Split DNS Configuration for IKEv2

Split DNS is a common configuration for secure tunnels, such as Virtual Private Networks in which host machines private to an organization can only be resolved using internal DNS resolvers . In such configurations, it is often desirable to only resolve hosts within a set of private domains using the tunnel, while letting resolutions for public hosts be handled by a device's default DNS configuration. The Internet Key Exchange protocol version 2 negotiates configuration parameters using Configuration Payload Attribute Types. This document defines two Configuration Payload Attribute Types that add support for trusted Split DNS domains. The INTERNAL_DNS_DOMAIN attribute type is used to convey one or more DNS domains that SHOULD be resolved only using the provided DNS nameserver IP addresses, causing these requests to use the IPsec connection. The INTERNAL_DNSSEC_TA attribute type is used to convey DNSSEC trust anchors for those domains. When only a subset of traffic is routed into a private network using an IPsec SA, these Configuration Payload options can be used to define which private domains are intended to be resolved through the IPsec connection without affecting the client's global DNS resolution. For the purposes of this document, DNS resolution servers accessible through an IPsec connection will be referred to as "internal DNS servers", and other DNS servers will be referred to as "external DNS servers". A client using these configuration payloads will be able to request and receive Split DNS configurations using the INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA configuration attributes. The client device can use the internal DNS server(s) for any DNS queries within the assigned domains. DNS queries for other domains SHOULD be sent to the regular external DNS server. Other tunnel-establishment protocols already support the assignment of Split DNS domains. For example, there are proprietary extensions to IKEv1 that allow a server to assign Split DNS domains to a client. However, the IKEv2 standard does not include a method to configure this option. This document defines a standard way to negotiate this option for IKEv2.

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 when, and only when, they appear in all captials, as shown here.

In order to negotiate which domains are considered internal to an IKEv2 tunnel, initiators indicate support for Split DNS in their CFG_REQUEST payloads, and responders assign internal domains (and DNSSEC trust anchors) in their CFG_REPLY payloads. When Split DNS has been negotiated, the existing DNS server configuration attributes will be interpreted as internal DNS servers that can resolve hostnames within the internal domains.

To indicate support for Split DNS, an initiator includes one more INTERNAL_DNS_DOMAIN attributes as defined in as part of the CFG_REQUEST payload. If an INTERNAL_DNS_DOMAIN attribute is included in the CFG_REQUEST, the initiator MUST also include one or more INTERNAL_IP4_DNS and INTERNAL_IP6_DNS attributes in the CFG_REQUEST. The INTERNAL_DNS_DOMAIN attribute sent by the initiator is usually empty but MAY contain a suggested domain name. The absence of INTERNAL_DNS_DOMAIN attributes in the CFG_REQUEST payload indicates that the initiator does not support or is unwilling to accept Split DNS configuration. To indicate support for DNSSEC, an initiator includes one or more INTERNAL_DNSSEC_TA attributes as defined in as part of the CFG_REQUEST payload. If an INTERNAL_DNSSEC_TA attribute is included in the CFG_REQUEST, the initiator MUST also include one or more INTERNAL_DNS_DOMAIN attributes in the CFG_REQUEST. If the initiator includes an INTERNAL_DNSSEC_TA attribute, but does not inclue an INTERNAL_DNS_DOMAIN attribute, the responder MAY still respond with both INTERNAL_DNSSEC_TA and INTERNAL_DNS_DOMAIN attributes. An initiator MAY convey its current DNSSEC trust anchors for the domain specified in the INTERNAL_DNS_DOMAIN attribute. If it does not wish to convey this information, it MUST use a length of 0. The absence of INTERNAL_DNSSEC_TA attributes in the CFG_REQUEST payload indicates that the initiator does not support or is unwilling to accept DNSSEC trust anchor configuration.

