Category Archives: Open Source

Open Source, Free Software, and similar

curl better – video

As so many other events in these mysterious times, the foss-north conference went online-only and on March 30, 2020 I was honored to be included among the champion speakers at this lovely conference and I talked about how to “curl better” there.

The talk is a condensed run-through of how curl works and why, and then a look into how some of the more important HTTP oriented command line options work and how they’re supposed to be used.

As someone pointed out: I don’t do a lot of presentations about the curl tool. Maybe I should do more of these.

curl is widely used but still most users only use a very small subset of options or even just copy their command line from somewhere else. I think more users could learn to curl better. Below is the video of this talk.

Doing a talk to a potentially large audience in front of your laptop in completely silence and not seeing a single audience member is a challenge. No “contact” with the audience and no feel for if they’re all going to sleep or seem interested etc. Still I have the feeling that this is the year we all are going to do this many times and hopefully get better at it over time…

curl is not removing FTP

FTP is going out of style.

The Chrome team has previously announced that they are deprecating and removing support for FTP.

Mozilla also announced their plan for the deprecation of FTP in Firefox.

Both browsers have paused or conditioned their efforts to not take the final steps during the Covid-19 outbreak, but they will continue and the outcome is given: FTP support in browsers is going away. Soon.


curl supported both uploads and downloads with FTP already in its first release in March 1998. Which of course was many years before either of those browsers mentioned above even existed!

In the curl project, we work super hard and tirelessly to maintain backwards compatibility and not break existing scripts and behaviors.

For these reasons, curl will not drop FTP support. If you have legacy systems running FTP, curl will continue to have your back and perform as snappy and as reliably as ever.

FTP the protocol

FTP is a protocol that is quirky to use over the modern Internet mostly due to its use of two separate TCP connections. It is unencrypted in its default version and the secured version, FTPS, was never supported by browsers. Not to mention that the encrypted version has its own slew of issues when used through NATs etc.

To put it short: FTP has its issues and quirks.

FTP use in general is decreasing and that is also why the browsers feel that they can take this move: it will only negatively affect a very minuscule portion of their users.


FTP is however still used in places. In the 2019 curl user survey, more than 29% of the users said they’d use curl to transfer FTP within the last two years. There’s clearly a long tail of legacy FTP systems out there. Maybe not so much on the public Internet anymore – but in use nevertheless.

Alternative protocols?

SFTP could have become a viable replacement for FTP in these cases, but in practice we’ve moved into a world where HTTPS replaces everything where browsers are used.


Train image by D Thory from Pixabay

curl + MQTT = true

This is the 25th transfer protocol added to curl. The first new addition since we added SMB and SMBS back in November 2014.


Back in early 2019, my brother Björn Stenberg brought a pull request to the curl project that added support for MQTT. I tweeted about it and it seemed people were interested in seeing this happen.

Time passed and Björn unfortunately didn’t manage to push his work forward and instead it grew stale and the PR eventually was closed due to that inactivity later the same year.

Roadmap 2020

In my work trying to go over and figure out what I want to see in curl the coming year and what we (wolfSSL) as a company would like to see being done, MQTT qualified as a contender for the list. See my curl roadmap 2020 video.

It’s happening again

I grabbed Björn’s old pull-request and rebased it onto git master, fixed a few minor conflicts and small cleanups necessary and then brought it further. I documented two of my early sessions on this, live-streamed on twitch. See MQTT in curl and MQTT part two below:


Björn’s code was an excellent start but didn’t take us all the way.

I wrote an MQTT test server, created a set of test cases, made sure the code worked for those test cases, made it more solid and more. It is still early days and the MQTT support is basic and comes with several caveats, but it’s slowly getting there.

MQTT – really?

When I say that MQTT almost fits the curl concepts and paradigms, I mean that you can consider what an MQTT client does to be “sending” and “receiving” and you can specify that with a URL.

Fetching an MQTT URL with curl means doing SUSCRIBE on a topic and waiting for that to arrive and get the payload sent to the output.

