also handily explains why I thought its behaviour was different. It used to be! There were still two things I didn’t understand: why was it enabled, and why didn’t I realise when I upgraded to Lego v5? Looking back, I think this line of the changelog is meant to cover the behaviour:

[lib,dnsprovider] New approach of the DNS calls

The migration guide also mentions the flag being renamed, but not the fact that the default setting also flipped. Feels like burying the lede a little, but hey ho.

As for why, there doesn’t seem to be any documented rationale. Given the recursive resolver check was added to fix a fairly niche problem, it seems like an odd decision to me. The new defaults mean that any recursive resolver that caches NXDOMAIN responses across query types will delay the propagation check, and likely fail it entirely if the TTL is longer than the timeout. That’s… unfortunate to say the least. I opened a discussion suggesting the default gets flipped back but it doesn’t sound like that will happen before v6.

For Centauri, at least, I’ve fixed it by unconditionally disabling the recursive resolver check, taking it back to the Lego v4 behaviour. And after all this, Simon’s original hunch turned out to be spot on: it was systemd-resolved caching, but it was doing the correct thing. Pesky old-man Lego was the one responsible for dropping a rake on the ground then immediately stepping on it.

The Case of the Unchanging Config

2026-04-09T00:00:00Z

Last week I was attempting to make it so I could share pictures on IRC directly from my client. This sounds simple, but it involves a bouncer¹ that proxies the request to a standalone image hosting service that I had to modify to be compatible. At one point my testing loop was:

Make a change to the hosting service
Commit it
Tag a new release
Wait for it to build
Update the version I’m running on my server
Reconfigure the bouncer to pass the new parameter or change the URL or whatever
Try uploading a photo from my phone
Realise I’ve overlooked something and go back to step one

I could have set up a local copy of everything and tested it sensibly. I should have set up a local copy of everything and tested it sensibly. But it seemed like such a trivial change, and setting up the whole environment seemed like such a pain. After the third or so iteration of failure I was pretty annoyed with myself, computers, and basically everything.

My biggest annoyance was that my bouncer would not pick up the new URL from the config when I changed it. It’s meant to reload the config when it receives a SIGHUP, and it claimed to in the logs, but I could clearly see it was still hitting the old URL. Restarting the bouncer to update the config is a pain, as it disconnects me from all the IRC networks, and has to reconnect to them all, reauthenticate, etc. It also mildly spams everyone who shares a channel with me².

When I finally got everything working I had a look at the bouncer source, and thought I’d spotted the issue. I raised a bug report, ending in this remark:

It looks like the config is reloaded properly, but the handler for uploads is created once at startup and has its own copy of the uploader, so effectively snapshots the config to whatever it is at startup:

fileUploadHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    cfg := srv.Config()
    h := fileupload.Handler{
        Uploader:    cfg.FileUploader,
        DB:          db,
        Auth:        cfg.Auth,
        HTTPOrigins: cfg.HTTPOrigins,
    }
    h.ServeHTTP(w, r)
})

I’m not quite sure what I thought I saw there, but that’s not right. I want to call it a hallucination, but that feels like a very overloaded word these days. I guess I was just seeing what I wanted to see, and that was a good excuse to stop investigating. Still, even if I misidentified the cause, the bug was still present, right? … Right?

The Science of Deduction

When the developer got back and said he couldn’t reproduce my issue, I went back to my install and immediately reproduced it. With a calmer head, I figured it was probably something with my particular set up. First thing to check: am I actually running the version I think I am?

Unfortunately the bouncer doesn’t actually seem to expose the version anywhere that I can see. It’s not in the logs, it doesn’t have a -version flag, and none of the IRC-based status commands seem to include it. But I know it’s a Go app, and I know Go embeds the version information. go version -m <binary> will dump it all out, but the binary is inside a Docker image, and the Docker image is one of my nice, minimal, artisanal ones so doesn’t ship a go binary. No problem, docker compose cp bouncer:/bnc ./bnc yoinks the binary out of the container, and then dumping the version shows that, yes, I am running the version I thought I was. Hmm.

The next tool I reached for in my toolbox was strace. Maybe it’s not actually reading the file for some reason? I immediately executed strace -p <pid> -e trace=openat,open,read,pread64 -f without having to look any part of that up. Yep. Definitely. Then I edited the config, HUP’d the bouncer, and saw that it was… reading the config file. As it’s meant to. By default strace truncates strings to 32 bytes, so I couldn’t actually see the line I’d changed. Some more definitely-not-RTFMing later, and rerunning it with an extra -s 65536 let me see the full config. Surprise! The config hadn’t changed!

