blog.jphoude.qc.ca

I may have missed something obvious, but it seems that the default ntp.conf shipped with RHEL 5 is not working…

I just want to configure my servers to sync time with a public NTP server. Using the default configuration (/etc/ntp.conf), ntpd starts correctly but never writes anything to /var/log/messages besides the first lines at startup.

What I have found is that the “restrict” lines in the default ntp.conf file do not seem to allow ntpd to connect to the servers that are listed there (*.centos.pool.ntp.org by default). Adding the following lines fix the problem :

restrict 0.centos.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 1.centos.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 2.centos.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
# the following is in the default ntp.conf file
server 0.centos.pool.ntp.org
server 1.centos.pool.ntp.org
server 2.centos.pool.ntp.org

Also (as an alternate fix) I had success with the following servers :

server ntp.isc.org
server clock.redhat.com

I’m not sure why, I suspect that’s because in the DNS they resolve to a single IP address, but I don’t really understand what would cause this.

Anyway, with these changes my NTP now sync properly, and I can see the “synchronized to 206.248.171.198, stratum 1″ lines in the /var/log/messages file.

This week I added a new hard drive to a server, meant as a spare drive for the RAID 1 array (linux software RAID). To activate the partitions as spares, I needed to recreate exactly the same partitions as the other disks (same size).

I first tried doing that manually with GNU parted, but that turned out to be hard to do, it seems I could never get the partitions to have exactly the same size…

So I searched a bit on Google, and found a way to make a backup of the partition table of one disk, and copy it to another disk very easily. As usual, the dd command is always a great tool for that kind of job. (more…)

There’s been a local root exploit discovered in the Linux kernel recently, which obliged me to manually compile new kernels on some of my servers. The problem with that : I rarely compile new kernels for these servers, so I’m not sure it will boot at all. Moreover, the servers are located in a datacenter I can’t access easily, so manually choosing the kernel in the grub boot menu is not an option. Fortunately, grub provides a feature to help with that. (more…)

SSH and rsync can be used as a powerful backup tool. Together with hard links, it is also possible to use it to make incremental backups using a simple script. (more…)

OK, so you downloaded an image from jailtime.org or you made your own image as described in my previous post, and now you want to install a Xen guest using it. Here’s how to do it :

(more…)