Summing up My 2019

2019 is special in my heart. 2019 was different than many other years to me in several ways. It was a great year! This is what 2019 was to me.

curl and wolfSSL

I quit Mozilla last year and in the beginning of the year I could announce that I joined wolfSSL. For the first time in my life I could actually work with curl on my day job. As the project turned 21 I had spent somewhere in the neighborhood of 15,000 unpaid spare time hours on it and now I could finally do it “for real”. It’s huge.

Still working from home of course. My commute is still decent.


Just in November 2018 the name HTTP/3 was set and this year has been all about getting it ready. I was proud to land and promote HTTP/3 in curl just before the first browser (Chrome) announced their support. The standard is still in progress and we hope to see it ship not too long into next year.


Focusing on curl full time allows a different kind of focus. I’ve landed more commits in curl during 2019 than any other year going back all the way to 2005. We also reached 25,000 commits and 3,000 forks on github.

We’ve added HTTP/3, alt-svc, parallel transfers in the curl tool, tiny-curl, fixed hundreds of bugs and much, much more. Ten days before the end of the year, I’ve authored 57% (over 700) of all the commits done in curl during 2019.

We ran our curl up conference in Prague and it was awesome.

We also (re)started our own curl Bug Bounty in 2019 together with Hackerone and paid over 1000 USD in rewards through-out the year. It was so successful we’re determined to raise the amounts significantly going into 2020.

Public speaking

I’ve done 28 talks in six countries. A crazy amount in front of a lot of people.

In media

Dagens Nyheter published this awesome article on me. I’m now shown on the internetmuseum. I was interviewed and highlighted in Bloomberg Businessweek’s “Open Source Code Will Survive the Apocalypse in an Arctic Cave” and Owen William’s Medium post The Internet Relies on People Working for Free.

When Github had their Github Universe event in November and talked about their new sponsors program on stage (which I am part of, you can sponsor me) this huge quote of mine was shown on the big screen.

Maybe not media, but in no less than two Mr Robot episodes we could see curl commands in a TV show!


I’ve participated in three podcast episodes this year, all in Swedish. Kompilator episode 5 and episode 8, and Kodsnack episode 331.


I’ve toyed with live-streamed programming and debugging sessions. That’s been a lot of fun and I hope to continue doing them on and off going forward as well. They also made me consider and get started on my libcurl video tutorial series. We’ll see where that will end…


I figure it can become another fun year too!

My 28 talks of 2019

CS3 Sthlm 2019

In 2019 I did more public speaking than I’ve ever than before in a single year: 28 public appearances. More than 4,500 persons have seen my presentations live at both huge events (like 1,200 in the audience at FOSDEM 2019) but also some very small and up-close occasions. Many thousands more have also seen video recordings of some of the talks – my most viewed youtube talk of 2019 has been seen over 58,000 times. Do I need to say that it was about HTTP/3, a topic that was my most common one to talk about through-out the year? I suspect the desire to listen and learn more about that protocol version is far from saturated out there…


Nordic APIs Summit 2019

During the year I’ve done presentations in

Barcelona, Brussels, Copenhagen, Gothenburg, Mainz, Prague, Stockholm and Umeå.

I’ve did many in Stockholm, two in Copenhagen.


Castor Software Days 2019

During the year I’ve done presentations in

Belgium, Czechia, Denmark, Germany, Spain and Sweden.

Most of my talks were held in Sweden. I did one streamed from my home office!


JAX 2019

14 of these talks had a title that included “HTTP/3” (example)

9 talks had “curl” in the title (one of them also had HTTP/3 in it) (example)

4 talks involved DNS-over-HTTPS (example)

2 talks were about writing secure code (example)

Talks in 2020


There will be talks by me in 2020 as well as the planning . Probably just a little bit fewer of them!

Invite me?

Sure, please invite me and I will consider it. I’ve written down some suggestions on how to do this the best way.

At GOTO10 early 2019

(The top image is from Fullstackfest 2019)


The Internet Museum translated to Swedish becomes “internetmuseum“. It is a digital, online-only, museum that collects Internet- and Web related historical information, especially focused on the Swedish angle to all of this. It collects stories from people who did the things. The pioneers, the ground breakers, the leaders, the early visionaries. Most of their documentation is done in the form of video interviews.

