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hopf
Elektronik GmbH
Nottebohmstr. 41
• D-58511 Lüdenscheid • Tel.: +49 (0)2351 9386-86 • Fax: +49 (0)2351 9386-93 • Internet: http://www.hopf.com • E-Mail: [email protected]
5.3.4.8 Symmetric Key
5.3.4.8.1 Why Authentication?
Most NTP users do not require authentication as the protocol contains several filters (for bad
time).
Despite this, however, the use of authentication is common. There are certain reasons for this:
•
Time should only be used from safe sources
•
An attacker broadcasts false time signals
•
An attacker poses as another time server
5.3.4.8.2 How is Authentication used in the NTP Service?
Client and server can execute an authentication whereby a code word is used on the client
side and a restriction on the server side.
NTP uses keys to implement the authentication. These keys are used when data are
exchanged between two machines.
In principle both sides must know this key. The key can generally be found in the
"*.*/etc/ntp.keys" directory. It is unencrypted and hidden from public view. This means that the
key has to be distributed on a safe route to all communication partners. The key can be
downloaded for distribution under "Downloads / Configuration Files" on the DEVICE tab. It is
necessary to be logged in as "Master" in order to access this.
The keyword of a client’s ntp.conf determines the key that is used to communicate with the
designated server (e.g. the
hopf
NTP Time Server 8030NTS/GPS). The key must be reliable
if time is to be synchronised. Authentication causes a delay. This delay is automatically taken
into account and adjusted in the current versions.
5.3.4.8.3 How is a key created?
A key is a sequence of up to 31 ASCII characters. Some characters with special significance
cannot be used (alphanumeric characters and the following symbols can be used:
[ ] ( ) * - _ !
$ % & / = ?
).
A new line can be inserted by pressing the
ADD
key. The key which is stored in the key file
is entered on this line. The key ID is used to identify the key and is in the range from 1
– 65534.
This means that 65534 different keys can be defined.