Responders MAY send one or more INTERNAL_DNS_DOMAIN attributes in their CFG_REPLY payload. If an INTERNAL_DNS_DOMAIN attribute is included in the CFG_REPLY, the responder MUST also include one or both of the INTERNAL_IP4_DNS and INTERNAL_IP6_DNS attributes in the CFG_REPLY. These DNS server configurations are necessary to define which servers can receive queries for hostnames in internal domains. If the CFG_REQUEST included an INTERNAL_DNS_DOMAIN attribute, but the CFG_REPLY does not include an INTERNAL_DNS_DOMAIN attribute, the initiator MUST behave as if Split DNS configurations are not supported by the server, unless the initiator has been configured with local polict to define a set of Split DNS domains to use by default. Each INTERNAL_DNS_DOMAIN represents a domain that the DNS servers address listed in INTERNAL_IP4_DNS and INTERNAL_IP6_DNS can resolve. If the CFG_REQUEST included INTERNAL_DNS_DOMAIN attributes with non-zero lengths, the content MAY be ignored or be interpreted as a suggestion by the responder. For each DNS domain specified in an INTERNAL_DNS_DOMAIN attribute, one or more INTERNAL_DNSSEC_TA attributes MAY be included by the responder. This attribute lists the corresponding internal DNSSEC trust anchor in the DNS presentation format of a DS record as specified in . The INTERNAL_DNSSEC_TA attribute MUST immediately follow the INTERNAL_DNS_DOMAIN attribute that it applies to.

All DNS servers provided in the CFG_REPLY MUST support resolving hostnames within all INTERNAL_DNS_DOMAIN domains. In other words, the INTERNAL_DNS_DOMAIN attributes in a CFG_REPLY payload form a single list of Split DNS domains that applies to the entire list of INTERNAL_IP4_DNS and INTERNAL_IP6_DNS attributes.

In this example exchange, the initiator requests INTERNAL_IP4_DNS and INTERNAL_DNS_DOMAIN attributes in the CFG_REQUEST, but does not specify any value for either. This indicates that it supports Split DNS, but has no preference for which DNS requests will be routed through the tunnel. The responder replies with two DNS server addresses, and two internal domains, "example.com" and "city.other.com". Any subsequent DNS queries from the initiator for domains such as "www.example.com" SHOULD use 198.51.100.2 or 198.51.100.4 to resolve.

In this example exchange, the initiator requests INTERNAL_IP4_DNS, INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA attributes in the CFG_REQUEST. Any subsequent DNS queries from the initiator for domains such as "www.example.com" or "city.other.com" would be DNSSEC validated using the DNSSEC trust anchor received in the CFG_REPLY. In this example, the initiator has no existing DNSSEC trust anchors would the requested domain. the "example.com" dommain has DNSSEC trust anchors that are returned, while the "other.com" domain has no DNSSEC trust anchors.

All multi-octet fields representing integers are laid out in big endian order (also known as "most significant byte first", or "network byte order").

Reserved (1 bit) - Defined in IKEv2 RFC . Attribute Type (15 bits) set to value 25 for INTERNAL_DNS_DOMAIN. Length (2 octets) - Length of domain name. Domain Name (0 or more octets) - A Fully Qualified Domain Name used for Split DNS rules, such as "example.com", in DNS presentation format and optionally using IDNA for Internationalized Domain Names. Implementors need to be careful that this value is not null-terminated.

An INTERNAL_DNSSEC_TA Configuration Attribute can either be empty, or it can contain one Trust Anchor by containing a non-zero Length with a DNSKEY Key Tag, DNSKEY Algorithm, Digest Type and Digest Data fields.

Reserved (1 bit) - Defined in IKEv2 RFC . Attribute Type (15 bits) set to value 26 for INTERNAL_DNSSEC_TA. Length (2 octets) - Set to 0 for an empty attribute.

