Least-Common Scope Communications
draft-mudric-6man-lcs-00

Abstract

This draft formulates a security problem statement. The problem arises when a Host uses its Global Unicast Address (GUA) to communicate with another Host situated on the same link.

To address this problem, we suggest to select and use addresses of a least scope that are common.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on February 19, 2021.

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Table of Contents

• 1. Terminology
• 2. Problem Statement
• 3. Least Common Scope Communications
• 4. LL Address Resolution
• 5. Security Considerations
• 6. IANA Considerations
• 7. Contributors
• 8. Acknowledgements
• 9. Normative References
• Appendix A. ChangeLog
• Authors' Addresses

1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

2. Problem Statement

Sockets listening on a global addresses are exposed to attacks. RFC6724 Rule 8 selects a candidate address with the smallest scope. Applications don't always have LL candidate address. They usually have a GUA address. If GUA is on a local link, an application will open a socket using GUA. To avoid using GUA on the local link, a sender needs to find a destination LL address. Currently SASA algorithm (RFC 6724 "Default Address Selection for Internet Protocol Version 6 (IPv6)") cannot use the smallest common scope, given destination GUA.

For security reasons, hosts should use an address with the smallest scope. To avoid these attacks, the host should use LL or ULA addresses.

These security reasons, in more detail, are described next. There is a security problem when a Host uses (one of) its Global Unicast Address(es) (GUA) to communicate to another Host situated on the same link. The problem appears even if that second Host uses its link-local address (LL) for this communication.

The problem is that the Host that uses the GUA to actively communicate with another Host situated on the same link opens a globally reachable entry point in its operating system kernel. This entry point appears when the GUA is assigned to a socket structure. Were that address be a LL, and not a GUA, that entry would not be globally reachable.

To realize communications between Hosts on the same link, it is sufficient to rather use LL addresses on both Hosts.

When a Host uses a GUA to communicate to another Host situated on the same link, it unnecessarily becomes an easy attack target. The attacker might be situated anywhere in the Internet (globally).

3. Least Common Scope Communications

It is recommended that a Host that needs to communicate with another Host that is situated in a particular scope, to use addresses of same scope, or of the least common scope.

For example, two Hosts situated on the same link should ideally use LL addresses to communicate to each other. An interpretation suggests that, given GUA and ULA, a least common 'scope' is the ULA scope (even though, formally, both ULA and GUA are of same global scope). But the global unicast addresses (GUAs) should not be used for two Hosts on the same link: the global scope is unnecessarily large; it unnecessarily opens doors to attacks.

4. LL Address Resolution

To facilitate LL communication on the local link, given a destination GUA or ULA:

• SASA needs to check if a destination is ON-LINK
• for ON-LINK destination, a host needs to resolve the GUA or ULA destination address into a destination host LL address,
• a socket needs to open a port for the source LL address, and
• send packets to the destination LL address.

If both GUA and ULA destinations are known, and ULA destination is not on the link, SASA SHOULD use ULA address.

The document RFC 6724 SASA needs to be updated to check ON-LINK status. The request for comments number 5014, which treats about socket APIs, needs to be updated to use the given destination GUA or ULA addresses for ON-LINK determination, prior to SASA address selection; it also needs to be be updated to specify to send packets using LL address while talking to ON-LINK destinations.

5. Security Considerations

Security

6. IANA Considerations

IANA

7. Contributors

Contributors.

8. Acknowledgements

Acks.

9. Normative References

Appendix A. ChangeLog

The changes are listed in reverse chronological order, most recent changes appearing at the top of the list.

-00: initial version, with Dusan's comments.

Authors' Addresses