Category Archives: Open Source

Open Source, Free Software, and similar

I’m leaving Mozilla

It’s been five great years, but now it is time for me to move on and try something else.

During these five years I’ve met and interacted with a large number of awesome people at Mozilla, lots of new friends! I got the chance to work from home and yet work with a global team on a widely used product, all done with open source. I have worked on internet protocols during work-hours (in addition to my regular spare-time working with them) and its been great! Heck, lots of the HTTP/2 development and the publication of that was made while I was employed by Mozilla and I fondly participated in that. I shall forever have this time ingrained in my memory as a very good period of my life.

I had already before I joined the Firefox development understood some of the challenges of making a browser in the modern era, but that understanding has now been properly enriched with lots of hands-on and code-digging in sometimes decades-old messy C++, a spaghetti armada of threads and the wild wild west of users on the Internet.

A very big thank you and a warm bye bye go to everyone of my friends at Mozilla. I won’t be far off and I’m sure I will have reasons to see many of you again.

My last day as officially employed by Mozilla is December 11 2018, but I plan to spend some of my remaining saved up vacation days before then so I’ll hand over most of my responsibilities way before.

The future is bright but unknown!

I don’t yet know what to do next.

I have some ideas and communications with friends and companies, but nothing is firmly decided yet. I will certainly entertain you with a totally separate post on this blog once I have that figured out! Don’t worry.

Will it affect curl or other open source I do?

I had worked on curl for a very long time already before joining Mozilla and I expect to keep doing curl and other open source things even going forward. I don’t think my choice of future employer should have to affect that negatively too much, except of course in periods.

With me leaving Mozilla, we’re also losing Mozilla as a primary sponsor of the curl project, since that was made up of them allowing me to spend some of my work days on curl and that’s now over.

Short-term at least, this move might increase my curl activities since I don’t have any new job yet and I need to fill my days with something…

What about toying with HTTP?

I was involved in the IETF HTTPbis working group for many years before I joined Mozilla (for over ten years now!) and I hope to be involved for many years still. I still have a lot of things I want to do with curl and to keep curl the champion of its class I need to stay on top of the game.

I will continue to follow and work with HTTP and other internet protocols very closely. After all curl remains the world’s most widely used HTTP client.

Can I enter the US now?

No. That’s unfortunately not related, and I’m not leaving Mozilla because of this problem and I unfortunately don’t expect my visa situation to change because of this change. My visa counter is now showing more than 214 days since I applied.

Get the CA cert for curl

When you use curl to communicate with a HTTPS site (or any other protocol that uses TLS), it will by default verify that the server is signed by a trusted Certificate Authority (CA). It does this by checking the CA bundle it was built to use, or instructed to use with the –cacert command line option.

Sometimes you end up in a situation where you don’t have the necessary CA cert in your bundle. It could then look something like this:

$ curl
curl: (60) SSL certificate problem: self signed certificate
More details here:

Do not disable!

A first gut reaction could be to disable the certificate check. Don’t do that. You’ll just make that end up in production or get copied by someone else and then you’ll spread the insecure use to other places and eventually cause a security problem.

Get the CA cert

I’ll show you four different ways to fix this.

1. Update your OS CA store

Operating systems come with a CA bundle of their own and on most of them, curl is setup to use the system CA store. A system update often makes curl work again.

This of course doesn’t help you if you have a self-signed certificate or otherwise use a CA that your operating system doesn’t have in its trust store.

2. Get an updated CA bundle from us

curl can be told to use a separate stand-alone file as CA store, and conveniently enough curl provides an updated one on the curl web site. That one is automatically converted from the one Mozilla provides for Firefox, updated daily. It also provides a little backlog so the ten most recent CA stores are available.

If you agree to trust the same CAs that Firefox trusts. This is a good choice.

3. Get it with openssl

Now we’re approaching the less good options. It’s way better to get the CA certificates via other means than from the actual site you’re trying to connect to!

This method uses the openssl command line tool. The servername option used below is there to set the SNI field, which often is necessary to tell the server which actual site’s certificate you want.

