Fall back to host DNS domain for container default

Author: 0xMH

Sources/Services/ContainerAPIService/Client/Utility.swift

@@ -223,7 +223,11 @@ public struct Utility {
         if management.dnsDisabled {
             config.dns = nil
         } else {
-            config.dns = dnsConfiguration(from: management.dns, defaults: containerSystemConfig.container.dns)
+            config.dns = dnsConfiguration(
+                from: management.dns,
+                defaults: containerSystemConfig.container.dns,
+                hostDomainFallback: containerSystemConfig.dns.domain
+            )
         }
 
         config.rosetta = management.rosetta || (Platform.current.architecture == "arm64" && requestedPlatform.architecture == "amd64")
@@ -326,13 +330,14 @@ public struct Utility {
 
     public static func dnsConfiguration(
         from flags: Flags.DNS,
-        defaults: ContainerDNSConfig
+        defaults: ContainerDNSConfig,
+        hostDomainFallback: String? = nil
     ) -> ContainerConfiguration.DNSConfiguration {
         let nameservers =
             flags.nameservers.isEmpty
             ? defaults.nameservers
             : flags.nameservers
-        let domain = flags.domain ?? defaults.domain
+        let domain = flags.domain ?? defaults.domain ?? hostDomainFallback
         let searchDomains =
             flags.searchDomains.isEmpty
             ? defaults.searchDomains

Tests/ContainerAPIClientTests/UtilityTests.swift

@@ -161,6 +161,36 @@ struct UtilityTests {
         #expect(result.options == ["ndots:2"])
     }
 
+    @Test
+    func testDNSConfigurationDomainFallsBackToHostDomain() {
+        let defaults = ContainerDNSConfig()
+        let flags = Flags.DNS(
+            domain: nil,
+            nameservers: [],
+            options: [],
+            searchDomains: []
+        )
+
+        let result = Utility.dnsConfiguration(from: flags, defaults: defaults, hostDomainFallback: "host.local")
+
+        #expect(result.domain == "host.local")
+    }
+
+    @Test
+    func testDNSConfigurationContainerDomainWinsOverHostDomain() {
+        let defaults = ContainerDNSConfig(domain: "container.local")
+        let flags = Flags.DNS(
+            domain: nil,
+            nameservers: [],
+            options: [],
+            searchDomains: []
+        )
+
+        let result = Utility.dnsConfiguration(from: flags, defaults: defaults, hostDomainFallback: "host.local")
+
+        #expect(result.domain == "container.local")
+    }
+
     @Test
     func testPublishPortParser() throws {
         let ports = try Parser.publishPorts([