cURL and libcurl

Why is curl different everywhere?

1 Comment

At a talk I did a while ago, someone from the back of the audience raised this question. I found it to be such a great question that I decided to spend a few minutes and explain how this happens and why.

In this blog post I’ll stick to discussing the curl command line tool. “curl” is often also used as a shortcut for the library but let’s focus on the tool here.

When you use a particular curl version installed in a system near you, chances are that it differs slightly from the curl your neighbor runs or even the one that you use in the machines at work.

Why is this?

Versions

We release a new curl version every eight weeks. On average we ship over thirty releases in a five-year period.

A lot of people use curl versions that are a few years old, some even many years old. There are easily more than 30 different curl version in active use at any given moment.

Not every curl release introduce changes and new features, but it is very common and all releases are at least always corrected a lot of bugs from previous versions. New features and fixed bugs make curl different between releases.

Linux/OS distributions tend to also patch their curl versions at times, and then they all of course have different criteria and work flows, so the exact same curl version built and shipped from two different vendors can still differ!

Platforms

curl builds on almost every platform you can imagine. When you build curl for your platform, it is designed to use features, native APIs and functions available and they will indeed differ between systems.

curl also relies on a number of different third party libraries. The set of libraries a particular curl build is set to use varies by platform, but even more so due to the decisions of the persons or group that built this particular curl executable. The exact set, and the exact versions of each of those third party libraries, will change curl’s feature set, from subtle and small changes up to large really noticeable differences.

TLS libraries

As a special third party library, I want to especially highlight the importance of the TLS library that curl is built to use. It will change not only what SSL and TLS versions curl supports, but also how to handle CA certificates, it provides crypto support for authentication schemes such as NTLM and more. Not to mention that of course TLS libraries also develop over time so if curl is built to use an older release, it probably has less support for later features and protocol versions.

Feature shaving

When building curl, you can switch features on and off to a very large extent, making it possible to quite literally build it in several million different combinations. The organizations, people and companies that build curl to ship with their operating systems or their package distribution systems decide what feature set they want or don’t want for their users. One builder’s decision and thought process certainly does not have to match the ones of the others’. With the same curl version, the same TLS library on the same operating system two curl builds might thus still end up different!

Build your own!

If you aren’t satisfied with the version or feature-set of your own locally installed curl – build your own!

cURL and libcurl

7.63.0 – another step down the endless path

This curl release was developed and put together over a period of six weeks (two weeks less than usual). This was done to accommodate to my personal traveling plans – and to avoid doing a release too close to Christmas in case we would ship any security fixes, but ironically, we have no security advisories this time!

Numbers

the 178th release
3 changes
42 days (total: 7,572)
79 bug fixes (total: 4,837)
122 commits (total: 23,799)
0 new public libcurl functions (total: 80)
1 new curl_easy_setopt() options (total: 262)
0 new curl command line option (total: 219)
51 contributors, 21 new (total: 1,829)
31 authors, 14 new (total: 646)
  0 security fixes (total: 84)

Changes

With the new CURLOPT_CURLU option, an application can now  pass in an already parsed URL to libcurl instead of a string.

When using libcurl’s URL API, introduced in 7.62.0, the result is held in a “handle” and that handle is what now can be passed straight into libcurl when setting up a transfer.

In the command line tool, the –write-out option got the ability to optionally redirect its output to stderr. Previously it was always a given file or stdout but many people found that a bit limiting.

Interesting bug-fixes

Weirdly enough we found and fixed a few cookie related bugs this time. I say “weirdly” because you’d think this is functionality that’s been around for a long time and should’ve been battle tested and hardened quite a lot already. As usual, I’m only covering some bugs here. The full list is in the changelog!

