Sudhakar's Linux Trivia

• ulimit   :   (RHL 7.3) Dont trust ulimit for corefile size.   To see corefile size, do ulimit -c.  ulimit can be unlimited, but still corefile size can be 0. BTW, for corefilesize, it is also good to check /etc/security/limits.conf
• If you ever want to write to physical memory, use /dev/mem and not /proc/kcore  /proc/kcore is a readonly image. BTW, this is a really risky operation.
• Building a diskless node. ("Live Linux CDROM") : 
• A copy of the HOWTO is here.  Just make sure that when you sepcify the CDROM device, it is not /dev/cdrom, but instead /dev/hdc or /dev/hdd or whatever /dev/cdrom sysmlinks to. I had  wasted some time on this.  Sigh!      
• I made an interesting mistake here. I forgot that the swap partition is on the disk. So even though I have a diskless filesystem, the machine still needs the disk to store the swap file. I still am trying to figure out how to avoid the problem. A dangerous case of the mistake is inserting the CD into a machine that has different partition table on the disk. Then, even though I am booting from the disk, I can trash parts of my disk through the swap space. BTW, I think I did this. Sigh!
• Checklist for creating a diskless that does not use the disk even for swap space, and  hence avoids the problem discussed in previous bullet. Edit the file /etc/rc.sysinit to make the following changes :
• remove the code that does fsck.
• remove the code that calls swapon.  There are 2 places where swapon is called.
• remove the code that remounts / partition in read-write mode.
• if you want to ssh to this machine running the diskless code, make sure you have ssh keys in $HOME/.ssh
• (for me only)  add Manish's sweep program. Sweep is a program that gives a root shell to a trusted user without a password. 
• edit /etc/inittab to run the in runlevel 1. This is useful when I want actually type into the console. This is to avoid the following problem. Sometimes, because the filesystem is read-only, somethings spew error messages at infinite rate, due to which 
• I cant really type anything on the screen
• init kills the virtual console because the console is "waking up too fast". So init disables the virtual console. Thus I have no way to type anything in the machine.

• Use "iwconfig" control the wireless card. It is the wireless equivalent of "ifconfig" 
• Use "logrotate" to control the number of weeks of (system) logs. See /etc/logrotate.d and /etc/logrotate.conf . (Change the how long log files are saved for.)
• Use "nmap" to see the ports open on a (remote) machine
• Use "xvidtune -next" to switch X resolution through command line.
• Use "netstat" to look at the network status of your machine.
• Use "ipsumdump" (from Kohler, UCLA) to look at the bandwidth consumption on your machine.
• Use "chvt N" to switch to virtual console N.
  
• sshd + cygwin : Sometime, I had to run sshd on a windows machine using cygwin. After following the install instructions (cache), when I do /usr/sbin/sshd, I get the error : /var/empty must be owned by root and not group or world-writable. To make sshd run, I had to edit the config file /etc/sshd_config so that "UsePrivilegeSeparation" is set to no. BTW, I also needed to do chmod og+r /etc/ssh_host_key, /etc/ssh_host_rsa_key and /etc/ssh_dsa_key. This is bad for security, but then, I got the job done.
  
• Use "getent" to read all the info from NIS server  (like "getent passwd" for the password file).
  

• /etc/inittab :

   '/sbin/mingetty --noclear tty1'   to prevent the login:
   prompt to clear the screen and see the end of the boot process

• /etc/sysctl.conf  :

  kernel.cores_uses_pid = 0       
          otherwise write lots of core.* files, instead of one. Tends to fill your
          disk too quickly if you do a lot of buggable program development and you forget
          to clean them. However, can be indispensible for debugging multithreaded apps.

• /etc/rc.d/init.d/pcmcia  : Dont beep PCMCIA  

  change "/sbin/cardmgr $CARDMGR_OPTS" to "/sbin/cardmgr -q $CARDMGR_OPTS" in /etc/rc.d/init.d/pcmcia