Doing the equivalent of a HTTP POST with curl, like with the command line’s -d option makes an MQTT PUBLISH and sends a payload to a topic.

Rough corners and wrong assumptions

I’m an MQTT rookie. I’m sure there will be mistakes and I will have misunderstood things. The MQTT will be considered experimental for a time forward so that people will get a chance to verify the functionality and we have a chance to change and correct the worst decisions and fatal mistakes. Remember that for experimental features in curl, we reserve ourselves the right to change behavior, API and ABI so nobody should ship such features enabled anywhere without first thinking it through very carefully!

If you’re a person who think MQTT in curl would be useful, good or just fun and you have use cases or ideas where you’d want to use this. Please join in and try and let us know how it works and what you think we should polish or fix to make it truly stellar!

The code is landed in the master branch since PR 5173 was merged. The code will be present in the coming 7.70.0 release, due to ship on April 29 2020.


As I write this, the MQTT support is still very basic. I want a first version out to users as early as possible as I want to get feedback and comments to help verify that we’re in the right direction and then work on making the support of the protocol more complete. TLS, authentication, QoS and more will come as we proceed. Of course, if you let me know what we must support for MQTT to make it interesting for you, I’ll listen! Preferably, you do the discussions on the curl-library mailing list.

We’ve only just started.


The initial MQTT patch that kicked us off was written by Björn Stenberg. I brought it forward from there, bug-fixed it, extended it, added a test server and test cases and landed the lot in the master branch.

The queuing top image by DaKub from Pixabay

curl ootw: –append

Previously mentioned command line options of the week.

--append is the long form option, -a is the short. The option has existed since at least May 1998 (present in curl 4.8). I think it is safe to say that if we would’ve created this option just a few years later, we would not have “wasted” a short option letter on it. It is not a very frequently used one.

Append remotely

The append in the option name is a require to the receiver to append to — rather than replace — a destination file. This option only has any effect when uploading using either FTP(S) or SFTP. It is a flag option and you use it together with the --upload option.

When you upload to a remote site with these protocols, the default behavior is to overwrite any file that happens to exist on the server using the name we’re uploading to. If you append this option to the command line, curl will instead instruct the server to append the newly uploaded data to the end of the remote file.

The reason this option is limited to just subset of protocols is of course that they are the only ones for which we can give that instruction to the server.


Append the local file “trailer” to the remote file called “begin”:

curl --append --upload trailer

A QQGameHall storm

Mar 31 2020, 11:13:38: I get a message from Frank in the #curl IRC channel over on Freenode. I’m always “hanging out” on IRC and Frank is a long time friend and fellow frequent IRCer in that channel. This time, Frank informs me that the curl web site is acting up:

“I’m getting 403s for some mailing list archive pages. They go away when I reload”

That’s weird and unexpected. An important detail here is that the curl web site is “CDNed” by Fastly. This means that every visitor of the web site is actually going to one of Fastly’s servers and in most cases they get cached content from those servers, and only infrequently do these servers come back to my “origin” server and ask for an updated file to send out to a web site visitor.

A 403 error for a valid page is not a good thing. I started checking out some of my logs – which then only are for the origin as I don’t do any logging at all at CDN level (more about that later) – and I could verify the 403 errors. So they’re in my log meaning it isn’t caused by (a misconfiguration of) the CDN. Why would a perfectly legitimate URL suddenly return 403 to have it go away again after a reload?

Why does he get a 403?

I took a look at Fastly’s management web interface and I spotted that the curl web site was sending out data at an unusual high speed at the moment. An average speed of around 50mbps, while we typically average at below 20. Hm… something is going on.

While I continued to look for the answers to these things I noted that my logs were growing really rapidly. There were POSTs being sent to the same single URL at a high frequency (10-20 reqs/second) and each of those would get some 225Kbytes of data returned. And they all used the same User-agent: QQGameHall. It seems this started within the last 24 hours or so. They’re POSTs so Fastly basically always pass them through to my server.

