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Monthly Meanderings: March 2026

2026-04-01T00:00:00Z

Since last month’s update I’ve been unpleasantly reminded that I’m middle aged, through the medium of a dodgy knee. There’s nothing quite like not being able to stand up without groaning to underscore that you’re not young any more. The ongoing game of “will this seemingly mundane activity make my knee go funny again?” is a delight to play, too. I’m great at it.

Website updates

Two new blog posts this month: the longest way to represent a date is a short thought experiment on absurd date formats, and modern CSS is fun goes through some neat new CSS features I’ve used recently for this site.

I did some design tweaks this month: headers now have a blue background behind them¹, and a lot of components have been redesigned to have a consistent style. You can see that component in action in the new “now playing” widget I’ve added:

VersaEmerge — Redesign Me Fixed at Zero Scrobbled 2h 39m ago

Instead of each component defining its own styles (which were mostly copied and pasted), there’s now a single class that handles the drop shadow, border, background colour, and the optional title. It also applies a consistent format to elements that are clickable: they use the blue accent colour for their borders, and have a glow effect shown on hover.

The now playing widget is part of a larger bit of work I did on importing some music stats. I’m pulling in the data from my Navidrome instance. A new music page shows my most listened albums and artists. Navidrome doesn’t store a complete play history, just the last play and the count, so I can’t do “what I listened to last month” stats until next month.

Other projects

Other than a minor bug-fix update to contempt, my Dockerfile templating/updating tool, I’ve not done much on my other open source projects. I’ve got a bit more work to do finish migrating all my repositories from GitHub to my private Forgejo instance. I’ve also been thinking about mirroring my public repositories to Codeberg so there’s a non-GitHub way to access them.

Entertainment

I apparently only watched a single film in March:

The Substance

2026-03-18

I really didn’t get on with this. It’s too long for what it is, and over-the-top in a way that just didn’t work for me.

Demi Moore and Margaret Qualley were great, but nothing else really was. The cinematography and sound design were too affected, the plot was pretty shallow and obvious, most of the body horror fell into the uncanny CGI valley for me.

It’s not completely without merit: there were around 40 minutes in the middle where I was really into it, but that’s not really enough given it’s closing in on a 2.5 hour runtime.

There’s something to be said for the message it’s presenting about women in Hollywood, too, but it was very heavy handed. Dennis Quaid’s character is even called Harvey; it’s not so much a subtle nod as a sledgehammer to the face.

Instead of films, I’ve been watching a bunch of TV. After devouring The Pitt last month, I struggled to find something to fill the hole it left. I settled on catching up on Chicago Fire and Chicago P.D.. They’re both mostly about getting things done without personal drama becoming the primary focus. They’re not quite as grounded as The Pitt, but at least they’ve not completely gone off the deep end like 9-1-1 where the first responders end up in space somehow?

I’ve also watched the first two episodes of SNL UK. I didn’t have very high expectations, and it gave me a pleasant surprise. Both episodes so far have some hilarious sketches, and are obviously written by British comics with the typical darker, more sardonic humour that separates us from the USA.

Last but not least, board games! I spent a weekend at a tabletop/LAN event with friends, so got to physically play some games for a change. I also kept up with some turn-based games on Board Game Arena, as usual.

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

Around the web

Warranty Void if Regenerated

Some really cool speculative fiction about a future where all software is written by LLMs, and the social effects that may have. Unfortunately the story itself is written with an LLM, and that becomes painfully obvious about a third of the way in. By that point I was already hooked, though.

My 2-step process for AI-free blogging

Speaking of LLM-generated text, this short piece by Manuel Moreale made me laugh. I’m by no means anti-LLM, but I really hate people publishing the output like it’s their own work. It breaks the social assumption that the writer put more effort in than the reader, and makes me wonder how much is hallucinated.

25 Years of Eggs

An interesting applied use of coding agents. I’m a sucker for personal statistics. I have no explicit desire to keep my receipts, or generate graphs of my egg consumption, but seeing the graphs makes me question that. The journey to get there is a great read, too.