Reserved (1 bit) - Defined in IKEv2 RFC . Attribute Type (15 bits) set to value 26 for INTERNAL_DNSSEC_TA. Length (2 octets) - Length of DNSSEC Trust Anchor data (4 octets plus the length of the Digest Data). DNSKEY Key Tag value (2 octets) - Delegation Signer (DS) Key Tag as specified in Section 5.1. DNSKEY Algorithm (1 octet) - DNSKEY algorithm value from the IANA DNS Security Algorithm Numbers Registry. Digest Type (1 octet) - DS algorithm value from the IANA Delegation Signer (DS) Resource Record (RR) Type Digest Algorithms Registry. Digest Data (1 or more octets) - The DNSKEY digest as specified in Section 5.1 in presentation format. Each INTERNAL_DNSSEC_TA attribute in the CFG_REPLY payload MUST immediately follow a corresponding INTERNAL_DNS_DOMAIN attribute. As the INTERNAL_DNSSEC_TA format itself does not contain the domain name, it relies on the preceding INTERNAL_DNS_DOMAIN to provide the domain for which it specifies the trust anchor. Any INTERNAL_DNSSEC_TA attribute that is not immediately preceded by an INTERNAL_DNS_DOMAIN or another INTERNAL_DNSSEC_TA attribute applying to the same domain name MUST be ignored and treated as a protocol error.

If a CFG_REPLY payload contains no INTERNAL_DNS_DOMAIN attributes, the client MAY use the provided INTERNAL_IP4_DNS or INTERNAL_IP6_DNS servers as the default DNS server(s) for all queries. If a client is configured by local policy to only accept a limited number of INTERNAL_DNS_DOMAIN values, the client MUST ignore any other INTERNAL_DNS_DOMAIN values. For each INTERNAL_DNS_DOMAIN entry in a CFG_REPLY payload that is not prohibited by local policy, the client MUST use the provided INTERNAL_IP4_DNS or INTERNAL_IP6_DNS DNS servers as the only resolvers for the listed domains and its sub-domains and it MUST NOT attempt to resolve the provided DNS domains using its external DNS servers. Other domain names SHOULD be resolved using some other external DNS resolver(s), configured independently from IKE. Queries for these other domains MAY be sent to the internal DNS resolver(s) listed in that CFG_REPLY message, but have no guarantee of being answered. For example, if the INTERNAL_DNS_DOMAIN attribute specifies "example.com", then "example.com", "www.example.com" and "mail.eng.example.com" MUST be resolved using the internal DNS resolver(s), but "anotherexample.com" and "ample.com" SHOULD NOT be resolved using the internal resolver and SHOULD use the system's external DNS resolver(s). The initiator SHOULD allow the DNS domains listed in the INTERNAL_DNS_DOMAIN attributes to resolve to special IP address ranges, such as those of , even if the initiator host is otherwise configured to block DNS answer containing these special IP addresses. When an IKE SA is terminated, the DNS forwarding MUST be unconfigured. This includes deleting the DNS forwarding rules; flushing all cached data for DNS domains provided by the INTERNAL_DNS_DOMAIN attribute, including negative cache entries; removing any obtained DNSSEC trust anchors from the list of trust anchors; and clearing the outstanding DNS request queue. INTERNAL_DNS_DOMAIN attributes SHOULD only be used on split tunnel configurations where only a subset of traffic is routed into a private remote network using the IPsec connection. If all traffic is routed over the IPsec connection, the existing global INTERNAL_IP4_DNS and INTERNAL_IP6_DNS can be used without creating specific DNS exemptions.