Cookie saving –  One cookie bug that we fixed was related to libcurl not saving a cookie jar when no cookies are kept in memory (any more). This turned out to be a changed behavior due to us doing more aggressive expiry of old cookies since a while back, and one user had a use case where they would load cookies from a cookie jar and then expect that the cookies would update and write to the jar again, overwriting the old one – although when no cookies were left internally it didn’t touch the file and the application thus reread the old cookies again on the next invoke. Since this was subtly changed behavior, libcurl will now save an empty jar in this situation to make sure such apps will note the blank jar.

Cookie expiry – For the received cookies that get ‘Max-Age=0’ set, curl would treat the zero value the same way as any number and therefore have the cookie continue to exist during the whole second it arrived (time() + 0 basically). The cookie RFC is actually rather clear that receiving a zero for this parameter is a special case and means that it should rather expire it immediately and now curl does.

Timeout handling – when calling curl_easy_perform() to do a transfer, and you ask libcurl to timeout that transfer after say 5.1 seconds, the transfer hasn’t completed in that time and the connection is in fact totally idle at that time, a recent regression would make libcurl not figure this out until a full 6 seconds had elapsed.

NSS – we fixed several minor  issues in the NSS back-end this time. Perhaps the most important issue was if the installed NSS library has been built with TLS 1.3 disabled while curl was built knowing about TLS 1.3, as then things like the ‘–tlsv1.2’ option would still cause errors. Now curl will fall back correctly. Fixes were also made to make sure curl again works with NSS versions back to 3.14.

OpenSSL – with TLS 1.3 session resumption was changed for TLS, but now curl will support it with OpenSSL.

snprintf – curl has always had its own implementation of the *printf() family of functions for portability reasons. First, traditionally snprintf() was not universally available but then also different implementations have different support for things like 64 bit integers or size_t fields and they would disagree on return values. Since curl’s snprintf() implementation doesn’t use the same return code as POSIX or other common implementations we decided we shouldn’t use the same name so that we don’t fool readers of code into believing that they are fully compatible. For that reason, we now also “ban” the use of snprintf() in the curl code.

URL parsing – there were several regressions from the URL parsing news introduced in curl 7.62.0. That was the first release that offers the new URL API for applications, and we also then switched the internals to use that new code. Perhaps the funniest error was how a short name plus port number (hello:80) was accidentally treated as a “scheme” by the parser and since the scheme was unknown the URL was rejected. The numerical IPv6 address parser was also badly broken – I take the blame for not writing good enough test cases for it which made me not realize this in time. Two related regressions that came from the URL  work broke HTTP Digest auth and some LDAP transfers.

DoH over HTTP/1 – DNS-over-HTTPS was simply not enabled in the build if HTTP/2 support wasn’t there, which was an unnecessary restriction and now h2-disabled builds will also be able to resolve host names using DoH.

Trailing dots in host name – an old favorite subject came back to haunt us and starting in this version, curl will keep any trailing dot in the host name when it resolves the name, and strip it off for all the rest of the uses where the name will be passed in: for cookies, for the HTTP Host: header and for the TLS SNI field. This, since most resolver APIs makes a difference between resolving “host” compared to “host.” and we wouldn’t previously acknowledge or support the two versions.

HTTP/2 – When we enabled HTTP/2 by default for more transfers in 7.62.0, we of course knew that could force more latent bugs to float up to the surface and get noticed. We made curl understand  HTTP_1_1_REQUIRED error when received over HTTP/2 and then retry over HTTP/1.1. and if NTLM is selected as the authentication to use curl now forces HTTP/1 use.

Next release

We have suggested new features already lined up waiting to get merged so the next version is likely to be called 7.64.0 and it is scheduled to happen on February 6th 2019.

Books, Network, Web

HTTP/3 Explained

I’m happy to tell that the booklet HTTP/3 Explained is now ready for the world. It is entirely free and open and is available in several different formats to fit your reading habits. (It is not available on dead trees.)

The book describes what HTTP/3 and its underlying transport protocol QUIC are, why they exist, what features they have and how they work. The book is meant to be readable and understandable for most people with a rudimentary level of network knowledge or better.

