Chameth.com - posts like debugging-beyond-the-debugger, just-a-nod but not adventures-in-3d-printing

Monthly Meanderings: February 2026

2026-03-01T00:00:00Z

It doesn’t feel like a whole month has gone past since I wrote the last instalment of Monthly Meanderings, even allowing for how short a month February is. For more context on this series, you can check out the introduction to the first edition.

Website updates

I only wrote one new blog post this month: Just a nod, which is about the “nod” button I added to the bottom of most pages. I did a lot of other little bits and pieces, though:

My blog posts are now automatically posted to a dedicated Bluesky account, and posts now include syndication links at the bottom.
The site now tracks how long each request takes to generate along with how many SQL queries were used. You can see the details for the current request by hovering/tapping over the pi symbol at the bottom right of a page. To understand why it’s a pi symbol, you should watch The Net!
Speaking of films, I redid the star rating rendering so instead of unicode stars it uses some hand-drawn pixel art stickers. They’re somewhat randomised, so they don’t look repetitive. This is what it now looks like: .
I spent some time tweaking various styles and layouts so everything works nicely on smaller screens. I try to make sure these things work when I make them, but it’s easy to miss a particular interaction, or make an innocent-seeming change that breaks tiny screens.
I added a new page that shows stats about my walks, based on my Apple Health data. I had a private view of this for a while, but decided there’s no reason it couldn’t be more public.
The films page has gained a search box, to directly find my review of a particular film. I used CSS anchor positioning for the results panel. Modern CSS is fun!
I fixed the 3D prints page being utterly broken in numerous ways. It showed up on my shiny new “how many SQL queries does each page do?” dashboard, as it was doing multiple queries for each individual print instead of just getting all the data in one go. Then I noticed a badly written redirect rule had broken all the images and download links. Whoops!
I did a lot of under-the-hood cleaning, most of which won’t be visible. Besides just making me happy, this means the contact form and nod buttons now work with Javascript disabled.
Finally, I added a page that shows my board game plays. The data comes from exports from bgstats. I have a bunch of ideas for other things to do with that data, but haven’t got around to it yet.

Phew. After writing all that out it feels like a really long month, not a short one!

Other projects

Not much going on with my other projects. The only big thing was goplum, my self-hosted monitoring/alerting tool, getting a 1.0 release. It adds a reminder feature so you can get alerted repeatedly if a service is down. It also reworks how plugins work so they’re compile-time instead of trying to be clever and dynamic, as that never really gave any benefits.

I’m still continuing my migration of projects to a private Forgejo instance. All of my personal repos are now sourced there, and mirrored out to GitHub, except one: my Dockerfiles. I’m not sure how I want to approach that yet, as the action that does the updating is pretty involved. I also have some repositories split off into their own GitHub organisations which I haven’t got around to mirroring yet.

Entertainment

Unlike the past few months, I didn’t touch many video games in February. The ones I’d been dabbling with didn’t really hook me enough to make me keep going back. I made up for it with lots of other media forms though. First up, a whole bunch of films:

I won’t write too much about them here — you can click through to read my reviews if you want — other than to call out the three excellent horror films I hadn’t seen before: Black Swan, The Cabin in the Woods, and Get Out. Not so long ago I would’ve said that I didn’t enjoy horror films, now they’re some my favourites.

I don’t have any TV show tracking on my website (yet?), as I haven’t really watched much recently. This month I started watching The Pitt though, and it rapidly become one of my favourite shows. By “started watching” I mean “watched every aired episode of both season 1 and season 2”. I highly recommend it. The only problem is that it’s hard to find anything to watch afterwards, as nothing else can quite measure up.

I’ve also been binging a book series: Beneath the Dragoneye Moons. It’s a LitRPG series: basically a fantasy setting that borrows systems from an RPG, like classes and levelling. Something about the genre really works for me, and Beneath the Dragoneye Moons is one of the more interesting series I’ve come across in the genre. I’ve been going through the series at the rate of about a book a day.

Finally, board games! Nothing particularly notable, but now I can embed a pretty display of what I actually played:

6 plays
4 plays
4 plays
4 plays
2 plays
2 plays
2 plays
2 plays
2 plays
2 plays
1 play
1 play
1 play
1 play
1 play
1 play
1 play
1 play
1 play
1 play
1 play

All of these were online via Board Game Arena. I haven’t really had time for in-person games with all the TV, films and reading!

