| Internet-Draft | DNS Storage of Tunnel Info | May 2023 |
| Eastlake & Song | Expires 25 November 2023 | [Page] |
A Domain Name System (DNS) Service Binding (SVCB) Service Parameter Type and a DNS Resource Record (RR) Type are specified for storing connection tunneling / encapsulation Information in the DNS.¶
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The Domain Name System (DNS) is a hierarchical, distributed, highly available database with a variety of security features used for bi-directional mapping between domain names and addresses, for email routing, and for other information [RFC1034] [RFC1035]. This data is formatted into resource records (RRs) whose content type and structure are indicated by the RR Type field. General familiarity with the DNS and its terminology [RFC8499] is assumed in this document.¶
It is common for there to be a requirement to use or some benefit from using a "tunnel" or encapsulation scheme when connecting to a service/host. For a reachability use case, see Section 1.3 of [RFC9012]. Typically, this involves taking a packet with a transport header addressed to the ultimate destination, adding a tunnel header to the packet, and then adding an outer transport header before transmitting the packet out of a network interface (port). The resulting packet is illustrated in Figure 1. (In some cases, such as IP-in-IP, the Tunneling Header may be null.)¶
+--------------------------------------------------------------+ | Outer Transport Header | +--------------------------------------------------------------+ | Tunneling Header | +--------------------------------------------------------------+ | Inner Transport Header | +--------------------------------------------------------------+ | Tunneled/Encapsulated Packet | | |
The addition of the Outer Transport and Tunneling Headers will lengthen packets which may result in the need for fragmentation. Some tunneling protocol support fragmentation but for those that do not, fragmentation of the Tunneled Packet before encapsulation may be required.¶
This document specifies a Domain Name System (DNS) Service Binding (SVCB) Service Parameter Type and a DNS Resource Record (RR) Type for storing connection tunneling / encapsulation information in the DNS. This enables the storage and retrieval of tunneling information that may be needed to connect to a remote service or host.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The following acronyms are used in this document:¶
The SVCB (Service Binding) RR is specified in [SVCBref]. It provides, when used in the "Service Mode", for the encoding of a variety of Service Parameters to assist in connecting to a service.¶
The "tunnel" SVCB Service Parameter, whose numeric key value is TBD1, has a value consisting of the Tunnel Type, Tunnel Parameters Length, and Tunnel Parameters TLVs as specified for the BGP Tunnel Encapsulation Attribute [RFC9012] and shown in Figure 2. The presentation format for this value is hexadecimal.¶
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tunnel Type | Tunnel Parameters Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / / Tunnel Parameters TLVs (variable length) / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
See further details on the fields in Figure 2 where fields of the same name are specified in Section 3.¶
The RDATA for this RR type includes tunneling information in the format used in the BGP Tunnel Encapsulation Attribute [RFC9012], a domain name that maps to the Inner Transport Header destination, and optional Outer Transport Header information, all as further explained below and illustrated in Figure 3.¶
The RRType Code for the TUNNEL RR is TBD2.¶
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Priority | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tunnel Type | Tunnel Parameters Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / / Tunnel Parameters TLVs (variable length) / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / Target Name (variable length) / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields in Figure 3 are as explained below. The contiguous Tunnel Type, Tunnel Parameters Length, and Tunnel Parameters TLV (Value) as a block of data are identical to the "Tunnel Encapsulation TLV" specified in [RFC9012] ("The BGP Tunnel Encapsulation Attribute").¶
- This field is a two-byte unsigned integer using network byte order that is the priority of using this tunnel to the target. A client MUST use the tunnel with the lowest priority RR that meets the following conditions:¶
- This field consists of "Tunnel Encapsulation Attribute Sub-TLVs" as specified in [RFC9012]. These TLVs can specify a variety of parameters, including the following, which may be useful in constructing the Outer Transport Header (Figure 1):¶
A client/application seeking to send packets to a host or service can query the DNS using the name of the host or service for the TUNNEL RR. If that name is found and one or more TUNNEL RRs are returned, it can use the highest priority TUNNEL RR for which it has implemented the Tunnel Type indicated in that RR to create and populate a header as shown in Figure 1. The Target Name in that TUNNEL RR can then be used to obtain an address for use in the Outer Transport Header as also shown in Figure 1.¶
Where a client/application is already using the SVCB RR, similar logic applies using the tunnel SVCB Service Parameter.¶
The suggestions and comments of the following persons are gratefully acknowledged:¶
tbd¶
IANA is requested to assign a value from the Service Parameter Keys Registry on the "DNS Service Bindings (SVCB)" IANA web page as follows:¶
Number Name Meaning Reference ------ ------ --------------------- --------------- TBD1 tunnel Tunneling information [this document]¶
IANA is requested to assign a TUNNEL RR Type (TBD2) as in the template in Appendix A.¶
A. Submission Date: tbd B.1 Submission Type: [X] New RRTYPE [ ] Modification to RRTYPE B.2 Kind of RR: [X] Data RR [ ] Meta-RR C. Contact Information for submitter (will be publicly posted): Name: Donald Eastlake Email Address: d3e3e3@gmail.com International telephone number: +1-508-333-2270 Other contact handles: D. Motivation for the new RRTYPE application. Need to store tunneling information in the DNS. E. Description of the proposed RR type. See draft-eastlake-dnsop-svcb-rr-tunnel F. What existing RRTYPE or RRTYPEs come closest to filling that need and why are they unsatisfactory? The SRV RR provides connection information for a service/host but not tunneling information. G. What mnemonic is requested for the new RRTYPE (optional)? TUNNEL H. Does the requested RRTYPE make use of any existing IANA registry or require the creation of a new IANA subregistry in DNS Parameters? If so, please indicate which registry is to be used or created. If a new subregistry is needed, specify the allocation policy for it and its initial contents. Makes use of the Border Gateway Protocol (BGP) Tunnel Encapsulation Registry and subsidiary Registries under tha encapsulation registry. Does not create a new registry. I. Does the proposal require/expect any changes in DNS servers/resolvers that prevent the new type from being processed as an unknown RRTYPE (see [RFC3597])? No. J. Comments: None.¶