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With this KIP, we would like to argue that it is desirable to make "use_all_dns_ips" or its behaviour as  as the default value for client.dns.lookup  for these reasons:

1. reduce connection failure rates by using all the possible IP addresses of a hostname
2. common expectation of applications dealing with hostname resolved to multiple IP addresses is to attempt connecting to all of them and use the one it can connect successfully, not to use the first one
3. resolving hostname to multiple IP addresses is becoming more common due to the rise of cloud and containerised environment

Public Interfaces

The behaviour of " default " value for of client.dns.lookup changes from "default" (using the first resolved IP" to ") to "use_all_dns_ips" (using one of the resolved IP's, whichever gets successful connection first").

To achieve the old behaviour of using the first resolved IP, add new possible value for explicitly set the value of client.dns.lookup configuration : "use_first_dns_ipto "default".

Note that this KIP preserves KIP-302 behaviour to only use multiple IPs of the same type (IPv4/IPv6) as the first one, to avoid any change in the network stack while trying multiple IPs.

Proposed Changes

Change ClientUtils#resolve toThe behaviour of ClientUtils#resolve stays the same, which is:

• If client.dns.lookup value is either "default" or "is "use_all_dns_ips" or "resolve_canonical_bootstrap_servers_only": Attempt connecting to each resolved IP addresses and use the first one that connects successfully.
• If client.dns.lookup value is "use_first_dns_ipis "default": Use the first resolved IP address.

Compatibility, Deprecation, and Migration Plan

Change the default value of client.dns.lookup configuration in AdminClientConfig, ProducerConfig and ConsumerConfig from "default" to "use_all_dns_ips".

Change all server, tool and test code that literally use "default" value to use "use_all_dns_ips" instead.

Print a warning if "default" value is used: "The 'client.dns.lookup' value 'default' is deprecated and will be removed in future version."

Compatibility, Deprecation, and Migration Plan

• The value "default
• If a hostname resolves to multiple IP addresses and connecting to the first IP failed, the client will attempt to connect to the other IP's instead of failing. Based on the common expectation, this is what is expected.
• If a hostname resolves to a single IP address and connecting to it failed, then the connection will fail.
• The value "use_first_dns_ip" will be marked as deprecated because of the above reason. And this value will be removed in version 3.0.
• If client.dns.lookup is set to "resolve_canonical_bootstrap_servers_only", it will use the behaviour of "use_all_dns_ips" when resolving the individual broker address, instead of using the first resolved IP address.

Please note that the concern about breaking SSL hostname verification raised in KIP-235 does not apply to this KIP because this KIP does not propose to modify ClientUtils#parseAndValidateAddresses. This KIP is proposing to only modify ClientUtils#resolve, which is used in ClusterConnectionStates#currentAddress, which is then used in NetworkClient#initiateConnect to establish the socket connection to the broker. In other words, this KIP only change the behaviour of resolving address to the IP address used by the socket to connect to the broker.

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• Keep the default behaviour of client.dns.lookup to connect to the first resolved IP. The reason this is rejected is because it does not match with the common expectation when a hostname resolves to multiple IP addresses.Remove "default" value from client.dns.lookup. There is a lot of places in the server code that uses "default" value, so removing "default" value would require changing many server code, which is high risk.