privacyguides.org/i18n/pl/advanced/dns-overview.md

title	icon	description
DNS Overview	material/dns	The Domain Name System is the "phonebook of the internet," helping your browser find the website it's looking for.

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.

Co to jest DNS?

Gdy odwiedzasz stronę internetową, zwracany jest adres w postaci dziesiętnej. Na przykład, gdy odwiedzasz privacyguides.org, zwracany jest adres 192.98.54.105.

DNS istnieje od wczesnych lat istnienia Internetu. Zapytania DNS wysyłane i odbierane z serwerów DNS zazwyczaj nie są szyfrowane. 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.

   === "Linux, macOS"

    ```
    dig +noall +answer privacyguides.org @1.1.1.1
    dig +noall +answer privacyguides.org @8.8.8.8
    ```
   

   === "Windows"

    ```
    nslookup privacyguides.org 1.1.1.1
    nslookup privacyguides.org 8.8.8.8
    ```
   

3. Next, we want to analyse the results:

   === "Wireshark"

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

   === "tshark"

    ```
    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	Wyszukiwarki	104	Standard query 0x58ba A privacyguides.org OPT
2	0.293395	1.1.1.1	192.0.2.1	Wyszukiwarki	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	Wyszukiwarki	104	Standard query 0xf1a9 A privacyguides.org OPT
4	2.154698	8.8.8.8	192.0.2.1	Wyszukiwarki	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. Nie zalecamy używania szyfrowanego DNS w tym celu. Zamiast tego skorzystaj z sieci Tor lub VPN. Jeśli korzystasz z sieci VPN, należy użyć serwerów DNS jej dostawcy. 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:

Adres IP

Najprostszym sposobem na określenie aktywności przeglądania może być sprawdzenie adresów IP, z którymi łączą się Twoje urządzenia. Na przykład, jeśli obserwator wie, że privacyguides.org znajduje się pod adresem 198.98.54.105, a Twoje urządzenie pobiera dane z adresu 198.98.54.105, istnieje duże prawdopodobieństwo, że odwiedzasz witrynę Privacy Guides.

Ta metoda jest użyteczna tylko wtedy, gdy adres IP należy do serwera, na którym znajduje się tylko kilka stron internetowych. Nie pomaga również to, jeśli witryna jest umieszczona na współdzielonej platformie (np. GitHub Pages, Cloudflare Pages, Netlify, WordPress, Blogger itd.). 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. 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.

Lista polecanych serwerów DNS{.md-button}

Co to jest DNSSEC?

Domain Name System Security Extensions (DNSSEC) to funkcja DNS uwierzytelniająca odpowiedzi na zapytania o nazwę domen. Nie zapewnia ona ochrony prywatności tych zapytań, ale uniemożliwia atakującym manipulowanie lub zatruwanie odpowiedzi na zapytania DNS.

Innymi słowy, DNSSEC podpisuje cyfrowo dane, aby zapewnić ich spójność. W celu zapewnienia bezpiecznego wyszukiwania, podpisywanie odbywa się na każdym poziomie procesu zapytania DNS. Dzięki temu wszystkie odpowiedzi z DNS są zaufane.

Proces podpisywania DNSSEC jest podobny do podpisywania dokumentu prawnego długopisem; osoba składająca podpis używa niepowtarzalnego podpisu, a ekspert sądowy może spojrzeć na ten podpis i zweryfikować, czy dokument został podpisany przez tę osobę. Te podpisy cyfrowe są gwarancją, że dane nie zostały naruszone.

DNSSEC wprowadza hierarchiczną politykę podpisywania cyfrowego we wszystkich warstwach 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.