I made a one-page notebook

The clever paper folding is interesting, but I really like some of the small touches on the page. There’s a nice little “in reply to” callout providing context at the top of the page, you can click to rotate the image further down, and there’s a details element containing a textual version of it. It has that great ‘small web’ vibe of someone who really cares about what they’re doing.

Workers who love ‘synergizing paradigms’ might be bad at their jobs

I haven’t read the full paper, but the summary given by the article is equal parts interesting and amusing. They developed a “Corporate Bullshit Receptivity Scale”, what more is there to say?

It’s meant to resemble painter’s tape, but I’m not sure quite how well it works. ↩︎

Modern CSS is fun

2026-03-17T00:00:00Z

I’ve been doing a bunch of CSS tweaking recently, and keep being surprised by how nice modern CSS is to work with. As someone grey-haired enough to remember writing HTML without CSS, it’s amazing to think how far along web technology has come¹. I wanted to demonstrate some of the handy bits and pieces I’ve used recently.

:has

The :has selector allows you to effectively query for child elements. While a span will match a <span> within an <a>, a:has(span) will match an <a> that contains a <span>. This really shines when combined with more complex selectors, for example:

input {
    border-radius: var(--border-radius);

    &:has(+ .results:not(:empty)) {
        border-bottom-left-radius: 0;
        border-bottom-right-radius: 0;
    }
}

This is part of the styling for my film search box. When the search box is presented initially it has fully rounded corners. But when it is followed by a non-empty results element, it removes the rounding on the bottom corners so that the border continues in a straight line down into the results. You can see it in action below; just enter a few characters (like “the”) to get some results:

You could achieve the same effect by having JavaScript add a class to the <input> element, but I’ll take a CSS solution over a JavaScript solution any day.

Nested rules

You probably spotted this in the example above. It’s what finally made me switch from SCSS to plain CSS. If you have a rule for .foo and a rule for .foo .bar you can just nest them. Not only does it save repeating yourself an awful lot, it keeps everything organised nicely. For example, the CSS for my film list embeds looks like this:

a.film-list {
    display: grid;
    grid-template-areas: "header images" "description images" "meta images";
    grid-template-columns: 1fr auto;
    grid-template-rows: auto 1fr auto;
    row-gap: var(--small-space);

    @media (width <= 800px) {
        grid-template-areas: "header" "description" "images" "meta";
        grid-template-columns: 1fr;
        grid-template-rows: auto auto auto auto;
    }

    & + & {
        margin-top: var(--medium-space);
    }

    h3 {
        font-size: var(--font-size-xxlarge);
    }

    .description {
        /* ... */
    }
}

Everything is wrapped up in a single bundle, including the @media rules for changing the layout on smaller screens, the lovely little & + & rule that adds some extra margin if there are two lists in a row, etc. The & syntax refers to the parent selector, so & + & in this case is the same as a.film-list + a.film-list: it’s very handy! The best part about this is that it’s the exact same syntax as SCSS, so converting is easy². The styled element looks like this:

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

Media range syntax

Once again, you might have spotted this in the previous snippet. I’m not deliberately teasing things, I promise! Back in the day, you did media queries like so:

@media (min-width: 1000px) and (max-width: 2000px) {
    /* some rules that work for screens at least 1000px wide and at most 2000px wide */
}

I always hated this syntax. I always got muddled up as to whether I wanted “min” or “max”, and whether they were inclusive or not. It’s easy to reason through, but it never came naturally. Fortunately you can now just use ranges:

@media (1000px <= width <= 2000px) {
    /* some rules that work for screens at least 1000px wide and at most 2000px wide */
}

I find this style so much easier to write and understand.

Anchor positioning

Trying to dynamically position one element next to another used to exclusively fall within the purview of JavaScript. Fortunately anchor positioning fixes all this. I use this in the film search field I showed above:

.film-search {
    input {
        anchor-name: --film-search-box;
    }
    .results {
        position: absolute;
        position-anchor: --film-search-box;
        position-area: bottom center;
        position-visibility: always;
        width: anchor-size();
    }
}

This does lots of fun things. The .results element is anchored to the input field (via the --film-search-box name), and it’s positioned on the bottom, directly under the anchor. The position-area rule describes a 3x3 grid, with the anchor at the center, so top left would put it diagonally adjacent to the anchor. Next, position-visibility keeps the .results element visible when the anchor isn’t; otherwise when you scroll the anchor off the screen the results would immediately vanish. Finally, the special anchor-size() function makes the .results element take up the same exact width as the anchor.