$ echo quit | openssl s_client -showcerts -servername server -connect server:443 > cacert.pem

A real world example, getting the certs for and then getting the main page with curl using them:

$ echo quit | openssl s_client -showcerts -servername -connect > cacert.pem

$ curl --cacert cacert.pem
4. Get it with Firefox

Suppose you’re browsing the site already fine with Firefox. Then you can do inspect it using the browser and export to use with curl.

Step 1 – click the i in the circle on the left of the URL in the address bar of your browser.

Step 2 – click the right arrow on the right side in the drop-down window that appeared.

Step 3 – new contents appeared, now click the “More Information” at the bottom, which pops up a new separate window…

Step 4 – Here you get security information from Firefox about the site you’re visiting. Click the “View Certificate” button on the right. It pops up yet another separate window.

Step 5 – in this window full of certificate information, select the “Details” tab…

Step 6 – when switched to the details tab, there’s the certificate hierarchy shown at the top and we select the top choice there. This list will of course look different for different sites

Step 7 – now click the “Export” tab at the bottom left and save the file (that uses a .crt extension) somewhere suitable.

If you for example saved the exported certificate using in /tmp, you could then use curl with that saved certificate something like this:

$ curl --cacert /tmp/GlobalSignRootCA-R3.crt

But I’m not using openssl!

This description assumes you’re using a curl that uses a CA bundle in the PEM format, which not all do – in particular not the ones built with NSS, Schannel (native Windows) or Secure Transport (native macOS and iOS) don’t.

If you use one of those, you need to then add additional command to import the PEM formatted cert into the particular CA store of yours.

A CA store is many PEM files concatenated

Just concatenate many different PEM files into a single file to create a CA store with multiple certificates.

curl 7.62.0 MOAR STUFF

This is a feature-packed release with more new stuff than usual.


the 177th release
10 changes
56 days (total: 7,419)

118 bug fixes (total: 4,758)
238 commits (total: 23,677)
5 new public libcurl functions (total: 80)
2 new curl_easy_setopt() options (total: 261)

1 new curl command line option (total: 219)
49 contributors, 21 new (total: 1,808)
38 authors, 19 new (total: 632)
  3 security fixes (total: 84)


New since the previous release is the dedicated curl bug bounty program. I’m not sure if this program has caused any increase in reports as it feels like a little too early to tell.

CVE-2018-16839 – an integer overflow case that triggers on 32 bit machines given extremely long input user name argument, when using POP3, SMTP or IMAP.

CVE-2018-16840 – a use-after-free issue. Immediately after having freed a struct in the easy handle close function, libcurl might write a boolean to that struct!

CVE-2018-16842 – is a vulnerability in the curl command line tool’s “warning” message display code which can make it read outside of a buffer and send unintended memory contents to stderr.

All three of these issues are deemed to have low severity and to be hard to exploit.

New APIs!

We introduce a brand new URL API, that lets applications parse and generate URLs, using libcurl’s own parser. Five new public functions in one go there! The link goes to the separate blog entry that explained it.

A brand new function is introduced (curl_easy_upkeep) to let applications maintain idle connections while no transfers are in progress! Perfect to maintain HTTP/2 connections for example that have a PING frame that might need attention.

More changes

Applications using libcurl’s multi interface will now get multiplexing enabled by default, and HTTP/2 will be selected for HTTPS connections. With these new changes of the default behavior, we hope that lots of applications out there just transparently and magically will start to perform better over time without anyone having to change anything!

We shipped DNS-over-HTTPS support. With DoH, your internet client can do secure and private name resolves easier. Follow the link for the full blog entry with details.

The good people at MesaLink has a TLS library written in rust, and in this release you can build libcurl to use that library. We haven’t had a new TLS backend supported since 2012!

Our default IMAP handling is slightly changed, to use the proper standards compliant “UID FETCH” method instead of just “FETCH”. This might introduce some changes in behavior so if you’re doing IMAP transfers, I advice you to mind your step into this upgrade.