To confirm my findings, I used docker compose cp again, this time yoinking the config file from inside the container. The inside config file was definitely different to the outside config file. What? My hypothesis at this point was “something something Docker nonsense”. I mount the config as read-only, and was wondering if that meant that Docker was doing something weird instead of just bind mounting it. A quick trip to /proc/<pid>/mounts showed that it was, in fact, not doing anything weird, and was just bind mounting it.

If the file is bind mounted, then surely it’s the same file? I ran stat on the file on the host, noted the inode number, then pondered how to actually run stat on the file inside the container, given the aforementioned awkwardly minimal image. The solution was easy: access it via /proc/<pid>/root/. I could’ve saved myself a bunch of docker compose cp if I’d thought about that earlier. Oh well. The inode of that file was different. What?

The answer was DNS. It’s always DNS. Oh, sorry, force of habit. I meant the answer was vim. I was editing the config in vim, and when it saves files, by default, it writes the new content to a temporary file and does an atomic rename. That’s normally a good thing: it prevents corruption if the write fails midway through³. In this case, though, that means the new file has a new inode. Bind mounting a file binds to the inode, so the container just keeps clutching onto the original config from when it was started, blissfully unaware that the party has relocated down the street.

This problem is likely to happen whenever you bind mount a file into a container. When I mentioned this to a friend, he immediately responded “oh yeah, never do that”, and went on to describe the horrible hacks he’s had to add to Ansible to sidestep the issue. The nicer solution is to just bind mount an entire directory if you can, as then it doesn’t matter what happens to the files within it. I really like having the config files sat alongside the Docker compose files, though; having to create a directory just to work around some bind mount weirdness upsets me.

Now I knew what the problem was, I found there was an issue raised against Docker twelve years ago⁴. The first response was “that’s expected”, and I fully agree with the author: “respectfully, that might be expected by you, but it was not expected by me”!⁵ As a result of the issue they added a nice note to the docs:

Note: Many tools used to edit files including vi and sed --in-place may result in an inode change. Since Docker v1.1.0, this will produce an error such as “sed: cannot rename ./sedKdJ9Dy: Device or resource busy”. In the case where you want to edit the mounted file, it is often easiest to instead mount the parent directory.

But in the intervening twelve years, both the note and the functionality described have gone missing. Ho-hum.

I found it interesting how I’ve only just hit this problem, given how long I’ve used Docker. But I realised that almost everything I run I’m happy to just restart. Cattle, not pets, and so on. My IRC bouncer is one of the few exceptions to that. The only other thing I regularly hot reloaded was Centauri, my reverse proxy, but that had a whole config directory mounted because it was shared between containers, so nicely sidestepped the foot-gun.

So lesson learnt: check for weird bind mount issues before raising issues about config hot reloading. It’ll join the esteemed company of “maybe the drive is full and causing completely unrelated problems?”, “perhaps everything is dog slow because the kernel ran out of entropy?”, and “did systemd sneakily take over that functionality while you weren’t looking?” in the troubleshooting checklist.

An IRC bouncer is basically an always-on proxy. It connects to the IRC networks for you, then your clients connect to your bouncer. The bouncer can then send incoming messages to all your different clients, cache them when you’re offline and replay them later, and lots of other nice things people take for granted in their chat apps these days. ↩︎
Join/part/quit spam is part of IRC, and clients have ways of handling it, but I still feel bad about doing it excessively. ↩︎
You can disable this in vim by setting backupcopy=yes. Good luck redoing all the debugging if you ever accidentally remove that from your vimrc, though! ↩︎
Which is weird, because Docker can’t possibly be that old. That would make me much older than I’m prepared to accept. ↩︎
If you read the thread they didn’t actually mean to sound so dismissive, but it’s still pretty funny. “Yes, it’s expected that the foot-gun causes your foot to hurt. Duh. What did you expect?” ↩︎

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. ↩︎

Exposing game servers over Tailscale

2025-12-07T00:00:00Z

I’ve recently been playing a lot of Factorio with a friend. I’ve been hosting, but my desktop computer is behind far too many layers of NAT, and I can’t be bothered dealing with setting up port forwards. Up until today we made do with Steam’s networking support, which in our case ended up relaying the connection via one of their servers. This is amazing as a free, no-hassle service, but the performance was so-so. We’d get random lag spikes, or the initial map download would crawl along at about 1/100th of the speed we should be able to get. We’re both fans of Tailscale though, so maybe there’s an easy solution there?

