Enabling Autokey Crypto in NTP on SLES10
The NTP protocol permits the use of crypto to authenticate clients and servers to each other, as well as between time servers. By default, SLES10 is set up to allow the v3 method of using symmetric keys, but not the v4 method that uses public/private keys. If you want to use the v4 method, this is the tip for you.
By default SLES runs NTP inside a chroot jail. This can be changed from the YaST NTP config screen if you wish. This is a more secure method of running NTP. The chroot jail’s root is at /var/lib/ntp/.
Additionally, ntp runs with an AppArmor profile loaded against it for added security.
Getting NTPv4 auth to work
There are 4 steps to get this to work.
- Copy the .rnd file to the chroot jail
- Run ntp-keygen
- Modify the AppArmor profile for /usr/sbin/ntpd to allow read access to the new files
- Modify the /etc/ntp.conf file to enable v4 auth.
Copy the .rnd file to the chroot jail
By default, there should be a .rnt file at /root/.rnd. If so, copy this to /var/lib/ntp/etc/.rnd. If there is no file there, one can be generated through use of openssl.
timehost:~ # openssl rand -out /var/lib/ntp/etc/.rnd 1
Change-directory to /var/lib/ntp/etc, and execute the following command:
timehost:~ # ntp-keygen -T
This will drop a pair of files in the directory you run it, so running it while in /var/lib/ntp/etc saves you the step of copying them to this directory.
Modify the AppArmor profile
This is done through YaST
- Launch YaST
- Go to the “Novell AppArmor” section, and enter the “Edit Profile” tool.
- Select “/usr/sbin/ntpd” and click Next.
- Click the “Add Entry” button and select File.
- Browse to /var/lib/ntp/etc/.rnd and click the “Read” permissions check-box, and click OK
- Repeat the previous two steps to add the two files created by ntp-keygen, named “ntpkey_cert_[hostname]” and “ntpkey_host_[hostname]”.
- Note: AppArmor behavior changes between Sp1 and Sp2. In Sp1 you can specify the link files, buti n Sp2 you have to specify the link-targets.
The YaST tool for NTP doesn’t allow for v4 configurations, so this has to be done on the command line. Open the /etc/ntp.conf file with your editor of choice, and insert the following lines before your “server” lines:
keysdir /var/lib/ntp/etc/ crypto randfile /var/lib/ntp/etc/.rnd
Then append the word “autokey” to the server and peer lines of your choice. At this point, you should be able to restart ntpd, and it will use authentication. This is a very basic NTPv4 configuration setup, but this should set the ground up for more complex configs.