I was approached and asked to be part of this – as an Internet Pioneer. Me? Internet Pioneer, really?

Internetmuseum’s page about me.

I’m humbled and honored to be considered and I certainly had a lot of fun doing this interview. To all my friends not (yet) fluent in Swedish: here’s your grand opportunity to practice, because this is done entirely in this language of curl founders and muppet chefs.

Photo from Internetmusuem

Back in the morning of October 18th 2019, two guys showed up as planned at my door and I let them in. One of my guests was a photographer who set up his gear in my living room for the interview, and then me and and guest number two, interviewer Jörgen, sat down and talked for almost an hour straight while being recorded.

The result can be seen here below.

The Science museum was first

This is in fact the second Swedish museum to feature me.

I have already been honored with a display about me, at the Tekniska Museet in Stockholm, the “Science museum” which has an exhibition about past Polhem Prize award winners.

Information displayed about me at the Swedish Science museum in Stockholm. I have a private copy of the cardboard posters.

(Top image by just-pics from Pixabay)

How randomly skipping tests made them better!

In the curl project we produce and ship a rock solid and reliable library for the masses, we must never exit, leak memory or do anything in an ungraceful manner. We must free all resources and error out nicely whatever problem we run into at whatever moment in the process.

To help us stay true to this, we have a way of testing we call “torture tests”. They’re very effective error path tests. They work like this:

Torture tests

They require that the code is built with a “debug” option.

The debug option adds wrapper functions for a lot of common functions that allocate and free resources, such as malloc, fopen, socket etc. Fallible functions provided by the system curl runs on.

Each such wrapper function logs what it does and can optionally either work just like it normally does or if instructed, return an error.

When running a torture test, the complete individual test case is first run once, the fallible function log is analyzed to count how many fallible functions this specific test case invoked. Then the script reruns that same test case that number of times and for each iteration it makes another of the fallible functions return error.

First make function 1 return an error. Then make function 2 return and error. Then 3, 4, 5 etc all the way through to the total number. Right now, a typical test case uses between 100 and 200 such function calls but some have magnitudes more.

The test script that iterates over these failure points also verifies that none of these invokes cause a memory leak or a crash.

Very slow

Running many torture tests takes a long time.

This test method is really effective and finds a lot of issues, but as we have thousands of tests and this iterative approach basically means they all need to run a few hundred times each, completing a full torture test round takes many hours even on the fastest of machines.

In the CI, most systems don’t allow jobs to run more than an hour.

The net result: the CI jobs only run torture tests on a few selected test cases and virtually no human ever runs the full torture test round due to lack of patience. So most test cases end up never getting “tortured” and therefore we miss out verifying error paths even though we can and we have the tests for it!

But what if…

It struck me that when running these torture tests on a large amount of tests, a lot of error paths are actually identical to error paths that were already tested and will just be tested again and again in subsequent tests.

If I could identify the full code paths that were already tested, we wouldn’t have to test them again. But getting that knowledge would require insights that our test script just doesn’t have and it will be really hard to make portable to even a fraction of the platforms we run and test curl on. Not the most feasible idea.

I went with something much simpler.

I simply estimate that most test cases actually have many code paths in common with other test cases. By randomly skipping a few iterations on each test, those skipped code paths might still very well be tested in another test. As long as the skipping is random and we do a large number of tests, chances are we cover most paths anyway. I say most because it certainly will not be all.

Random skips

In my first shot at this (after I had landed to change that allows me to control the torture tests this way) I limited the number of errors to 40 per test case. Now suddenly the CI machines can actually blaze through the test cases at a much higher speed and as a result, they ran torture tests on tests we hadn’t tortured in a long time.

I call this option to the script --shallow.

Already on this my first attempt in doing this, I struck gold and the script highlighted code paths that would create memory leaks or even crashes!

As a direct result of more test cases being tortured, I found and fixed nine independent bugs in curl already before I could land this in the master branch, and there seems to be more failures that pop up after the merge too! The randomness involved may of course delay the detection of some problems.

