Set mtu to old Containerization default (apple#1110)

The old mtu default in containerization was 1280 to account for some
alpine/musl images that have issues with
1500. This changed in the last couple tags to be modifiable, but the new
default is the standard 1500. Ideally we eventually allow supplying the
mtu to be used when you create a network (or possibly a container
creation setting), but for now just default in here back to what CZ used
to use.

Author: dcantah

Sources/Helpers/RuntimeLinux/IsolatedInterfaceStrategy.swift

@@ -25,6 +25,12 @@ import Containerization
 struct IsolatedInterfaceStrategy: InterfaceStrategy {
     public func toInterface(attachment: Attachment, interfaceIndex: Int, additionalData: XPCMessage?) -> Interface {
         let ipv4Gateway = interfaceIndex == 0 ? attachment.ipv4Gateway : nil
-        return NATInterface(ipv4Address: attachment.ipv4Address, ipv4Gateway: ipv4Gateway, macAddress: attachment.macAddress)
+        return NATInterface(
+            ipv4Address: attachment.ipv4Address,
+            ipv4Gateway: ipv4Gateway,
+            macAddress: attachment.macAddress,
+            // https://github.com/apple/containerization/pull/38
+            mtu: 1280
+        )
     }
 }

Sources/Helpers/RuntimeLinux/NonisolatedInterfaceStrategy.swift

@@ -44,6 +44,13 @@ struct NonisolatedInterfaceStrategy: InterfaceStrategy {
 
         log.info("creating NATNetworkInterface with network reference")
         let ipv4Gateway = interfaceIndex == 0 ? attachment.ipv4Gateway : nil
-        return NATNetworkInterface(ipv4Address: attachment.ipv4Address, ipv4Gateway: ipv4Gateway, reference: networkRef, macAddress: attachment.macAddress)
+        return NATNetworkInterface(
+            ipv4Address: attachment.ipv4Address,
+            ipv4Gateway: ipv4Gateway,
+            reference: networkRef,
+            macAddress: attachment.macAddress,
+            // https://github.com/apple/containerization/pull/38
+            mtu: 1280
+        )
     }
 }