Basics of DNS

Domain Name System (DNS) translates human-readable domain names (like <www.example.com>) into IP addresses that computers use to identify each other on the network.

  • Domain Name: A readable name for an IP address (e.g., <www.example.com>).
  • IP Address: A numerical label assigned to each device connected to a computer network (e.g., 192.168.1.1 for IPv4 or 2001:0db8:85a3:0000:0000:8a2e:0370:7334 for IPv6).

Key DNS Components

  • DNS Resolver: The client-side part of DNS; it queries the DNS server.
  • DNS Server: The server-side part of DNS; it responds to queries with IP addresses.
  • Root Name Servers: The top-level DNS servers that direct queries to appropriate TLD (Top-Level Domain) servers.
  • TLD Name Servers: Servers that store information for all domains within a top-level domain (like .com, .org).
  • Authoritative DNS Servers: Servers that contain DNS records for specific domains.

Common DNS Record Types

  • A Record: Maps a domain name to an IPv4 address.
  • AAAA Record: Maps a domain name to an IPv6 address.
  • CNAME Record: Canonical Name record, which maps an alias name to the true or canonical domain name.
  • MX Record: Mail Exchange record, which specifies the mail server responsible for receiving email.
  • NS Record: Name Server record, which specifies the authoritative DNS servers for the domain.
  • TXT Record: Text record, often used to provide additional information such as SPF records for email verification.
  • SRV Record: Service record, which defines the location of servers for specified services.
  • PTR Record: Pointer record, used for reverse DNS lookups.

DNS Query Types

  • Recursive Query: The DNS resolver queries multiple DNS servers until it finds the final IP address.
  • Iterative Query: The DNS resolver queries each DNS server in turn; each server may refer the resolver to another server.
  • Non-recursive Query: The DNS resolver queries the DNS server, and if the DNS server has the requested record, it responds immediately.

DNS Caching

  • DNS Cache: Temporary storage of DNS query results to improve speed and reduce the load on DNS servers.
  • TTL (Time To Live): The duration that a DNS record is cached by a DNS resolver or server.

DNS Resolution Process

  1. Query Initiation: A user types a domain name into a browser.
  2. DNS Resolver: The resolver checks its cache. If the IP is not cached, it sends a query to a root DNS server.
  3. Root DNS Server: Responds with the address of a TLD DNS server.
  4. TLD DNS Server: Responds with the address of the authoritative DNS server for the domain.
  5. Authoritative DNS Server: Provides the IP address for the domain.
  6. DNS Resolver: Returns the IP address to the user’s browser.
  7. Browser: Connects to the server using the provided IP address.

DNS Providers

The DNS providers below focus on showcasing their key features and services so you can choose the one that best suits your needs. If you can’t decide or can’t figure out which one best suits your needs, Homelab-Alpha recommends Quad9.



Quad9

NameQuad9
DescriptionNon-profit, free security and privacy-focused DNS
Privacy PolicyLink
IPv4 Addresses9.9.9.9, 149.112.112.112
IPv6 Addresses2620:fe::fe, 2620:fe::9
DNS over UDP/TCP (Do53)Yes
DNS over HTTPS (DoH)https://dns.quad9.net/dns-query
DNS over TLS (DoT)tls://dns.opendns.com
DNS over QUIC (DoQ)Not supported
DNSCryptYes
DNSSECYes
EDNS PaddingYes
FiltersMalware, Phishing
RemarksFocused on blocking malicious domains

AdGuard DNS

NameAdGuard DNS
DescriptionDNS service focused on blocking ads and trackers
Privacy PolicyLink
IPv4 Addresses94.140.14.14, 94.140.15.15
IPv6 Addresses2a10:50c0::ad1:ff, 2a10:50c0::ad2:ff
DNS over UDP/TCP (Do53)Yes
DNS over HTTPS (DoH)https://dns.adguard-dns.com/dns-query
DNS over TLS (DoT)tls://dns.adguard-dns.com
DNS over QUIC (DoQ)quic://dns.adguard-dns.com
DNSCryptNo
DNSSECYes
EDNS PaddingYes
FiltersAd blocking, Tracking
RemarksIdeal for ad and tracker blocking