All-in-all, this puts the results exactly where I want them, without having to deal with any JavaScript at all. I can see how it’d also be amazingly useful trying to do tooltips or other forms of ‘floating’ content too. You can even leave the positioning up to the browser, giving it hints about which order to try, or what property to optimise for (e.g. you can say “anchor to the left or right, whichever has more horizontal space”).

attr() function

This one blew my mind a little. In some of the elements on the site, I’ve added a little label to the top. It looks something like this:

It’s meant to just be a visual flourish, not a semantic title or anything, so I initially just added a ::before selector to each element, customising the content to have the right value. I didn’t like the duplication, though. Now, instead, I do this:

&::before {
    content: attr(data-title);
}

Each component defines its title in a data attribute: <div class="raised-box" data-title="an example">, and the attr function plucks it out and puts it in the content rule. It’s worth noting that this is only widely supported for the content rule, you can’t yet use it for colours or dimensions or other things. It still feels a bit magical, though. Like you’re making a reusable, customisable component with just HTML and CSS.

layers

I’d come across layers a few times, but I never found a need for them. It seemed like something you’d only really need with complicated design systems, or something. Then I had a problem and layers were the perfect solution!

The headings on this site have a whole bunch of CSS attached to them. Currently they look a bit like blue painter’s tape with handwriting on them. This style is applied to all headers, and then the various places that don’t want it had to manually reset everything. I got fed up with that, so decided to add a plain-header class which would “disable” the extra styling:

h2,
h3,
h4,
h5,
h6 {
    /* common styles for all headers, regardless */
    font-family: "Chris Hand", sans-serif;
    font-size: var(--font-size-xxlarge);
    color: var(--text-highlight-colour);
    margin: var(--medium-space) 0;
    line-height: 1;

    &:not(.plain-header) {
        /* do fancy stuff if it _doesn't_ have the .plain-header class */
        transform: rotate(-0.15deg);
        padding: var(--smedium-space);
        font-weight: 800;
        filter: drop-shadow(3px 3px 5px rgb(0, 0, 0));

        a {
            background: none;
            text-decoration-skip-ink: auto;
        }
    }
}

This might seem a bit backwards but I want the default to be styled, as those are far more common. I added this change, sprinkled around some “plain-header” classes, tidied up a lot of duplicate CSS, and things mostly worked. Mostly. Some things stopped working. In a few places the headers had slight tweaks, and they stopped working entirely.

It turns out in moving the rules from a plain h2 selector to a h2:not(.plain-header), I’d made them more specific. Previously a selector like h2.special would have been more specific, so its properties would override those from the less specific h2 selector. My first reaction was to try and hack around it. Changing all the overrides to body h2.special would make them more specific again, but I didn’t want to have to remember to do that forever more.

Instead, I defined some custom layers, and put the header definitions in one:

@layer reset, links, headings

@layer headings {
    h2,
    h3 {
        /* etc */
    }
}

The first @layer rule defines some layers that will be handled in a specific order: first the “reset” layer, then the “links” layer, then the “headings” layer, and then anything not in a layer after that. Rules in one layer don’t have to worry about beating the specificity of rules in another layer, because they’re handled separately.

Note that I couldn’t just put the headings in a layer and call it a day: their rules have to come after the CSS reset, and after the normal link styles, as headers have a bit of extra styling for nested links. If the CSS reset weren’t in a layer, then the * { margin: 0; padding: 0; } type reset would apply over the rules from the headings layer.

Using layers is definitely a bit fiddly, and is not necessary for a lot of sites, but it’s a much cleaner alternative to ugly specificity hacking.

