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Introduction to DNS

The Domain Name System is the 'phonebook of the Internet'. DNS translates domain names to IP addresses so browsers and other services can load Internet resources, through a decentralized network of servers.

What is DNS?

When you visit a website, a numerical address is returned. For example, when you visit privacyguides.org, the address 192.98.54.105 is returned.

DNS has existed since the early days of the Internet. DNS requests made to and from DNS servers are not generally encrypted. In a residential setting, a customer is given servers by the ISP via DHCP.

Unencrypted DNS requests are able to be easily surveilled and modified in transit. In some parts of the world, ISPs are ordered to do primitive DNS filtering. When you request the IP address of a domain that is blocked, the server may not respond or may respond with a different IP address. As the DNS protocol is not encrypted, the ISP (or any network operator) can use DPI to monitor requests. ISPs can also block requests based on common characteristics, regardless of which DNS server is used. Unencrypted DNS always uses port 53 and always uses UDP.

Below, we discuss and provide a tutorial to prove what an outside observer may see using regular unencrypted DNS and encrypted DNS.

Unencrypted DNS

  1. Using tshark (part of the Wireshark project) we can monitor and record internet packet flow. This command records packets that meet the rules specified:

    tshark -w /tmp/dns.pcap udp port 53 and host 1.1.1.1 or host 8.8.8.8
    
  2. We can then use dig (Linux, MacOS etc) or nslookup (Windows) to send the DNS lookup to both servers. Software such as web browsers do these lookups automatically, unless they are configured to use encrypted DNS.

    dig +noall +answer privacyguides.org @1.1.1.1
    dig +noall +answer privacyguides.org @8.8.8.8
    
    nslookup privacyguides.org 1.1.1.1
    nslookup privacyguides.org 8.8.8.8
    
  3. Next, we want to analyse the results:

    wireshark -r /tmp/dns.pcap
    
    tshark -r /tmp/dns.pcap
    

If you run the Wireshark command above, the top pane shows the "frames", and the bottom pane shows all the data about the selected frame. Enterprise filtering and monitoring solutions (such as those purchased by governments) can do the process automatically, without human interaction, and can aggregate those frames to produce statistical data useful to the network observer.

No. Time Source Destination Protocol Length Info
1 0.000000 192.0.2.1 1.1.1.1 DNS 104 Standard query 0x58ba A privacyguides.org OPT
2 0.293395 1.1.1.1 192.0.2.1 DNS 108 Standard query response 0x58ba A privacyguides.org A 198.98.54.105 OPT
3 1.682109 192.0.2.1 8.8.8.8 DNS 104 Standard query 0xf1a9 A privacyguides.org OPT
4 2.154698 8.8.8.8 192.0.2.1 DNS 108 Standard query response 0xf1a9 A privacyguides.org A 198.98.54.105 OPT

An observer could modify any of these packets.

What is "encrypted DNS"?

Encrypted DNS can refer to one of a number of protocols, the most common ones being:

DNSCrypt

DNSCrypt was one of the first methods of encrypting DNS queries. DNSCrypt operates on port 443 and works with both the TCP or UDP transport protocols. DNSCrypt has never been submitted to the Internet Engineering Task Force (IETF) nor has it gone through the Request for Comments (RFC) process, so it has not been used widely outside of a few implementations. As a result, it has been largely replaced by the more popular DNS over HTTPS.

DNS over TLS (DoT)

DNS over TLS is another method for encrypting DNS communication that is defined in RFC 7858. Support was first implemented in Android 9, iOS 14, and on Linux in systemd-resolved in version 237. Preference in the industry has been moving away from DoT to DoH in recent years, as DoT is a complex protocol and has varying compliance to the RFC across the implementations that exist. DoT also operates on a dedicated port 853 which can be blocked easily by restrictive firewalls.

DNS over HTTPS (DoH)

DNS over HTTPS as defined in RFC 8484 packages queries in the HTTP/2 protocol and provides security with HTTPS. Support was first added in web browsers such as Firefox 60 and Chrome 83.

Native implementation of DoH showed up in iOS 14, macOS 11, Microsoft Windows, and Android 13 (however, it won't be enabled by default). General Linux desktop support is waiting on the systemd implementation so installing third-party software is still required.

What can an outside party see?

In this example we will record what happens when we make a DoH request:

  1. First, start tshark:

    tshark -w /tmp/dns_doh.pcap -f "tcp port https and host 1.1.1.1"
    
  2. Second, make a request with curl:

    curl -vI --doh-url https://1.1.1.1/dns-query https://privacyguides.org
    
  3. After making the request, we can stop the packet capture with CTRL + C.

  4. Analyse the results in Wireshark:

    wireshark -r /tmp/dns_doh.pcap
    

We can see the connection establishment and TLS handshake that occurs with any encrypted connection. When looking at the "application data" packets that follow, none of them contain the domain we requested or the IP address returned.

Why shouldn't I use encrypted DNS?

In locations where there is internet filtering (or censorship), visiting forbidden resources may have its own consequences which you should consider in your threat model. We do not suggest the use of encrypted DNS for this purpose. Use Tor or a VPN instead. If you're using a VPN, you should use your VPN's DNS servers. When using a VPN, you are already trusting them with all your network activity.

When we do a DNS lookup, it's generally because we want to access a resource. Below, we will discuss some of the methods that may disclose your browsing activities even when using encrypted DNS:

IP Address

The simplest way to determine browsing activity might be to look at the IP addresses your devices are accessing. For example, if the observer knows that privacyguides.org is at 198.98.54.105, and your device is requesting data from 198.98.54.105, there is a good chance you're visiting Privacy Guides.

This method is only useful when the IP address belongs to a server that only hosts few websites. It's also not very useful if the site is hosted on a shared platform (e.g. Github Pages, Cloudflare Pages, Netlify, WordPress, Blogger, etc). It also isn't very useful if the server is hosted behind a reverse proxy, which is very common on the modern Internet.

Server Name Indication (SNI)

Server Name Indication is typically used when a IP address hosts many websites. This could be a service like Cloudflare, or some other Denial-of-service attack protection.

  1. Start capturing again with tshark. We've added a filter with our IP address so you don't capture many packets:

    tshark -w /tmp/pg.pcap port 443 and host 198.98.54.105
    
  2. Then we visit https://privacyguides.org.

  3. After visiting the website, we want to stop the packet capture with CTRL + C.

  4. Next we want to analyze the results:

    wireshark -r /tmp/pg.pcap
    

    We will see the connection establishment, followed by the TLS handshake for the Privacy Guides website. Around frame 5. you'll see a "Client Hello".

  5. Expand the triangle ▸ next to each field:

    ▸ Transport Layer Security
      ▸ TLSv1.3 Record Layer: Handshake Protocol: Client Hello
        ▸ Handshake Protocol: Client Hello
          ▸ Extension: server_name (len=22)
            ▸ Server Name Indication extension
    
  6. We can see the SNI value which discloses the website we are visiting. The tshark command can give you the value directly for all packets containing a SNI value:

    tshark -r /tmp/pg.pcap -Tfields -Y tls.handshake.extensions_server_name -e tls.handshake.extensions_server_name
    

This means even if we are using "Encrypted DNS" servers, the domain will likely be disclosed through SNI. The TLS v1.3 protocol brings with it Encrypted Client Hello, which prevents this kind of leak.

Governments, in particular China and Russia, have either already started blocking it or expressed a desire to do so. Recently, Russia has started blocking foreign websites that use the HTTP/3 standard. This is because the QUIC protocol that is a part of HTTP/3 requires that ClientHello also be encrypted.

Online Certificate Status Protocol (OCSP)

Another way your browser can disclose your browsing activities is with the Online Certificate Status Protocol. When visiting an HTTPS website, the browser might check to see if the website's certificate has been revoked. This is generally done through the HTTP protocol, meaning it is not encrypted.

The OCSP request contains the certificate "serial number", which is unique. It is sent to the "OCSP responder" in order to check its status.

We can simulate what a browser would do using the openssl command.

  1. Get the server certificate and use sed to keep just the important part and write it out to a file:

    openssl s_client -connect privacyguides.org:443 < /dev/null 2>&1 |
        sed -n '/^-*BEGIN/,/^-*END/p' > /tmp/pg_server.cert
    
  2. Get the intermediate certificate. Certificate Authorities (CA) normally don't sign a certificate directly; they use what is known as an "intermediate" certificate.

    openssl s_client -showcerts -connect privacyguides.org:443 < /dev/null 2>&1 |
        sed -n '/^-*BEGIN/,/^-*END/p' > /tmp/pg_and_intermediate.cert
    
  3. The first certificate in pg_and_intermediate.cert is actually the server certificate from step 1. We can use sed again to delete until the first instance of END:

    sed -n '/^-*END CERTIFICATE-*$/!d;:a n;p;ba' \
        /tmp/pg_and_intermediate.cert > /tmp/intermediate_chain.cert
    
