Use Sendable DNS types.

- Closes #1268.
- The types we were using weren't very usable
  with Swift 6 structured concurrency.
- Use notImplemented instead of formatError for
  unknown record types.
- Fold case on domain names.
- Reject self-referencing compression pointers.
- Use pure actor for LocalhostDNSHandler now
  that we have sendable types.

Author: jglogan

Package.resolved

@@ -47,7 +47,6 @@ let package = Package(
         .library(name: "TerminalProgress", targets: ["TerminalProgress"]),
     ],
     dependencies: [
-        .package(url: "https://github.com/Bouke/DNS.git", from: "1.2.0"),
         .package(url: "https://github.com/apple/containerization.git", exact: Version(stringLiteral: scVersion)),
         .package(url: "https://github.com/apple/swift-argument-parser.git", from: "1.3.0"),
         .package(url: "https://github.com/apple/swift-collections.git", from: "1.2.0"),
@@ -56,7 +55,6 @@ let package = Package(
         .package(url: "https://github.com/apple/swift-protobuf.git", from: "1.29.0"),
         .package(url: "https://github.com/apple/swift-system.git", from: "1.4.0"),
         .package(url: "https://github.com/grpc/grpc-swift.git", from: "1.26.0"),
-        .package(url: "https://github.com/orlandos-nl/DNSClient.git", from: "2.4.1"),
         .package(url: "https://github.com/swift-server/async-http-client.git", from: "1.20.1"),
         .package(url: "https://github.com/swiftlang/swift-docc-plugin.git", from: "1.1.0"),
     ],
@@ -427,17 +425,15 @@ let package = Package(
             dependencies: [
                 .product(name: "NIOCore", package: "swift-nio"),
                 .product(name: "NIOPosix", package: "swift-nio"),
-                .product(name: "DNSClient", package: "DNSClient"),
-                .product(name: "DNS", package: "DNS"),
                 .product(name: "Logging", package: "swift-log"),
+                .product(name: "ContainerizationExtras", package: "containerization"),
                 .product(name: "ContainerizationOS", package: "containerization"),
             ]
         ),
         .testTarget(
             name: "DNSServerTests",
             dependencies: [
-                .product(name: "DNS", package: "DNS"),
-                "DNSServer",
+                "DNSServer"
             ]
         ),
         .testTarget(

Package.swift

@@ -14,14 +14,14 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
-import DNS
+import ContainerizationExtras
 
 /// Handler that uses table lookup to resolve hostnames.
 public struct HostTableResolver: DNSHandler {
-    public let hosts4: [String: IPv4]
+    public let hosts4: [String: IPv4Address]
     private let ttl: UInt32
 
-    public init(hosts4: [String: IPv4], ttl: UInt32 = 300) {
+    public init(hosts4: [String: IPv4Address], ttl: UInt32 = 300) {
         self.hosts4 = hosts4
         self.ttl = ttl
     }
@@ -48,28 +48,11 @@ public struct HostTableResolver: DNSHandler {
             }
             // If hostname doesn't exist, return nil which will become NXDOMAIN
             return nil
-        case ResourceRecordType.nameServer,
-            ResourceRecordType.alias,
-            ResourceRecordType.startOfAuthority,
-            ResourceRecordType.pointer,
-            ResourceRecordType.mailExchange,
-            ResourceRecordType.text,
-            ResourceRecordType.service,
-            ResourceRecordType.incrementalZoneTransfer,
-            ResourceRecordType.standardZoneTransfer,
-            ResourceRecordType.all:
-            return Message(
-                id: query.id,
-                type: .response,
-                returnCode: .notImplemented,
-                questions: query.questions,
-                answers: []
-            )
         default:
             return Message(
                 id: query.id,
                 type: .response,
-                returnCode: .formatError,
+                returnCode: .notImplemented,
                 questions: query.questions,
                 answers: []
             )
@@ -93,6 +76,6 @@ public struct HostTableResolver: DNSHandler {
             return nil
         }
 
-        return HostRecord<IPv4>(name: question.name, ttl: ttl, ip: ip)
+        return HostRecord<IPv4Address>(name: question.name, ttl: ttl, ip: ip)
     }
 }

