More HTTP framing attempts

Monday, March 2nd, 2015

Previously, in my exciting series “improving the HTTP framing checks in Firefox” we learned that I landed a patch, got it backed out, struggled to improve the checks and finally landed the fixed version only to eventually get that one backed out as well.

And now I’ve landed my third version. The amendment I did this time:

When receiving HTTP content that is content-encoded and compressed I learned that when receiving deflate compression there is basically no good way for us to know if the content gets prematurely cut off. They seem to lack the footer too often for it to make any sense in checking for that. gzip streams however end with a footer so they are easier to reliably detect when they are incomplete. (As was discovered before, the Content-Length: is far too often not updated by the server so it is instead wrongly showing the uncompressed size.)

This (deflate vs gzip) knowledge is now used by the patch, meaning that deflate compressed downloads can be cut off without the browser noticing…

Will this version of the fix actually stick? I don’t know. There’s lots of bad voodoo out there in the HTTP world and I’m putting my finger right in the middle of some of it with this change. I’m pretty sure I’ve not written my last blog post on this topic just yet… If it sticks this time, it should show up in Firefox 39.


curl, smiley-URLs and libc

Tuesday, February 24th, 2015

Some interesting Unicode URLs have recently been seen used in the wild – like in this billboard ad campaign from Coca Cola, and a friend of mine asked me about curl in reference to these and how it deals with such URLs.


(Picture by stevencoleuk)

I ran some tests and decided to blog my observations since they are a bit curious. The exact URL I tried was ‘www.πŸ˜ƒ.ws’ (not the same smiley as shown on this billboard: πŸ˜‚) – it is really hard to enter by hand so now is the time to appreciate your ability to cut and paste! It appears they registered several domains for a set of different smileys.

These smileys are not really allowed IDN (where IDN means International Domain Names) symbols which make these domains a bit different. They should not (see below for details) be converted to punycode before getting resolved but instead I assume that the pure UTF-8 sequence should or at least will be fed into the name resolver function. Well, either way it should either pass in punycode or the UTF-8 string.

If curl was built to use libidn, it still won’t convert this to punycode and the verbose output says “Failed to convert www.πŸ˜ƒ.ws to ACE; String preparation failed

curl (exact version doesn’t matter) using the stock threaded resolver

  • Debian Linux (glibc 2.19) – FAIL
  • Windows 7 - FAIL
  • Mac OS X 10.9 – SUCCESS

But then also perhaps to no surprise, the exact same results are shown if I try to ping those host names on these systems. It works on the mac, it fails on Linux and Windows. Wget 1.16 also fails on my Debian systems (just as a reference and I didn’t try it on any of the other platforms).

My curl build on Linux that uses c-ares for name resolving instead of glibc succeeds perfectly. host, nslookup and dig all work fine with it on Linux too (as well as nslookup on Windows):

$ host www.πŸ˜ƒ.ws
www.\240\159\152\ has address
$ ping www.πŸ˜ƒ.ws
ping: unknown host www.πŸ˜ƒ.ws

While the same command sequence on the mac shows:

$ host www.πŸ˜ƒ.ws
www.\240\159\152\ has address
$ ping www.πŸ˜ƒ.ws
PING www.πŸ˜ƒ.ws ( 56 data bytes
64 bytes from icmp_seq=0 ttl=44 time=191.689 ms
64 bytes from icmp_seq=1 ttl=44 time=191.124 ms

Slightly interesting additional tidbit: if I rebuild curl to use gethostbyname_r() instead of getaddrinfo() it works just like on the mac, so clearly this is glibc having an opinion on how this should work when given this UTF-8 hostname.

Pasting in the URL into Firefox and Chrome works just fine. They both convert the name to punycode and use “” which then resolves to the same IPv4 address.

Update: as was pointed out in a comment below, the “″ IP address is not the correct IP for the site. It is just the registrar’s landing page so it sends back that response to any host or domain name in the .ws domain that doesn’t exist!

What do the IDN specs say?

The U-263A smileyThis is not my area of expertise. I had to consult Patrik FΓ€ltstrΓΆm here to get this straightened out (but please if I got something wrong here the mistake is still all mine). Apparently this smiley is allowed in RFC 3940 (IDNA2003), but that has been replaced by RFC 5890-5892 (IDNA2008) where this is DISALLOWED. If you read the spec, this is 263A.

So, depending on which spec you follow it was a valid IDN character or it isn’t anymore.

What does the libc docs say?

The POSIX docs for getaddrinfo doesn’t contain enough info to tell who’s right but it doesn’t forbid UTF-8 encoded strings. The regular glibc docs for getaddrinfo also doesn’t say anything and interestingly, the Apple Mac OS X version of the docs says just as little.