Around the web

Start all of your commands with a comma

An old post that was doing the rounds again on Hacker News. It’s an interesting idea. I recently aliased cc to launch Claude Code with some custom parameters, and it horribly backfired when I tried to compile a C program and remembered that cc was part of GCC. I haven’t transitioned all my personal commands to use a prefix yet, but I might.

Tactical tornado is the new default

An interesting post about software developers who act as “tactical tornadoes”, and how that’s basically the role that all LLMs fill. Whenever I use coding agents, I often find myself following behind them in my IDE cleaning up bits and pieces. It’s often faster than prompting them to do it. I definitely get the “cleaning up after a tornado” metaphor.

Amusingly, the agents will sometimes notice that you’re tidying up after them and ascribe it to an automated process, outputting something like “the automated linter hasn’t run yet” if you fall behind. There’s probably a deeper philosophical discussion about what it means when the agent thinks that the human is the automated part of the system…

My smart sleep mask broadcasts users’ brainwaves to an open MQTT broker

Another one for the “the ’s’ in ‘IoT’ stands for security” books. I find it hard to actually comprehend how these things get to market. Does nobody actually think about it, or do they know full well how bad it is and just launch it anyway?

The IndieWeb and Small web

Ask three people to define ‘IndieWeb’ and ‘Small web’ and you’ll get about 20 different answers. This post by Christiano almost exactly nails my own views on the terms, and explains them well. I think most personal sites (my own included) straddle both terms to some extent, but having the labels is useful in discussions.

Just a nod

2026-02-11T00:00:00Z

Last month I added a new feature to the bottom of almost all pages on this site: a “nod” button. It’s a bit like the upvote buttons you see on bear blog posts, or the ubiquitous like button you get on social media, with one key difference: it doesn’t track or count anything. Before I get into the whys and hows, this is what it currently looks like:

The nod button in all its glory. If you want a clickable version you’ll need to scroll a bit more.

So why bother? I like hearing from people who have looked at my website. I have a contact form that’s also embedded at the bottom of all my blog posts; it’s always a delight when people reach out. Actually writing a message is a pretty high bar that most visitors won’t clear, though. A like/upvote/whatever-you-want-to-call-it is a much less demanding interaction.

And why no counting? I’d been thinking about some kind of upvote button for a while, and counting was always the thing that put me off. Having a number attached to a post like that feels like I’d end up invoking Goodhart’s law and start aiming for a certain number of upvotes or likes or whatever. I don’t want to be anxiously watching numbers, or feel pressured to make a post “good enough” (I already self-filter too much…).

As for not tracking people, that feels like table stakes to me: I’m an individual concerned about privacy, not a giant mega-corporation who wants to slurp up all the data to serve ads or engagement bait feeds. I don’t really need to know someone’s identity to receive a message from them. In the olden days of the internet, you just knew everyone by screen names which could change on a whim and didn’t reveal anything about the real you. On the internet, nobody knows you’re a dog, as Peter Steiner said.

All these constraints felt insurmountable for a while. I considered just making a button that sent a contact form submission with a pre-filled message, but those end up in my e-mail inbox. I don’t really want to have to process them, especially if there’s a big traffic spike like when Hacker News notices one of my posts. I could store the button presses in a database, but then how would I actually know they happened, especially if I didn’t want to watch a number go up?

An Amazon Dash button

So I did what I usually do with these types of problems: ignored it for a bit. The idea rattles around at the back of my head, and sometimes I’ll come up with an answer. I’ve seen this called diffuse thinking. Eventually the idea bounced to the forefront of my head while I was looking at my IRC client, and the solution suddenly clicked. If you’ve not heard of it, IRC is a chat protocol that’s almost as old as I am. Daniel Stenberg has compiled a nice history of IRC that I won’t repeat here. I use IRC to stay in touch with friends, hang around the channels for some open source projects, and I also run a private server with just me and some notification bots.

The notifications I have on IRC are all “transient”: they’re handy to see in the moment, but it doesn’t matter if I miss them. I have some high-volume news feeds, log messages from some self-hosted services, details about git commits, and so on. Basically just background noise of things that are happening that don’t need action. Perfect for the kind of interaction I want!

