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Dāvis's tech blog
Office where Mac computers couldn't browse HTTPS sites at 2011.11.15. 23:20
Recently I was invited to troubleshoot network problems in an office where all Mac computers virtually couldn't browse any HTTPS web sites. They reported that opening online banking site tool about 4 minutes on Mac computer and only a couple of seconds on Windows PC. Meanwhile the same Mac computer was able to open the same online banking site in seconds when connected to internet at another office.
My first suspicion was lowered (less that 1500) path MTU and ICMPs blocked by ISP firewall, however this turned out to be false (by running mturoute from a Windows machine). Also running ping with 1472 bytes of ICMP data (and DF flag set) to some hosts on the Internet verified that MTU was 1500 and showed no packet loss.
Running Wireshark on one of affected Mac computer while opening a HTTPS website and analysis of gathered dump showed a few percent packet loss in TCP connection to HTTPS web site. In some cases (I assume, when TCP was in slow-start phase after retransmissions) these lost packets led to state when web server didn't send any more data for several seconds (after server continued sending data, a packet or two later TCP fast retransmissions were triggered and normal TCP operation resumed). After some more investigation it turned out that some ISPs configure traffic shaping with very small queues and burst limits that can be exceeded even in some moments of web page loading (by traffic created by web page download and other internet usage at those moments) causing packet loss in TCP connections. I wasn't provided with packed dump from Windows computer, so I couldn't investigate why that delay didn't happen on Windows computers.
The most notable difference between TCP connections initiated by Mac and Windows computers was TCP timestamps (MacOS by default sends them, while Windows doesn't). When TCP timestamps (as well as TCP window scaling) were temporary (until reboot) turned off in Mac computer by invoking:
sudo sysctl -w net.inet.tcp.rfc1323=0
HTTPS web pages opened in it as fast as in Windows computers. Afterwards the changes were made permanent by invoking:
sudo sh -c 'echo "net.inet.tcp.rfc1323=0" >> /etc/sysctl.conf'
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Problems with old PSAPI.DLL and Internet Explorer 7 on Windows XP at 2009.10.24. 19:42
Recently I faced a computer with Windows XP Service Pack 3 and Internet Explorer 8 Beta. Internet Explorer 8 Beta didn't work properly with computer's owner favorite social portal site, so we decided to downgrade IE.
After uninstall of IE 8 we had some minor problems with IE 6, so I decided to install Internet Explorer 7. Real problems started with IE 7. I wasn't able to launch it normally - double clicking desktop icon created a shortcut to Internet Explorer on desktop. Somehow default action (Open Home Page) and also Start Without Add-ons were missing from IE desktop icon and the first action has become Create Shortcut. After typing HTTP URL in Windows Explorer address bar it showed error The specified procedure could not be found. and applications trying to initialize IE components displayed error message with Entry Point Not Found in title and text The procedure entry point GetProcessImageFileNameW could not be located in the dynamic link library PSAPI.DLL.
Cause and resolution of this problem were quite simple - there was an older version of PSAPI.DLL in %SystemRoot%\system32 folder. Copying this file from another Windows XP SP3 machine resolved the problem (this file also could be extracted from Service Pack 3 installation or Windows XP SP3 install CD). The file PSAPI.DLL is opened by different Windows components and some other software, so the old file can't just be deleted or overwritten - it has to be renamed. If PSAPI.DLL is extracted from Windows installation CD or Service Pack setup, it must be extracted from psapi.dl_ by invoking expand psapi.dl_ %SystemRoot%\system32\psapi.dll in Command Prompt.
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Problems installing Apache HTTP Server at 2009.03.10. 20:45
A few days ago I wanted to install Apache web server on a computer running Windows XP (development system), but .msi installer of current version (2.2.11) didn't initialize properly - after some seconds message consisting of Installation Wizard Interrupted and The Installation Wizard was interrupted before Apache HTTP Server 2.2.11 could be completely installed. appeared on installation wizard and the only enabled button was Finish.
I created Windows Installer log file by invoking MSIEXEC /I apache_2.2.11-win32-x86-openssl-0.9.8i.msi /L*v LogFileName.txt. In log file I immediately noticed these two lines:
Action ended 16:50:09: ResolveServerName. Return value 3.
MSI (c) (D0:18) [16:50:09:437]: Doing action: SetupCompleteError
I tried to enumerate areas where this particular computer was different from an average workstation/server and tried to find out what exactly could cause such error. I paid special attention to network configuration, so one of first things that came into my mind was seven network interfaces (some physical, some virtual) of this workstation. About five network interfaces were online (Connected), so I disabled (simply right clicked on these network interfaces in Control Panel - Network Connections and selected Disable) some of them, leaving only two NICs online. This solved my problem - Apache web server installation wizard initialized properly and I had no further problems installing Apache HTTPD. After installation I, of course, re-enabled those network interfaces.
1 comment

Finding broken video card cooler by analyzing crash dump at 2008.11.03. 20:16
A few days ago one of my workmates had strange problems with his workstation, including several Blue Screens per day.
I opened two crash dumps with Debugging Tools for Windows and both had video card driver on the stack. After telling this to my workmate he correctly suspected that video card cooler has failed. So we fixed his computer by replacing video card.
With this post I wanted to accent that also hardware problems can be noticed by looking at crash dumps. For those who yet don't know how simple it is to open a crash dump I can recommend this article.
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Problems with Visual Studio 2008 database projects at 2008.10.05. 19:09
Recently I faced a situation where a software developer using Visual Studio 2008 wasn't able to do virtually anything in database projects.
It was possible to create a new database project, but on many operations, including Import Database Schema and New Schema Comparison an error message stating Object reference not set to an instance of an object was shown.
I was told that reinstall of Visual Studio doesn't help. Also System Restore made situation only worse.
Accidentally it was discovered that this is a per-user problem (only developers account was affected). After some research I discovered that most likely some of Visual Studio components have had some problems previously so they were skipped from loading.
So a simple "%ProgramFiles%\Microsoft Visual Studio 9.0\Common7\IDE\devenv.exe" /ResetSkipPkgs in Command Prompt of developers account solved this problem.
Most likely this method (resetting skipped packages) can also solve some other "cryptic" per-user problems of Visual Studio 2008.
2 comments

Security of Windows (PPTP based) VPNs at 2008.02.02. 14:35
Recently I have faced two interesting articles about security of PPTP (most common VPN type natively supported by Windows). PPTP requires virtually no configuration on client side (only server name/IP address, username and password are required to connect), so it is easy to set up, deploy and use. I have always had several questions about security of PPTP and they are answered here:

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Removing Direct3D updates at 2007.10.21. 15:28
A few days ago I installed Direct3D updates for DirectX 9.0c (required by a game) on one laptop computer with Windows XP SP2.
I was pretty scared, because after installing these updates any fullscreen Direct3D application (e.g. most games) crashed notebook's video driver in a few seconds (sometimes with a bluescreen, but more often with Windows - Display Driver Stopped Responding error message). As far as I know Microsoft doesn't provide users with any opportunity to uninstall DirectX (and it's updates). There are some third party DirectX removal tools, but I wasn't willing to risk to use them, so I made an experiment on virtual machine.
It was a nice surprise, that Direct3D updates (at least those I had installed) don't replace any files, neither they create any noticeable registry entries. So I simply deleted Direct3D update files from the laptop by invoking ren %SystemRoot%\system32\d3dx9_*.dll *.dll.bak in Command Prompt. After deleting these files Direct3D started again to work normally.
I assume that similar methods may be used to remove updates for other DirectX components as well.
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