Resolving DNS using IPv4 DNS server not working despite NAT64 working correctly

[agners]

Hello,

We are trying to use the DNS resolver capabilities of the OTBR. When hosting the OTBR in an environment where the host has IPv6, resolving a DNS name works just fine:

root@ha-fujitsu:/usr/src# ot-ctl dns config
Server: [2001:4860:4860:0:0:0:0:8888]:53
ResponseTimeout: 6000 ms
MaxTxAttempts: 3
RecursionDesired: yes
ServiceMode: srv_txt_opt
Nat64Mode: allow
Done
root@ha-fujitsu:/usr/src# ot-ctl dns resolve4 example.com
DNS response for example.com. - fdf8:76ab:ebd6:2:0:0:5db8:d822 TTL:14385 
Done

However, trying to resolve using an IPv4 address doesn't seem to work:

root@ha-fujitsu:/usr/src# ot-ctl dns resolve4 example.com 8.8.8.8
Synthesized IPv6 DNS server address: fdf8:76ab:ebd6:2:0:0:808:808
DNS response for example.com. - 
Error 28: ResponseTimeout

Pinging the same IPv4 address seems to work however:

ot-ctl ping 8.8.8.8
Pinging synthesized IPv6 address: fdf8:76ab:ebd6:2:0:0:808:808
16 bytes from fdf8:76ab:ebd6:2:0:0:808:808: icmp_seq=1 hlim=111 time=3ms
1 packets transmitted, 1 packets received. Packet loss = 0.0%. Round-trip min/avg/max = 3/3.0/3 ms.
Done

I am quite certain that I don't have any firewall active which would prevent the IPv4 DNS request. So far I haven't further looked into the issue (I didn't analyze network traffic yet), but before digging deeper, I was wondering if this should be working in general.

I am also a bit confused about the DNS64 option. It seems that this simply adds a local bind server which forwards requests to IPv4 addresses. Is this really necessary? With NAT64, and from the description of the dns commands it sounds like OTBR can use IPv4 addresses, which will be synthesized to an IPv6 address using the preferred NAT64 prefix, just fine. So is this option required to reach IPv4 DNS server when NAT64 is already working fine?

[jwhui]

The dns resolve4 command should work. If you can help grab pcaps for both the Thread and infrastructure interfaces, we can help take a look.

The DNS64 option is not necessary any more. In Thread, the client is responsible for directly retrieving A records and synthesizing IPv6 addresses.