  4. Get the OCSP responder for the server certificate:

    openssl x509 -noout -ocsp_uri -in /tmp/pg_server.cert
    

    Our certificate shows the Lets Encrypt certificate responder. If we want to see all the details of the certificate we can use:

    openssl x509 -text -noout -in /tmp/pg_server.cert
    
  5. Start the packet capture:

    tshark -w /tmp/pg_ocsp.pcap -f "tcp port http"
    
  6. Make the OCSP request:

    openssl ocsp -issuer /tmp/intermediate_chain.cert \
                 -cert /tmp/pg_server.cert \
                 -text \
                 -url http://r3.o.lencr.org
    
  7. Open the capture:

    wireshark -r /tmp/pg_ocsp.pcap
    

    There will be two packets with the "OCSP" protocol: a "Request" and a "Response". For the "Request" we can see the "serial number" by expanding the triangle ▸ next to each field:

    ▸ Online Certificate Status Protocol
      ▸ tbsRequest
        ▸ requestList: 1 item
          ▸ Request
            ▸ reqCert
              serialNumber
    

    For the "Response" we can also see the "serial number":

    ▸ Online Certificate Status Protocol
      ▸ responseBytes
        ▸ BasicOCSPResponse
          ▸ tbsResponseData
            ▸ responses: 1 item
              ▸ SingleResponse
                ▸ certID
                  serialNumber
    
  8. Or use tshark to filter the packets for the Serial Number:

    tshark -r /tmp/pg_ocsp.pcap -Tfields -Y ocsp.serialNumber -e ocsp.serialNumber
    

If the network observer has the public certificate, which is publicly available, they can match the serial number with that certificate and therefore determine the site you're visiting from that. The process can be automated and can associate IP addresses with serial numbers. It is also possible to check Certificate Transparency logs for the serial number.

Should I use encrypted DNS?

We made this flow chart to describe when you should use encrypted DNS:

graph TB
    Start[Start] --> anonymous{Trying to be<br> anonymous?}
    anonymous--> | Yes | tor(Use Tor)
    anonymous --> | No | censorship{Avoiding<br> censorship?}
    censorship --> | Yes | vpnOrTor(Use<br> VPN or Tor)
    censorship --> | No | privacy{Want privacy<br> from ISP?}
    privacy --> | Yes | vpnOrTor
    privacy --> | No | obnoxious{ISP makes<br> obnoxious<br> redirects?}
    obnoxious --> | Yes | encryptedDNS(Use<br> encrypted DNS<br> with 3rd party)
    obnoxious --> | No | ispDNS{Does ISP support<br> encrypted DNS?}
    ispDNS --> | Yes | useISP(Use<br> encrypted DNS<br> with ISP)
    ispDNS --> | No | nothing(Do nothing)

Encrypted DNS with a third-party should only be used to get around redirects and basic DNS blocking when you can be sure there won't be any consequences or you're interested in a provider that does some rudimentary filtering.

List of recommended DNS servers

What is DNSSEC?

Domain Name System Security Extensions (DNSSEC) is a feature of DNS that authenticates responses to domain name lookups. It does not provide privacy protections for those lookups, but rather prevents attackers from manipulating or poisoning the responses to DNS requests.

In other words, DNSSEC digitally signs data to help ensure its validity. In order to ensure a secure lookup, the signing occurs at every level in the DNS lookup process. As a result, all answers from DNS can be trusted.

The DNSSEC signing process is similar to someone signing a legal document with a pen; that person signs with a unique signature that no one else can create, and a court expert can look at that signature and verify that the document was signed by that person. These digital signatures ensure that data has not been tampered with.

DNSSEC implements a hierarchical digital signing policy across all layers of DNS. For example, in the case of a privacyguides.org lookup, a root DNS server would sign a key for the .org nameserver, and the .org nameserver would then sign a key for privacyguides.org’s authoritative nameserver.

Adapted from DNS Security Extensions (DNSSEC) overview by Google and DNSSEC: An Introduction by Cloudflare, both licensed under CC BY 4.0.

What is QNAME minimization?

A QNAME is a "qualified name", for example privacyguides.org. QNAME minimisation reduces the amount of information sent from the DNS server to the authoritative name server.

Instead of sending the whole domain privacyguides.org, QNAME minimization means the DNS server will ask for all the records that end in .org. Further technical description is defined in RFC 7816.

What is EDNS Client Subnet (ECS)?

The EDNS Client Subnet is a method for a recursive DNS resolver to specify a subnetwork for the host or client which is making the DNS query.

It's intended to "speed up" delivery of data by giving the client an answer that belongs to a server that is close to them such as a content delivery network, which are often used in video streaming and serving JavaScript web apps.

This feature does come at a privacy cost, as it tells the DNS server some information about the client's location.