Things I’m looking forward to

There are a few things that aren’t (widely) available yet, that particularly interest me:

sibling-index() and sibling-count()

These new functions are available in WebKit and Blink based browsers, but not Firefox. They give you the index of the element within its siblings, or the count of siblings. For the film list component above, I currently have this abomination for the overlapping posters:

&:nth-child(1) {
    z-index: 5;
    left: 0;
}
&:nth-child(2) {
    z-index: 4;
    left: calc(133px * var(--overlap) * 1);
}
&:nth-child(3) {
    z-index: 3;
    left: calc(133px * var(--overlap) * 2);
}
&:nth-child(4) {
    z-index: 2;
    left: calc(133px * var(--overlap) * 3);
}
&:nth-child(5) {
    z-index: 1;
    left: calc(133px * var(--overlap) * 4);
}

These could be replaced with something like:

z-index: calc(1 + sibling-count() - sibling-index());
left: calc(133px * var(--overlap) * (sibling-index() - 1));

As an added bonus, it would scale to any number of posters. index being 1-based not 0-based is a bit unfortunate as it makes using it in calculations awkward (see 1 offset in both of those rules!), but it’s leagues better than writing 5 separate rules.

random()

Currently only available in Safari. There are a few places where I’d like to have slight random variations of the style. Things like elements that are rotated slightly for aesthetics; they look a bit silly if they’re all identically positioned. I also randomise the icons and positions of my rating stars to break up the visual monotony. Here are a few examples:

Currently I do the stars by hardcoding a bunch of classes and having the backend randomly apply one when it generates the markup for stars. Being able to do it in CSS would be great, though.

Mixins

This one’s so far off it’s not even listed on caniuse.com yet. Mixins are another handy feature of SCSS, that allow you to define reusable blocks of rules, then import them when needed:

@mixin fancy-background {
    background: /*...*/;
}

.some-element {
    @include fancy-background;
}

#other-element {
    @include fancy-background;
}

This would be particularly useful if you can’t easily control the markup to add classes to everything. For example, my footnotes are rendered by a markdown plugin, and don’t easily have a way to add extra classes to them. To style it the same as another element, I currently duplicate a bunch of rules between them both. Mixins would allow me to define those rules once, and then import them in both places.

Aside from JavaScript, which seems to just endlessly reinvent new frameworks and ways to make the most sprawling dependency tree possible. ↩︎
It also means that if your website’s syntax highlighting library doesn’t seem to understand nested rules, you can just pretend your perfectly valid CSS is actually SCSS and it’ll magically work. Grumble, grumble. ↩︎

How to break everything by fuzz testing

2020-04-26T00:00:00Z

Fuzz testing is a bit like the infinite monkey theorem, but instead of Shakespeare you get crashes.

Fuzz testing, if you’re not aware, is a form of testing that uses procedurally generated random inputs to see how a program behaves. For instance, if you were fuzz testing a web page renderer you might generate a bunch of HTML - some valid, and some not - and make sure the rendering process didn’t unexpectedly crash.

Fuzz testing doesn’t readily lend itself to all types of software, but it particularly shines in cases where some kind of complex user input is accepted and processed in some way - like the aforementioned web page renderer. I was recently adding a library to parse EXIF data to images to an Internet-facing service and realised it was a perfect opportunity to do some fuzz testing. Even if I didn’t find any issues, I’d improve my confidence that the library was safe enough to expose to the Internet.

Breaking my EXIF library

I wrote a quick harness to run go-fuzz on the library, and gave it some pre-existing demo files as sample input. The way go-fuzz works is that it instruments your code and then mutates the inputs to try to improve the coverage. For example, if I had some sample data that had an EXIF tag with a value of 1 then go-fuzz might change it to a 2 and see if the code follows a different path. In most cases it won’t but when it does, they tend to be very interesting cases.

One of the first issues that go-fuzz found was that some values in a maker note field would cause the library to panic (i.e., crash). This happened because there was a check to see if the first six characters were “Nikon” and a null byte, without first checking to see if there were actually six characters available. This is a kind of bug that doesn’t happen much with “real” data - as the field is either not present or completed correctly - but could easily be exploited once this code is exposed to the Internet.

