Network Address Translation (NAT) Good Practice Guideline

Transcription

1 Programme NPFIT Document Record ID Key Sub-Prog / Project Infrastructure Security NPFIT-FNT-TO-IG-GPG Prog. Director Chris Wilber Status Approved Owner James Wood Version 2.0 Author Mike Farrell Version Date 04/01/2010 Network Address Translation (NAT) Good Practice Guideline Crown Copyright 2010

2 Amendment History: Version Date Amendment History 0.1 First draft for comment /01/2006 Updated formatting /01/2006 Additions /03/2005 Technical Author /03/2006 Approved /11/2009 Document refreshed /11/2009 Incorporating changes suggested by the Infrastructure Security Team /11/2009 Incorporating further change suggested by the Infrastructure Security Team /11/2009 Incorporating changes suggested by Matt Ballinger /01/2010 Approved for Issue, incorporating minor changes suggested by Head of IT Security Forecast Changes: Anticipated Change When Annual Review Jan 2011 Reviewers: This document must be reviewed by the following: Name Signature Title / Responsibility Date Version Infrastructure Security Team Matt Ballinger Junior Project Manager - Technology Office Approvals: This document must be approved by the following: Name Signature Title / Responsibility Date Version James Wood Head of IT Security 2.0 Distribution: NHS Connecting for Health Infrastructure Security Website Crown Copyright 2010 Page 2 of 19

3 Document Status: This is a controlled document. Whilst this document may be printed, the electronic version maintained in FileCM is the controlled copy. Any printed copies of the document are not controlled. Related Documents: These documents will provide additional information. Ref no Doc Reference Number Title Version 1 NPFIT-SHR-QMS-PRP-0015 Glossary of Terms Consolidated.doc Latest 2 NPFIT-FNT-TO-INFR-SEC-0001 Glossary of Security Terms Latest Glossary of Terms: List any new terms created in this document. Mail the NPO Quality Manager to have these included in the master glossary above [1]. Term Acronym Definition Crown Copyright 2010 Page 3 of 19

4 Contents 1 About this Document Purpose Audience Content Disclaimer Introduction Background Network Address Translation Port Address Translation (PAT) Overview Audit and Administration Considerations Full Cone NAT Overview Audit and Administration Considerations Restricted Cone NAT Overview Audit and Administration Considerations Symmetric NAT Overview NAT with Virtual Private Networks (VPNs) NAT Traversal (NAT-T) IPv Overview IPv4 to IPv6 Transition Appendix A. N3 IP Address Space Crown Copyright 2010 Page 4 of 19

5 1 About this Document 1.1 Purpose The purpose of this guide is to address the major challenges associated with Network Address Translation (NAT), the various forms of NAT and the advantages and disadvantages of each type. The following information covers all environments anticipated to interact with the NHS Care Records Service (NCRS). This document contains guidance for New NHS Network (N3) connected systems and networks, in conformance with the Information Governance Statement of Compliance (IGSoC). The information contained in this document should be used as an informed assessment of NAT. However it is the sole responsibility of network owners to ensure that any network solutions that they deploy are sufficiently secure to fully satisfy their own risk assessment. 1.2 Audience This document has been written for readers who have a good level of experience and familiarity with local and wide area networks. 1.3 Content This document comprises this following sections / topics: - Introduction and Background Network Address Translation Port Address Translation Full Cone NAT Restricted Cone NAT Symmetric NAT NAT with Virtual Private Networks, and NAT Traversal (NAT-T) IPv6 Appendix A. N3 IP Address Space Crown Copyright 2010 Page 5 of 19

6 1.4 Disclaimer Reference to any specific commercial product, process or service by trade name, trademark manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favouring by National Health Service Connecting for Health (NHS CFH). The views and opinions of authors expressed within this document shall not be used for advertising or product endorsement purposes. Any party relying on or using any information contained in this document and/or relying on or using any system implemented based upon information contained in this document should do so only after performing a risk assessment. It is important to note that a risk assessment is a prerequisite for the design of effective security countermeasures. A correctly completed risk assessment enables an NHS organisation to demonstrate that a methodical process has been undertaken which can adequately describe the rationale behind any decisions made. Risk assessments should include the potential impact to live services of implementing changes. This means that changes implemented following this guidance are done so at the implementers risk. Misuse or inappropriate use of this information can only be the responsibility of the implementer. Crown Copyright 2010 Page 6 of 19