Starting in 7.62.0, applications can now set the buffer size libcurl will use for uploads. The buffers used for download and upload are separate and applications have been able to specify the download buffer size for a long time already and now they can finally do it for uploads too. Most applications won’t need to bother about it, but for some edge case uses there are performance gains to be had by bumping this size up. For example when doing SFTP uploads over high latency high bandwidth connections.

curl builds that use libressl will now at last show the correct libressl version number in the “curl -V” output.

Deprecating legacy

CURLOPT_DNS_USE_GLOBAL_CACHE is deprecated! If there’s not a massive complaint uproar, this means this option will effectively be made pointless in April 2019. The global cache isn’t thread-safe and has been called obsolete in the docs since 2002!

HTTP pipelining support is deprecated! Starting in this version, asking for pipelining will be ignored by libcurl. We strongly urge users to switch to and use HTTP/2, which in 99% of the cases is the better alternative to HTTP/1.1 Pipelining. The pipelining code in libcurl has stability problems. The impact of disabled pipelining should be minimal but some applications will of course notice. Also note the section about HTTP/2 and multiplexing by default under “changes” above.

To get an overview of all things marked for deprecation in curl and their individual status check out this page.

Interesting bug-fixes

TLS 1.3 support for GnuTLS landed. Now you can build curl to support TLS 1.3 with most of the TLS libraries curl supports: GnuTLS, OpenSSL, BoringSSL, libressl, Secure Transport, WolfSSL, NSS and MesaLink.

curl got Windows VT Support and UTF-8 output enabled, which should make fancy things like “curl” to render nice outputs out of the box on Windows as well!

The TLS backends got a little cleanup and error code use unification so that they should now all return the same error code for the same problem no matter which backend you use!

When you use curl to do URL “globbing” as for example “curl http://localhost/[1-22]” to fetch a range or a series of resources and accidentally mess up the range, curl would previously just say that it detected an error in the glob pattern. Starting now, it will also try to show exactly where in which pattern it found the error that made it stop processing it.


The curl for Windows CI builds on AppVeyor are now finally also running the test suite! Actually making sure that the Windows build is intact in every commit and PR is a huge step forward for us and our aim to keep curl functional. We also build several additional and different build combinations on Windows in the CI than we did previously. All in an effort to reduce regressions.

We’ve added four new checks to travis (that run on every pull-request and commit):

  1. The “tidy” build runs clang-tidy on all sources in src/ and lib/.
  2. a –disable-verbose build makes sure this configure option still builds curl warning-free
  3. the “distcheck” build now scans all files for accidental unicode BOM markers
  4. a MesaLink-using build verifies this configuration

CI build times

We’re right now doing 40 builds on every commit, spending around 12 hours of CPU time for a full round. With >230 landed commits in the tree that originated from 150-something pull requests,  with a lot of them having been worked out using multiple commits, we’ve done perhaps 500 full round CI builds in these 56 days.

This of course doesn’t include all the CPU time developers spend locally before submitting PRs or even the autobuild system that currently runs somewhere in the order of 50 builds per day. If we assume an average time spent for each build+test to take 20 minutes, this adds another 930 hours of CI hours done from the time of the previous release until this release.

To sum up, that’s about 7,000 hours of CI spent in 56 days, equaling about 520% non-stop CPU time!

We are grateful for all the help we get!

Next release

The next release will ship on December 12, 2018 unless something urgent happens before that.

Note that this date breaks the regular eight week release cycle and is only six weeks off. We do this since the originally planned date would happen in the middle of Christmas when “someone” plans to be off traveling…

The next release will probably become 7.63.0 since we already have new changes knocking on the door waiting to get merged that will warrant another minor number bump. Stay tuned for details!

curl up 2019 will happen in Prague

The curl project is happy to invite you to the city of Prague, the Czech Republic, where curl up 2019 will take place.

curl up is our annual curl developers conference where we gather and talk Internet protocols, curl’s past, current situation and how to design its future. A weekend of curl.

Previous years we’ve gathered twenty-something people for an intimate meetup in a very friendly atmosphere. The way we like it!

In a spirit to move the meeting around to give different people easier travel, we have settled on the city of Prague for 2019, and we’ll be there March 29-31.

