Andryan’s Random Notes

Since I don’t use it often, I keep forgetting the shortcut keys on taking screenshots in Mac OS X. Here is the list:

• Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop
• Command-Shift-4, then select an area: Take a screenshot of an area and save it as a file on the desktop
• Command-Shift-4, then space, then click a window: Take a screenshot of a window and save it as a file on the desktop
• Command-Control-Shift-3: Take a screenshot of the screen, and save it to the clipboard
• Command-Control-Shift-4, then select an area: Take a screenshot of an area and save it to the clipboard
• Command-Control-Shift-4, then space, then click a window: Take a screenshot of a window and save it to the clipboard

References:
http://guides.macrumors.com/Taking_Screenshots_in_Mac_OS_X

In a situation, I had to extend my PPPoE network over a wireless bridge. I do not want to have nor maintain 2 PPPoE servers. I didn’t bother failed to understand how PPPoE works and made a wireless bridge without WDS. When one user log in from the bridged side of the network, it works flawlessly. When more users are trying to log in, the user which logged in earlier gets disconnected. Apparently this is caused by MAC address problem[1]. From the PPPoE server’s side, users logging in from the bridged side of the network have the same MAC address which is the bridge device’s MAC address. From the users’ side, they are able to see correct MAC address of every device on the other side of the network. The PPPoE server gets confused when it sends PPPoE packets because multiple users have the same MAC address and there is no way for the server to direct reply to individual user.

[1]Wireless AP is connected to the side of the network where the PPPoE server is connected to, and wireless client is connected to the other side of the network. Every device connected to the same side of the wireless AP gets the wireless client’s MAC address for every device connected over the wireless bridge, however every device over the bridge gets to see the real MAC address of every device connected to the same side of the wireless AP. I believe that if I interchange the wireless AP and client (so now PPPoE server is connected to the wireless client instead), it may work properly since the PPPoE server will get the correct MAC address of all devices over the bridge. Devices connected to the wireless AP obviously will get the same MAC address for the PPPoE server and PPPoE users connected to the same side of the wireless client, but the most important thing is that the PPPoE users over the bridge can communicate properly with the PPPoE server. Since this is a one-to-many and many-to-one situation, this should work, but not for many-to-many situation. This explanation sounds quite confusing due to my limited English. If you could rewrite this part, please let me know.

If connectivity-wise there is no problem, then check system date and time. Most likely system date and time is way off. Synchronize if necessary!

Third party NTP clients (such as Automachron) are better than Windows’ built-in one since they don’t need port 123 (UDP) incoming to be open.

Check advanced settings of IE, something to do with image link or stuff. Apparently YM uses Internet Explorer component for the chat windows. Probably to ease display formatting since HTML features are already built-in.

Oracle JInitiator, Windows Live Messenger, Internet Explorer 7 – possible conflict causes IE to crash, disable Windows Live Messenger add-on on IE to fix.
Java applet error (cache failed) requires the correct expected Java version to be installed.

To determine which shell is currently being used, do echo $0.

Previously I wrote a post which discusses real-time CPU statistics utility. Now if you need historical data of CPU utilization statistics, use sysstat’s sar. For Fedora and CentOS, there is an official RPM package for sysstat.

Once you have installed sysstat, leave it for a few minutes until you get some records. The data collector runs from /etc/cron.d every 10 minutes (by default) and records all CPU usage. If you have SMP systems, then it will record each CPU’s utilization statistics. The default behaviour of sar is to print ALL CPU statistics combined, use -p to select individual CPU. Have fun watching your CPU utilization stats!

I had a task to allow root login via telnet on RHEL 4.3 servers. I tried my luck on Google and found this. Once I have done exactly as mentioned, I still couldn’t login as root via telnet.

After researching a little bit more on Google, I finally found the answer! Apparently krb5-workstation‘s /etc/xinetd.d/krb5-telnet is not the telnet-server package that I have been looking for. telnet-server‘s telnetd is actually another package which is mentioned in the document I found earlier. I disabled krb5-telnet and enabled telnet in /etc/xinetd.d/.

Voila! Now it allows root login via telnet. Red Hat should have written a note about this in the document.

PS: Please enable telnet-server ONLY if you need it and you know what you’re doing. I do NOT recommend the use of telnet-server.

References:
http://kbase.redhat.com/faq/FAQ_45_453.shtm
http://forums11.itrc.hp.com/service/forums/questionanswer.do?admit=109447626+1210184407758+28353475&threadId=1035531