7 2 Introduction The following information provides a knowledge-based framework that will help maintain good practice values within any NHS organisation. The reader will find good practice guidance for the use of NAT within their organisational network infrastructure. This includes: - What NAT is, plus the advantages and disadvantages of its use. The impact on auditing and security of using NAT. 2.1 Background Network Address Translation is a widely used technology that permits the manipulation of IP traffic. Further details can be found in Request for Comments (RFC) For instance, NAT can both reduce costs and provide an organisation with a central point of communication by hiding a number of internal machines behind a single public Internet Protocol (IP) address. The masking of internal network resources from an external network, such as the Internet, is also an important security feature NAT may also prove useful if it becomes necessary to restrict traffic flows to individual machines, while still allowing the majority of connected machines with a shared IP address to access the external network. In using NAT it may be necessary to consider the practicalities of logging, as well as source/destination access control policies, as NAT manipulates the headers of IP packets, and effectively breaks the end to end Transmission Control Protocol/Internet Protocol (TCP/IP) connection. If considering using NAT it is prudent to establish full logging and auditing policies beforehand, to ensure compliance with good practice guidelines for auditing the use of shared IP addresses. 1 Crown Copyright 2010 Page 7 of 19

8 3 Network Address Translation Network Address Translation is a technology that is prevalent in Internet Protocol version 4 (IPv4) networks, where IPv4 public Internet addresses are a limited resource. Because of the continuing expansion of the World Wide Web (WWW), and other internet based services demanding IPv4 addresses, it is no longer an option for organisations to obtain additional public IPv4 address space to interface public facing systems without a significant need, and so NAT has become a necessity for many network designs. Network Address Translation typically takes place at the boundary between an organisation s internal network and any external network gateway, and allows a multitude of private (RFC 1918) 2 IP addresses to use a limited pool of public IP addresses, or a single address if necessary. NHS organisations typically use NAT to interface between their local sites and the N3 network, whilst home workers may well use NAT within their local router to interface to their Internet Service Provider (ISP). The main security benefit in this case being that any number of local devices can be hidden from the Internet. There are many types of NAT offering many different benefits as well as limitations. I.e. the types of compatible applications or the levels of auditing that are applicable at the end service level. With NAT the border device, typically a router or firewall, uses stateful translation tables to map the private hidden IP addresses to the single address (or pool) and then rewrites the outgoing IP packets on exit so that they appear to originate from the border device. In the reverse communications path responses are mapped back to the originating IP address using the rules (or state ) stored in the translation tables. The translation table rules established in this fashion are flushed after a pre-determined period, unless new traffic refreshes their state. The border device can contain two types of NAT table entries, dependent on the NAT method in use: - Dynamic entries - where multiple internal (private) IP addresses are translated into a single external IP address, or a pool of external IP addresses. Static Entries where internal and external IP addresses are mapped one-toone. In large deployments the masking of unauthorised use of the network, using NAT, can be of serious concern. When faced with possible illegal activities external to the local source network, investigation and discovery of the originating machines within the network can be extremely difficult if detailed logs are not kept. 2 Crown Copyright 2010 Page 8 of 19

9 Notwithstanding future legislation, local policy should determine the minimum retention period for NAT logs, balancing the imperatives of: - Too long = significant storage overhead. Too short = logs may expire before an incident can be investigated The Home Office s voluntary code of practice (Retention of Communications 3 Data Under Part 11: Anti-Terrorism, Crime & Security Act 2001) 3 provides a valuable reference. It suggests a maximum retention period of 12 months but does not prejudice longer periods 3 Crown Copyright 2010 Page 9 of 19