Sign up now!

Symposium on the Future of HTTP

This year, we’re starting off the Friday afternoon with a Symposium dedicated to “the future of HTTP” which is aimed to be less about curl and more about where HTTP is and where it will go next. Suitable for a slightly wider audience than just curl fans.

That’s Friday the 29th of March, 2019.

Program and talks

We are open for registrations and we would love to hear what you would like to come and present for us – on the topics of HTTP, of curl or related matters. I’m sure I will present something too, but it becomes a much better and more fun event if we distribute the talking as much as possible.

The final program for these days is not likely to get set until much later and rather close in time to the actual event.

The curl up 2019 wiki page is where you’ll find more specific details appear over time. Just go back there and see.

Helping out and planning?

If you want to follow the planning, help out, offer improvements or you have questions on any of this? Then join the curl-meet mailing list, which is dedicated for this!

Free or charge thanks to sponsors

We’re happy to call our event free, or “almost free” of charge and we can do this only due to the greatness and generosity of our awesome sponsors. This year we say thanks to Mullvad, Sticker Mule, Apiary and Charles University.

There’s still a chance for your company to help out too! Just get in touch.

curl up 2019 with logos

10,000 stars

On github, you can ‘star’ a project. It’s a fairly meaningless way to mark your appreciation of a project hosted on that site and of course, the number doesn’t really mean anything and it certainly doesn’t reflect how popular or widely used or unused that particular software project is. But here I am, highlighting the fact that today I snapped the screenshot shown above when the curl project just reached this milestone: 10,000 stars.

In the great scheme of things, the most popular and starred projects on github of course have magnitudes more stars. Right now, curl ranks as roughly the 885th most starred project on github. According to github themselves, they host an amazing 25 million public repositories which thus puts curl in the top 0.004% star-wise.

There was appropriate celebration going on in the Stenberg casa tonight and here’s a photo to prove it:

I took a photo when we celebrated 1,000 stars. It doesn’t feel so long ago but was a little over 1500 days ago.

August 12 2014

Onwards and upwards!

The Polhem prize, one year later

On September 25th 2017, I received the email that first explained to me that I had been awarded the Polhem Prize.

Du har genom ett omfattande arbete vaskats fram som en värdig mottagare av årets Polhemspris. Det har skett genom en nomineringskommitté och slutligen ett råd med bred sammansättning. Priset delas ut av Kungen den 19 oktober på Tekniska muséet.

My attempt of an English translation:

You have been selected as a worthy recipient of this year's Polhem prize through extensive work. It has been through a nomination committee and finally a council of broad composition. The prize is awarded by the King on October 19th at the Technical Museum.

A gold medal

At the award ceremony in October 2017 I received the gold medal at the most fancy ceremony I could ever wish for, where I was given the most prestigious award I couldn’t have imagined myself even being qualified for, handed over by no other than the Swedish King.

An entire evening with me in focus, where I was the final grand finale act and where my life’s work was the primary reason for all those people being dressed up in fancy clothes!

Things have settled down since. The gold medal has started to get a little dust on it where it lies here next to me on my work desk. I still glance at it every once in a while. It still feels surreal. It’s a fricking medal in pure gold with my name on it!

I almost forget the money part of the prize. I got a lot of money as well, but in retrospect it is really the honors, that evening and the gold medal that stick best in my memory. Money is just… well, money.

So did the award and prize make my life any different? Yes sure, a little, and I’ll tell you how.

What’s all that time spent on?

My closest surrounding of friends and family got a better understanding of what I’ve actually been doing all these long hours, all these years and more than one phrase in the style of “oh, so you actually did something useful?!” have been uttered.

Certainly I’ve tried to explain to them before, but nothing works as good as a gold medal from an award committee to say that what I do is actually appreciated “out there” and it has made a serious impact on the world.

I think I’m considered a little less weird now when I keep spending night hours in front of my computer when the house is otherwise dark and silent. Well, maybe still weird, but at least my weirdness has proven to result in something useful for mankind and that’s more than many other sorts of weird do… We all have hobbies.

