Testing the Implementation

Without validation, configuration changes are just optimistic adjustments. By testing, you ensure that your system uses the specified DNS servers and also that the DNS queries are encrypted, guaranteeing the privacy and security boost DoT promises.

To check the active DNS server, enter

resolvectl status | grep 'Current DNS Server'

If the output matches the specified servers (e.g., 1.1.1.1), it's a preliminary indication things are on track.

Monitoring DNS Traffic for Encryption

DoT is a fundamental security feature for modern networks, but like all systems, DoT requires vigilance. One cannot simply enable and forget; it must be actively monitored.

Before you can monitor traffic on your interface, you need to determine its name using the ip a or ip addr show command:

ip a

This command provides a list of all network interfaces. Typically, wired connections are represented as ethX (e.g., eth0), while wireless connections are often labeled as wlanX (e.g., wlan0).

Look for the interface that has an Inet address (IPv4) or Inet6 address (IPv6) associated with it and is UP. This is typically the active network interface. For instance, the output might look like Listing 1.

01 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
02 inet 192.168.1.10/24 brd 192.168.1.255 scope global dynamic eth0

In Listing 1, eth0 is the active network interface.

Monitoring with tcpdump

Now that you've identified your active network interface, utilize tcpdump to monitor DNS traffic. With DoT, monitoring becomes critical to ensure encryption is active. To monitor your DNS traffic, use

sudo tcpdump -i eth0 -n 'port 853'

Replace eth0 with your identified interface.

With the above command, tcpdump intercepts and logs network traffic. If DoT is functional, the domain names accessed won't appear in plaintext. DoT operates via port 853, with encrypted queries passing through it.

While tcpdump is active, access a few websites. Instead of recognizable domain names, you'll largely see encrypted data, verifying the effectiveness of your DoT setup.

The importance of regular traffic monitoring can't be overstated. Monitoring helps identify unusual traffic patterns and potential security breaches, while also ensuring that DoT operates without hitches. Of course, tools like DoT are only part of the solution for maintaining a secure environment. You're still only as safe as your system is, which means you can never escape the need for regular audits and system updates (see the box entitled "Keeping the System Up to Date").

sudo apt update && sudo apt upgrade