C.2 Impact of IP Addresses for DNS Resolver on UE

23.5483GPP5G System Enhancements for Edge ComputingRelease 17Stage 2TS

The UE can be configured by the 5GC with an IP address for the DNS resolver using ePCO or IPv6 Router Advertisement (RA), DHCPv4 or DHCPv6 as described in clause 5.8.2 of TS 23.501 [2]. 5GC can reconfigure the DNS resolver IP address using NAS or IPv6 Router Advertisement (RA). In case of anycast IP address is used for the DNS resolver, the 5GC can use UL CL/BP to branch out the DNS messages and the DN is responsible to route them to the closest instance of the MNO DNS resolver without having to reconfigure the DNS resolver IP address in the UE.

NOTE: 5GC is likely not to be able to reconfigure the DNS resolver IP address when DHCP is used to configure this information on the UE, e.g. in case of UE split. Applications in the UE can request the DNS resolver configured on the UE to resolve an FQDN. However, applications can also be configured with their own DNS resolver addresses and can use encrypted messaging based e.g. on DNS over HTTPS (DoH) or, DNS over TLS (DoT). Configuration of application DNS resolvers is out of scope of 5GC. DNS messages delivered over DoT, or DoH might be forwarded transparently to the destination addresses of the messages. The application DNS resolver can be operated by the 5GC operator or by a third party.

A network interface change, or NAS SM EAS rediscovery indication (explicitly as described in clause 6.2.3.3) or reconfiguration of DNS server address in NAS SM message that implicitly indicating EAS rediscovery as described in 6.2.3.2.3 can and should result in the UE OS/application clearing name/IP address translations in its DNS cache.

If a network interface change or NAS SM EAS rediscovery explicit indication or reconfiguration of DNS server address using NAS SM (i.e. implicit EAS rediscovery indication) does not result in the UE OS/application clearing name to IP address translations in its DNS cache, a subsequent DNS EAS address resolution request can result to address of old EAS.

During EPC to 5GC mobility without N26 interface, the UE can receive a new DNS server address different from the one received from the SMF+PGW-C during the PDN connection initiated in EPC. This can result in the UE OS clearing name/IP address translations in its DNS cache. During 5GC to EPC mobility without N26 interface, the UE can perform the same if it receives a new DNS server address from the SMF+PGW-C.