What is curl?

Family and friends have gotten a rudimentary level of understanding of what curl is and what it does. I’m not suggesting they fully grasp it or know what an “internet protocol” is now, but at least a lot of people understand that it works with “internet transfers”. It’s not like people were totally uninterested before, but when I was given this prize – by a jury of engineers no less – that says this is a significant invention and accomplishment with a value that “can not be overestimated“, it made them more interested. The little video that was produced helped:

Some mysteries remain

People in general still have a hard time to grasp the reach of the project, how much time I’ve spent so far on it, how I can find motivation to keep up the work and not the least how this is all given away for free for everyone.

The simple fact that these are all questions that I’ve been asked I think is a small reward in itself. I think the fact that I was awarded this prize for my work on Open Source is awesome and I feel honored to be a person who introduces this way of thinking to some of the people who previously would think that you have to sell proprietary things or earn a lot of money for your products in order to impact and change society as a whole.

Not widely known

The Polhem prize is not widely known in Sweden among the general populace and thus neither is the fact that I won it. Only a very special subset of people know about this. Of course it is even less known outside of Sweden and in fact the information about the prize given in English is very sparse.

Next year’s winner

The other day I received my invitation to participate in this year’s award ceremony on November 14. Of course I’ll happily accept that and I will be there and celebrate the winner this year!

The curl project

How did the prize affect the project itself, the project that I was awarded for having cared for this long?

It hasn’t affected it much at all (as far as I can tell). The project has moved along like before and we’ve worked on fixing bugs and added features and cool things over time after my award just as we did before it. That’s how it has felt like. Business as usual.

If anything, I think I might have gotten some renewed energy and interest in the project and the commit author statistics actually show that my commit frequency has gone up since around the time I got the award. Our gitstats show that I’ve done more than half of the commits every single month the last year, most of this time even more than 70% of the commits.

I may have served twenty years here, but I’m not done yet!

More curl bug bounty

Together with Bountygraph, the curl project now offers money to security researchers for report security vulnerabilities to us.

The idea is that sponsors donate money to the bounty fund, and we will use that fund to hand out rewards for reported issues. It is a way for the curl project to help compensate researchers for the time and effort they spend helping us improving our security.

Right now the bounty fund is very small as we just started this project, but hopefully we can get a few sponsors interested and soon offer “proper” rewards at decent levels in case serious flaws are detected and reported here.

If you’re a company using curl or libcurl and value security, you know what you can do…

Already before, people who reported security problems could ask for money from Hackerone’s IBB program, and this new program is in addition to that – even though you won’t be able to receive money from both bounties for the same issue.

After I announced this program on twitter yesterday, I did an interview with Arif Khan for Here’s what I had to say:

A few questions

Q: You have launched a self-managed bug bounty program for the first time. Earlier, IBB used to pay out for most security issues in libcurl. How do you think the idea of self-management of a bug bounty program, which has some obvious problems such as active funding might eventually succeed?

First, this bounty program is run on so I wouldn’t call it “self-managed” since we’re standing on a lot of infra setup and handled by others.

To me, this is an attempt to make a bounty program that is more visible as clearly a curl bounty program. I love Hackerone and the IBB program for what they offer, but it is A) very generic, so the fact that you can get money for curl flaws there is not easy to figure out and there’s no obvious way for companies to sponsor curl security research and B) they are very picky to which flaws they pay money for (“only critical flaws”) and I hope this program can be a little more accommodating – assuming we get sponsors of course.

Will it work and make any differences compared to IBB? I don’t know. We will just have to see how it plays out.

Q: How do you think the crowdsourcing model is going to help this bug bounty program?

It’s crucial. If nobody sponsors this program, there will be no money to do payouts with and without payouts there are no bounties. Then I’d call the curl bounty program a failure. But we’re also not in a hurry. We can give this some time to see how it works out.

My hope is though that because curl is such a widely used component, we will get sponsors interested in helping out.

Q: What would be the maximum reward for most critical a.k.a. P0 security vulnerabilities for this program?