Now that I’d finally figured out that the transient-notification-shape goes in the transient-notification-hole, I just needed to actually implement it. As part of that private IRC setup I already have a bot that accepts messages over HTTP and sends them to a channel, so the backend was pretty simple. For the frontend I was imagining something like the ill-fated Amazon Dash buttons. My first attempt was… underwhelming:

Flat UIs are in right now, right?… Right?

I guess I shouldn’t give up the day job to become a graphic designer. Fortunately, I came across this codepen from Thierry Michel with some very fun skeuomorphic arcade buttons. I spent some time and LLM tokens on converting the SCSS to CSS, changing the colour, and making a few other tweaks. I really love how this looks and feels.

This is what it looks like in my IRC client when someone nods (and when containers get pushed to the GitHub Container Registry for my projects…):

Receiving some nods on IRC via a bot

When I added the button I wasn’t sure if I’d keep it, but I’ve very much enjoyed seeing the “nods” come in every now and then. I’ve been surprised by how many come in for the various snippets and other non-blog pages on the site. It’s a nice reminder that people are out there browsing the site, without all the baggage of monitoring analytics or tracking likes.

Monthly Meanderings: January 2026

2026-02-01T00:00:00Z

Welcome to the second edition of my monthly meanderings. For a bit of context, you can check out the introduction to the first edition.

Website updates

It’s been a pretty busy month for chameth.com. Three blog posts: The Meaning of Life — an entry into the IndieWeb carnival where I mostly review a book on Stoicism — Surge Protectors: Marketing vs Reality which is a dump of a research rabbit hole I fell into, and An Interesting Tailscale + Docker Gotcha which documents a fun problem I encountered.

I’ve also made a new films section, which is basically my Letterboxd data but free range instead of cooped up in a silo. I imported my 200-odd reviews, and for the last few I wrote the review on my site and then copied it over to Letterboxd. That’s what the IndieWeb folks call POSSE: publish on your own site, syndicate elsewhere. I like the idea that my website is the canonical source of truth for all the public things I generate. I want to expand this to some other areas in the future: music, video games, board games, etc.

I made most of the film components into “shortcodes”, so I can drop them into arbitrary pages or posts, like so:

Watched films ranked

Every film I’ve watched since I started logging, ranked.

Obviously super subjective, and subject to change often and arbitrarily.

264 films

Finally, I added a new widget to the bottom of most pages: a “nod” button. It’s effectively a “like” or “kudos” button but deliberately does not track how many times it’s been pressed, has no social network connotations, and won’t flood my inbox or hit rate limits if it gets spammed. If you click the button, it simply sends a message to a private IRC channel that says “Someone nodded at $URL”. I’ll probably write a bit more about it in a blog post in the future.

Other projects

I only have one small non-chameth.com update: a new project called pigeonhole which provides a really basic web service that lets you leave messages in a named cubby, and retrieve them at a later date. I’m using it to automate adding things to my Obsidian notes from my Apple Watch. There’s lots of reasons why that isn’t as simple as it sounds. I went through a bunch of iterations to try and make it work before just writing code to fix it.

I wanted to do this because I’m trying to replace Todoist with just having tasks in Obsidian. I already used Obsidian for longer form notes, but I’ve now also started using it for drafting blog posts (I’m typing this in Obsidian right now!), and trying to keep more useful day-to-day notes. I’ll probably write a blog post with more details once I’ve figured everything out.

Finally, I’ve been moving a bunch of projects to a private Forgejo instance, with the ultimate goal of having all of my projects live there. I’m going to mirror the contents out to GitHub and maybe other places, but Forgejo will be the single source of truth, and where all the workflows/dependency wrangling/etc happens. I’ve got Renovate configured to do dependency bumps, and I’m very much liking the whole setup so far. I’m saying it a lot but again: I’ll blog about it in more detail when it’s a bit further along.

Entertainment

I mentioned last month that I was playing Hogwarts: Legacy. I finished that off, and have been struggling to find something to scratch the same itch. I’ve tried a whole bunch of open world games, and basically none of them work for me. They have cumbersome control schemes (yay console ports), really awkward or sluggish movement, and so on. I think I’m just too fussy, but if I’m going to sink dozens of hours into a game it shouldn’t feel annoying or like a chore!

I did play through Thank Goodness You’re Here, which is delightfully funny but sadly only took 3.6 hours to finish with 100% of the achievements. Still highly recommend picking it up, especially when it’s on sale for under ‘10 bob’. I’ve also started playing through Control, and I’m enjoying the ambience. There’s an insanely sharp difficulty spike for the first boss, though, which nearly made me put it down.

