3. Hostname Resolution Query Handling
When a client has both IPv4 and IPv6 connectivity and is trying to establish a connection with a named host, it needs to send out both AAAA and A DNS queries. Both queries SHOULD be made as soon after one another as possible, with the AAAA query made first and immediately followed by the A query.
Implementations SHOULD NOT wait for both families of answers to return before attempting connection establishment. If one query fails to return or takes significantly longer to return, waiting for the second address family can significantly delay the connection establishment of the first one. Therefore, the client SHOULD treat DNS resolution as asynchronous. Note that if the platform does not offer an asynchronous DNS API, this behavior can be simulated by making two separate synchronous queries on different threads, one per address family.
The algorithm proceeds as follows: if a positive AAAA response (a response with at least one valid AAAA record) is received first, the first IPv6 connection attempt is immediately started. If a positive A response is received first due to reordering, the client SHOULD wait a short time for the AAAA response to ensure that preference is given to IPv6 (it is common for the AAAA response to follow the A response by a few milliseconds). This delay will be referred to as the "Resolution Delay". The recommended value for the Resolution Delay is 50 milliseconds. If a positive AAAA response is received within the Resolution Delay period, the client immediately starts the IPv6 connection attempt. If a negative AAAA response (no error, no data) is received within the Resolution Delay period or the AAAA response has not been received by the end of the Resolution Delay period, the client SHOULD proceed to sorting addresses (see Section 4) and staggered connection attempts (see Section 5) using any IPv4 addresses returned so far. If the AAAA response arrives while these connection attempts are in progress but before any connection has been established, then the newly received IPv6 addresses are incorporated into the list of available candidate addresses (see Section 6) and the process of connection attempts will continue with the IPv6 addresses added, until one connection is established.
3.1. Handling Multiple DNS Server Addresses
If multiple DNS server addresses are configured for the current network, the client may have the option of sending its DNS queries over IPv4 or IPv6. In keeping with the Happy Eyeballs approach, queries SHOULD be sent over IPv6 first (note that this is not referring to the sending of AAAA or A queries, but rather the address of the DNS server itself and IP version used to transport DNS messages). If DNS queries sent to the IPv6 address do not receive responses, that address may be marked as penalized and queries can be sent to other DNS server addresses.
As native IPv6 deployments become more prevalent and IPv4 addresses are exhausted, it is expected that IPv6 connectivity will have preferential treatment within networks. If a DNS server is configured to be accessible over IPv6, IPv6 should be assumed to be the preferred address family.
Client systems SHOULD NOT have an explicit limit to the number of DNS servers that can be configured, either manually or by the network. If such a limit is required by hardware limitations, the client SHOULD use at least one address from each address family from the available list.