Right now we have a total of 500 USD to hand out. If you report a p0 bug now, I suppose you’ll get that. If we just get sponsors, I’m hoping we should be able to raise that reward level significantly. I might be very naive, but I think we won’t have to pay for very many critical flaws.

It goes back to the previous question: this model will only work if we get sponsors.

Q: Do you feel there’s a risk that bounty hunters could turn malicious?

I don’t think this bounty program particularly increases or reduces that risk to any significant degree. Malicious hunters probably already exist and I would assume that blackhat researchers might be able to extract more money on the less righteous markets if they’re so inclined. I don’t think we can “outbid” such buyers with this program.

Q: How will this new program mutually benefit security researchers as well as the open source community around curl as a whole?

Again, assuming that this works out…

Researchers can get compensated for the time and efforts they spend helping the curl project to produce and provide a more secure product to the world.

curl is used by virtually every connected device in the world in one way or another, affecting every human in the connected world on a daily basis. By making sure curl is secure we keep users safe; users of countless devices, applications and networked infrastructure.

Update: just hours after this blog post, Dropbox chipped in 32,768 USD to the curl bounty fund…

The world’s biggest curl installations

curl is quite literally used everywhere. It is used by a huge number of applications and devices. But which applications, devices and users are the ones with the largest number of curl installations? I’ve tried to come up with a list…

I truly believe curl is one of the world’s most widely used open source projects.

If you have comments, other suggestions or insights to help me polish this table or the numbers I present, please let me know!

Some that didn’t make the top-10

10 million Nintendo Switch game consoles all use curl, more than 20 million Chromebooks have been sold and they have curl as part of their bundled OS and there’s an estimated 40 million printers (primarily by Epson and HP) that aren’t on the top-10. To reach this top-list, we’re looking at 50 million instances minimum…

10. Internet servers: 50 million

There are many (Linux mainly) servers on the Internet. curl and libcurl comes pre-installed on some Linux distributions and for those that it doesn’t, most users and sysadmins install it. My estimate says there are few such servers out there without curl on them.

This source says there were 75 million servers “hosting the Internet” back in 2013.

curl is a default HTTP provider for PHP and a huge percentage of the world’s web sites run at least parts with PHP.

9. Sony Playstation 4: 75 million

Bundled with the Operating system on this game console comes curl. Or rather libcurl I would expect. Sony says 75 million units have been sold.

curl is given credit on the screen Open Source software used in the Playstation 4.

8. Netflix devices: 90 million

I’ve been informed by “people with knowledge” that libcurl runs on all Netflix’s devices that aren’t browsers. Some stats listed on the Internet says 70% of the people watching Netflix do this on their TVs, which I’ve interpreted as possible non-browser use. 70% of the total 130 million Netflix customers makes 90.

libcurl is not used for the actual streaming of the movie, but for the UI and things.

7. Grand Theft Auto V: 100 million

The very long and rarely watched ending sequence to this game does indeed credit libcurl. It has also been recorded as having been sold in 100 million copies.

There’s an uncertainty here if libcurl is indeed used in this game for all platforms GTA V runs on, which then could possibly reduce this number if it is not.

6. macOS machines: 100 million

curl has shipped as a bundled component of macOS since August 2001. In April 2017, Apple’s CEO Tim Cook says that there were 100 million active macOS installations.

Now, that statement was made a while ago but I don’t have any reason to suspect that the number has gone down notably so I’m using it here. No macs ship without curl!

5. cars: 100 million

I wrote about this in a separate blog post. Eight of the top-10 most popular car brands in the world use curl in their products. All in all I’ve found curl used in over twenty car brands.

Based on that, rough estimates say that there are over 100 million cars in the world with curl in them today. And more are coming.

4. Fortnite: 120 million

This game is made by Epic Games and credits curl in their Third Party Software screen.

In June 2018, they claimed 125 million players. Now, I supposed a bunch of these players might not actually have their own separate device but I still believe that this is the regular setup for people. You play it on your own console, phone or computer.