Room for polishing

The test script right now uses a fixed random seed so that repeated invokes will make it work exactly the same. Which is good when you want to reproduce it elsewhere. It is bad in the way that each test will have the exact same tests skipped every test round – as long as the set of fallible functions are unmodified.

The seed can be set by a command line argument so I imagine a future improvement would be to set the random seed based on the git commit hash at the point where the tests are run, or something. That way, torture tests on subsequent commits would get a different random spread.

Alternatively, I will let the CI systems use a true random seed to make it test a different set every time independent of git etc – as when it detects an error the informational output will still be enough for a user to reproduce the problem without the need of a seed.

Further, I’ve started out running --shallow=40 (on the Ubuntu version) which is highly unscientific and arbitrary. I will experiment altering this amount both up and down a bit to see what I learn from that.

Torture via strace?

Another idea that’s been brewing in my head for a while but I haven’t yet actually attempted to do this.

The next level of torture testing is probably to run the tests with strace and use its error injection ability, as then we don’t even need to build a debug version of our code and we don’t need to write wrapper code etc.


Dice image by Erik Stein from Pixabay

Reporting documentation bugs in curl got easier

After I watched a talk by Marcus Olsson about docs as code (at foss-sthlm on December 12 2019), I got inspired to provide links on the curl web site to make it easier for users to report bugs on documentation.

Starting today, there are two new links on the top right side of all libcurl API function call documentation pages.

File a bug about this page – takes the user directly to a new issue in the github issue tracker with the title filled in with the name of the function call, and the label preset to ‘documentation’. All there’s left is for the user to actually provide a description of the problem and pressing submit (and yeah, a github account is also required).

View man page source – instead takes the user over to browsing that particular man pages’s source file in the github source code repository.

Since this is also already live on the site, you can also browse the documentation there. Like for example the curl_easy_init man page.

If you find mistakes or omissions in the docs – no matter how big or small – feel free to try out these links!


Image by Pexels from Pixabay

BearSSL is curl’s 14th TLS backend

curl supports more TLS libraries than any other software I know of. The current count stops at 14 different ones that can be used to power curl’s TLS-based protocols (HTTPS primarily, but also FTPS, SMTPS, POP3S, IMAPS and so on).

The beginning

The very first curl release didn’t have any TLS support, but already in June 1998 we shipped the first version that supported HTTPS. Back in those days the protocol was still really SSL. The library we used then was called SSLeay. (No, I never understood how that’s supposed to be pronounced)

The SSLeay library became OpenSSL very soon after but the API was brought along so curl supported it from the start.

More than one

In the spring of 2005 we merged the first support for building curl with a different TLS library. It was GnuTLS, which comes under a different license than OpenSSL and had a slightly different feature set. The race had began.


A short while ago and in time to get shipped in the coming 7.68.0 release (set to ship on January 8th 2020), the 14th TLS backend was merged into the curl source tree in the shape of support for BearSSL. BearSSL is a TLS library aimed at smaller devices and is perhaps lacking a bit in features (like no TLS 1.3 for example) but has still been requested by users in the past.


Since September 2017, you can even build libcurl to support one or more TLS libraries in the same build. When built that way, users can select which TLS backend curl should use at each start-up. A feature used and appreciated by for example git for Windows.

Time line

Below is an attempt to visualize how curl has grown in this area. Number of supported TLS backends over time, from the first curl release until today. The image comes from a slide I intend to use in a future curl presentation. A notable detail on this graph is the removal of axTLS support in late 2018 (removed in 7.63.0). PolarSSL is targeted to meet the same destiny in February 2020 since it gets no updates anymore and has in practice already been replaced by mbedTLS.

Click the image to enjoy the full resolution version!


If you’ve heard me talk about HTTP/3 (h3) and QUIC (like my talk at Full Stack Fest 2019), you already know that QUIC needs new APIs from the TLS libraries.

For h3 support to become reality in curl shipped in distros etc, the TLS library curl is set to use needs to provide a QUIC compatible API and the QUIC/h3 library curl uses then needs to support that.

