Access denied when TiProxy and TiDB are in localhost

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Start a TiDB cluster and TiProxy in localhost.
2. Create user u1@localhost.
3. Connect to TiProxy with u1.

2. What did you expect to see? (Required)

Connect succeeds.

3. What did you see instead (Required)

Connect failed: Access denied for user.

4. What is your version? (Required)

master.

The reason:
The Config.Host of TiDB is by default 0.0.0.0. TiDB registers to PD with address 192.168.0.101 instead of 127.0.0.1 and so TiProxy connects to 192.168.0.101. In this case, the TiDB sees that the TiProxy address is also 192.168.0.101 and so doesn't match the address DNS response contained records which contain invalid names in net.LookupAddr(host).

When TiDB starts with --host=127.0.0.1, TiDB registers to PD with address 127.0.0.1 and so TiProxy connects to 127.0.0.1. The TiDB sees that the TiProxy address is 127.0.0.1 and it matches localhost and succeeds.

Start by reproducing the localhost setup with a TiDB cluster, TiProxy, and user u1@localhost, comparing the default host configuration with --host=127.0.0.1. Trace how TiDB registers its address through PD and how TiProxy connects and handles the address during authentication. Done means the documented localhost setup allows u1 to connect successfully without relying on the alternate host configuration.