These protocols are not done yet, there aren’t even any implementation of these protocols in the main browsers yet! The book will be updated and extended along the way when things change, implementations mature and the protocols settle.

If you find bugs, mistakes, something that needs to be explained better/deeper or otherwise want to help out with the contents, file a bug!

It was just a short while ago I mentioned the decision to change the name of the protocol to HTTP/3. That triggered me to refresh my document in progress and there are now over 8,000 words there to help.

The entire HTTP/3 Explained contents are available on github.

If you haven’t caught up with HTTP/2 quite yet, don’t worry. We have you covered for that as well, with the http2 explained book.

cURL and libcurl, Mozilla, Network, Work

I’m leaving Mozilla

8 Comments

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.

Network, Web

HTTP/3

33 Comments

The protocol that’s been called HTTP-over-QUIC for quite some time has now changed name and will officially become HTTP/3. This was triggered by this original suggestion by Mark Nottingham.

The QUIC Working Group in the IETF works on creating the QUIC transport protocol. QUIC is a TCP replacement done over UDP. Originally, QUIC was started as an effort by Google and then more of a “HTTP/2-encrypted-over-UDP” protocol.

When the work took off in the IETF to standardize the protocol, it was split up in two layers: the transport and the HTTP parts. The idea being that this transport protocol can be used to transfer other data too and its not just done explicitly for HTTP or HTTP-like protocols. But the name was still QUIC.

People in the community has referred to these different versions of the protocol using informal names such as iQUIC and gQUIC to separate the QUIC protocols from IETF and Google (since they differed quite a lot in the details). The protocol that sends HTTP over “iQUIC” was called “hq” (HTTP-over-QUIC) for a long time.

Mike Bishop scared the room at the QUIC working group meeting in IETF 103 when he presented this slide with what could be thought of almost a logo…

On November 7, 2018 Dmitri of Litespeed announced that they and Facebook had successfully done the first interop ever between two HTTP/3 implementations. Mike Bihop’s follow-up presentation in the HTTPbis session on the topic can be seen here. The consensus in the end of that meeting said the new name is HTTP/3!

No more confusion. HTTP/3 is the coming new HTTP version that uses QUIC for transport!

cURL and libcurl, Firefox, Security

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 https://example.com/
curl: (60) SSL certificate problem: self signed certificate
More details here: https://curl.se/docs/sslcerts.html

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 daniel.haxx.se and then getting the main page with curl using them:

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

$ curl --cacert cacert.pem https://daniel.haxx.se
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 https://curl.se

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 and libcurl

curl 7.62.0 MOAR STUFF

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

Numbers

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)

Security

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 wttr.in” 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.

CI

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 and libcurl, Network, Open Source

curl up 2019 will happen in Prague

2 Comments

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

Network, Security

DNS-over-HTTPS is RFC 8484

The protocol we fondly know as DoH, DNS-over-HTTPS, is now  officially RFC 8484 with the official title “DNS Queries over HTTPS (DoH)”. It documents the protocol that is already in production and used by several client-side implementations, including Firefox, Chrome and curl. Put simply, DoH sends a regular RFC 1035 DNS packet over HTTPS instead of over plain UDP.

I’m happy to have contributed my little bits to this standard effort and I’m credited in the Acknowledgements section. I’ve also implemented DoH client-side several times now.

Firefox has done studies and tests in cooperation with a CDN provider (which has sometimes made people conflate Firefox’s DoH support with those studies and that operator). These studies have shown and proven that DoH is a working way for many users to do secure name resolves at a reasonable penalty cost. At least when using a fallback to the native resolver for the tricky situations. In general DoH resolves are slower than the native ones but in the tail end, the absolutely slowest name resolves got a lot better with the DoH option.

To me, DoH is partly necessary because the “DNS world” has failed to ship and deploy secure and safe name lookups to the masses and this is the one way applications “one layer up” can still secure our users.

cURL and libcurl

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!

curl, open source and networking