With this complete lack of guidance, it is hardly any additional surprise that the glibc gethostbyname docs also doesn’t mention what it does in this case but clearly it doesn’t do the same as getaddrinfo in the glibc case at least.

What’s on the actual site?

A redirect to which shows a rather boring page.


Who’s right?

I don’t know. What do you think?

Tightening Firefox’s HTTP framing – again

Thursday, February 12th, 2015

An old http1.1 frameCall me crazy, but I’m at it again. First a little resume from our previous episodes in this exciting saga:

Chapter 1: I closed the 10+ year old bug that made the Firefox download manager not detect failed downloads, simply because Firefox didn’t care if the HTTP 1.1 Content-Length was larger than what was actually saved – after the connection potentially was cut off for example. There were additional details, but that was the bigger part.

Chapter 2: After having been included all the way to public release, we got a whole slew of bug reports immediately when Firefox 33 shipped and we had to revert parts of the fix I did.

Chapter 3.

Will it land before it turns 11 years old? The bug was originally submitted 2004-03-16.

Since chapter two of this drama brought back the original bugs again we still have to do something about them. I fully understand if not that many readers of this can even keep up of all this back and forth and juggling of HTTP protocol details, but this time we’re putting back the stricter frame checks with a few extra conditions to allow a few violations to remain but detect and react on others!

Here’s how I addressed this issue. I wanted to make the checks stricter but still allow some common protocol violations.

In particular I needed to allow two particular flaws that have proven to be somewhat common in the wild and were the reasons for the previous fix being backed out again:

A – HTTP chunk-encoded responses that lack the final 0-sized chunk.

B – HTTP gzipped responses where the Content-Length is not the same as the actual contents.

So, in order to allow A + B and yet be able to detect prematurely cut off transfers I decided to:

  1. Detect incomplete chunks then the transfer has ended. So, if a chunk-encoded transfer ends on exactly a chunk boundary we consider that fine. Good: This will allow case (A) to be considered fine. Bad: It will make us not detect a certain amount of cut-offs.
  2. When receiving a gzipped response, we consider a gzip stream that doesn’t end fine according to the gzip decompressing state machine to be a partial transfer. IOW: if a gzipped transfer ends fine according to the decompressor, we do not check for size misalignment. This allows case (B) as long as the content could be decoded.
  3. When receiving HTTP that isn’t content-encoded/compressed (like in case 2) and not chunked (like in case 1), perform the size comparison between Content-Length: and the actual size received and consider a mismatch to mean a NS_ERROR_NET_PARTIAL_TRANSFER error.

Firefox BallPrefs

When my first fix was backed out, it was actually not removed but was just put behind a config string (pref as we call it) named “network.http.enforce-framing.http1“. If you set that to true, Firefox will behave as it did with my original fix applied. It makes the HTTP1.1 framing fairly strict and standard compliant. In order to not mess with that setting that now has been around for a while (and I’ve also had it set to true for a while in my browser and I have not seen any problems with doing it this way), I decided to introduce my new changes pref’ed behind a separate variable.

network.http.enforce-framing.soft” is the new pref that is set to true by default with my patch. It will make Firefox do the detections outlined in 1 – 3 and setting it to false will disable those checks again.

Now I only hope there won’t ever be any chapter 4 in this story… If things go well, this will appear in Firefox 38.


But how do they solve these problems in the Chromium project? They have slightly different heuristics (with the small disclaimer that I haven’t read their code for this in a while so details may have changed). First of all, they do not allow a missing final 0-chunk. Then, they basically allow any sort of misaligned size when the content is gzipped.

Update: this patch was subsequently backed out again due to several bug reports about it. I have yet to analyze exactly what went wrong.

HTTP/2 is at 5%

Tuesday, February 10th, 2015

http2 logoHere follow some numbers extracted from my recent HTTP/2 presentation.

First: HTTP/2 is not finalized yet and it is not yet in RFC status, even though things are progressing nicely within the IETF. With some luck we reach RFC status within Q1 this year.

On January 13th 2015, Firefox 35 was released with HTTP/2 enabled by default. Firefox was already running it enabled before that in beta and development versions.

Chrome has also been sporting HTTP/2 support in development versions since many moths back where it could easily be manually enabled. Chrome 40 was the first main release shipped with HTTP/2 enabled by default, but it has so far only been enabled for a very small fraction of the user-base.