Sources/DNSServer/Handlers/HostTableResolver.swift

@@ -14,8 +14,6 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
-import DNS
-
 /// Handler that returns NXDOMAIN for all hostnames.
 public struct NxDomainResolver: DNSHandler {
     private let ttl: UInt32
@@ -35,29 +33,11 @@ public struct NxDomainResolver: DNSHandler {
                 questions: query.questions,
                 answers: []
             )
-        case ResourceRecordType.nameServer,
-            ResourceRecordType.alias,
-            ResourceRecordType.startOfAuthority,
-            ResourceRecordType.pointer,
-            ResourceRecordType.mailExchange,
-            ResourceRecordType.text,
-            ResourceRecordType.host6,
-            ResourceRecordType.service,
-            ResourceRecordType.incrementalZoneTransfer,
-            ResourceRecordType.standardZoneTransfer,
-            ResourceRecordType.all:
-            return Message(
-                id: query.id,
-                type: .response,
-                returnCode: .notImplemented,
-                questions: query.questions,
-                answers: []
-            )
         default:
             return Message(
                 id: query.id,
                 type: .response,
-                returnCode: .formatError,
+                returnCode: .notImplemented,
                 questions: query.questions,
                 answers: []
             )

Sources/DNSServer/Handlers/NxDomainResolver.swift

@@ -0,0 +1,100 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 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 Foundation
+
+// TODO: Look for a way that we can make use of the
+// bit-fiddling types from ContainerizationExtras, instead
+// of copying them here.
+
+/// Errors that can occur during DNS message serialization/deserialization.
+public enum DNSBindError: Error, CustomStringConvertible {
+    case marshalFailure(type: String, field: String)
+    case unmarshalFailure(type: String, field: String)
+
+    public var description: String {
+        switch self {
+        case .marshalFailure(let type, let field):
+            return "failed to marshal \(type).\(field)"
+        case .unmarshalFailure(let type, let field):
+            return "failed to unmarshal \(type).\(field)"
+        }
+    }
+}
+
+/// Protocol for types that can be serialized to/from a byte buffer.
+protocol Bindable: Sendable {
+    /// The fixed size of this type in bytes, if applicable.
+    static var size: Int { get }
+
+    /// Serialize this value into the buffer at the given offset.
+    /// - Returns: The new offset after writing.
+    func appendBuffer(_ buffer: inout [UInt8], offset: Int) throws -> Int
+
+    /// Deserialize this value from the buffer at the given offset.
+    /// - Returns: The new offset after reading.
+    mutating func bindBuffer(_ buffer: inout [UInt8], offset: Int) throws -> Int
+}
+
+extension [UInt8] {
+    /// Copy a value into the buffer at the given offset.
+    /// - Returns: The new offset after writing, or nil if the buffer is too small.
+    package mutating func copyIn<T>(as type: T.Type, value: T, offset: Int = 0) -> Int? {
+        let size = MemoryLayout<T>.size
+        guard self.count >= size + offset else {
+            return nil
+        }
+        return self.withUnsafeMutableBytes {
+            $0.baseAddress?.advanced(by: offset).assumingMemoryBound(to: T.self).pointee = value
+            return offset + size
+        }
+    }
+
+    /// Copy a value out of the buffer at the given offset.
+    /// - Returns: A tuple of (new offset, value), or nil if the buffer is too small.
+    package func copyOut<T>(as type: T.Type, offset: Int = 0) -> (Int, T)? {
+        let size = MemoryLayout<T>.size
+        guard self.count >= size + offset else {
+            return nil
+        }
+        return self.withUnsafeBytes {
+            guard let value = $0.baseAddress?.advanced(by: offset).assumingMemoryBound(to: T.self).pointee else {
+                return nil
+            }
+            return (offset + size, value)
+        }
+    }
+
+    /// Copy a byte array into the buffer at the given offset.
+    /// - Returns: The new offset after writing, or nil if the buffer is too small.
+    package mutating func copyIn(buffer: [UInt8], offset: Int = 0) -> Int? {
+        guard offset + buffer.count <= self.count else {
+            return nil
+        }
+        self[offset..<offset + buffer.count] = buffer[0..<buffer.count]
+        return offset + buffer.count
+    }
+
+    /// Copy bytes out of the buffer into another buffer.
+    /// - Returns: The new offset after reading, or nil if the buffer is too small.
+    package func copyOut(buffer: inout [UInt8], offset: Int = 0) -> Int? {
+        guard offset + buffer.count <= self.count else {
+            return nil
+        }
+        buffer[0..<buffer.count] = self[offset..<offset + buffer.count]
+        return offset + buffer.count
+    }
+}