Merge pull request #271 from AdguardTeam/agm-2240-add-info-about-edns

Update linux.md

docs/private-dns/connect-devices/mobile-and-desktop/linux.md

@@ -102,6 +102,28 @@ If you see a notification that you are not connected to AdGuard DNS, most likely
 
 :::
 
+## Use EDNS (Extended DNS)
+
+EDNS extends the DNS protocol, enabling larger UDP packets to carry additional data. In AdGuard DNS, it allows passing DeviceID in plain DNS using an extra parameter.
+
+DeviceID, an eight-digit hexadecimal identifier (e.g., `1a2b3c4d`), helps link DNS requests to specific devices. For encrypted DNS, this ID is part of the domain (e.g., `1a2b3c4d.d.adguard-dns.com`). For unencrypted DNS, EDNS is required to transfer this identifier.
+
+AdGuard DNS uses EDNS to retrieve DeviceID by looking for option number `65074`. If such an option exists, it will read DeviceID from there. For this, you can use the `dig` command on the terminal:
+
+```sh
+dig @94.140.14.49 'www.example.com' A IN +ednsopt=65074:3031323334353637
+```
+
+Here, `65074` is the option ID, and `3031323334353637` is its value in hex format (DeviceID: `01234567`).
+
+All done! DeviceID should be displayed.
+
+:::note
+
+The `dig` command is merely an example, you can use any DNS software with an ability to add EDNS options to perform this action.
+
+:::
+
 ## Use plain DNS
 
 If you prefer not to use extra software for DNS configuration, you can opt for unencrypted DNS. You have two choices: using linked IPs or dedicated IPs: