Tag Archives: Network

localhost hack on Windows

There's no place like 127.0.0.1Readers 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.

NASed and RAID1ed

Synology DS211jI finally got my act together and bought myself a Synology DS211j NAS  with two 2TB drives. I’ll use it as a shared network disk at home and I intend to backup to it – as I also have a home office it’ll feel better to be able to also backup company related data somewhat more safely. My previous backup was only copying data from one HDD to another within the same physical machine.

To make it slightly more resistant to disk and hardware errors I’ve configured the disks to do RAID1 (all data is stored on both disks simultaneously).

The GPLv2 license was provided printed on paper in the package.

Add latency to localhost

Pádraig Brady taught me a great trick in a comment to a previous blog post and it was so neat I feel a need to highlight it further as it also makes it easier for me to find it again later!

To simulate a far away server, add RTT time to the localhost device. For example if we add 100 milliseconds (which then makes 200ms ping time to localhost):

$ tc qdisc add dev lo root handle 1:0 netem delay 100msec
Restore it back to normal again with:
$ tc qdisc del dev lo root
tc qdisc add dev lo root handle 1:0 netem delay 100msec

Restore it back to normal again with:

tc qdisc del dev lo root

In addition, add a random packet loss. 500ms latency with 2.5% packet loss:

tc qdisc add dev lo root handle 1:0 netem delay 250msec loss 2.5%


Future transports

On Sunday morning during FSCONS 2010, in the room “Torg 4 South” I did a 30 minute talk about a few future, potentially coming network protocols for transport. A quick look at the current state, some problems of today and 4 different technologies that have been and are being developed to solve the problem.

I got a fair amount of questions and several persons approached me afterwards to make sure they got a copy of my slides.

The video recording is hopefully going to be made available later on, but until then you can read the slides below and imagine my Swedish  accent talking about these matters!

Future transports

You can also download the slides directly as a PDF.

Scalable application layer transfers

At FSCONS 2010 I had the pleasure to do a talk about how to make your client-side networking applications really scale when upping the number of simultaneous connections. Including some details that libcurl will support you all the way!

My talk was named “Scalable application layer transfers” and the slides from it is available online. See below. Hopefully the video recording of it will appear later and I’ll post a  follow-up then. A little extra bonus material as background would be my poll vs select vs event-based article.

As I mentioned in a previous post, the room was shock full when I started preparing my equipment for the talk since the session before me was a keynote, but by the time I actually starter presenting there were only the limited set of hardcore geeks left.

In the FSCONS program there were several talks over the weekend about women in FOSS and so on, while I on the other hand certainly only contributed to enforcing the stereotypes by being white, male, middle-aged, very techy and I delivered my two speeches for audiences in which I believe not a single woman attended. Whether I am part of the problem or the solution we can discuss in a separate post later on… 🙂

pNFS is my kind of toy

Ok, so NFS has never really been my cup of tea. Complicated and the problem with the root user and locking and what not have always made me get all itchy when thinking about or using NFS.

Enter pNFS, the NFS 4.1 invention that truly suddenly makes the NFS technology so much interesting for high performance solutions. I also think  it is a bit unknown so I thought I’d help to share the knowledge about this to you my dear readers. The p in pNFS stands for parallel. The whole idea is that the single NFS server just provides meta data back to the client, with enough information to allow the client to read the actual payload data directly from the storage server(s), that then supposedly are different ones than the main meta data server

img_pnfs_standard

(picture from www.nexenta.org)

As you can see on this fancy picture, it allows each client to speak directly to the storage device to get or send the data. This allows them to avoid using a single bottleneck NFS server.

NFS 4.1 and pNFS are IETF standards, RFC5661 to RFC5664. The first one being 616 pages long and one of the largest RFCs I’m aware of.

poll vs select

I’m a person working a lot with networking and development around it. I mostly do this on Linux, often involving drivers or otherwise very close to the operating system and C and the core libraries.

The other day I once again fell over some random inaccuracy about poll compared to select and instead of trying to whine on some IRC channel or complain on their mailing list, I decided I would instead strike back by writing up and presenting a web page of my own. It details as much as possible about poll vs select and related event-based functions. I want it to become a placeholder for everything that is relevant to say about poll and select in a comparison aspect and when comparing them to event-based alternatives like libevent and libev.

So the next time I face someone not quite understanding this whole situation or perhaps when someone reiterates something that isn’t quite true, I have a resource to point to.

Not to mention that I think this new poll vs select page fits in nicely with my other “X vs Y” articles and docs pages I’ve written the last few years.

If you find flaws, or miss details or have questions about this page. Please do not hesitate to comment here, or to mail me about it or tweet me on twitter or whatever method you prefer. I appreciate your feedback!

poll vs select

My Debian Black-out – the price of bleeding edge

Ok, I admit it. I run Debian Unstable so I know I deserve to get hit really bad at times when things turn really ugly. It is called unstable for a reason.

The other day I decided it was about time I did a dist-upgrade. When I did that, I got a remark that I better restart my gnome session as otherwise apps would crash. So I logged out and… I couldn’t login again. In fact, neither my keyboard nor mouse (both on USB) worked anymore! I sighed, and rebooted (for the first time in many months) only to find out that 1) it didn’t fix the problem, both input devices were still non-functional and perhaps even more important 2) the wifi network didn’t work either so I couldn’t login to it from one of my other computers either!

Related to this story is the fact that I’ve been running an older kernel, 2.6.26, since that was the last version that built my madwifi drivers correctly and kernels after that I was supposed to use ath5k for my Atheros card, but I’ve not been very successful with ath5k and thus remained using the latest kernel I had a fine madwifi for.

I rebooted again and tried a more recent kernel (2.6.30). Yeah, then the keyboard and mouse worked again, but the ath5k didn’t get the wifi up properly. I think I basically was just lacking the proper tools to check the wifi network and set the desired ssid etc, but without network that’s a bit of a pain. Also, when I logged in on my normal gnome setup, it mentioned a panel something being broken and logged me out again! 🙁

Grrr. Of course I could switch to my backup – my laptop – but it was still highly annoying to end up being locked out from your computer.

Today I bought myself 20 meter cat5e cable and made my desktop wired so I can reach the network with the existing setup, I dist-upgraded again (now at kernel 2.6.31) and when I tried to login it just worked. Phew. Back in business. I think I’ll leave myself with the cable connected now that I’ve done the job on that already.

The lesson? Eeeh… when things break, fix them!

libssh2 upped a notch

There have been some well-founded criticism against libssh2 for a long time for its bad transfer performance when doing SCP and SFTP based transfers. Tests have proved it to be significantly slower than the openssh based alternatives in comparisons done in similar conditions. We’re talking down to a tenth(!) of the speed for SFTP.

Luckily I have a unnamed (by agreement) sponsor who pays me for improving this.

Giving it some love

I basically started out reading the SFTP code and cleaned it up as I went over it, and I added some clarifying comments etc. I found some irregularities that I fixed. Soon I could spot an obvious performance boost, like perhaps 3-4 times the previous speed. But since SFTP was painfully slow originally, this was still very crappy compared to openssh.

I then switched over to plain SCP tests. SCP is basically just an “scp” command sent over SSH and then streaming the data over a plain SSH “channel”, while SFTP is a whole additional protocol layer on top. Thus SCP is more low-level, on the actual SSH level, and the foundation on which SFTP runs anyway so getting SCP faster was fundamental.

Make it speedier

My initial tests with libssh2 1.0 showed libssh2 to download data at roughly 25% of the speed of openssh when SCPing a 1GB file from an openssh server running on localhost. The openssh client shows roughly 40MB/sec on my test box.

Also, just checking my CPU load meter while doing the libssh2 transfers showed that it certainly wasn’t hitting the roof or anything. It was barely even noticeable! Of course something was really wrong but what was it?

SSH has a lower protocol layer that does the entire encryption thing, the transport layer, but on top of that is the “channel layer” that is packet based for sending data back and forth over the transport layer. This channel thing has a receive window concept, much like TCP itself has, which tells the remote side how much data it is allowed to send until it gets further notice.

libssh2 1.0 had a very conservative windowing logic. It started with a default window size of 64KB and it upped it at every read with the same amount that was read (which then could be 1K to 16KB something depending on the app).

