chore: import upstream snapshot with attribution

Sources/DNSServer/Handlers/HostTableResolver.swift

@@ -0,0 +1,95 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import ContainerizationExtras
+
+/// Handler that uses table lookup to resolve hostnames.
+///
+/// Keys in `hosts4` are normalized to `DNSName` on construction, so lookups
+/// are case-insensitive and trailing dots are optional.
+public struct HostTableResolver: DNSHandler {
+    public let hosts4: [DNSName: IPv4Address]
+    private let ttl: UInt32
+
+    /// Creates a resolver backed by a static IPv4 host table.
+    ///
+    /// - Parameter hosts4: A dictionary mapping domain names to IPv4 addresses.
+    ///   Keys are normalized to `DNSName` (lowercased, trailing dot stripped), so
+    ///   `"FOO."`, `"foo."`, and `"foo"` all refer to the same entry.
+    /// - Parameter ttl: The TTL in seconds to set on answer records (default is 300).
+    /// - Throws: `DNSBindError.invalidName` if any key is not a valid DNS name.
+    public init(hosts4: [String: IPv4Address], ttl: UInt32 = 300) throws {
+        self.hosts4 = try Dictionary(uniqueKeysWithValues: hosts4.map { (try DNSName($0.key), $0.value) })
+        self.ttl = ttl
+    }
+
+    public func answer(query: Message) async throws -> Message? {
+        guard let question = query.questions.first else {
+            return nil
+        }
+        let n = question.name.hasSuffix(".") ? String(question.name.dropLast()) : question.name
+        let key = try DNSName(labels: n.isEmpty ? [] : n.split(separator: ".", omittingEmptySubsequences: false).map(String.init))
+        let record: ResourceRecord?
+        switch question.type {
+        case ResourceRecordType.host:
+            record = answerHost(question: question, key: key)
+        case ResourceRecordType.host6:
+            // Return NODATA (noError with empty answers) for AAAA queries ONLY if A record exists.
+            // This is required because musl libc has issues when A record exists but AAAA returns NXDOMAIN.
+            // musl treats NXDOMAIN on AAAA as "domain doesn't exist" and fails DNS resolution entirely.
+            // NODATA correctly indicates "no IPv6 address available, but domain exists".
+            if hosts4[key] != nil {
+                return Message(
+                    id: query.id,
+                    type: .response,
+                    returnCode: .noError,
+                    questions: query.questions,
+                    answers: []
+                )
+            }
+            // If hostname doesn't exist, return nil which will become NXDOMAIN
+            return nil
+        default:
+            return Message(
+                id: query.id,
+                type: .response,
+                returnCode: .notImplemented,
+                questions: query.questions,
+                answers: []
+            )
+        }
+
+        guard let record else {
+            return nil
+        }
+
+        return Message(
+            id: query.id,
+            type: .response,
+            returnCode: .noError,
+            questions: query.questions,
+            answers: [record]
+        )
+    }
+
+    private func answerHost(question: Question, key: DNSName) -> ResourceRecord? {
+        guard let ip = hosts4[key] else {
+            return nil
+        }
+
+        return HostRecord<IPv4Address>(name: question.name, ttl: ttl, ip: ip)
+    }
+}