Before I could figure out Franks’s 403s, I decided to slow down this madness by temporarily forbidding this user-agent access so that the bot or program or whatever would notice it starts to fail, and it would of course then stop bombarding the site.


Ok, a quick deny of the user-agent made my server start responding with 403s to all those requests and instead of a 225K response it now sent back 465 bytes per request. The average bandwidth on the site immediately dropped down to below 20Mbps again. Back to looking for Frank’s 403-problem

First the 403s seen due to the ratelimiting, then I removed the ratelmiting and finally I added a block of the user-agent. Screenshotted error rates from Fastly’s admin interface. This is errors per minute.

The answer was pretty simple and I didn’t have to search a lot. The clues existed in the error logs and it turned out we had “mod_evasive” enabled since another heavy bot load “attack” a while back. It is a module for “rate limiting” incoming requests and since a lot of requests to our server now comes from Fastly’s limited set of IP addresses and we had this crazy QQ thing hitting us, my server would return a 403 every now and then when it considered the rate too high.

I whitelisted Fastly’s requests and Frank’s 403 problems were solved.

Deny a level up

The bot traffic showed no sign of slowing down. Easily 20 requests per second, to the same URL and they all get an error back and obviously they don’t care. I decided to up my game a little so with help, I moved my blocking of this service to Fastly. I now block their user-agent already there so the traffic doesn’t ever reach my server. Phew, my server was finally back to its regular calm state. They way it should be.

It doesn’t stop there. Here’s a follow-up graph I just grabbed, a little over a week since I started the blocking. 16.5 million blocked requests (and counting). This graph here shows number of requests/hour on the Y axis, peeking at almost 190k; around 50 requests/second. The load is of course not actually a problem, just a nuisance now. QQGameHall keeps on going.

Errors per hour over the period of several days.


What we know about this.

Friends on Twitter and googling for this name informs us that this is a “game launcher” done by Tencent. I’ve tried to contact them via Twitter (as I have no means of contacting them otherwise that seems even remotely likely to work).

I have not checked what these user-agent POSTs, because I didn’t log that. I suspect it was just a zero byte POST.

The URL they post to is the CA cert bundle file with provide on the curl CA extract web page. The one we convert from the Mozilla version into a PEM for users of the world to enjoy. (Someone seems to enjoy this maybe just a little too much.)

The user-agents seemed to come (mostly) from China which seems to add up. Also, the look of the graph when it goes up and down could indicate an eastern time zone.

This program uses libcurl. Harry in the #curl channel found files in Virus Total and had a look. It is, I think, therefore highly likely that this “storm” is caused by an application using curl!

My theory: this is some sort of service that was deployed, or an upgrade shipped, that wants to get an updated CA store and they get that from our site with this request. Either they get it far too often or maybe there are just a very large amount them or similar. I cannot understand why they issue a POST though. If they would just have done a GET I would never have noticed and they would’ve fetched perfectly fine cached versions from the CDN…

Feel free to speculate further!

Logging, privacy, analytics

I don’t have any logging of the CDN traffic to the curl site. Primarily because I haven’t had to, but also because I appreciate the privacy gain for our users and finally because handling logs at this volume pretty much requires a separate service and they all seem to be fairly pricey – for something I really don’t want. So therefore I don’t see the source IP addresses these things. (But yes, I can ask Fastly to check and tell me if I really really wanted to know.)

Also: I don’t run any analytics (Google or otherwise) on the site, primarily for privacy reasons. So that won’t give me that data or other clues either.

Update: it has been proposed I could see the IP address in the X-Forwarded-For: headers and it seems accurate. Of course I didn’t log that header during this period but I will consider starting doing it for better control and info in the future.

Update 2: As of May 18 2020, this flood has not diminished. Logs show that we still block about 5 million requests/day from this service, peaking at over 100 requests/minute.


Top image by Elias Sch. from Pixabay

curl ootw: -v is for verbose

(Previous entries in the curl option of the week series.)

This is one of the original 24 command line options that existed already in the first ever curl release in the spring of 1998. The -v option’s long version is --verbose.

