Tag Archives: cURL and libcurl

c-ares, cURL and libcurl

libcurl’s name resolving

Recently we’ve put in some efforts into remodeling libcurl’s code that handles name resolves, and then in particular the two asynchronous name resolver backends that we support: c-ares and threaded.

Name resolving in general in libcurl

libcurl can be built to do name resolves using different means. The primary difference between them is that they are either synchronous or asynchronous. The synchronous way makes the operation block during name resolves and there’s no “decent” way to abort the resolves if they take longer time than the program wants to allow it (other than using signals and that’s not what we consider a decent way).

Asynch resolving in libcurl

This is done using one of two ways: by building libcurl with c-ares support or by building libcurl and tell it to use threads to solve the problem. libcurl can be built using either mechanism on just about all platforms, but on Windows the build defaults to using the threaded resolver.

The c-ares solution

c-ares’ primary benefit is that it is an asynchronous name resolver library so it can do name resolves without blocking without requiring a new thread. It makes it use less resources and remain a perfect choice even if you’d scale up your application up to and beyond an insane number of simultaneous connections. Its primary drawback is that since it isn’t based on the system default name resolver functions, they don’t work exactly like the system name resolver functions and that causes trouble at times.

The threaded solution

By making sure the system functions are still used, this makes name resolving work exactly as with the synchronous solution, but thanks to the threading it doesn’t block. The downside here is of course that it uses a new thread for every name resolve, which in some cases can become quite a large number and of course creating and killing threads at a high rate is much more costly than sticking with the single thread.

Pluggable

Now we’ve made sure that we have an internal API that both our asynchronous name resolvers implement, and all code internally use this API. It makes the code a lot cleaner than the previous #ifdef maze for the different approaches, and it has the side-effect that it should allow much easier pluggable backends in case someone would like to make libcurl support another asynchronous name resolver or system.

This is all brand new in the master branch so please try it out and help us polish the initial quirks that may still exist in the code.

There is no current plan to allow this plugging to happen run-time or using any kind of external plugins. I don’t see any particular benefit for us to do that, but it would give us a lot more work and responsibilities.

cURL

cURL and libcurl, Network, Web

HTTP transfer compression

HTTP is a protocol that looks simple in its simplest form and its readability can easily fool you into believing an implementation is straight forward and quickly done.

That’s not the reality though. HTTP is a very big protocol with lots of corners and twisting mazes that one can get lost in. Even after having been the primary author of curl for 13+ years, there are still lots of HTTP things I don’t master.

To name an example of an area with little known quirks, there’s a funny situation when it comes to how HTTP supports and doesn’t support compression of data and compression of data in transfer.

No header compression

A little flaw in HTTP in regards to compression is that there’s no way to compress headers, in either direction. No matter what we do, we must send the text as-is and both requests and responses are sometimes very big these days. Especially taken into account how cookies are always inserted in requests if they match. Anyway, this flaw is nothing we can do anything about in HTTP 1.1 so we need to live with it.

On the other side, compression of the response body is supported.

Compressing data

Compression of data can be done in two ways: either the actual transfer is compressed or the body data is compressed. The difference is subtle, but when the body data is compressed there’s really nothing that mandates that the client has to uncompress it for the end user, and if the transfer is compressed the receiver must uncompress it in order to deal with the transfer properly.

For reasons that are unknown to me, HTTP clients and servers started out supporting compression only using the Content-Encoding style. It means that the client tells the server what kind of content encodings it supports (using Accept-Encoding:) and the server then sends the response data using one of the supported encodings. The client then decides on its own that if it gets the content in one of the compressed formats that it said it can handle, it will automatically uncompress that on arrival.

The HTTP protocol designers however intended this kind of automatic compression and subsequent uncompress to be done using Transfer-Encoding, as the end result is the completely transparent and the uncompress action is implied and intended by the protocol design. This is done by the client telling the server what transfer encodings it supports with the TE: header and the server adds a Transfer-Encoding: header in the response telling how the transfer is encoded.

HTTP 1.1 introduced a mandatory encoding that all servers can use whenever they feel like it: chunked encoding, so all HTTP 1.1 clients already have to deal with Transfer-Encoding to some degree.

Surely curl is better than all those other guys, right?