10 4 Port Address Translation (PAT) 4.1 Overview Port Address Translation (PAT), or Network Address Port Translation (NAPT) as it is also known, is a common form of IPv4 NAT. Also known as a hide NAT, PAT maps connections from many internal addresses to a single external IP address by using multiple ports that create and handle connections. These connections are held in a state table to preserve and maintain this connectivity. Because of the design of the TCP/IP protocol, well known ports (0-1023) are not used, leaving ports 1024 to to be mapped against a single external IP address. Whilst over connections could be mapped against a single IP address it is considered good practice not to exceed If this limit is regularly exceeded performance issues may be encountered, at which point the use of a second IP address, or pool of IP addresses, should be considered. It can sometimes be difficult to retrospectively build this into an existing solution, therefore it should be factored into the design from the outset. As a result of this mapping process it is not possible for an external host to create a connection directly to an internal host, because the end-to-end connection is effectively terminated at the border device. Although in the first instance this can appear to be a limiting factor for the usefulness of PAT, this process also has its benefits. It provides a very simple yet effective method of protecting internal hosts from external attack at the network level. If correctly configured the border device can remove all traces of information from the initiated connection, thereby restricting the amount of information disclosed to any malicious user external to the network. 4.2 Audit and Administration Considerations PAT is often utilised in home environments or in large scale deployments. From an administrative point of view PAT is the simplest to implement, only requiring the entry of a static rule to run effectively. Auditing, on the other hand, can generate large log files dependent on the level of information required and the amount of traffic passing through the border device. Without these detailed logs it is very difficult to track individual connections made through PAT. In addition restrictions at the destination service may be difficult to enforce. Crown Copyright 2010 Page 10 of 19

11 5 Full Cone NAT 5.1 Overview Also known as One-to-One NAT or Static NAT, Full Cone NAT creates a static entry on the NAT device. This maps a single internal IP address to a single external IP address. In a typical installation this process also directly maps all the ports on a one to one basis. As this form of translation is static the translating device maintains only basic connection information, because the translation is applied directly at the initiation of each connection, by matching the source and destination IP addresses. Typically this form of NAT is utilised when connections are not only initiated from the private network, but also when connections need to be initiated into the private network. E.g. for access to a specific system from an external network. Fig. 1 provides an example of the translation undertaken by the border router when a user within a private network initiates a connection to a server on the Internet. (1) A packet is created with the following information Src Address: Dest Address: (5) The response is received (2) The router performs the translation to: Src Address: Dest Address: (4) The router performs the translation to: Src Address: Dest Address: (3) The server responds with a packet: Src Address: Dest Address: User Machine NAT Router Server on Internet Fig. 1: Outbound NAT. In this scenario the ports are not illustrated as there is no port translation. Should the server initiate the connection to the user machine, the reverse of the connection process described above would be applicable Crown Copyright 2010 Page 11 of 19

12 5.2 Audit and Administration Considerations This form of NAT can prove useful in cases when other forms of NAT may already be in use for the masking of multiple internal IP addresses, and where certain machines require external identification. This could be an audit requirement, or be part of an access enforcement policy by a service which restricts access by IP address. If undertaking auditing at the service endpoint, this form of NAT provides a direct mapping of an external IP address to an internal IP address, which can be linked in case of investigation. The discovery of the internal address, together with its associated machine and user, is dependent on the source organisation s disclosure of this information. Crown Copyright 2010 Page 12 of 19

13 6 Restricted Cone NAT 6.1 Overview Restricted Cone NAT is very similar to Full Cone NAT in its operation but distinguishes itself by not allowing incoming connections, unless the private machine (internal to the network) has previously initiated a connection to the external destination address. Enhancements to Restricted Cone NAT can create a Port Restricted Cone NAT. This can also be utilised in enforcing policy by using the port to restrict access. 6.2 Audit and Administration Considerations This form of NAT has similar issues as Full Cone NAT. However with the addition of Port Restricted Cone NAT, further security measures at the service end can be utilised to restrict connections to individual ports. Crown Copyright 2010 Page 13 of 19

14 7 Symmetric NAT 7.1 Overview Also known as bi-directional NAT, symmetric NAT uses a rule that directs each request from the same internal IP address and port to a specific destination IP address and port to be mapped to a unique external source IP address and port. If the internal IP and port is utilised to connect to a different destination IP and port, a different mapping is used. Only an external host that receives a packet from an internal host can send a packet back. Please note that there are some problems associated with Symmetric NAT that may cause issues with User Datagram Protocol (UDP) traffic and the combination of IPv4 and IPv6 network traffic. Crown Copyright 2010 Page 14 of 19