DNS records can be used to publish specific records containing trust anchors for applications. The most common record type is the TLSA record specified in . This DNS record type publishes which CA certificate or EE certificate to expect for a certain host name. These records are protected by DNSSEC and thus can be trusted by the application. Whether to trust TLSA records instead of the traditional WebPKI depends on the local policy of the client. By accepting an INTERNAL_DNSSEC_TA trust anchor via IKE from the remote IKE server, the IPsec client might be allowing the remote IKE server to override the trusted certificates for TLS. Similar override concerns apply to other public key or fingerprint based DNS records, such as OPENPGPKEY, SMIMEA or IPSECKEY records. Thus, installing an INTERNAL_DNSSEC_TA trust anchor can be seen as the equivalent of installing an Enterprise Certificate Agency (CA) certificate. It allows the remote IKE/IPsec server to modify DNS answers including its DNSSEC cryptographic signatures by overriding existing DNS information with trust anchor conveyed via IKE and (temporarilly) installed on the IKE client. Of specific concern is the overriding of based TLSA records, which represent a confirmation or override of an existing WebPKI TLS certificate. Other DNS record types that convey cryptographic materials (public keys or fingerprints) are OPENPGPKEY, SMIMEA, SSHP and IPSECKEY records. IKE clients willing to accept INTERNAL_DNSSEC_TA attributes MUST use a whitelist of one or more domains that can be updated out of band. IKE clients with an empty whitelist MUST NOT use any INTERNAL_DNSSEC_TA attributes received over IKE. Such clients MAY interpret receiving an INTERNAL_DNSSEC_TA attribute for a non-whitelisted domain as an indication that their local configuration may need to be updated out of band. IKE clients should take care to only whitelist domains that apply to internal or managed domains, rather than to generic Internet traffic. The DNS root zone (".") MUST NOT be whitelisted. Other generic or public domains, such as top-level domains, similarly SHOULD NOT be whitelisted. Any updates to this whitelist of domain names MUST happen via explicit human interaction to prevent invisible installation of trust anchors. IKE clients SHOULD accept any INTERNAL_DNSSEC_TA updates for subdomain names of the whitelisted domain names. For example, if "example.net" is whitelisted, then INTERNAL_DNSSEC_TA received for "antartica.example.net" SHOULD be accepted. IKE clients MAY interpret an INTERNAL_DNSSEC_TA for domain that was not preconfigured as an indication that it needs to update its IKE configuration (out of band). The client MUST NOT use such a INTERNAL_DNSSEC_TA to reconfigure its local DNS settings. IKE clients MUST ignore any received INTERNAL_DNSSEC_TA requests for a FDQN for which it did not receive and accept an INTERNAL_DNS_DOMAIN Configuration Payload. In most deployment scenario's, the IKE client has an expectation that it is connecting, using a split-network setup, to a specific organisation or enterprise. A recommended policy would be to only accept INTERNAL_DNSSEC_TA directives from that organization's DNS names. However, this might not be possible in all deployment scenarios, such as one where the IKE server is handing out a number of domains that are not within one parent domain.

The use of Split DNS configurations assigned by an IKEv2 responder is predicated on the trust established during IKE SA authentication. However, if IKEv2 is being negotiated with an anonymous or unknown endpoint (such as for Opportunistic Security ), the initiator MUST ignore Split DNS configurations assigned by the responder. If a host connected to an authenticated IKE peer is connecting to another IKE peer that attempts to claim the same domain via the INTERNAL_DNS_DOMAIN attribute, the IKE connection SHOULD only process the DNS information if the two connections are part of the same logical entity. Otherwise, the client SHOULD refuse the DNS information and potentially warn the end-user. If the initiator is using DNSSEC validation for a domain in its public DNS view, and it requests and receives an INTERNAL_DNS_DOMAIN attribute without an INTERNAL_DNSSEC_TA, it will need to reconfigure its DNS resolver to allow for an insecure delegation. It SHOULD NOT accept insecure delegations for domains that are DNSSEC signed in the public DNS view, for which it has not explicitely requested such deletation by specifying the domain specifically using a INTERNAL_DNS_DOMAIN(domain) request. Deployments that configure INTERNAL_DNS_DOMAIN domains should pay close attention to their use of indirect reference RRtypes such as CNAME, DNAME, MX or SRV records so that resolving works as intended when all, some, or none of the IPsec connections are established. The content of INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA may be passed to another (DNS) program for processing. As with any network input, the content SHOULD be considered untrusted and handled accordingly.

This document defines two new IKEv2 Configuration Payload Attribute Types, which are allocated from the "IKEv2 Configuration Payload Attribute Types" namespace.