Another interesting bug that go-fuzz found was that if a tag had a particularly large count, the library would try to allocate an obscene amount of memory and die. There was already a check in the code that was meant to avoid this exact scenario, but go-fuzz managed to find a way around it. Each tag has a size (for example an integer tag takes a fixed number of bytes) and a count; the existing check multiplied the two together and made sure that the result wasn’t too large. For most cases this was fine but go-fuzz found a case where the count was so large that when multiplied by the size of the tag it overflowed the integer and became negative, thus passing the sanity check but then subsequently failing when it came around to actually allocating the memory.

The final bug of note that go-fuzz found was the most interesting. EXIF data is stored in IFDs (“Image File Directories”), and each IFD provides what is effectively a pointer (a byte offset) to the next one. The EXIF library already had a check to make sure that these didn’t loop, but it only checked the immediately preceding IFD - so if IFD 1 linked to IFD 2, it would catch IFD 2 linking back to IFD 1 and break the loop with an error. Go-fuzz found that having three interlinked IFDs had the same issue, though, and the guard code wasn’t triggered. This created an infinite loop, maxing out a CPU core until the process was eventually killed - one of the worst kind of bugs you could have in an Internet-facing service which doesn’t deal with private data! The fix for this was fairly straightforward - I just made the library keep a record of the previously visited IFDs and bail out if it found a loop.

Breaking my IDE

When go-fuzz detects an issue it outputs not only the details of the problem (the stack trace, error message, and so forth) but also the input that generated the problem. This is useful for reproducing and making sure the issue is fixed, but it also makes it really easy to write a test to ensure that the behaviour never regresses in the future.

As I was working through fixing the bugs that go-fuzz found, I dutifully added new tests where needed. After adding the sample input with looping IFDs to the project, I switched to IDEA to write a test to use it. I clicked on the input file to copy the file name, and then the entire IDE hung and had to be restarted. Uh oh! When I restarted IDEA, it immediately began indexing the project and again hung. It turns out IDEA parses EXIF data (presumably, even if it does nothing else with the data, to get the rotation property for images), and the library they use - an independent one written on Java - had the same bug as the Go library I was using.

In order to stop IDEA from indexing the file and becoming unusable I renamed it from a ‘.tif’ extension to ‘.dat’, and everything went back to normal. I thought I’d best report the bug to JetBrains, though, so they could put a proper fix in.

Breaking YouTrack

JetBrains use their own issue tracker called YouTrack for reporting bugs in IDEA. I dutifully went over and described the problem, attaching the log files from the IDE, a description of how the file was malformed, and carefully selected the .dat version of the file to upload so that it wouldn’t cause anyone else the same immediate problem.

After trying to upload the file I got a strange error back. Uh oh! I submitted the IDEA issue as it stood, unable to see if the attachments had even uploaded, and went and wrote up an issue for YouTrack itself about the error message. While I was doing that, YouTrack seemed to slow down and become really annoying to use. I had a sinking feeling the exact same thing was probably happening as with IDEA and my library - but this time YouTrack had content-sniffed the file instead of relying on the file extension. In hindsight, I should’ve put the file in a passworded archive to ensure no automated tools got hold of it. I marked the issue as a security problem as in a service like YouTrack it presents a denial-of-service opportunity¹ (remember when I said it was one of the worst kinds of bugs you could have in an Internet-facing service?…)

Shortly after I raised my YouTrack ticket, a notice appeared at the top of the page saying they were investigating the current performance issues. Uh oh! I was holding out hope that this was unrelated to me uploading the buggy dat file, but the timing all seemed a bit suspect. I shot support an e-mail saying I think I might be the root cause for their performance issues and linked to the ticket. In the time it took me to e-mail them, the entire site had been put into maintenance mode. I got an e-mail back a few hours later confirming the outage was in fact all my fault, as I’d feared. Within the space of days the JetBrains security team had fixed the issue in YouTrack, which was a pretty nice turnaround.

So if you were trying to access YouTrack at the start of March and couldn’t - I’m sorry, I didn’t mean to! Also, if you’re building an Internet-facing service that takes user input you should really consider running a fuzz tester against it!

“We have a problem”. “Remember, there are no such things as problems, only opportunities”. “Well then we have a DDoS opportunity.” – @J4vv4d ↩︎

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).