DNS — Domain Name System

DNS is the distributed database that maps human-readable domain names to machine-readable IP addresses. Without DNS, you'd need to memorize IP addresses for every website.

DNS is hierarchical and distributed — no single server knows everything. The hierarchy:
• Root nameservers (13 sets, labeled a-m.root-servers.net) — know where all TLD servers are
• TLD nameservers (.com, .org, .in, .io) — know where all second-level domain servers are
• Authoritative nameservers — know the actual DNS records for a specific domain
• Recursive resolver (your ISP or 8.8.8.8) — does the legwork of querying the hierarchy on your behalf

DNS Resolution walk-through for "google.com":
① Your browser checks its DNS cache → not found
② OS checks /etc/hosts file → not found
③ OS asks your recursive resolver (8.8.8.8)
④ Resolver checks its cache → not found
⑤ Resolver asks a Root nameserver: "Where's .com?"
⑥ Root says: "Ask ns1.verisign.net (the .com TLD server)"
⑦ Resolver asks .com TLD: "Where's google.com?"
⑧ TLD says: "Ask ns1.google.com (Google's authoritative server)"
⑨ Resolver asks Google's NS: "What's the IP for google.com?"
⑩ Google's NS responds: "142.250.80.46, TTL=300"
⑪ Resolver caches this, returns to your browser
⑫ Browser connects to 142.250.80.46

All this happens in ~50–100ms on first query. Cached queries: <1ms.

DNS Record Types:
• A → hostname to IPv4 address
• AAAA → hostname to IPv6 address
• CNAME → alias (one name points to another name)
• MX → mail exchange servers (for email delivery)
• TXT → arbitrary text (used for SPF, DKIM, domain verification)
• NS → nameserver records (which servers are authoritative)
• SOA → Start of Authority (metadata about the zone)
• PTR → reverse DNS (IP to hostname)

TTL (Time To Live): every DNS record has a TTL in seconds. Resolvers and browsers cache records until TTL expires. Changing DNS doesn't take effect instantly — you must wait for old TTL to expire across all caches globally. This is "DNS propagation."

1import socket
2import dns.resolver   # pip install dnspython
3
4# ── Basic DNS lookup ───────────────────────────────────
5hostname = "google.com"
6ip = socket.gethostbyname(hostname)
7print(f"{hostname} → {ip}")
8
9# ── Rich DNS queries with dnspython ───────────────────
10def query_dns(domain: str, record_type: str):
11    try:
12        answers = dns.resolver.resolve(domain, record_type)
13        print(f"
14{domain} {record_type} records:")
15        for rdata in answers:
16            print(f"  {rdata}")
17        print(f"  TTL: {answers.rrset.ttl} seconds")
18    except Exception as e:
19        print(f"  {record_type}: {e}")
20
21query_dns("google.com", "A")
22query_dns("google.com", "AAAA")
23query_dns("google.com", "MX")
24query_dns("google.com", "TXT")
25query_dns("github.com", "NS")
26
27# ── Reverse DNS (IP → hostname) ────────────────────────
28def reverse_dns(ip: str):
29    try:
30        hostname = socket.gethostbyaddr(ip)[0]
31        print(f"{ip} → {hostname}")
32    except socket.herror:
33        print(f"{ip} → No reverse DNS")
34
35reverse_dns("8.8.8.8")       # dns.google
36reverse_dns("1.1.1.1")       # one.one.one.one
37reverse_dns("142.250.80.46") # some google server
38
39# ── DIY DNS resolver using raw UDP ────────────────────
40import struct
41
42def build_dns_query(domain: str) -> bytes:
43    # Header: ID=1234, QR=0, Opcode=0, RD=1 (recursion desired)
44    header = struct.pack(">HHHHHH", 1234, 0x0100, 1, 0, 0, 0)
45    # Question: encode domain as length-prefixed labels
46    labels = b""
47    for part in domain.split("."):
48        labels += bytes([len(part)]) + part.encode()
49    labels += b""          # root label
50    question = labels + struct.pack(">HH", 1, 1)  # QTYPE=A, QCLASS=IN
51    return header + question
52
53query = build_dns_query("google.com")
54sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
55sock.settimeout(2)
56sock.sendto(query, ("8.8.8.8", 53))
57response, _ = sock.recvfrom(512)
58print(f"
59Raw DNS response ({len(response)} bytes): {response.hex()[:80]}...")
60sock.close()