vminitd: Fall back to /proc/net/pnp for unset nameservers or domain

When configureDns is called with an empty nameservers list or no domain,
read /proc/net/pnp (written by the kernel IP_PNP DHCP client) and use
any nameserver and domain lines found there. The two are filled in
independently, so an explicit nameserver does not prevent the domain
from being read from pnp. This provides automatic DNS configuration
for bridge-mode containers without a new RPC or proto change.

Author: torarnv

vminitd/Sources/vminitd/Server+GRPC.swift

@@ -1267,13 +1267,12 @@ extension Initd: Com_Apple_Containerization_Sandbox_V3_SandboxContext.SimpleServ
         request: Com_Apple_Containerization_Sandbox_V3_ConfigureDnsRequest,
         context: GRPCCore.ServerContext
     ) async throws -> Com_Apple_Containerization_Sandbox_V3_ConfigureDnsResponse {
-        let domain = request.hasDomain ? request.domain : nil
         log.debug(
             "configureDns",
             metadata: [
                 "location": "\(request.location)",
                 "nameservers": "\(request.nameservers)",
-                "domain": "\(domain ?? "")",
+                "domain": "\(request.hasDomain ? request.domain : "")",
                 "searchDomains": "\(request.searchDomains)",
                 "options": "\(request.options)",
             ])
@@ -1282,8 +1281,25 @@ extension Initd: Com_Apple_Containerization_Sandbox_V3_SandboxContext.SimpleServ
             let etc = URL(fileURLWithPath: request.location).appendingPathComponent("etc")
             try FileManager.default.createDirectory(atPath: etc.path, withIntermediateDirectories: true)
             let resolvConf = etc.appendingPathComponent("resolv.conf")
+            var nameservers = request.nameservers
+            var domain = request.hasDomain ? request.domain : nil
+            if (nameservers.isEmpty || domain == nil),
+               let pnp = try? String(contentsOfFile: "/proc/net/pnp", encoding: .utf8)
+            {
+                let lines = pnp.split(separator: "\n")
+                if nameservers.isEmpty {
+                    nameservers = lines
+                        .filter { $0.hasPrefix("nameserver") }
+                        .compactMap { $0.split(separator: " ").dropFirst().first.map(String.init) }
+                }
+                if domain == nil {
+                    domain = lines
+                        .first { $0.hasPrefix("domain") }
+                        .flatMap { $0.split(separator: " ").dropFirst().first.map(String.init) }
+                }
+            }
             let config = DNS(
-                nameservers: request.nameservers,
+                nameservers: nameservers,
                 domain: domain,
                 searchDomains: request.searchDomains,
                 options: request.options