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| 1 | +package proxy |
| 2 | + |
| 3 | +import ( |
| 4 | + "encoding/binary" |
| 5 | + "net" |
| 6 | + "sync" |
| 7 | + "time" |
| 8 | +) |
| 9 | + |
| 10 | +// ReverseDNSCache maps IP addresses to hostnames based on observed DNS |
| 11 | +// responses. This enables the SOCKS5 handler to recover the original hostname |
| 12 | +// when tun2proxy sends IP-only CONNECT requests (tun2proxy operates at the |
| 13 | +// network level and only sees resolved IPs, not hostnames). |
| 14 | +// |
| 15 | +// Entries expire after 10 minutes. The cache is bounded to 10000 entries. |
| 16 | +type ReverseDNSCache struct { |
| 17 | + mu sync.RWMutex |
| 18 | + entries map[string]reverseDNSEntry |
| 19 | +} |
| 20 | + |
| 21 | +type reverseDNSEntry struct { |
| 22 | + hostname string |
| 23 | + addedAt time.Time |
| 24 | +} |
| 25 | + |
| 26 | +const ( |
| 27 | + reverseDNSTTL = 10 * time.Minute |
| 28 | + reverseDNSMaxSize = 10000 |
| 29 | +) |
| 30 | + |
| 31 | +// NewReverseDNSCache creates a new reverse DNS cache. |
| 32 | +func NewReverseDNSCache() *ReverseDNSCache { |
| 33 | + return &ReverseDNSCache{ |
| 34 | + entries: make(map[string]reverseDNSEntry, 256), |
| 35 | + } |
| 36 | +} |
| 37 | + |
| 38 | +// Store adds an IP -> hostname mapping to the cache. |
| 39 | +func (c *ReverseDNSCache) Store(ip, hostname string) { |
| 40 | + c.mu.Lock() |
| 41 | + defer c.mu.Unlock() |
| 42 | + |
| 43 | + if len(c.entries) >= reverseDNSMaxSize { |
| 44 | + c.evictExpiredLocked() |
| 45 | + } |
| 46 | + if len(c.entries) >= reverseDNSMaxSize { |
| 47 | + return |
| 48 | + } |
| 49 | + |
| 50 | + c.entries[ip] = reverseDNSEntry{ |
| 51 | + hostname: hostname, |
| 52 | + addedAt: time.Now(), |
| 53 | + } |
| 54 | +} |
| 55 | + |
| 56 | +// Lookup returns the hostname for an IP, or empty string if not found or expired. |
| 57 | +func (c *ReverseDNSCache) Lookup(ip string) string { |
| 58 | + c.mu.RLock() |
| 59 | + defer c.mu.RUnlock() |
| 60 | + |
| 61 | + entry, ok := c.entries[ip] |
| 62 | + if !ok { |
| 63 | + return "" |
| 64 | + } |
| 65 | + if time.Since(entry.addedAt) > reverseDNSTTL { |
| 66 | + return "" |
| 67 | + } |
| 68 | + return entry.hostname |
| 69 | +} |
| 70 | + |
| 71 | +// PopulateFromResponse parses a DNS response and extracts A/AAAA records, |
| 72 | +// storing the IP -> hostname mappings. |
| 73 | +func (c *ReverseDNSCache) PopulateFromResponse(hostname string, resp []byte) { |
| 74 | + if len(resp) < dnsHeaderLen { |
| 75 | + return |
| 76 | + } |
| 77 | + |
| 78 | + flags := binary.BigEndian.Uint16(resp[2:4]) |
| 79 | + if flags&dnsFlagQR == 0 { |
| 80 | + return // not a response |
| 81 | + } |
| 82 | + if flags&0x000F != 0 { |
| 83 | + return // error response |
| 84 | + } |
| 85 | + |
| 86 | + qdcount := int(binary.BigEndian.Uint16(resp[4:6])) |
| 87 | + ancount := int(binary.BigEndian.Uint16(resp[6:8])) |
| 88 | + if ancount == 0 { |
| 89 | + return |
| 90 | + } |
| 91 | + |
| 92 | + // Skip question section. |
| 93 | + offset := dnsHeaderLen |
| 94 | + for i := 0; i < qdcount; i++ { |
| 95 | + _, newOffset, err := parseDNSName(resp, offset) |
| 96 | + if err != nil { |
| 97 | + return |
| 98 | + } |
| 99 | + offset = newOffset + 4 |
| 100 | + if offset > len(resp) { |
| 101 | + return |
| 102 | + } |
| 103 | + } |
| 104 | + |
| 105 | + // Parse answer records. |
| 106 | + for i := 0; i < ancount; i++ { |
| 107 | + _, newOffset, err := parseDNSName(resp, offset) |
| 108 | + if err != nil { |
| 109 | + return |
| 110 | + } |
| 111 | + offset = newOffset |
| 112 | + if offset+10 > len(resp) { |
| 113 | + return |
| 114 | + } |
| 115 | + |
| 116 | + rrType := binary.BigEndian.Uint16(resp[offset : offset+2]) |
| 117 | + rdLength := binary.BigEndian.Uint16(resp[offset+8 : offset+10]) |
| 118 | + offset += 10 |
| 119 | + |
| 120 | + if offset+int(rdLength) > len(resp) { |
| 121 | + return |
| 122 | + } |
| 123 | + |
| 124 | + switch rrType { |
| 125 | + case dnsTypeA: |
| 126 | + if rdLength == 4 { |
| 127 | + ip := net.IPv4(resp[offset], resp[offset+1], resp[offset+2], resp[offset+3]) |
| 128 | + c.Store(ip.String(), hostname) |
| 129 | + } |
| 130 | + case dnsTypeAAAA: |
| 131 | + if rdLength == 16 { |
| 132 | + ip := net.IP(resp[offset : offset+16]) |
| 133 | + c.Store(ip.String(), hostname) |
| 134 | + } |
| 135 | + } |
| 136 | + |
| 137 | + offset += int(rdLength) |
| 138 | + } |
| 139 | +} |
| 140 | + |
| 141 | +func (c *ReverseDNSCache) evictExpiredLocked() { |
| 142 | + now := time.Now() |
| 143 | + for ip, entry := range c.entries { |
| 144 | + if now.Sub(entry.addedAt) > reverseDNSTTL { |
| 145 | + delete(c.entries, ip) |
| 146 | + } |
| 147 | + } |
| 148 | +} |
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