Lecture 30

More on Bind

What was wrong with delorean?

We had two default routes. That is, two different addresses that were treated as the place to send packets whose IP address is not on our network.

The second route was introduced as a result of the change we made to /etc/network/interfaces. We had added a stanza as follows: 

iface eth0:1 inet static
  address 192.168.67.1
  netmask 255.255.255.0
  network 192.168.67.0
  broadcast 192.168.67.255
  gateway 128.227.170.67
  up route add -net 192.168.0.0 netmask 255.255.0.0 gw 192.168.67.1 metric 0
The line in italics caused the problem. It added another default route, this time to our own machine! We couldn't do anything. We removed that default route and deleted that line from /etc/network/interfaces, fixing the problem.

An investigation involving nineteeneightyfive.

Let's see how we can fix someone's bind problem.

ntpd: the Network Time Protocol Daemon

In order for a network of computers to operate corectly, especially when multiple machines may update time-critical files, all machines in the network need to maintain nearly synchronized clocks.

Several years ago, a network I was using had time gaps between machines. Scripts that I had written to maintain a collection of files in a consistent state started failing. These scripts ran processes on several machines that mounted the same file systems and they required that there be no more than a second or so difference between their clocks. Although the system administrators on that network probably thought I was being a pain, by solving my problem they improved their system reliability markedly as a result.

NTP allows client computers to maintain their system time in synchronization with the U.S. Naval Observatory Master Clocks in Washingon, DC and Colorado Springs, CO. This means that you can not only synchronize the clocks on machines in your own network, you can synchronize with all other machines in this larger NTP network.

The nominal accuracy of an NTP client is between 1 and 30 ms depending on the properties of he internet roue between the client and server.

NTP servers are organized into strata from zero (0) to sixteen (16). A stratum 0 server is directly connected to a time source (atomic clock). For n = 1 to 14, stratum n+1 server is directly connected to a stratum n server. Stratum 16 indicates a broken server.

Fortunately for you, it just so happens that the nearest USNO NTP stratum one time server is in our building, namely, ntp-s1.cise.ufl.edu:

If you apt-get install ntp-server, then you'll need to modify /etc/ntp.conf to have a line specifying 

server ntp-s1.cise.ufl.edu