Cloudflare

NameCloudflare
DescriptionFast, privacy-first DNS resolver
Privacy PolicyLink
IPv4 Addresses1.1.1.1, 1.0.0.1
IPv6 Addresses2606:4700:4700::1111, 2606:4700:4700::1001
DNS over UDP/TCP (Do53)Yes
DNS over HTTPS (DoH)https://dns.cloudflare.com/dns-query
DNS over TLS (DoT)tls://1dot1dot1dot1.cloudflare-dns.com
DNS over QUIC (DoQ)Not supported
DNSCryptNo
DNSSECYes
EDNS PaddingYes
FiltersMalware, Adult Content
RemarksKnown for speed and privacy

Google Public DNS

NameGoogle Public DNS
DescriptionFree, global DNS resolution service
Privacy PolicyLink
IPv4 Addresses8.8.8.8, 8.8.4.4
IPv6 Addresses2001:4860:4860::8888, 2001:4860:4860::8844
DNS over UDP/TCP (Do53)Yes
DNS over HTTPS (DoH)https://dns.google/dns-query
DNS over TLS (DoT)tls://dns.google
DNS over QUIC (DoQ)Not supported
DNSCryptNo
DNSSECYes
EDNS PaddingYes
FiltersNo
RemarksFast, reliable, no filters

Cisco OpenDNS

NameOpenDNS
DescriptionDNS service with customizable security and filtering options
Privacy PolicyLink
IPv4 Addresses208.67.222.222, 208.67.220.220
IPv6 Addresses2620:119:35::35, 2620:119:53::53
DNS over UDP/TCP (Do53)Yes
DNS over HTTPS (DoH)https://doh.opendns.com/dns-query
DNS over TLS (DoT)doh.opendns.com
DNS over QUIC (DoQ)Not supported
DNSCryptNo
DNSSECYes
EDNS PaddingNo
FiltersMalware, Phishing, Custom
RemarksGreat for families and small businesses

Common DNS Tools

dig

A network administration command-line tool for querying DNS name servers.

dig example.com
dig +short example.com
dig example.com MX
dig -x 192.168.1.1

nslookup

A network administration command-line tool for querying DNS to obtain domain name or IP address mapping.

nslookup example.com
nslookup -type=MX example.com
nslookup 192.168.1.1

host

A simple utility for performing DNS lookups.

host example.com
host -t MX example.com
host 192.168.1.1

Advanced DNS Concepts

  • DNSSEC (DNS Security Extensions): Enhances security by adding cryptographic signatures to DNS data.
  • Anycast Routing: A network addressing and routing methodology in which a single destination address has multiple routing paths.
  • Split-Horizon DNS: Provides different DNS responses based on the source of the DNS query, often used for internal vs. external users.

DNS Best Practices

  1. Regular Updates: Ensure your DNS records are up-to-date and accurate.
  2. Use DNSSEC: Implement DNSSEC to protect against DNS spoofing and cache poisoning.
  3. Monitor TTL Values: Set appropriate TTL values to balance between speed and freshness of DNS data.
  4. Implement Redundancy: Use multiple DNS servers to ensure reliability and availability.
  5. Regular Audits: Conduct periodic DNS audits to detect and rectify configuration errors.

Troubleshooting DNS Issues

  1. Check Local DNS Cache: Clear your DNS cache to resolve any local caching issues.

    On Linux

    sudo systemd-resolve --flush-caches

    On Windows

    ipconfig /flushdns

    On macOS (Monterey, Big Sur and Catalina)

    sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

    On macOS (Mojave, High Sierra and Sierra)

    sudo killall -HUP mDNSResponder

  2. Verify DNS Configuration: Use tools like dig, nslookup, or host to verify DNS records.

  3. Check Network Connectivity: Ensure you have a stable network connection.

  4. Review DNS Server Logs: Examine logs on your DNS servers for any error messages or unusual activity.

  5. Consult ISP: If issues persist, contact your Internet Service Provider for assistance.


Conclusion

Understanding DNS is crucial for anyone involved in network administration or web development. This cheat sheet provides a comprehensive overview of the essential aspects of DNS, from basic concepts to advanced configurations and troubleshooting. Use it as a reference to ensure your DNS setup is optimized and secure.

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Last updated 01 Sep 2024, 10:22 CEST . history