In other entertainment, I’ve been playing a load of turn-based games on Board Game Arena , and I’ve watched a bunch of films after barely watching any last month. Here’s another of those shortcodes to show them off:

Around the web

Remotely unlocking an encrypted hard disk

A fun post documenting using Tailscale in a very weird way to solve a relatable problem. Very much my cup of tea.

I charged $18,000 for a static HTML page

This one’s actually from 2019, but it’s the first time I’ve come across it, and it’s a fun story. As a contractor, deciding what to bill for and what not to can be a bit messy. Downtime between meetings? The ten minutes in the shower you spent thinking through a bug? It’s a lot more clear cut if the client wants you to sit in an office, twiddling your thumbs, though.

Oxide RFD: Using LLMs at Oxide

A wonderfully well thought out piece about using LLMs in a respectful, sensible manner. I’d been thinking about writing something similar, but it wouldn’t have been anywhere near as well crafted as this. While I basically agree with it all, the part about LLM-generated text really resonates:

LLM-generated prose undermines a social contract of sorts: absent LLMs, it is presumed that of the reader and the writer, it is the writer that has undertaken the greater intellectual exertion.

Your app subscription is now my weekend project

Roberto points out that monthly SaaS subscriptions start looking even less appealing when you can just prompt an alternative into existence. I talked a bit about using LLMs for one-person software before, and it becomes even more appealing when the alternative is paying monthly for a service that keeps shoe-horning in features you don’t want, making you agree to share data with 329 “trusted partners”, and/or repeatedly ratcheting up the price.

Ekul’s Internet Space

I love the aesthetics of this personal site, and the data tracking that goes along with it. I’m also slightly aghast at the amount of Monster consumed! Makes me want to track more things and expose them on my own site.

Monthly Meanderings: December 2025

2026-01-01T00:00:00Z

For a while I’ve been idly thinking about a way to get smaller bits of content onto my website without it being too annoying for me, or too hard to consume. Things like interesting links, small project updates, and so on.

I didn’t immediately come up with anything I was happy with, so just sat on it. Ignoring the problem seems to work surprisingly well for things like this. I’d been meaning to redesign the about page for a while, but wasn’t sure what to do with all the interests I wanted to list. After ignoring the problem for a while I came across the interests directory and realised I could just bundle all those things into their own page.

This seems amazingly obvious in hindsight, but I think I needed a bit of a push to accept that a page just about my interests was… well… interesting. Anyway, as the end of December rolled around, I got a similar dose of inspiration: a flurry of monthly round-up posts from various blogs I follow. If I bundle up all the small bits and bobs, it’ll make something vaguely blog post shaped, and writing and consuming those is already a solved problem. Again, pretty obvious in hindsight, and again I think I just needed a nudge to shut up the little voice in my head saying “but those people all have interesting things to say… do you, really?”

So without further ado, welcome to the inaugural post in my new monthly meanderings series. If you have any feedback on the format or contents, please drop me a note with the contact form at the bottom!

Website updates

As already mentioned, I reworked the about page and split out a new interests page. The old about page had a few lists of interests show up in thought bubbles when you hovered over things, which was a cool effect on desktop, but barely worked at all on mobile. The new page gives me a lot more room for waffle, and I do like waffling.

Only a single new blog post this month: exposing game servers over Tailscale. Just a quick how-to I wrote up, having not found it documented usefully anywhere before. I have a bunch of ideas for posts I want to write, but none of them have really sparked joy quite yet, so I’m once again employing the “sit on them for a bit and maybe inspiration will strike” approach.

Other projects

I created a small collection of Docker related projects this month: containuum is a go library that helps with listing containers, and keeping that list up-to-date. On top of that I built centauri-docker-confd which generates a config for centauri (my reverse proxy) based on running Docker containers. I already had a way of doing that, but containuum is designed to be a bit more robust, and I made it so the config can be passed over a network connection instead of having to write it to disk and then signal the proxy to reload.