It is likely that some TLS libraries are going to be fast with providing such APIs and some are going be (very) slow. Their particular individual abilities combined with the desire to ship curl with h3 support is likely going to affect what TLS library you will see used by curl in your distro will affect what TLS library you will build your own curl builds to use in the future.


The recently added BearSSL backend was written by Michael Forney. Top image by LEEROY Agency from Pixabay

Mr Robot curls

The Mr Robot TV series features a security expert and hacker lead character, Elliot.

Season 4, episode 8

Vasilis Lourdas reported that he did a “curl sighting” in the show and very well I took a closer peek and what do we see some 37 minutes 36 seconds into episode 8 season 4…

(I haven’t followed the show since at some point in season two so I cannot speak for what actually has happened in the plot up to this point. I’m only looking at and talking about what’s on the screenshots here.)

Elliot writes Python. In this Python program, we can see two curl invokes, both unfortunately a blurry on the right side so it’s hard to see them exactly (the blur is really there in the source and I couldn’t see/catch a single frame without it). Fortunately, I think we get some additional clues later on in episode 10, see below.

He invokes curl with -i to see the response header coming back but then he makes some questionable choices. The -k option is the short version of --insecure. It truly makes a HTTPS connection insecure since it completely switches off the CA cert verification. We all know no serious hacker would do that in a real world use.

Perhaps the biggest problem for me is however the following -X POST. In itself it doesn’t have to be bad, but when taking the second shot from episode 10 into account we see that he really does combine this with the use of -d and thus the -X is totally superfluous or perhaps even wrong. The show technician who wrote this copied a bad example…

The -b that follows is fun. That sets a specific cookie to be sent in the outgoing HTTP request. The random look of this cookie makes it smell like a session cookie of some sorts, which of course you’d rarely then hard-code it like this in a script and expect it to be of use at a later point. (Details unfold later.)

Season 4, episode 10

Lucas Pardue followed-up with this second sighting of curl from episode 10, at about 23:24. It appears that this might be the python program from episode 8 that is now made to run on or at least with a mobile phone. I would guess this is a session logged in somewhere else.

In this shot we can see the command line again from episode 8.

We learn something more here. The -b option didn’t specify a cookie – because there’s no = anywhere in the argument following. It actually specified a file name. Not sure that makes anything more sensible, because it seems weird to purposely use such a long and random-looking filename to store cookies in…

Here we also see that in this POST request it passes on a bank account number, a “coin address” and amountOfCoins=3684210526.31579 to this URL:, and it gets 200 OK back from a HTTP/1.1 server.

I tried it

curl -i -k -X POST -d bankAccountNumber=8647389203882 -d coinAddress=1MbwAEKJCtPYpLPxEkUmZxwjk63nQrpbXo -d amountOfCoins=3684210526.31579

I don’t have the cookie file so it can’t be repeated completely. What did I learn?

First: OpenSSL 1.1.1 doesn’t even want to establish a TLS connection against this host and says dh key too small. So in order to continue this game I took to a curl built with a TLS library that didn’t complain on this silly server.

Next: I learned that the server responding on this address (because there truly is a HTTPS server there) doesn’t have this host name in its certificate so -k is truly required to make curl speak to this host!

Then finally it didn’t actually do anything fun that I could notice. How boring. It just responded with a 301 and Location: Notice how it redirects away from HTTPS.

What’s on that site? A rather good-looking fake cryptocurrency market site. The links at the bottom all go to various NBC Universal and USA Network URLs, which I presume are the companies behind the TV series. I saved a screenshot below just in case it changes.

This is your wake up curl

curl_multi_wakeup() is a new friend in the libcurl API family. It will show up for the first time in the upcoming 7.68.0 release, scheduled to happen on January 8th 2020.

Sleeping is important

One of the core functionalities in libcurl is the ability to do multiple parallel transfers in the same thread. You then create and add a number of transfers to a multi handle. Anyway, I won’t explain the entire API here but the gist of where I’m going with this is that you’ll most likely sooner or later end up calling the curl_multi_poll() function which asks libcurl to wait for activity on any of the involved transfers – or sleep and don’t return for the next N milliseconds.