On January 28th 2015, Google reported to me by email that they saw HTTP/2 being used in 5% of their global traffic (que all relevant disclaimers that this is not statistically safe numbers). This, close after a shaky period with Google having had their HTTP/2 services disabled through parts of the Christmas holidays (due to bugs) – and as explained above, there’s been no time for any mainstream browser to use HTTP/2 by default for very long!

Further data points: Mozilla collects telemetry data from Firefox users who opted-in to it, and it collects numbers on “HTTP Protocol Version Used on Response”. On February 10, it reports that Firefox 35 users have got their responses to report HTTP/2 in 9% of all responses (out of more than 340 billion reported responses). The Telemetry for Firefox Nightly 38 even reports HTTP/2 in 14% of all responses (based on a much smaller sample collection), which I guess could very well be because users on such a bleeding edge version are more experimental by nature.

In these Firefox stats we see that recently, the number of HTTP/2 responses outnumber the HTTP/1.0 responses 9 to 1.

Changing networks with Linux

Friday, January 16th, 2015

A rather long time ago I blogged about my work to better deal with changing networks while Firefox is running, and the change was then pushed for Android and I subsequently pushed the same functionality for Firefox on Mac.

Today I’ve landed yet another change, which detects network changes on Firefox OS and Linux.

Firefox Nightly screenshotAs Firefox OS uses a Linux kernel, I ended up doing the same fix for both the Firefox OS devices as for Firefox on Linux desktop: I open a socket in the AF_NETLINK family and listen on the stream of messages the kernel sends when there are network updates. This way we’re told when the routing tables update or when we get a new IP address etc. I consider this way better than the NotifyIpInterfaceChange() API Windows provides, as this allows us to filter what we’re interested in. The windows API makes that rather complicated and in fact a lot of the times when we get the notification on windows it isn’t clear to me why!

The Mac API way is what I would consider even more obscure, but then I’m not at all used to their way of doing things and how you add things to the event handlers etc.

The journey to the landing of this particular patch was once again long and bumpy and full of sweat in this tradition that seem seems to be my destiny, and this time I ran into problems with the Firefox OS emulator which seems to have some interesting bugs that cause my code to not work properly and as a result of that our automated tests failed: occasionally data sent over a pipe or socketpair doesn’t end up in the receiving end. In my case this means that my signal to the child thread to die would sometimes not be noticed and thus the thread wouldn’t exit and die as intended.

I ended up implementing a work-around that makes it work even if the emulator eats the data by also checking a shared should-I-shutdown-now flag every once in a while. For more specific details on that, see the bug.

Changing networks on Mac with Firefox

Thursday, October 30th, 2014

Not too long ago I blogged about my work to better deal with changing networks while Firefox is running. That job was basically two parts.

A) generic code to handle receiving such a network-changed event and then

B) a platform specific part that was for Windows that detected such a network change and sent the event

Today I’ve landed yet another fix for part B called bug 1079385, which detects network changes for Firefox on Mac OS X.

mac miniI’ve never programmed anything before on the Mac so this was sort of my christening in this environment. I mean, I’ve written countless of POSIX compliant programs including curl and friends that certainly builds and runs on Mac OS just fine, but I never before used the Mac-specific APIs to do things.

I got a mac mini just two weeks ago to work on this. Getting it up, prepared and my first Firefox built from source took all-in-all less than three hours. Learning the details of the mac API world was much more trouble and can’t say that I’m mastering it now either but I did find myself at least figuring out how to detect when IP addresses on the interfaces change and a changed address is a pretty good signal that the network changed somehow. episode 8

Monday, October 27th, 2014

Today I hesitated to make my new weekly video episode. I looked at the viewers number and how they basically have dwindled the last few weeks. I’m not making this video series interesting enough for a very large crowd of people. I’m re-evaluating if I should do them at all, or if I can do something to spice them up…

… or perhaps just not look at the viewers numbers at all and just do what think is fun?

I decided I’ll go with the latter for now. After all, I enjoy making these and they usually give me some interesting feedback and discussions even if the numbers are really low. What good is a number anyway?

This week’s episode:






  • I’m offering two talks for FOSDEM


  • release next Wednesday
  • bug fixing period
  • security advisory is pending


Stricter HTTP 1.1 framing good bye

Sunday, October 26th, 2014

I worked on a patch for Firefox bug 237623 to make sure Firefox would use a stricter check for “HTTP 1.1 framing”, checking that Content-Length is correct and that there’s no broken chunked encoding pieces. I was happy to close an over 10 years old bug when the fix landed in June 2014.

The fix landed and has not caused any grief all the way since June through to the actual live release (Nightlies, Aurora, Beta etc). This change finally shipped in Firefox 33 and I had more or less already started to forget about it, and now things went south really fast.