The next project has a bit of backstory: after reading I got hacked, my server started mining Monero this morning [warning: the article is interesting but a bit LLM-sloppy] I realised that I, too, was running a vulnerable version of Umami. Luckily, as far as I can tell, it wasn’t exploited. But it got me looking into automated vulnerability scanning. I don’t put much stock into it normally, as you get so many false alerts, but this is one case where it would have definitely helped. There didn’t seem to be any good solution for my “middle-of-the-road” approach to containers (lots of containers, but not needing Kubernetes), so I wrote purser. It takes every image used by a running container, runs them through Trivy, and generates a combined report. I’ll add some form of active notifications to it in the future.

Entertainment

This month I’ve been playing a lot of Hogwarts: Legacy, after picking it up for free on Epic Games. I grew up with the Harry Potter books coming out, and while I’m not a super-fan, it still has a special appeal to me. Obviously she-who-must-not-be-named is doing her best to out-evil Voldemort, but I think you can still enjoy the universe despite that. Especially when you don’t have to contribute any money that might go towards her.

Anyway, the game is surprisingly fun. They did a really good job of making it feel like an open world. You want to walk around from place to place to see everything, find all the various collectables, and so on. Compared to Far Cry or Cyberpunk the world is a lot smaller but that means it’s far more densely packed with stuff to do. I initially didn’t vibe with the way combat works: it’s pretty heavily reliant on dodging or shielding against incoming attacks, which reminded me a lot of Clair Obscur: Expedition 33. Fortunately, it’s a lot more forgiving in Hogwarts: Legacy and I got used to it after not too long.

It’s been a relatively sedate month for other forms of entertainment. I saw Fackham Hall and Wicked: For Good at the cinema. The former was a funny romp, vaguely reminiscent of Airplane or The Naked Gun, as you’d expect from something penned by Jimmy Carr. Wicked was… fine? I wasn’t overly enamoured with part one, so it’s not a surprise I didn’t vibe with the follow-up. If you smushed them into a single normal-length feature I think they would have made a really good film, but two films both over two hours long is a bit much for what actually happens.

Around the web

Wikipedia: List of citogenesis incidents

A list of incidents where (often false) information on Wikipedia has ended up becoming its own source. That led to Mark Dominus’ blog post titled Imaginary Albanian eggplant festivals… IN SPACE where he discovers that a mountain on Ceres got named after a made-up festival (and then got it fixed!)

Not relying on Wikipedia as a source was drummed into us at school, but it feels like this sort of thing is only going to get worse and worse now LLMs are on the scene. I assume we’re already at the point where LLM hallucinations are prevalent enough that they’ll find their way into the training data for future iterations.

Simon Collison’s website

I really like the design here. Both the homepage and how well every journal entry is formatted. It makes me feel bad for the many walls of text that adorn this site. The archived v4 design is also wonderful.

My Dinner with Skinner

A 43-minute-long feature film that is “just” the Steamed Hams gag from The Simpsons done in the style of My Dinner with Andre. There’s a (much shorter) behind-the-scenes video that goes over the extreme amount of effort that was put into it, too.

I’m still trying to wrap my head around this. It’s… something.

Delete Spotify? Sure, But Don’t Just Replace it With Another Subscription

A similar take to my own escaping Spotify the hard way, but it avoids getting lost in the technical weeds like I did. (In my defence, they’re very fun weeds to get lost in.) Stephanie is also one of the people that does monthly wrap-ups, and I was pleasantly surprised when the latest one popped up in my RSS reader and I saw my own site linked.

Debugging beyond the debugger

2019-05-08T00:00:00Z

Real-life debugging tools

Most programming — and sysadmin — problems can be debugged in a fairly straight forward manner using logs, print statements, educated guesses, or an actual debugger. Sometimes, though, the problem is more elusive. There’s a wider box of tricks that can be employed in these cases but I’ve not managed to find a nice overview of them, so here’s mine. I’m mainly focusing on Linux and similar systems, but there tend to be alternatives available for other Operating Systems or VMs if you seek them out.

Networking

tcpdump

tcpdump prints out descriptions of packets on a network interface. You can apply filters to limit which packets are displayed, chose to dump the entire content of the packet, and so forth.