3. Television sets: 380 million

We know curl is used in television sets made by Sony, Philips, Toshiba, LG, Bang & Olufsen, JVC, Panasonic, Samsung and Sharp – at least.

The wold market was around 229 million television sets sold in 2017 and about 760 million TVs are connected to the Internet. Counting on curl running in 50% of the connected TVs (which I think is a fair estimate) makes 380 million devices.

2. Windows 10: 500 million

Since a while back, Windows 10 ships curl bundled by default. I presume most Windows 10 installations actually stay fairly updated so over time most of the install base will run a version that bundles curl.

In May 2017, one number said 500 million Windows 10 machines.

1. Smart phones: 3000 million

I posit that there are almost no smart phones or tablets in the world that doesn’t run curl.

curl is bundled with the iOS operating system so all iPhones and iPads have it. That alone is about 1.3 billion active devices.

curl is bundled with the Android version that Samsung, Xiaomi and OPPO ship (and possibly a few other flavors too). According to some sources, Samsung has something like 30% market share, and Apple around 20% – for mobile phones. Another one billion devices seems like a fair estimate.

Further, curl is used by some of the most used apps on phones: Youtube, Instagram, Skype, Spotify etc. The three first all boast more than one billion users each, and in Youtube’s case it also claims more than one billion app downloads on Android. I think it’s a safe bet that these together cover another 700 million devices. Possibly more.

Same users, many devices

Of course we can’t just sum up all these numbers and reach a total number of “curl users”. The fact is that a lot of these curl instances are used by the same users. With a phone, a game console, a TV and some more an ordinary netizen runs numerous different curl instances in their daily lives.


Did I ever expect this level of success? No.


libcurl gets a URL API

libcurl has done internet transfers specified as URLs for a long time, but the URLs you’d tell libcurl to use would always just get parsed and used internally.

Applications that pass in URLs to libcurl would of course still very often need to parse URLs, create URLs or otherwise handle them, but libcurl has not been helping with that.

At the same time, the under-specification of URLs has led to a situation where there’s really no stable document anywhere describing how URLs are supposed to work and basically every implementer is left to handle the WHATWG URL spec, RFC 3986 and the world in between all by themselves. Understanding how their URL parsing libraries, libcurl, other tools and their favorite browsers differ is complicated.

By offering applications access to libcurl’s own URL parser, we hope to tighten a problematic vulnerable area for applications where the URL parser library would believe one thing and libcurl another. This could and has sometimes lead to security problems. (See for example Exploiting URL Parser in Trending Programming Languages! by Orange Tsai)

Additionally, since libcurl deals with URLs and virtually every application using libcurl already does some amount of URL fiddling, it makes sense to offer it in the “same package”. In the curl user survey 2018, more than 40% of the users said they’d use an URL API in libcurl if it had one.

Handle based

Create a handle, operate on the handle and then cleanup the handle when you’re done with it. A pattern that is familiar to existing users of libcurl.

So first you just make the handle.

/* create a handle */
CURLU *h = curl_url();

Parse a URL

Give the handle a full URL.

/* "set" a URL in the handle */
curl_url_set(h, CURLUPART_URL,
    "", 0);

If the parser finds a problem with the given URL it returns an error code detailing the error.  The flags argument (the zero in the function call above) allows the user to tweak some parsing behaviors. It is a bitmask and all the bits are explained in the curl_url_set() man page.

A parsed URL gets split into its components, parts, and each such part can be individually retrieved or updated.

Get a URL part

Get a separate part from the URL by asking for it. This example gets the host name:

/* extract host from the URL */
char *host;
curl_url_get(h, CURLUPART_HOST, &host, 0);

/* use it, then free it */

As the example here shows, extracted parts must be specifically freed with curl_free() once the application is done with them.

The curl_url_get() can extract all the parts from the handle, by specifying the correct id in the second argument. scheme, user, password, port number and more. One of the “parts” it can extract is a bit special: CURLUPART_URL. It returns the full URL back (normalized and using proper syntax).

curl_url_get() also has a flags option to allow the application to specify certain behavior.