15 8 NAT with Virtual Private Networks (VPNs) Owing to the nature of the packet manipulation carried out by NAT, there are several issues with attempting to pass Internet Protocol Security (IPSec) Virtual Private Network (VPN) traffic across devices that perform NAT functions. VPN tunnels gain protection through authentication headers, and use checksums to validate the encapsulated traffic. The NAT packet manipulation alters the checksum of the packet therefore rendering any protection invalid. In these cases technologies such as NAT Traversal (NAT-T) can be utilised. This uses UDP traffic along with the VPN traffic, thus allowing the creation of a VPN across the NAT device. 8.1 NAT Traversal (NAT-T) IPSec VPN users can run into trouble when traversing a NAT-ing device, such as a firewall or router, because: - 1) The TCP/UDP header within an IPSec Encapsulated Security Payload (ESP) packet is encrypted, preventing the mapping of ports by the NAT-ing device. 2) NAT changes the IP and TCP/UDP headers carried within IP packets, invalidating IPSec s integrity check. VPN Pass-through, usually found in home routers that support PAT, addresses 1) above by NAT-ing encrypted packets without mapping ports inside the TCP/IP payload. However VPN Pass-through is not a standard, and behaviour varies between vendors products. NAT Traversal (NAT-T) refers to a series of Internet Engineering Task Force (IETF) drafts 4 that fix 2) above, by wrapping encrypted IPSec packets inside a clear text UDP wrapper. Any NAT-ing device can translate both the source IP address and source UDP port of the clear text wrapper without changing any part of the encrypted IPSec packet carried inside. It is essential though that the devices at both ends of the IPSec tunnel support the same version of NAT-T, be able to detect when to use NAT-T, and keep the NAT mapping alive for the lifetime of the tunnel. Fig 2 below provides an example of IPSec NAT Traversal and Crown Copyright 2010 Page 15 of 19

16 Fig 2 Example of IPSec NAT Traversal Crown Copyright 2010 Page 16 of 19

17 9 IPv6 9.1 Overview IPv6 is designated as the successor to IPv4, with the main driving force for its design being the expected depletion of the IPv4 public address space. Where IPv4 uses 32 bit addresses IPv6 uses 128 bits, resulting in an immensely larger address space than IPv4 (around 79 Octillion times the IPv4 address space), with the IPv6 subnet size standardised at 64 bits. This expanded address space eliminates the primary need for network address translation (NAT), from the network design point of view, as increased flexibility in IP address allocation and routing is provided by IPv6. As well as increased IP address space IPv6 provides several key benefits over IPv4, including: - Simpler packet headers. IPv6 specifies a new packet format, designed to minimise packet-header processing. IPv6 provides better capabilities to support auto-configuration, such as Dynamic Host Configuration Protocol (DHCP), multicasting, traffic engineering, and zero configuration (Zeroconf) 5 networking. Mandatory IPsec support. IPsec was originally developed for IPv IPv4 to IPv6 Transition Because of the large number of IPv4 deployments throughout the world it is likely that the two protocols will co-exist for a number of years, if not decades. Many systems now support dual-stack TCP/IP functions and can communicate in IPv4 and/or IPv6. The three main transition techniques envisaged are as follows: - Dual-stack network, where hosts and routers implement both IPv4 and IPv6 protocols. This enables the network to support both IPv4 and IPv6 services and applications. Tunnelling, this enables the interconnection of IP clouds. For instance, separate IPv6 networks can be interconnected through a native IPv4 network by means of a tunnel. IPv6 packets would be encapsulated by a border router before transportation across the IPv4 network, and decapsulated at the border of the receiving IPv6 network. Tunnels could also be used to interconnect remaining IPv4 clouds through the IPv6 infrastructure. 5 Crown Copyright 2010 Page 17 of 19

18 A translation mechanism, required where it is necessary for an IPv6 only host has to communicate with an IPv4 host. Like tunnelling techniques translation can be implemented in border routers and hosts. From the security point of view the main benefit of NAT, whether it be in an IPv4 or IPv6 environment, is the concealment of private computing and network infrastructure from an external network, such as the Internet. It should be noted that currently IPv6 does not adequately support NAT, although there has been discussion within the IETF around it Crown Copyright 2010 Page 18 of 19

19 Appendix A. N3 IP Address Space N3 is an IPv4 wide area network which primarily routes RFC IP addresses allocated to NHS and 3 rd Party sites. This address space is controlled by the N3 Service Provider (N3SP) and comprises: - The class A private address range ( to ) The class B private address range ( to ) NB: The Class C private address range ( ) is not routable across the N3 network. It is recommended for internal Local Area Network (LAN) use. Information on current N3 IP network addressing policy can be found at: Crown Copyright 2010 Page 19 of 19