Typical usage might look something like:

# tcpdump -nSi eth0 port 80
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
16:03:35.577781 IP6 2001:db8::1.54742 > 2001:db8::2.80: Flags [S], seq 2815779044, win 64800, options [mss 1440,sackOK,TS val 2378811665 ecr 0,nop,wscale 7], length 0
16:03:35.586853 IP6 2001:db8::2.80 > 2001:db8::1.54742: Flags [S.], seq 1522609102, ack 2815779045, win 28560, options [mss 1440,sackOK,TS val 3063610173 ecr 2378811665,nop,wscale 7], length 0
16:03:35.586877 IP6 2001:db8::1.54742 > 2001:db8::2.80: Flags [.], ack 1522609103, win 507, options [nop,nop,TS val 2378811674 ecr 3063610173], length 0
16:03:35.620678 IP6 2001:db8::1.54742 > 2001:db8::2.80: Flags [P.], seq 2815779045:2815779399, ack 1522609103, win 507, options [nop,nop,TS val 2378811708 ecr 3063610173], length 354: HTTP: GET / HTTP/1.1

Here you can see the start of a plaintext HTTP request: the three-way handshake as the TCP connection is established followed by a GET request. Even if the data is encrypted as it will be in most cases, it’s often useful to see the “shape” of the transmissions: did the client start sending data when it connected, did the server ever respond, etc.

Daniel Miessler has a good tutorial on tcpdump if you’re not familiar with it and don’t want to jump straight into the man page.

… with Docker

Docker sets up separate network namespaces for each container. To see the traffic across the interfaces of a single container you can nsenter the container’s network namespace:

# nsenter -t $(docker inspect --format '{{.State.Pid}}' my_container) -n tcpdump -nS port 80

This retrieves the PID for the container, and tells nsenter to enter the network (-n) namespace from the given target (-t) PID, and then run the given command (in this case tcpdump ...).

openssl s_client / s_server

When a connection is using TLS it’s often useful to try connecting to the server and see what certificate it presents, algorithms it negotiates, and so forth. OpenSSL offers two useful subcommands which can help with this: s_client for connecting as a client, and s_server for listening to connections.

For example, using s_client to connect to google.com on the standard HTTPS port shows us details about the server cert and its verification status:

$ openssl s_client -connect google.com:443
CONNECTED(00000003)
depth=2 OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
verify return:1
depth=1 C = US, O = Google Trust Services, CN = Google Internet Authority G3
verify return:1
depth=0 C = US, ST = California, L = Mountain View, O = Google LLC, CN = *.google.com
verify return:1
---
Certificate chain
 0 s:C = US, ST = California, L = Mountain View, O = Google LLC, CN = *.google.com
   i:C = US, O = Google Trust Services, CN = Google Internet Authority G3
 1 s:C = US, O = Google Trust Services, CN = Google Internet Authority G3
   i:OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
---
# ...

Whereas connecting to my webserver and providing an unknown host in the SNI field results in an SSL alert 112 (“The server name sent was not recognized”) and no server certificate is sent:

$ openssl s_client -connect chameth.com:443 -servername example.com
CONNECTED(00000003)
140384831313024:error:14094458:SSL routines:ssl3_read_bytes:tlsv1 unrecognized name:../ssl/record/rec_layer_s3.c:1536:SSL alert number 112
---
no peer certificate available
---
# ...

Often if you hit this kind of alert in an application the exact error will be lost somewhere in the many layers between the SSL library and the logs, so being able to directly connect and test can help diagnose a lot of issues.

Once a connection is established you can read and write plain text and it will be encrypted and decrypted automatically.

Java apps

If a Java app is involved in the connection, you can enable a lot of built-in debugging with a simple JVM property: javax.net.debug. You can tweak what exactly gets logged, but the easiest thing to do is just set the property to all and you’ll see information about certificate chains, verification, and packet dumps:

$ java -Djavax.net.debug=all -jar ....
# ...
found key for : duke
chain [0] = [
[
  Version: V1
  Subject: CN=Duke, OU=Java Software, O="Sun Microsystems, Inc.",
  L=Cupertino, ST=CA, C=US
# ...

More information about Java’s debugging options is available on docs.oracle.com.

Thread and core dumps

Higher-level languages frequently provide an interactive way to dump the current execution state of all of their threads (a “thread dump”). This is useful to spot deadlocks, some types of race conditions, and as a quick and dirty method of investigating hangs or excessive CPU usage.

With both Java and Go applications you can send a QUIT signal to have a thread dump printed out; Go applications will quit after doing so, Java ones will carry on running. At most terminals you can hit Ctrl and \ to send a QUIT signal.

