1 files changed, 48 insertions, 0 deletions
diff --git a/DomainDig/ReverseDNSService.swift b/DomainDig/ReverseDNSService.swift
new file mode 100644
index 0000000..caab0f7
--- /dev/null
+++ b/DomainDig/ReverseDNSService.swift
@@ -0,0 +1,48 @@
+import Foundation
+
+struct ReverseDNSService {
+    /// Look up the PTR record for an IPv4 address via Cloudflare DoH.
+    static func lookup(ip: String) async -> String? {
+        let octets = ip.split(separator: ".")
+        guard octets.count == 4 else { return nil }
+
+        let reversed = octets.reversed().joined(separator: ".")
+        let ptrDomain = "\(reversed).in-addr.arpa"
+
+        // PTR record type = 12
+        do {
+            let records = try await lookupPTR(domain: ptrDomain)
+            return records.first
+        } catch {
+            return nil
+        }
+    }
+
+    private static func lookupPTR(domain: String) async throws -> [String] {
+        var components = URLComponents(string: "https://cloudflare-dns.com/dns-query")!
+        components.queryItems = [
+            URLQueryItem(name: "name", value: domain),
+            URLQueryItem(name: "type", value: "12") // PTR
+        ]
+
+        var request = URLRequest(url: components.url!)
+        request.setValue("application/dns-json", forHTTPHeaderField: "Accept")
+
+        let (data, response) = try await URLSession.shared.data(for: request)
+
+        guard let httpResponse = response as? HTTPURLResponse,
+              httpResponse.statusCode == 200 else {
+            throw URLError(.badServerResponse)
+        }
+
+        let dnsResponse = try JSONDecoder().decode(CloudflareDNSResponse.self, from: data)
+
+        guard let answers = dnsResponse.Answer else {
+            return []
+        }
+
+        return answers
+            .filter { $0.type == 12 }
+            .map { $0.data.trimmingCharacters(in: CharacterSet(charactersIn: "\"")) }
+    }
+}