My remake of this was to simplify the logic, read data from the network more evenly distributed over time, update the window size much less frequent and increase the window size by magnitudes! I found that when using a window size of 38MB (600 times the previous default size!!) things started flying.

Improved

With these modifications, libssh2 transfers SCP at close to 40MB/sec! SFTP is still left behind at a “mere” 14MB/sec on the same test setup but it has its own set of problems and solutions. Now this discussion on the libssh2 list is more about how to sensibly size the window to work the best way for different situations.

SFTP is a protocol that works more on file operations. The client sends OPEN, READ and CLOSE requests and the server replies with status and data. The READ request asks for N bytes starting at offset Z so a simple implementation like libssh2 asks for chunk after chunk in a serial manner, increasing the offset as it loops over the range. This causes a back-and-forth effect that certainly does not make optimized use of the network bandwidth.

SFTP ping pong

openssh has a nifty approach to enhance throughput for SFTP: it sends off and handles multiple outstanding READ requests in parallel so that it better can keep things busy (and the reverse when doing uploads). That concept is slightly harder to do with an API like the one libssh2 offers but it is of course still quite doable. I suspect that we might achieve results somewhat faster by simply use multiple connections as then we can remain using this simplistic approach but still use the full bandwidth. (Yes, I realize multiple connections may not be feasible for all applications.)

Previous tests we’ve done with SFTP uploads using multiple connections have proven libssh2 to be on par or even better than competitors on both Windows and Mac.

Please test

I’ll leave it like this for now. I’ll be very happy if people could test this version and report findings so that we make sure this is working and stable enough to release soonish. We’ll need to do something that offers window size controlling to apps, but we’ll discuss that further on the mailing list. Join in!

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A stream of streamings

I’m a last.fm fan. I love its ability to not only stream music without needing a dedicated client installed (yes a flash application I think suits a purpose) and I think it’s ability to provide music I might also like is amazingly nice. I’m a “random it all” kind of guy when I listen to my local music collection in most situations as well. It is not specifly well suited for listening on exact the songs you want, as if you select a specific song it won’t even play the full-length version of it.

Lately there’s been a lot of buzz in Swedish tech media about spotify, which is a similar idea (at the moment still an on invitation-only thing in Sweden). They stream music, but only to a proprietary Windows or Mac client and currently they offer free listening with ads (embedded in the audio and visible in the client) or 99 SEK (== 9 Euros == 11 USD) per month. The client is highly focused on specific songs or artists and it has nothing in the way of “random artitists I generally like and similar ones”. I’m not too thrilled.

Spotify offers its service in several places, and I hear in the UK it’s not even invitation-only (which of course is useful for the more forward-thinking hacking kind of guys who thus use a UK based proxy to reach them). There’s however no sign of a Linux client. We’re forced to run their windows client with Wine.

I’ve gotten the impression that Pandora is a similar concept to play with if you happen to be based in the US. I’m in Sweden and Pandora just shows me a “We are deeply, deeply sorry to say that due to licensing constraints, we can no longer allow access to Pandora for listeners located outside of the U.S.”

The other day despotify.se showed up. A bunch of clever hackers reverse engineered the Spotify protocol and stream and offer a full unofficial open sourced ncurses/libvorbis/pulse-audio/gstreamer/expat/zlib/openssl-based player! Reading the code shows that these guys certainly had to crack some hard nuts, but the activity in their IRC channel seems fierce and the code is rather clean so I expect it to turn out to eventually become a fine player if Spotify just doesn’t decide to play hard ball with them. Unfortunately, despotify hasn’t yet been able to produce a single sound for me since it has just died on assert()s on basically any attempts I’ve tried. The interface is also a bit… strange and not the easiest to figure out. (It should be noted that the despotify client still requires you who have an actual spotify account.)

It’ll be interesting to see how Spotify, or perhaps the big media companies owning all the music rights, will act on this initiative. This client does open up abilities for new fancy features. How about ripping the stream? How about re-distributing the stream like as a proxy? And of course it being open, it does open up for adding features I want to add.

Update: just hours after I posted this, Spotify closed access to their service using the despotify client as long as you’re not a “premium” (paying) user…