DNS Resolution When You Type a URL

# Full iterative trace from root servers down
# This shows every step the recursive resolver takes
dig +trace example.com

# Output (simplified):
# .                  IN NS   a.root-servers.net.    <- Step 1: Root
# com.               IN NS   a.gtld-servers.net.    <- Step 2: TLD
# example.com.       IN NS   ns1.example.com.       <- Step 3: Authoritative
# example.com.       IN A    93.184.216.34          <- Step 4: Answer

# Check which resolver you are using
dig whoami.akamai.net

# See the TTL (time-to-live) for a domain
# The TTL tells caches how long to keep this answer
dig example.com | grep -E 'ANSWER SECTION' -A 2
# example.com.    3600  IN  A  93.184.216.34
#                 ^^ TTL = 3600 seconds (1 hour)

# Query a specific DNS server directly
dig @8.8.8.8 example.com

# Check all the DNS record types for a domain
dig example.com ANY +noall +answer

import dns.resolver  # pip install dnspython
import time

def trace_dns(domain: str):
    """Walk through DNS resolution and measure timing."""
    print(f"Resolving: {domain}\n")

    # Step 1: Check what the default resolver returns
    start = time.time()
    answers = dns.resolver.resolve(domain, 'A')
    elapsed = (time.time() - start) * 1000

    for rdata in answers:
        print(f"IP Address: {rdata.address}")
    print(f"Resolution time: {elapsed:.1f}ms")
    print(f"TTL: {answers.rrset.ttl} seconds")

    # Step 2: Check NS records (who is authoritative)
    ns_answers = dns.resolver.resolve(domain, 'NS')
    print(f"\nAuthoritative name servers:")
    for ns in ns_answers:
        print(f"  {ns.target}")

    # Step 3: Resolve again to show caching effect
    start = time.time()
    dns.resolver.resolve(domain, 'A')
    cached_elapsed = (time.time() - start) * 1000
    print(f"\nSecond query (cached): {cached_elapsed:.1f}ms")

trace_dns("example.com")

# Typical output:
# IP Address: 93.184.216.34
# Resolution time: 45.2ms
# TTL: 3600 seconds
#
# Authoritative name servers:
#   ns1.example.com.
#   ns2.example.com.
#
# Second query (cached): 0.3ms

Browser Cache -> OS Cache -> Recursive Resolver Cache -> [Full Resolution]

Full Resolution (iterative, only when all caches miss):

  Your Machine           Recursive Resolver        Root Server
  -----------           ------------------        -----------
  "example.com?" -----> checks cache... MISS
                        "example.com?" ---------> "Go ask .com TLD"
                                                   (referral)

                        .com TLD Server
                        ---------------
                        "example.com?" ---------> "Go ask ns1.example.com"
                                                   (referral)

                        Authoritative Server
                        --------------------
                        "example.com?" ---------> "93.184.216.34, TTL=3600"
                                                   (final answer)

                        Caches answer (TTL=3600)
  <---- 93.184.216.34 --
  Caches answer
  Connects to 93.184.216.34