Note that this uses the lowercase ‘v’. The uppercase -V option shows detailed version information.

So use it!

In a blog post series of curl command line options you’d think that this option would basically be unnecessary to include since it seems to basic, so obvious and of course people know of it and use it immediately to understand why curl invokes don’t behave as expected!

Time and time again the first response to users with problems is to please add –verbose to the command line. Many of those times, the problem is then figured out, understood and sorted out without any need of further help.

--verbose should be the first action to try for everyone who runs a curl command that fails unexplainably.

What verbose shows

First: there’s only one verbosity level in curl. There’s normal and there’s verbose. Pure binary; on or off. Adding more -v flags on the same line won’t bring you more details. In fact, adding more won’t change anything at all other than making your command line longer. (And yes, you have my permission to gently taunt anyone you see online who uses more than one -v with curl.)

Verbose mode shows outgoing and incoming headers (or protocol commands/responses) as well as “extra” details that we’ve deemed sensible in the code.

For example you will get to see which IP addresses curl attempts to connect to (that the host name resolved to), it will show details from the server’s TLS certificate and it will tell you what TLS cipher that was negotiated etc.

-v shows details from the protocol engine. Of course you will also see different outputs depending on what protocol that’s being used.

What verbose doesn’t show

This option is meant to help you understand the protocol parts but it doesn’t show you everything that’s going on – for example it doesn’t show you the outgoing protocol data (like the HTTP request body). If -v isn’t enough for you, then the --trace and --trace-ascii options are there for you.

If you are in the rare situation where the trace options aren’t detailed enough, you can go all-in with full SSLKEYLOGFILE mode and inspect curl’s network traffic with Wireshark.

Less verbose?

It’s not exactly “verbose level” but curl does by default for example show a progress meter and some other things. You can silence curl completely by using -s (--silent) or use the recently introduced option --no-progress-meter.


curl -v

Google Open Source Peer Bonus award 2020

I’m honored to – once again – be a recipient of this award Google hands out to open source contributors, annually. I was previously awarded this in 2011.

I don’t get a lot of awards. Getting this token of appreciation feels awesome and I’m humbled and grateful I was not only nominated but also actually selected as recipient. Thank you, Google!

Nine years ago I got 350 USD credits in the Google store and I got my family a set of jackets using them – my kids have grown significantly since then, so to them those black beauties are now just a distant memory, but I still actually wear mine from time to time!

curl beers and curl stickers!

This time, the reward comes with a 250 USD “payout” (that’s the gift mentioned in the mail above), as a real money transfer that can be spent on other things than just Google merchandise!

I’ve decided to accept the reward and the money and I intend to spend it on beer and curl stickers for my friends and fans. As I prefer to view it:

The Google Open Source Beer Bonus.

Thank you Google and thank you Gaspar!

Update: the Google Open Source blog post about it.

curl ootw: –proxy-basic

Previous command line options of the week.

--proxy-basic has no short option. This option is closely related to the option --proxy-user, which has as separate blog post.

This option has been provided and supported since curl 7.12.0, released in June 2004.


In curl terms, a proxy is an explicit middle man that is used to go through when doing a transfer to or from a server:

curl <=> proxy <=> server

curl supports several different kinds of proxies. This option is for HTTP(S) proxies.

HTTP proxy authentication

Authentication: the process or action of proving or showing something to be true, genuine, or valid.

When it comes to proxies and curl, you typically provide name and password to be allowed to use the service. If the client provides the wrong user or password, the proxy will simply deny the client access with a 407 HTTP response code.

curl supports several different HTTP proxy authentication methods, and the proxy can itself reply and inform the client which methods it supports. With the option of this week, --proxy-basic, you ask curl to do the authentication using the Basic method. “Basic” is indeed very basic but is the actual name of the method. Defined in RFC 7616.


The Basic method sends the user and password in the clear in the HTTP headers – they’re just base64 encoded. This is notoriously insecure.

If the proxy is a HTTP proxy (as compared to a HTTPS proxy), users on your network or on the path between you and your HTTP proxy can see your credentials fly by!