Tailscale even have an article on sharing a Factorio server but it’s a ten minute long video¹, and focused on setting up an actual dedicated server in Docker rather than just clicking the convenient “Host” button in the game client. What we did instead was:

I shared my desktop’s Tailscale node with my friend, by creating a sharing link in the Admin console
My friend accepted the link, and could then see my machine in their admin panel
I used tailscale lock to sign my friend’s desktop’s node key, as I have tailnet lock enabled
I added a grant to my tailnet’s ACL to permit access from my friend to the specific IP/port on my tailnet:

		{
			"src": ["friend@example.com"],
			"dst": ["100.0.0.42", "fd7a::42"],
			"ip":  ["udp:34197"],
		}

After this, my friend could connect to Factorio by entering the Tailscale IP address, or the Tailscale hostname (as they use MagicDNS). Running tailscale status shows that Tailscale managed to establish a direct connection despite the many layers of NAT involved, and the performance improvements were significant and immediate.

We did a bit of testing, and confirmed that no other services on my machine are accessible. And sharing a machine by default only allows it to accept incoming connections, so I can’t get unwanted access to anything on my friend’s Tailnet, either.

I’ve written before about all the different ways I use Tailscale; this is yet another new way it’s solved a problem for me.

I’m not a fan of video tutorials at the best of times, but especially not when I want to do something as quickly as possible so I can get back to ~~my addiction~~ growing the factory. ↩︎

Avoiding the Consequences of Dumb Laws with Tailscale

2025-09-30T00:00:00Z

More and more sites are implementing privacy-invading age checks or just completely blocking the UK thanks to the Online Safety Act.

Protecting kids from some content online is certainly a noble goal, but the asinine guidance from Ofcom, threats of absolutely disproportionate fines, and the stupidly broad categories of content have resulted in companies just giving up or going through a tick-box exercise that offers very little protection but lots of inconvenience and a complete invasion of privacy.

Instead of uploading my ID to some third party company, I’ve taken to proxying my traffic through to a country that doesn’t have such stupid laws. Thankfully, Tailscale makes this really easy. I’ve discussed how I use Tailscale before, but not really covered app connectors. I find Tailscale’s description of these pretty confusing, but they basically amount to automatic, DNS-based subnet routing configurations (or, to put it another way, a per-website exit node). You can safely ignore all references to ‘SaaS apps’ in their docs.

I create a custom app connector, and give it the domains to be included:

App configuration in the Tailscale admin console

Tailscale then magically resolves those domains, and has the ‘connector’ advertise routes for them. Any client that accepts routes will start sending requests to the connector, which passes them onto the Internet at large. Any other traffic is left alone, unlike when you use an exit node.

The special bit here is how you can specify wildcard domains. Tailscale proxies the DNS requests from clients (so it can inject responses for nodes on your tailnet), which means it can dynamically update the routes as you resolve new domains. I tried to set this up more manually, and quickly came unstuck: despite using the same DNS servers, my server and my desktop would get different responses for the same query as it varied by geography. Trying to get the full set of IPs (and keeping them updated) would have been a nightmare. Tailscale expanding the wildcards nicely sidesteps all of that.

At first I was just proxying the traffic to one of my servers, but just today I added a new connector for Imgur and found I was still blocked, just for different reasons. They not only block my entire country but also a load of known datacenter IP ranges. Hmph. I fixed this by hacking up a new side project: tsv. It’s a simple Go app that accepts traffic from the tailnet (advertising itself as both an app connector and an exit node), and passes it on to another VPN.

There are lots of other ways you could accomplish this, but this makes it so all my devices can still access services without any additional configuration. As long as Tailscale is installed, the Internet will still work as it’s meant to, without all the nonsense. If I come across a site that doesn’t work, adding it is trivial: I just make a new app connector in Tailscale.

Obvious disclaimer: the laws in the UK are binding on the service providers, not the end user. Doing this sort of thing in other countries might be illegal. I don’t know; do your own research! Also all of this is a workaround for something that should be fixed at a legislative level, but I’m not holding my breath.