chore: import upstream snapshot with attribution

Sources/Containerization/DNSConfiguration.swift

@@ -0,0 +1,91 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization 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 ContainerizationError
+import ContainerizationExtras
+
+/// DNS configuration for a container. The values will be used to
+/// construct /etc/resolv.conf for a given container.
+public struct DNS: Sendable {
+    /// The set of default nameservers to use if none are provided
+    /// in the constructor.
+    public static let defaultNameservers = ["1.1.1.1"]
+
+    /// The nameservers a container should use.
+    public var nameservers: [String]
+    /// The DNS domain to use.
+    public var domain: String?
+    /// The DNS search domains to use.
+    public var searchDomains: [String]
+    /// The DNS options to use.
+    public var options: [String]
+
+    public init(
+        nameservers: [String] = defaultNameservers,
+        domain: String? = nil,
+        searchDomains: [String] = [],
+        options: [String] = []
+    ) {
+        self.nameservers = nameservers
+        self.domain = domain
+        self.searchDomains = searchDomains
+        self.options = options
+    }
+
+    /// Validates the DNS configuration.
+    ///
+    /// Ensures that all nameserver entries are valid IPv4 or IPv6 addresses.
+    /// Arbitrary hostnames are not permitted as nameservers.
+    ///
+    /// - Throws: ``ContainerizationError`` with code `.invalidArgument` if
+    ///   any nameserver is not a valid IP address.
+    public func validate() throws {
+        for nameserver in nameservers {
+            let isValidIPv4 = (try? IPv4Address(nameserver)) != nil
+            let isValidIPv6 = (try? IPv6Address(nameserver)) != nil
+            if !isValidIPv4 && !isValidIPv6 {
+                throw ContainerizationError(
+                    .invalidArgument,
+                    message: "nameserver '\(nameserver)' is not a valid IPv4 or IPv6 address"
+                )
+            }
+        }
+    }
+}
+
+extension DNS {
+    public var resolvConf: String {
+        var text = ""
+
+        if !nameservers.isEmpty {
+            text += nameservers.map { "nameserver \($0)" }.joined(separator: "\n") + "\n"
+        }
+
+        if let domain {
+            text += "domain \(domain)\n"
+        }
+
+        if !searchDomains.isEmpty {
+            text += "search \(searchDomains.joined(separator: " "))\n"
+        }
+
+        if !options.isEmpty {
+            text += "options \(options.joined(separator: " "))\n"
+        }
+
+        return text
+    }
+}