Not really. Not yet anyway.

curl has a long history of copying its behavior from what the browsers do, in order to allow users to basically script anything imaginable that is HTTP-like with curl. In this vein, we implemented compression support the same way as all the browsers did it: the content encoding style. (I have reason to believe that at least Opera actually supports or used to support compressed Transfer-Encoding.)

Starting now (code pushed to git repo just after the 7.21.5 release), we’ve taken steps to improve things. We’re changing gears and we’re introducing support for asking for and using compressed Transfer-Encoding. This will start out as an optional feature/flag (–tr-encoding / CURLOPT_TRANSFER_ENCODING) so that we can start out and see how servers in the wild behave and that we can deal with them properly. Then possibly we can switch the default in the future to always ask for compressed transfers. At least for the command line tool.

We know from the little tests we are aware of, that there are at least one known little problem or shall we call it a little detail to keep on eye at, with introducing compressed Transfer-Encoding. As has been so fine reported several years ago in the opera blog Browser sniffing gone wrong (again): Cars.com, there are cases where this may cause the server to send data that gets compressed twice (using both Content and Transfer Encoding) and that needs to be taken care of properly by the client.

At the time of this writing, I’ve not yet taken care of the double-compress case in the code, but I intend to get on to it within shortly.

I’m otherwise very interested in hearing what kind of experience people will have from this. What servers and sites will support this as documented and intended?

cURL and libcurl

Shipping curl 7.21.5

I don’t usually post anything here when we do curl releases, pretty much because we do them bimonthly on a fairly steady schedule so there should be little surprise to anyone interested by the time they get public.

But hey, this is hard work and just to remind you all what’s going on I thought I’d throw in a mention of what we’ve spent the last two months doing. curl and libcurl 7.21.5 is released today.

The five notable changes introduced this time include:

The CURLOPT_SOCKOPTFUNCTION callback can now return information back to libcurl that the socket libcurl operates on is already connected. This is useful for applications that do a lot of fiddling on their own and possibly provide its own socket to start with using the CURLOPT_OPENSOCKETFUNCTION.

curl the tool got support for the –netrc-file option, that allows a user to point out a specific .netrc file instead of always forcing the user to use the fixed $HOME/.netrc one.

Brand new support for building libcurl with the cyassl library for SSL/TLS support. Previously curl only had support for the older OpenSSL emulation API that cyassl used to provide, but starting now we’re using cyassl directly and it is now a proper SSL citizen among the seven SSL libraries curl supports.

Since the previous release when we shipped the first support for TLS-SRP that required GnuTLS, the OpenSSL project accepted patches that introduced TLS-SRP into their official version as well and accordingly we have received patches that now allow users to use TLS-SRP with libcurl built against (a new enough) OpenSSL as well.

We have started to re-use two error codes a bit differently within libcurl, so that it now can return: CURLE_NOT_BUILT_IN (4) when an application tries to use a feature that was missing or was explicitly disabled at build-time and CURLE_UNKNOWN_OPTION (48) when the application has passed in an option that isn’t known or recognized.

And we’re counting more than 40 bugfixes worth mentioning. The most important ones are possibly:

If using the multi interface doing RTSP, libcurl could crash when trying to re-use a previous connection.

POP3 didn’t do TLS properly, it issued the wrong command to start TLS and it didn’t send the password correctly once it did switch to TLS!

When using the multi interface, there could be times when the timeout didn’t trigger so it wouldn’t close lingering connections even when asked to do so.

SFTP and SFP with the multi_socket interface were not working correctly and would very easily end up with stalled transfers due to the application being told to wait for the wrong action (or none at all).

If told to use the CCC command (which is used with FTP-SSL when the client asks the server to switch off from an SSL connection back to plain TCP again), curl would disable SSL on the connection but then use the wrong socket reader function and crash.

… but of course, if you’ve suffered from a particular bug in a previous release I’m sure you’ll consider the exact bug fix that corrects your problem to be the most important one!

Not to forget, the great people apart from yours truly that have contributed with code and insights since the previous release. Without them, the above list of changes and bugfixes just wouldn’t exist. The friends we have to thank are (in no particular order):

Mike Crowe, Kamil Dudka, Julien Chaffraix, Hoi-Ho Chan, Ben Noordhuis, Dan Fandrich, Henry Ludemann, Karl M, Manuel Massing, Marcus Sundberg, Stefan Krause, Todd A Ouska, Saqib Ali, Andre Guibert de Bruet, Tor Arntsen, Vincent Torri, Dave Reisner, Chris Smowton, Tinus van den Berg, Hongli Lai, Gisle Vanem, Andrei Benea, Mehmet Bozkurt

… and now back to working towards the next release. To be expected in roughly two months. Repeat.

cURL and libcurl, Open Source

curlyears plus equals one

BirthdaycakeThirteen years ago I released the first version of curl to the world – on March 20 1998. curl is now a teenage project and there’s no slowdown or end in sight.

