| 1 | // SPDX-License-Identifier: GPL-3.0-or-later |
| 2 | |
| 3 | package snmptopology |
| 4 | |
| 5 | import ( |
| 6 | "context" |
| 7 | "net" |
| 8 | "net/netip" |
| 9 | "sort" |
| 10 | "strings" |
| 11 | "sync" |
| 12 | "time" |
| 13 | ) |
| 14 | |
| 15 | const ( |
| 16 | topologyReverseDNSTimeout = 50 * time.Millisecond |
| 17 | topologyReverseDNSCacheTTL = 10 * time.Minute |
| 18 | topologyReverseDNSNegTTL = 30 * time.Second |
| 19 | ) |
| 20 | |
| 21 | type topologyReverseDNSCacheEntry struct { |
| 22 | name string |
| 23 | expiresAt time.Time |
| 24 | } |
| 25 | |
| 26 | type topologyReverseDNSResolver struct { |
| 27 | mu sync.RWMutex |
| 28 | timeout time.Duration |
| 29 | ttl time.Duration |
| 30 | cache map[string]topologyReverseDNSCacheEntry |
| 31 | } |
| 32 | |
| 33 | func newTopologyReverseDNSResolver(timeout, ttl time.Duration) *topologyReverseDNSResolver { |
| 34 | return &topologyReverseDNSResolver{ |
| 35 | timeout: timeout, |
| 36 | ttl: ttl, |
| 37 | cache: make(map[string]topologyReverseDNSCacheEntry), |
| 38 | } |
| 39 | } |
| 40 | |
| 41 | // lookupCached returns the cached result for ip without performing any network I/O. |
| 42 | // Returns "" when the IP has never been resolved or its cache entry has expired. |
| 43 | func (r *topologyReverseDNSResolver) lookupCached(ip string) string { |
| 44 | if r == nil { |
| 45 | return "" |
| 46 | } |
| 47 | addr, err := netip.ParseAddr(strings.TrimSpace(ip)) |
| 48 | if err != nil || !addr.IsValid() { |
| 49 | return "" |
| 50 | } |
| 51 | ip = addr.Unmap().String() |
| 52 | |
| 53 | r.mu.RLock() |
| 54 | entry, ok := r.cache[ip] |
| 55 | r.mu.RUnlock() |
| 56 | if ok && time.Now().Before(entry.expiresAt) { |
| 57 | return entry.name |
| 58 | } |
| 59 | return "" |
| 60 | } |
| 61 | |
| 62 | func (r *topologyReverseDNSResolver) lookup(ip string) string { |
| 63 | if r == nil { |
| 64 | return "" |
| 65 | } |
| 66 | addr, err := netip.ParseAddr(strings.TrimSpace(ip)) |
| 67 | if err != nil || !addr.IsValid() { |
| 68 | return "" |
| 69 | } |
| 70 | ip = addr.Unmap().String() |
| 71 | now := time.Now() |
| 72 | |
| 73 | r.mu.RLock() |
| 74 | entry, ok := r.cache[ip] |
| 75 | r.mu.RUnlock() |
| 76 | if ok && now.Before(entry.expiresAt) { |
| 77 | return entry.name |
| 78 | } |
| 79 | |
| 80 | ctx, cancel := context.WithTimeout(context.Background(), r.timeout) |
| 81 | defer cancel() |
| 82 | names, err := net.DefaultResolver.LookupAddr(ctx, ip) |
| 83 | resolved := "" |
| 84 | if err == nil { |
| 85 | resolved = topologyNormalizeReverseDNSName(names) |
| 86 | } |
| 87 | ttl := r.ttl |
| 88 | if resolved == "" && topologyReverseDNSNegTTL > 0 { |
| 89 | ttl = topologyReverseDNSNegTTL |
| 90 | } |
| 91 | |
| 92 | r.mu.Lock() |
| 93 | r.cache[ip] = topologyReverseDNSCacheEntry{ |
| 94 | name: resolved, |
| 95 | expiresAt: now.Add(ttl), |
| 96 | } |
| 97 | r.mu.Unlock() |
| 98 | |
| 99 | return resolved |
| 100 | } |
| 101 | |
| 102 | func topologyNormalizeReverseDNSName(names []string) string { |
| 103 | if len(names) == 0 { |
| 104 | return "" |
| 105 | } |
| 106 | seen := make(map[string]struct{}, len(names)) |
| 107 | out := make([]string, 0, len(names)) |
| 108 | for _, name := range names { |
| 109 | name = strings.TrimSpace(name) |
| 110 | name = strings.TrimSuffix(name, ".") |
| 111 | name = strings.ToLower(name) |
| 112 | if name == "" { |
| 113 | continue |
| 114 | } |
| 115 | if _, ok := seen[name]; ok { |
| 116 | continue |
| 117 | } |
| 118 | seen[name] = struct{}{} |
| 119 | out = append(out, name) |
| 120 | } |
| 121 | if len(out) == 0 { |
| 122 | return "" |
| 123 | } |
| 124 | sort.Strings(out) |
| 125 | return out[0] |
| 126 | } |
| 127 | |
| 128 | var defaultTopologyReverseDNSResolver = newTopologyReverseDNSResolver(topologyReverseDNSTimeout, topologyReverseDNSCacheTTL) |
| 129 | |
| 130 | // resolveTopologyReverseDNSName performs a live DNS lookup (with cache). |
| 131 | // Used during the collector's Collect() cycle to warm the cache. |
| 132 | func resolveTopologyReverseDNSName(ip string) string { |
| 133 | return defaultTopologyReverseDNSResolver.lookup(ip) |
| 134 | } |
| 135 | |
| 136 | // resolveTopologyReverseDNSNameCached returns a cached DNS name if available, |
| 137 | // or an empty string if the IP has not been resolved yet. Never blocks on network I/O. |
| 138 | // Used during function responses to avoid external calls. |
| 139 | func resolveTopologyReverseDNSNameCached(ip string) string { |
| 140 | return defaultTopologyReverseDNSResolver.lookupCached(ip) |
| 141 | } |