OPS.1.2.6 NTP Time Synchronisation
IntroductionNetworked IT systems often require synchronous states. In most cases, the time serves as a reference. However, the internal clocks of IT systems can deviate from the actual time. The Network Time Protocol (NTP) is used to regularly determine a reference time of central timers via network connections and to adjust internal clocks accordingly. In networks, precise time synchronisation makes it possible to apply uniform time stamps to information—in order to arrange data chronologically, compare data, or limit access rights, for example. This is the only way to achieve things like correlating time sequences from log data of different IT systems. Accurate time information is also important in the field of cryptographic protocols. Furthermore, synchronising all timers accurately is essential in OT networks. NTP clients obtain time information from NTP servers. NTP servers can in turn obtain time information from other NTP servers as NTP clients. This creates a hierarchical time distribution (in “strata”). At the top are NTP servers that obtain their time from precise sources (e.g. an atomic clock or a GPS or DCF77 receiver). These NTP servers are called stratum-1. The NTP service uses methods to determine the deviation of a system clock from external time sources even in the case of deviating responses from different time sources. For example, it ignores time information from a source that suddenly deviates significantly from its own system time. Control messages allow clients to request status information or change the behaviour of an NTP server, including across the respective network. NTP messages are usually transmitted in an unsecured form. NTP does, however, offer the option to protect a message with cryptographic keys so that the message cannot be changed without authorisation. ObjectiveThe aim of this module is to secure NTP servers and clients in such a way that the IT systems in a given information domain can reliably determine the time and adjust their clocks accordingly. Scoping and ModellingOPS.1.2.6 NTP Time Synchronisation must be applied to every IT system that uses NTP in the information domain at hand. In order to create an IT-Grundschutz model for a specific information domain, all the modules must be considered in their entirety. As a rule, several modules must be applied to a given topic or target object. This module covers: - Planning for the use of the NTP protocol
- Operation of NTP servers
- Operation of NTP clients
The following content is also significant, but dealt with elsewhere: - General requirements for the operation of servers (see SYS.1.1 General Server)
- General requirements for the operation of clients (see SYS.2.1 General Client)
Useful ResourcesNo further information is available for module OPS.1.2.6 NTP Time Synchronisation.
1. Panoramica
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OPS.1.2.6.A1 Planning the Use of NTPs (B)
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The IT Operation Department MUST plan where and how NTP is used. This SHOULD be documented fully. In the process, the applications that rely on accurate time information MUST be identified. The requirements of the information domain at hand regarding the accurate time of IT systems MUST be defined and documented. The IT Operation Department MUST define which NTP servers are to be used by which NTP clients. Whether the NTP servers are to operate in client-server or broadcast mode MUST be defined.
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OPS.1.2.6.A2 Secure Use of External Time Sources (B)
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If time information is obtained from an NTP server outside the network of the organisation in question, the IT Operation Department MUST assess whether the operator's NTP server is sufficiently reliable. The IT Operation Department MUST ensure that only NTP servers classified as reliable are used. The IT Operation Department MUST know and observe the operator's usage regulations.
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OPS.1.2.6.A3 Secure Configuration of NTP Servers (B)
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The IT Operation Department MUST configure the NTP server in such a way that clients can only change NTP server settings if this is explicitly intended. Furthermore, it MUST be ensured that only trusted clients can request status information. If the respective organisation's internal NTP servers do not use sufficiently accurate time sources themselves, the IT Operation Department MUST configure these NTP servers to regularly request accurate time information from external NTP servers.
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OPS.1.2.6.A4 Failure to Account for Unsolicited Time Information (B)
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The IT Operation Department MUST configure all NTP clients to discard unsolicited time information received from other IT systems.
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OPS.1.2.6.A5 Use of Client-Server Mode for NTP (S)
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The IT Operation Department SHOULD configure all IT systems to use the NTP service in client-server mode. NTP servers SHOULD only send time information to clients when they actively request it.
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OPS.1.2.6.A6 Monitoring of IT Systems Using NTP (S)
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The IT Operation Department SHOULD monitor the availability, capacity, and system time of internal NTP servers. The IT Operation Department SHOULD configure IT systems that synchronise their time via NTP to log the following events: - Unexpected restarts of the IT system
- Unexpected restarts of the NTP service
- Errors related to the NTP service
- Unusual time information
If an NTP server regularly sends time information on its own (broadcast mode), the IT Operation Department SHOULD monitor the NTP clients to see if they receive unusual time information.
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OPS.1.2.6.A7 Secure Configuration of NTP Clients (S)
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The IT Operation Department SHOULD define which time information an IT system should use when it has been restarted. The IT Operation Department SHOULD define which time information an IT system should use when its NTP service has been restarted. The IT Operation Department SHOULD determine how NTP clients will react to significantly deviating time information. In particular, a decision SHOULD be taken as to whether strongly deviating time information will be accepted by NTP servers after a system restart. The IT Operation Department SHOULD set limits for time information that deviates significantly. The IT Operation Department SHOULD ensure that NTP clients will still receive sufficient time information even if an NTP server asks them to send fewer requests (or none at all).
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OPS.1.2.6.A8 Use of Secure Time Synchronisation Protocols (S)
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The IT Operation Department SHOULD check whether secure protocols can be used for time synchronisation (e.g. Network Time Security, NTS). Secure protocols SHOULD be used whenever possible.
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OPS.1.2.6.A9 Sufficient Availability of Accurate Time Sources (H)
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If accurate system times are of significant importance, an organisation SHOULD have multiple stratum-1 NTP servers on its network. The IT systems of an information domain with an NTP service SHOULD use stratum-1 NTP servers directly or indirectly as a time reference. The stratum-1 servers SHOULD each have different time sources.
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OPS.1.2.6.A10 Internal NTP Servers Only (H)
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Each IT system in an information domain with an NTP service SHOULD receive its time information exclusively from NTP servers within the respective organisation's network.
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OPS.1.2.6.A11 Redundant NTP Servers (H)
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IT systems for which the accuracy of the system time is of significant importance SHOULD obtain their time information from at least four independent NTP servers.
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OPS.1.2.6.A12 NTP Servers with Authenticated Information (H)
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NTP servers SHOULD authenticate themselves to clients when communicating. This SHOULD also apply to the servers from which NTP servers receive their own time information. NTP clients SHOULD only accept authenticated NTP data.
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1.1 Riferimenti
1.2 Requisiti identificati
1.3 Regolamentazioni correlate
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