Set a URL part

/* set a URL part */
curl_url_set(h, CURLUPART_PATH,
  "/index.html", 0);

curl_url_set() lets the user set or update all and any of the individual parts of the URL.

curl_url_set() can also update the full URL, which also accepts a relative URL in case an existing one was already set. It will then apply the relative URL onto the former one and “transition” to the new absolute URL. Like this;

/* first an absolute URL */
curl_url_set(h, CURLUPART_URL,
  "", 0);

/* .. then we set a relative URL "on top" */
curl_url_set(h, CURLUPART_URL,
   "../new/place", 0);

Duplicate a handle

It might be convenient to setup a handle once and then make copies of that…

CURLU *n = curl_url_dup(h);

Cleanup the handle

When you’re done working with this URL handle, free it and all its related resources.



This API is marked as experimental for now and ships for the first time in libcurl 7.62.0 (October 31, 2018). I will happily read your feedback and comments on how it works for you, what’s missing and what we should fix to make it even more usable for you and your applications!

We call it experimental to reserve the right to modify it slightly  going forward if necessary, and as soon as we remove that label the API will then be fixed and stay like that for the foreseeable future.

See also

The URL API wiki page.

DoH in curl

DNS-over-HTTPS (DoH) is being designed (it is not an RFC quite yet but very soon!) to allow internet clients to get increased privacy and security for their name resolves. I’ve previously explained the DNS-over-HTTPS functionality within Firefox that ships in Firefox 62 and I did a presentation about DoH and its future in curl at curl up 2018.

We are now introducing DoH support in curl. I hope this will not only allow users to start getting better privacy and security for their curl based internet transfers, but ideally this will also provide an additional debugging tool for DoH in other clients and servers.

Let’s take a look at how we plan to let applications enable this when using libcurl and how libcurl has to work with this internally to glue things together.

How do I make my libcurl transfer use DoH?

There’s a primary new option added, which is the “DoH URL”. An application sets the CURLOPT_DOH_URL for a transfer, and then libcurl will use that service for resolving host names. Easy peasy. There should be nothing else in the transfer that changes or appears differently. It’ll just resolve the host names over DoH instead of using the default resolver!

What about bootstrap, how does libcurl find the DoH server’s host name?

Since the DoH URL itself typically is given using a host name, that first host name will be resolved using the normal resolver – or if you so desire, you can provide the IP address for that host name with the CURLOPT_RESOLVE option just like you can for any host name.

If done using the resolver, the resolved address will then be kept in libcurl’s DNS cache for a short while and the DoH connection will be kept in the regular connection pool with the other connections, making subsequent DoH resolves on the same handle much faster.

How do I use this from the command line?

Tell curl which DoH URL to use with the new –doh-url command line option:

$ curl --doh-url

How do I make my libcurl code use this?

curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL,
curl_easy_setopt(curl, CURLOPT_DOH_URL,
res = curl_easy_perform(curl);


Internally, libcurl itself creates two new easy handles that it adds to the existing multi handles and they are then performing two HTTP requests while the original transfer sits in the “waiting for name resolve” state. Once the DoH requests are completed, the original transfer’s state can progress and continue on.

libcurl handles parallel transfers perfectly well already and by leveraging the already existing support for this, it was easy to add this new functionality and still work non-blocking and even event-based correctly depending on what libcurl API that is being used.

We had to add a new little special thing that makes libcurl handle the end of a transfer in a new way since there are now easy handles that are created and added to the multi handle entirely without the user’s knowledge, so the code also needs to remove and delete those handles when they’re done serving their purposes.

Was this hard to add to a 20 year old code base?

Actually, no. It was surprisingly easy, but then I’ve also worked on a few different client-side DoH implementations already so I had gotten myself a clear view of how I wanted the functionality to work plus the fact that I’m very familiar with the libcurl internals.

Plus, everything inside libcurl is already using non-blocking code and the multi interface paradigms so the foundation for adding parallel transfers like this was already in place.

The entire DoH patch for curl, including documentation and test cases, was a mere 1500 lines.


This is merged into the master branch in git and is planned to ship as part of the next release: 7.62.0 at the end of October 2018.