DNS response case preservation

[mpcaddy]

Issue Details

• Version of AdGuard Home server:
  • v0.106.3
• How did you install AdGuard Home:
  • Homeassistant Addon Docker Contaoner
• How did you setup DNS configuration:
  • Router DHCP
• If it's a router or IoT, please write device model:
  • EdgeRouter
• CPU architecture:
  • x64
• Operating system and version:
  • Ubuntu 20.04.1

Expected Behavior

RFC4343 for dns states that the reply must match the request. The case for the reply must match the case for the request.

Actual Behavior

The reply for the dns query is now returning lowercase domain name.

Additional Information

With the changes made in PR3115 the DNS reply is now invalid as it is lowercasing the domain. Which whilst this is fine and great for filtering the reply should maintain the original case.

This problem caused one of my devices to go offline as it could no longer resolve the address it was sending data to and after an exchange with their support and running a tcpdump, found the dns reply and request was different. The IoT device has very limited configuration options and no static configuration it gets its settings over dhcp so as a workaround I've set it to return the router as a dns server which looks up via adguard as the router is returning a valid response even though the response it gets from adguard is invalid (which suggests the routers dns client is implemented incorrectly but that's another matter).

From looking at the code I believe it is line 26 in filter.go that is the cause of this as if I've read it correctly that replaces the original name with a lowercase one.

[ainar-g]

Thanks for the report. We'll try to return the case preservation in the next few days.

[ainar-g]

@mpcaddy, this should be fixed as of edge snapshot c95acf7. Can you please check if the case is now retained for you? Thanks!

[mpcaddy]

Hi thanks, from looking at the code I think that looks fine.
I'm running it using the Home Assistant addon and don't see a way to get it to run the edge version from there.

[ainar-g]

I see. I don't know if Home Assistant has a way to switch to an edge revision, as it is an unofficial plugin, if I recall correctly. I'll close this issue, of you don't mind. Feel free after the final v0.107.0 release if the issue persists.