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8.9 NTP Authentication
NTP provides two variants for authentication in version 4:
•
NTP symmetric keys (i.e. symmetric keys)
•
NTP autokeys
NTP authentication assures a correct time source and prevents manipulation of NTP
information. NTP data itself is, however, not encoded.
8.9.1 NTP symmetric keys
A 32-bit key ID and a cryptographic 64/128-bit check sum of the packet is attached to
each NTP IP packet.
The following algorithms are used for this purpose:
•
Data Encryption Standard (DES)
(partly restricted in North America and no longer integrated into new NTP variants
(>V4.2))
•
Message Digest (MD5)
The DTS 4160 only supports the MD5 procedure.
The receiving NTP service calculates the check sum with an algorithm and compares it
with the one contained in the packet. Both NTP services must have the same
encryption key and the same corresponding key ID for this purpose.
Packets with a wrong key or wrong check sum will not be used for synchronization .
The DTS 4160 must be correspondingly configured to be able to use NTP
authentication (chapter 6.5.12 NTP ). The NTP service of the other equipment (e.g.
server, PC...) must also be configured. In the case of standard NTP, this occurs via the
ntp.conf file:
# path for key file
keys /etc/ntp/ntp.keys
trustedkey 1 2 3 4 5 6# define trusted keys
requestkey 4 # key (7) for accessing server variables
controlkey 5 # key (6) for accessing server variables
server ntp1.test.org key 2
server ntp2.test.org key 6
server 192.168.23.5 key 3
The description of the ntp.conf file can be accessed via the corresponding man-page,
or consulted at http://www.eecis.udel.edu/~mills/ntp/html/authopt.html
The authentication mode is automatically activated when a key is used and the paths
for the keys have been correspondingly configured.
trustedkey
defines all keys currently permitted
requestkey
defines the key for the ntpq help tool.
controlkey
defines the key for the ntpdc help tool.
The keys are located in the ntp.keys file defined with
keys
. This has the following
format:
1 M TestTest
2 M df2ab658
15 M I_see!
498 M NTPv4.98