So what does a project like ours introduce after having existed for so long? The recent year has been full of activities in the project, and here’s a run down with some of the stuff that has been going on:

We switched source code versioning system from CVS to git

gopher support got back into curl

support for RTMP was added

Two additional SSL libraries are now supported: PolarSSL and axTLS, making it a total of seven

70 persons provided code into the git repository, making the THANKS document now list 854 names.

About 1000 commits were made – out of a total of almost 14000 (counted from Dec 1999) so it makes this year slightly under average in terms of commit rate.

6 releases were shipped with a total of 179 bugfixes

1 security flaw was found and fixed

More than 4900 mails were posted to the curl mailing lists

We introduced a unit test system

Over 1400GB of data was downloaded from the curl web site

During the end of this period, 45% of our web visitors used Firefox, 23% used Chrome and less than 20% used IE. 75% were on Windows, 13% on Linux and 10% on Mac.

cURL

Network, Windows

localhost hack on Windows

There's no place like 127.0.0.1

Readers of my blog and friends in general know that I’m not really a Windows guy. I never use it and I never develop things explicitly for windows – but I do my best in making sure my portable code also builds and runs on windows. This blog post is about a new detail that I’ve just learned and that I think I could help shed the light on, to help my fellow hackers. The other day I was contacted by a user of libcurl because he was using it on Windows and he noticed that when wanting to transfer data from the loopback device (where he had a service of his own), and he accessed it using “localhost” in the URL passed to libcurl, he would spot a DNS request for the address of that host name while when he used regular windows tools he would not see that! After some mails back and forth, the details got clear:

Windows has a default /etc/hosts version (conveniently instead put at “c:\WINDOWS\system32\drivers\etc\hosts”) and that default  /etc/hosts alternative used to have an entry for “localhost” in it that would point to 127.0.0.1.

When Windows 7 was released, Microsoft had removed the localhost entry from the /etc/hosts file. Reading sources on the net, it might be related to them supporting IPv6 for real but it’s not at all clear what the connection between those two actions would be.

getaddrinfo() in Windows has since then, and it is unclear exactly at which point in time it started to do this, been made to know about the specific string “localhost” and is documented to always return “all loopback addresses on the local computer”.

So, a custom resolver such as c-ares that doesn’t use Windows’ functions to resolve names but does it all by itself, that has been made to look in the /etc/host file etc now suddenly no longer finds “localhost” in a local file but ends up asking the DNS server for info about it… A case that is far from ideal. Most servers won’t have an entry for it and others might simply provide the wrong address.

I think we’ll have to give in and provide this hack in c-ares as well, just the way Windows itself does.

Oh, and as a bonus there’s even an additional hack mentioned in the getaddrinfo docs: On Windows Server 2003 and later if the pNodeName parameter points to a string equal to “..localmachine”, all registered addresses on the local computer are returned.

cURL and libcurl, libssh2, Network

Fosdem 2011: my libcurl talk on video

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Kai Engert was good enough to capture all the talks in the security devroom at Fosdem 2011, and while I’m seeding the full torrent I’ve made my own talk available as a direct download from here:

Fosdem 2011: security-room at 14:15 by Daniel Stenberg

The thing is about 107MB big, 640×480 resolution and is roughly 26 minutes playing time. WebM format.

cURL and libcurl, libssh2, Open Source

libcurl, seven SSL libs and one SSH lib

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I did a talk today at Fosdem with this title. The room only had 48 seats and it was completely packed with people standing everywhere it was possible around the seated guys.

The English slides from my talk are below. It was also recorded on video so I hope I’ll be able to post once it becomes available online

libcurl, seven SSL libraries and one SSH library
cURL and libcurl, Haxx, Open Source, Rockbox

Going to FOSDEM 2011

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Fosdem 2011We’re going to FOSDEM again. This year we’ll ship over the entire company (all three of us) and we’ll join up with a few fellow Rockbox hackers and spend a weekend in Brussels among thousands of fellow free software and open source hackers.

