Table of Contents
This program generates a file containing keys that can be used in NTP’s symmetric key cryptography.
The program produces a file containing ten pseudo-random printable ASCII strings suitable for the MD5 message digest algorithm. It also produces an additional ten hex-encoded random bit strings suitable for the SHA1 and other message digest algorithms.
The keys file must be distributed and stored using secure means beyond the scope of NTP itself. The keys can also be used as passwords for the ntpq utility program.
Command Line Options
- -M, --md5key
Dummy option for backward compatibility in old scripts. This program always runs in -M mode.
Running the program
The simplest way to run the ntpkeygen program is logged in directly as root. The recommended procedure is to change to the keys directory, usually /etc/ntp/, then run the program. Then chown the output file to ntp:ntp. It should be mode 400.
|ntpkeygen uses the system randomness source. On a POSIX system this is usually /dev/urandom. Immediately after a reboot, on any OS, there may not be sufficient entropy available for this program to perform well. Do not run this program from any startup scripts. Only run this program on an active host with a lot of available entropy.|
Key file access and location
File names begin with the prefix ntpkey and end with the postfix hostname.filestamp, where hostname is the owner name, usually the string returned by the Unix gethostname() routine, and filestamp is the NTP seconds when the file was generated, in decimal digits.
ntpkeygen also makes a soft link from ntp.keys to the generated file. ntp.keys is the normal file used in ntp.conf.
Random Seed File
All key generation schemes must have means to randomize the entropy seed used to initialize the internal pseudo-random number generator used by the library routines.
It is important to understand that entropy must be evolved for each generation, for otherwise the random number sequence would be predictable. Various means dependent on external events, such as keystroke intervals, can be used to do this and some systems have built-in entropy sources.
This implementation uses Python’s random.SystemRandom class, which relies on os.urandom(). The security of os.urandom() is improved in Python 3.5+.
Cryptographic Data Files
Since the file contains private shared keys, it should be visible only to root or ntp.
In order to use a shared key, the line to be used must also be setup on the target server.
This file is also used to authenticate remote configuration commands used by the ntpq(1) utility.
Comments may appear in the file, and are preceded with the # character.
Following any headers the keys are entered one per line in the format:
Positive integer in the range 1-65,535
MD5 or SHA1, type of key
the actual key, printable ASCII or hex
Figure 1. Typical Symmetric Key File
Figure 1 shows a typical symmetric keys file used by the reference implementation. Each line of the file contains three fields, first keyno an integer between 1 and 65534, inclusive, representing the key identifier used in the server configuration commands. Next is the key type for the message digest algorithm, which can be any message digest algorithm supported by the OpenSSL library.
For details, see ntp.keys(5).
ntpkeygen just makes a sample file with good random keys. You can edit it by hand to change the keyno or keytype and/or copy lines to other key files.