If the proxy is a HTTPS proxy however, the connection to it is protected by TLS and everything is encrypted over the wire and then the credentials that is sent in HTTP are protected from snoopers.

Also note that if you pass in credentials to curl on the command line, they might be readable in the script where you do this from. Or if you do it interactively in a shell prompt, they might be viewable in process listings on the machine – even if curl tries to hide them it isn’t supported everywhere.


Use a proxy with your name and password and ask for the Basic method specifically. Basic is also the default unless anything else is asked for.

curl --proxy-user daniel:password123 --proxy-basic --proxy http://myproxy.example


With --proxy you specify the proxy to use, and with --proxy-user you provide the credentials.

Also note that you can of course set and use entirely different credentials and HTTP authentication methods with the remote server even while using Basic with the HTTP(S) proxy.

There are also other authentication methods to selected, with --proxy-anyauth being a very practical one to know about.

A curl dashboard

When I wrote up my looong blog post for the curl’s 22nd anniversary, I vacuumed my home directories for all the leftover scripts and partial hacks I’d used in the past to produce graphs over all sorts of things in the curl project. Being slightly obsessed with graphs, that means I got a whole bunch of them.

I made graphs with libreoffice

I dusted them off and made sure they all created a decent CSV output that I could use. I imported that data into libreoffice’s calc spreadsheet program and created the graphs that way. That was fun and I was happy with the results – and I could also manually annotate them with additional info. I then created a new git repository for the purpose of hosting the statistics scripts and related tools and pushed my scripts to it. Well, at least all the ones that seemed to work and were the most fun.

Having done the hard work once, it felt a little sad to just have that single moment snapshot of the project at the exact time I created the graphs, just before curl’s twenty-second birthday. Surely it would be cooler to have them updated automatically?

How would I update them automatically?

I of course knew of gnuplot since before as I’ve seen it used elsewhere (and I know its used to produce the graphs for the curl gitstats) but I had never used it myself.

How hard can it be?

I have a set of data files and there’s a free tool available for plotting graphs – and one that seems very capable too. I decided to have a go.

Of course I struggled at first when trying to get the basic concepts to make sense, but after a while I could make it show almost what I wanted and after having banged my head against it even more, it started to (partially) make sense! I’m still a gnuplot rookie, but I managed to tame it enough to produce some outputs!

The setup

I have a set of (predominantly) perl scripts that output CSV files. One output file for each script basically.

The statistics scripts dig out data using git from the source code repository and meta-data from the web site repository, and of course process that data in various ways that make sense. I figured a huge benefit of my pushing the scripts to a public repository is that they can be reviewed by anyone and the output should be possible to be reproduced by anyone – or questioned if I messed up somewhere!

The CSV files are then used as input to gnuplot scripts, and each such gnuplot script outputs its result as an SVG image. I selected SVG to make them highly scalable and yet be fairly small disk-space wise.

Different directory names

To spice things up a little, I decided that each new round of generated graph images should be put in a newly created directory with a random piece of string in its name. This, to make sure that we can cache the images on the curl web site very long and still not have a problem when we update the dashboard page on the site.


On the web site itself, the update script runs once every 24 hours, and it first updates its own clone of the source repo, and the stats code git repo before it runs over twenty scripts to make CSV files and the corresponding SVGs.

A dashboard

I like to view the final results as a dashboard. With over 20 up-to-date graphs showing the state of development, releases, commits, bug-fixes, authors etc it truly gives the reader an idea of how the project is doing and what the trends look like.

I hope to keep adding to and improving the graphs over time. If you have ideas of what to visualize and add to the collection, by all means let me know!


At the time of me writing this, the dashboard page looks like below. Click the image to go to the live dashboard.

For other projects?

Nothing in this effort makes my scripts particularly unique for curl so they could all be used for other projects as well – with little to a lot of hands on required. My data extraction scripts of course get and use data that we have stored, collected and keep logged in the project etc, and that data and those logs are highly curl specific.