During this conference, 5-6 February, I’ve submitted a libcurl-related talk to the embedded-room that wasn’t accepted into the regular program, but I’ve agreed to still prepare it and I then might get a slot in case someone gets sick or something. A bit ungrateful as now I still have to prepare my slides for the talk but there’s a big risk that I’ve done it in vain! I’ve also submitted a suggestion for a second talk in the opensc/security room (also related to stuff in the curl project) but as of now (with but 16 days left) that schedule is yet to be announced so I don’t know if I’ll do a talk there or not.

So, I might do no talks. I might do two. I just don’t know. We’ll see.

If you’re a friend of mine and you’re going to FOSDEM this year, please let me know and we can meet and have a chat or whatever. I love getting faces to all the names, nicks and email addresses I otherwise only see of many people.

Update: My talk in the security room is titled “libcurl: Supporting seven SSL libraries and one SSH library” and will start at 14:15 on Saturday the 5th of February.

cURL and libcurl, Mail

“Hacking me”

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If you ever wonder how clever it was of me to make an FTP tool that used the default anonymous password curl_by_daniel@... once upon a time and you want to know why I changed that to ftp@example.com instead? Here’s a golden snippet to just absorb and enjoy:

Date: Thu, 23 Dec 2010 22:56:00
From: iHack3r <hidden>
To: info@[my company]
Subject: Hacking me

To the idiot named Daniel, Please stop brute force attacking my FTP client. I do not appreciate it, i have an anonymous account set up for the general public to access my files that i want them to access, QUIT trying to hack the admin because 1. DISABLED unless i am leaving to go somewhere without my computer 2: THE PASSWORD is random letters and numbers.

-iHack3r

The password was changed at Feb 13 2007 in curl version 7.16.2, but there are a surprisingly large amount of older curls still around out there…

Update: as the person responded again after having read this blog post and still didn’t get it, I felt the urge to speak up in even more clear terms:

I didn’t have anything to do with any “hacker attack” on any site. Not yours, and not anyone else’s. The fact that almost-my-email address appeared in your logs is because I wrote the FTP client. It is a general FTP client that is being used by a very very large amount of people all over the world. If I ever would attack a site, why on earth would I send along my real name or email address?

cURL and libcurl, Network

Byte ranges for FTP

In the IETF ftpext2 working group there have been some talks around clients’ and servers’ ability to do and support “ranged” file transfers, that is transferring only a piece of any given file. FTP supports the REST command and has done so since the dawn of man (RFC765 – June 1980), and using that command, a client can set the starting point for a transfer but there is no way to set the end point. HTTP has supported the Range: header since the first HTTP 1.1 spec back in January 1997, and that supports both a start and an end point. The HTTP header does in fact support multiple ranges within the same header, but let’s not overdo it here!

Currently, to avoid getting an entire file a client would simply close the data connection when it has got all the data it wants. The unfortunate reality is that some servers don’t notice clients doing this, so in order for this to work reliably a client also has to send ABOR, and after this command has been sent there is no way for the client to reliably figure out the state of the control connection so it has to get closed as well (which is crap in case more files are to be transferred to or from the same host). It primarily becomes unreliable because when ABOR is sent, the client gets one or two responses back due to a race condition between the closing and the actual end of transfer etc, and it isn’t possible to tell exactly how to continue.

A solution for the future is being worked on. I’ve joined up the effort to write a spec that will suggest a new FTP command that sets the end point for a transfer in the same vein REST sets the start point. For the moment, we’ve named our suggested command RANG (as short for range). “We” in this context means Tatsuhiro Tsujikawa, Anthony Bryan and myself but we of course hope to get further valuable feedback by the great ftpext2 people.

There already are use cases that want range request for FTP. The people behind metalinks for example want to download the same file from many servers, and then it makes sense to be able to download little pieces from different sources.

The people who found the libcurl bugs I linked to above use libcurl as part of the Fedora/Redhat installer Anaconda, and if I understand things right they use this feature to just get the beginning of some files to check them out and avoid having to download the full file before it knows it truly wants it. Thus it saves lots of bandwidth.

In short, the use-cases for ranged FTP retrievals are quite likely pretty much the same ones as they are for HTTP!

The first RANG draft is now available.