Calling this waiting function (or using the older curl_multi_wait() or even doing a select() or poll() call “manually”) is crucial for a well-behaving program. It is important to let the code go to sleep like this when there’s nothing to do and have the system wake up it up again when it needs to do work. Failing to do this correctly, risk having libcurl instead busy-loop somewhere and that can make your application use 100% CPU during periods. That’s terribly unnecessary and bad for multiple reasons.

Wakey wakey

When your application calls libcurl to say “sleep for a second or until something happens on these N transfers” and something happens and the application for example needs to shut down immediately, users have been asking for a way to do a wake up call.

– Hey libcurl, wake up and return early from the poll function!

You could achieve this previously as well, but then it required you to write quite a lot of extra code, plus it would have to be done carefully if you wanted it to work cross-platform etc. Now, libcurl will provide this utility function for you out of the box!


This function explicitly makes a curl_multi_poll() function return immediately. It is designed to be possible to use from a different thread. You will love it!


This is the only call that can be woken up like this. Old timers may recognize that this is also a fairly new function call. We introduced it in 7.66.0 back in September 2019. This function is very similar to the older curl_multi_wait() function but has some minor behavior differences that also allow us to introduce this new wakeup ability.


This function was brought to us by the awesome Gergely Nagy.

Top image by Wokandapix from Pixabay

A 25K commit gift

The other day we celebrated curl reaching 25,000 commits, and just days later I received the following gift in the mail.

The text found in that little note is in Swedish and a rough translation to English makes it:

Twenty-five thousand thanks for curl. The cake delivery failed so here is a commit mascot and some new bugs to squash.

I presumed the cake reference was a response to a tweet of mine suggesting cake would be a suitable way to celebrate this moment.

The gift arrived without any clue who sent it, but when I tweeted this image, my mystery friend Filip soon revealed himself:

Thank you Filip!

(Top image by Arek Socha from Pixabay)

curl speaks etag

The ETag HTTP response header is an identifier for a specific version of a resource. It lets caches be more efficient and save bandwidth, as a web server does not need to resend a full response if the content has not changed. Additionally, etags help prevent simultaneous updates of a resource from overwriting each other (“mid-air collisions”).

That’s a quote from the mozilla ETag documentation. The header is defined in RFC 7232.

In short, a server can include this header when it responds with a resource, and in subsequent requests when a client wants to get an updated version of that document it sends back the same ETag and says “please give me a new version if it doesn’t match this ETag anymore”. The server will then respond with a 304 if there’s nothing new to return.

It is a better way than modification time stamp to identify a specific resource version on the server.

ETag options

Starting in version 7.68.0 (due to ship on January 8th, 2020), curl will feature two new command line options that makes it easier for users to take advantage of these features. You can of course try it out already now if you build from git or get a daily snapshot.

The ETag options are perfect for situations like when you run a curl command in a cron job to update a file if it has indeed changed on the server.

--etag-save <filename>

Issue the curl command and if there’s an ETag coming back in the response, it gets saved in the given file.

--etag-compare <filename>

Load a previously stored ETag from the given file and include that in the outgoing request (the file should only consist of the specific ETag “value” and nothing else). If the server deems that the resource hasn’t changed, this will result in a 304 response code. If it has changed, the new content will be returned.

Update the file if newer than previously stored ETag:

curl --etag-compare etag.txt --etag-save etag.txt -o saved-file

If-Modified-Since options

The other method to accomplish something similar is the -z (or --time-cond) option. It has been supported by curl since the early days. Using this, you give curl a specific time that will be used in a conditional request. “only respond with content if the document is newer than this”:

curl -z "5 dec 2019" https:/

You can also do the inversion of the condition and ask for the file to get delivered only if it is older than a certain time (by prefixing the date with a dash):

curl -z "-1 jan 2003"

Perhaps this is most conveniently used when you let curl get the time from a file. Typically you’d use the same file that you’ve saved from a previous invocation and now you want to update if the file is newer on the remote site:

curl -z saved-file -o saved-file

Tool, not libcurl

It could be noted that these new features are built entirely in the curl tool by using libcurl correctly with the already provided API, so this change is done entirely outside of the library.


The idea for the ETag goodness came from Paul Hoffman. The implementation was brought by Maros Priputen – as his first contribution to curl! Thanks!