Tag: CentOS 5

IT

PXEboot with CentOS 5.6 and dnsmasq

Install the required software:
yum install dnsmasq

Open up the firewall:
/sbin/iptables -I INPUT -m udp -p udp --dport 67 -j ACCEPT
/sbin/iptables -I INPUT -m udp -p udp --dport 53 -j ACCEPT

Then run:
dnsmasq -d --interface=br0 --dhcp-range=181.30.68.111,static --dhcp-host=84:2b:2b:78:30:79,181.30.68.112 --dhcp-option=option:router,181.30.68.65 --dhcp-boot=pxelinux.0 --enable-tftp --tftp-root=/var/ftpd

Put all required files, such as pxelinux.0 from the package syslinux in /var/ftpd

Ubuntu/Debian

I'm running RedHat-based software on all of my machines. Above information may be useful for Ubuntu/Debian users, but it's not tested and I'm not supporting it.

Servers: RedHat Enterprise Linux/CentOS is more suitable for servers, as there's a lot of professional level support available. I think that's important, because if I say, get a car accident, I want the servers to be managable by another professional.

Desktops/Laptops: RPM packages are pretty exchangable between RedHat-based platforms. That's a good reason to run Fedora on the desktop.
© GeekLabInfo PXEboot with CentOS 5.6 and dnsmasq is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

Syslog kills Postfix performance

This night, some scriptkiddie decided it'd be fun to overload one of my servers by bruteforcing the FTP. The FTP server taking 90% of the CPU made the whole server slow. Especially the Postfix mailserver experienced a huge delay.

Anyway, I just firewalled out the scriptkiddie and Postfix could start processing the queue. But for some reason, it delivered no more than 5-10 mails a second. That would mean that processing the queue of 18.000 mails would take 30-60 minutes.

I've tried to figure out why Postfix was so slow before. Without any luck. Using the well known tuning pages didn't help me at all. Today, I opened up /usr/bin/top, to see that the system was doing a lot of I/O. I accidently found out why: I like to have syslog write *.* to /var/log/allmessages.

This simple rule caused so much extra I/O, that postfix was slowed down 90%. After disabling "allmessages" performance went up, and the queue was processed within minutes. Makes me wonder what happens if I...

  • disable logging at all, which is a bad idea as you can no longer spot abuse
  • find a way to enable write-cache - the default syslog of CentOS 5 does not support this AFAIK
  • write logs to another device

Interesting...

© GeekLabInfo Syslog kills Postfix performance is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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