The amount of broken servers ended up too massive for us and we had to backpedal. The largest amount of problems can be split up in these two categories:

  1. Servers that deliver gzipped content and sends a Content-Length: for the uncompressed data. This seems to be commonly done with old mod_deflate and mod_fastcgi versions on Apache, but we also saw people using IIS reporting this symptom.
  2. Servers that deliver chunked-encoding but who skip the final zero-size chunk so that the stream actually never really ends.

We recognize that not everyone can have the servers fixed – even if all these servers should still be fixed! We now make these HTTP 1.1 framing problems get detected but only cause a problem if a certain pref variable is set (network.http.enforce-framing.http1), and since that is disabled by default they will be silently ignored much like before. The Internet is a more broken and more sad place than I want to accept at times.

We haven’t fully worked out how to also make the download manager (ie the thing that downloads things directly to disk, without showing it in the browser) happy, which was the original reason for bug 237623…

Although the code may now no longer alert anything about HTTP 1.1 framing problems, it will now at least mark the connection not due for re-use which will be a big boost compared to before since these broken framing cases really hurt persistent connections use. The partial transfer return codes for broken SPDY and HTTP/2 transfers remain though and I hope to be able to remain stricter with these newer protocols.

This partial reversion will land ASAP and get merged into patch releases of Firefox 33 and later.

Finally, to top this off. Here’s a picture of an old HTTP 1.1 frame so that you know what we’re talking about.

An old http1.1 frame

Pretending port zero is a normal one

Saturday, October 25th, 2014

Speaking the TCP protocol, we communicate between “ports” in the local and remote ends. Each of these port fields are 16 bits in the protocol header so they can hold values between 0 – 65535. (IPv4 or IPv6 are the same here.) We usually do HTTP on port 80 and we do HTTPS on port 443 and so on. We can even play around and use them on various other custom ports when we feel like it.

But what about port 0 (zero) ? Sure, IANA lists the port as “reserved” for TCP and UDP but that’s just a rule in a list of ports, not actually a filter implemented by anyone.

In the actual TCP protocol port 0 is nothing special but just another number. Several people have told me “it is not supposed to be used” or that it is otherwise somehow considered bad to use this port over the internet. I don’t really know where this notion comes from more than that IANA listing.

Frank Gevaerts helped me perform some experiments with TCP port zero on Linux.

In the Berkeley sockets API widely used for doing TCP communications, port zero has a bit of a harder situation. Most of the functions and structs treat zero as just another number so there’s virtually no problem as a client to connect to this port using for example curl. See below for a printout from a test shot.

Running a TCP server on port 0 however, is tricky since the bind() function uses a zero in the port number to mean “pick a random one” (I can only assume this was a mistake done eons ago that can’t be changed). For this test, a little iptables trickery was run so that incoming traffic on TCP port 0 would be redirected to port 80 on the server machine, so that we didn’t have to patch any server code.

Entering a URL with port number zero to Firefox gets this message displayed:

This address uses a network port which is normally used for purposes other than Web browsing. Firefox has canceled the request for your protection.

… but Chrome accepts it and tries to use it as given.

The only little nit that remains when using curl against port 0 is that it seems glibc’s getpeername() assumes this is an illegal port number and refuses to work. I marked that line in curl’s output in red below just to highlight it for you. The actual source code with this check is here. This failure is not lethal for libcurl, it will just have slightly less info but will still continue to work. I claim this is a glibc bug.

$ curl -v -H "Host:"
* Rebuilt URL to:
* Hostname was NOT found in DNS cache
* Trying
* getpeername() failed with errno 107: Transport endpoint is not connected
* Connected to () port 0 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.38.1-DEV
> Accept: */*
> Host:
< HTTP/1.1 200 OK
< Date: Fri, 24 Oct 2014 09:08:02 GMT
< Server: Apache/2.4.10 (Debian)
< Last-Modified: Fri, 24 Oct 2014 08:48:34 GMT
< Content-Length: 22
< Content-Type: text/html


Why doing this experiment? Just for fun to to see if it worked.

(Discussion and comments on this post is also found at Reddit.)

FOSS them students

Thursday, October 16th, 2014

On October 16th, I visited DSV at Stockholm University where I had the pleasure of holding a talk and discussion with students (and a few teachers) under the topic Contribute to Open Source. Around 30 persons attended.

Here are the slides I use, as usual possibly not perfectly telling stand-alone without the talk but there was no recording made and I talked in Swedish anyway…

Contribute to Open Source from Daniel Stenberg