For Java you can also use the jstack tool from the JDK to dump threads by PID; this can be useful if the application is running in the background or has redirected sysout:

$ jstack 8321
Attaching to process ID 8321, please wait...
Debugger attached successfully.
Client compiler detected.

Thread t@5: (state = BLOCKED)
 - java.lang.Object.wait(long) @bci=-1107318896 (Interpreted frame)
 - java.lang.Object.wait(long) @bci=0 (Interpreted frame)
 - java.lang.ref.ReferenceQueue.remove(long) @bci=44, line=116 (Interpreted frame)
 - java.lang.ref.ReferenceQueue.remove() @bci=2, line=132 (Interpreted frame)
 - java.lang.ref.Finalizer$FinalizerThread.run() @bci=3, line=159 (Interpreted frame)

# ...

A core dump provides more complete information about the state of a process, but is often more complex to interpret. The gcore utility from GDB will create a core dump of a process with a given PID. You can then generally load the core file using your normal debugger, depending on the language in question.

System calls

strace is the swiss army knife for seeing what a process is doing. It details each system call made by a program (you can filter them down, of course). For example:

$ strace -e read curl https://google.com/
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0 \236\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P!\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\200l\2\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20Q\0\0\0\0\0\0"..., 832) = 832
# ...
read(3, "\0\0\0\0\0\0\0\4\25\345\366\302\273sE6\365wI\225\321|\3435Z\362\216\372\215\251aO"..., 253) = 253
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="https://www.google.com/">here</A>.
</BODY></HTML>
read(3, "\27\3\3\0!", 5)                = 5
# ...

Brendan Gregg has a nice guide on strace and alternatives.

… with docker

When the application is running in docker you can usually just strace it from the host with the correct PID (from e.g. docker inspect --format '{{.State.Pid}}' my_container). Sometimes you may need to trace the startup of an application though, which is a bit trickier. Instead you can run a new container using the same PID namespace as your target, and the permissions needed to strace:

$ docker run --rm -it --pid=container:my_container \
  --net=container:my_container \
  --cap-add sys_admin \
  --cap-add sys_ptrace \
  alpine

From within the new container you can install strace, and trace any running program within the target container using strace -p as normal. To start a new program you need access to the target container’s filesystem, which you can get to via /proc/1/root (PID 1 being the main process that docker started in the target container).

Files

Sometimes the problem might relate to file access. There are a couple of straight forward — but nonetheless useful — tools which might help here. inotifywait uses the Linux inotify subsystem to watch files or directories for operations. For example:

$ inotifywait -mr site/content
Setting up watches.  Beware: since -r was given, this may take a while!
Watches established.
site/content/post/ MODIFY 2019-05-08-debugging-beyond-the-debugger.md
site/content/post/ OPEN 2019-05-08-debugging-beyond-the-debugger.md
site/content/post/ MODIFY 2019-05-08-debugging-beyond-the-debugger.md
site/content/post/ MODIFY 2019-05-08-debugging-beyond-the-debugger.md
site/content/post/ CLOSE_WRITE,CLOSE 2019-05-08-debugging-beyond-the-debugger.md
# ...

Here the -m switch makes inotifywait monitor the files forever (instead of exiting on the first modification, which is the normal behaviour) and r makes it recurse into the directory and monitor each file and subdirectory in there.

If you want to see what processes currently have a file open, fuser is the go-to tool. For example:

$ fuser -v /
                     USER PID ACCESS COMMAND
/:                   root     kernel mount /
                     chris      2961 .rc.. systemd
                     chris      2986 .r... gdm-x-session
                     chris      2994 .r... dbus-daemon
                     chris      3001 .r... gnome-session-b
# ...

Honourable mentions

These aren’t really debugging tools, but I feel it’s worth mentioning as they often feature somewhere along the debugging-of-weird-problems journey.

I’ve seen some weird and wonderful problems happen because a disk is full, so a quick df early on in the debugging process never hurts. Some apps may hang, some may corrupt their config, some may fall over and die; sometimes the manner in which they fail doesn’t obviously point to a disk space issue.

Another issue that comes up now and then — especially inside VMs or other environment that don’t have a decent amount of “noise” happening — is entropy exhaustion. A quick look at /proc/sys/kernel/random/entropy_avail should be enough to confirm that everything is ticking along nicely. If it’s exceedingly low then you may find that anything involving random number generation stalls (TLS connections for example).