DNS & Domain Resolution

1// ============================================
2// DNS Concepts — Practical Demonstrations
3// ============================================
4
5// ---------- DNS Lookup Simulation ----------
6
7// Simulating the DNS resolution hierarchy
8class DNSResolver {
9  constructor() {
10    // Each cache level has different TTLs
11    this.browserCache = new Map();    // TTL: ~1 minute
12    this.osCache = new Map();         // TTL: ~5 minutes
13    this.recursiveCache = new Map();  // TTL: varies (30s to 24h)
14
15    // Authoritative records (the "truth")
16    this.authoritativeRecords = {
17      'example.com': {
18        A: ['93.184.216.34'],
19        AAAA: ['2606:2800:220:1:248:1893:25c8:1946'],
20        MX: ['mail.example.com'],
21        NS: ['ns1.example.com', 'ns2.example.com'],
22        TTL: 3600, // 1 hour
23      },
24      'api.example.com': {
25        // Round-robin: multiple A records for load distribution
26        A: ['10.0.1.1', '10.0.1.2', '10.0.1.3', '10.0.1.4'],
27        TTL: 60, // Short TTL for faster failover
28      },
29    };
30  }
31
32  resolve(domain, recordType = 'A') {
33    const cacheKey = `\${domain}:\${recordType}`;
34
35    // Level 1: Browser cache
36    if (this.browserCache.has(cacheKey)) {
37      console.log(`  ✅ Browser cache HIT for \${domain}`);
38      return this.browserCache.get(cacheKey);
39    }
40
41    // Level 2: OS cache
42    if (this.osCache.has(cacheKey)) {
43      console.log(`  ✅ OS cache HIT for \${domain}`);
44      const result = this.osCache.get(cacheKey);
45      this.browserCache.set(cacheKey, result);
46      return result;
47    }
48
49    // Level 3: Recursive resolver cache
50    if (this.recursiveCache.has(cacheKey)) {
51      console.log(`  ✅ Recursive resolver cache HIT for \${domain}`);
52      const result = this.recursiveCache.get(cacheKey);
53      this.osCache.set(cacheKey, result);
54      this.browserCache.set(cacheKey, result);
55      return result;
56    }
57
58    // Cache MISS — full resolution needed
59    console.log(`  ❌ Cache MISS — querying authoritative servers for \${domain}`);
60    const record = this.authoritativeRecords[domain];
61    if (!record || !record[recordType]) {
62      throw new Error(`NXDOMAIN: \${domain} not found`);
63    }
64
65    const result = record[recordType];
66    // Cache at all levels
67    this.recursiveCache.set(cacheKey, result);
68    this.osCache.set(cacheKey, result);
69    this.browserCache.set(cacheKey, result);
70    return result;
71  }
72}
73
74// ---------- Geo-DNS Load Balancing ----------
75
76class GeoDNS {
77  constructor() {
78    this.regionMap = {
79      'us-east': { ip: '10.1.1.1', datacenter: 'Virginia' },
80      'us-west': { ip: '10.2.1.1', datacenter: 'Oregon' },
81      'eu-west': { ip: '10.3.1.1', datacenter: 'Ireland' },
82      'ap-south': { ip: '10.4.1.1', datacenter: 'Mumbai' },
83    };
84  }
85
86  resolve(domain, clientRegion) {
87    const target = this.regionMap[clientRegion];
88    if (!target) {
89      // Fallback to nearest region
90      return this.regionMap['us-east'];
91    }
92    console.log(
93      `Routing \${domain} from \${clientRegion} → \${target.datacenter} (\${target.ip})`
94    );
95    return target;
96  }
97}
98
99// ---------- DNS-based Health Check & Failover ----------
100
101class HealthCheckDNS {
102  constructor(endpoints) {
103    this.endpoints = endpoints; // [{ ip, healthy }]
104    this.checkInterval = 30000; // Check every 30 seconds
105  }
106
107  getHealthyEndpoints() {
108    return this.endpoints.filter(ep => ep.healthy).map(ep => ep.ip);
109  }
110
111  resolve(domain) {
112    const healthy = this.getHealthyEndpoints();
113    if (healthy.length === 0) {
114      console.error('🚨 ALL endpoints are DOWN!');
115      return this.endpoints[0].ip; // Return first as last resort
116    }
117    // Return only healthy IPs
118    console.log(`Healthy servers for \${domain}: \${healthy.join(', ')}`);
119    return healthy;
120  }
121
122  // Simulate a server going down
123  markUnhealthy(ip) {
124    const endpoint = this.endpoints.find(ep => ep.ip === ip);
125    if (endpoint) {
126      endpoint.healthy = false;
127      console.log(`🔴 Server \${ip} marked UNHEALTHY`);
128    }
129  }
130}
131
132// ---------- Demo Usage ----------
133const resolver = new DNSResolver();
134console.log('First lookup (cache miss):');
135console.log('IP:', resolver.resolve('example.com'));
136console.log('\nSecond lookup (cache hit):');
137console.log('IP:', resolver.resolve('example.com'));
138
139const geoDns = new GeoDNS();
140geoDns.resolve('api.example.com', 'eu-west');
141geoDns.resolve('api.example.com', 'ap-south');