Installing and Administering Internet Services

Chapter 7 221
Configuring NTP
Configuration
Configuring Authentication
Authentication is a mechanism that helps protect against
unauthorized access to time servers. Authentication is enabled on a
system-by-system basis. Once enabled on a system, authentication
applies to all NTP relationships configured on the system. When
authentication is enabled on a host, only those time servers that send
messages encrypted with a configured key are considered as candidates
to which the host would be synchronized.
In authenticated mode, each NTP packet transmitted by a host has
appended to it a key number and an encrypted checksum of the
packet contents. The key number is specified in the peer, server, or
broadcast statement for the remote host. You specify either the Data
Encryption Standard (DES) or the Message Digest (MD5) algorithm to
be used for the encryption of NTP packets.
Upon receipt of an encrypted NTP packet, the receiving host recomputes
the checksum and compares it with the one included in the packet. Both
the sending and receiving systems must use the same encryption key,
defined by the key number.
When authentication is enabled on a host, the following time servers will
not be considered by the host for synchronization:
• Time servers that send unauthenticated NTP packets.
• Time servers that send authenticated packets that the host is unable
to decrypt.
• Time servers that send authenticated packets encrypted using a
non-trusted key.
An authentication key file is specified on the host. The key file
contains a list of keys and their corresponding key numbers. Each
key-key number combination is further defined by a key format, which
determines the encryption method being used. See the xntpd man page
for more information about the content of the authentication key file. A
sample key file is provided in /usr/newconfig/etc/ntp.keys. The
recommended location for the key file is /etc/ntp.keys. The key file
should be secured to allow only the system administrator to have read
and write access (mode 600).
While the key file can contain many keys, you can declare a subset of
these keys as trusted keys. Trusted keys are used to determine if a time
server is “trusted” as a potential synchronization candidate. Only time