diff -Naur /nix/store/32wd1lrf55ymaz1aysrqffpxfgkwl6m4-source/src/libstore/build/local-derivation-goal.cc nix3/src/libstore/build/local-derivation-goal.cc
--- /nix/store/32wd1lrf55ymaz1aysrqffpxfgkwl6m4-source/src/libstore/build/local-derivation-goal.cc	1970-01-01 08:00:01.000000000 +0800
+++ nix3/src/libstore/build/local-derivation-goal.cc	2021-04-24 16:29:52.493166702 +0800
@@ -395,6 +395,8 @@
     additionalSandboxProfile = parsedDrv->getStringAttr("__sandboxProfile").value_or("");
 #endif
 
+    networked = parsedDrv->getBoolAttr("__networked");
+
     /* Are we doing a chroot build? */
     {
         auto noChroot = parsedDrv->getBoolAttr("__noChroot");
@@ -412,7 +414,7 @@
         else if (settings.sandboxMode == smDisabled)
             useChroot = false;
         else if (settings.sandboxMode == smRelaxed)
-            useChroot = !(derivationIsImpure(derivationType)) && !noChroot;
+            useChroot = !allowNetwork() && !(derivationIsImpure(derivationType)) && !noChroot;
     }
 
     auto & localStore = getLocalStore();
@@ -623,7 +625,7 @@
                 "nogroup:x:65534:\n", sandboxGid()));
 
         /* Create /etc/hosts with localhost entry. */
-        if (!(derivationIsImpure(derivationType)))
+        if (!allowNetwork() && !(derivationIsImpure(derivationType)))
             writeFile(chrootRootDir + "/etc/hosts", "127.0.0.1 localhost\n::1 localhost\n");
 
         /* Make the closure of the inputs available in the chroot,
@@ -810,7 +812,7 @@
            us.
         */
 
-        if (!(derivationIsImpure(derivationType)))
+        if (!allowNetwork() && !(derivationIsImpure(derivationType)))
             privateNetwork = true;
 
         userNamespaceSync.create();
@@ -1066,7 +1068,7 @@
        to the builder is generally impure, but the output of
        fixed-output derivations is by definition pure (since we
        already know the cryptographic hash of the output). */
-    if (derivationIsImpure(derivationType)) {
+    if (allowNetwork() || derivationIsImpure(derivationType)) {
         for (auto & i : parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings()))
             env[i] = getEnv(i).value_or("");
     }
@@ -1702,7 +1704,7 @@
             /* Fixed-output derivations typically need to access the
                network, so give them access to /etc/resolv.conf and so
                on. */
-            if (derivationIsImpure(derivationType)) {
+            if (allowNetwork() || derivationIsImpure(derivationType)) {
                 // Only use nss functions to resolve hosts and
                 // services. Don’t use it for anything else that may
                 // be configured for this system. This limits the
@@ -1943,7 +1945,7 @@
 
                 sandboxProfile += "(import \"sandbox-defaults.sb\")\n";
 
-                if (derivationIsImpure(derivationType))
+                if (allowNetwork() || derivationIsImpure(derivationType))
                     sandboxProfile += "(import \"sandbox-network.sb\")\n";
 
                 /* Add the output paths we'll use at build-time to the chroot */
diff -Naur /nix/store/32wd1lrf55ymaz1aysrqffpxfgkwl6m4-source/src/libstore/build/local-derivation-goal.hh nix3/src/libstore/build/local-derivation-goal.hh
--- /nix/store/32wd1lrf55ymaz1aysrqffpxfgkwl6m4-source/src/libstore/build/local-derivation-goal.hh	1970-01-01 08:00:01.000000000 +0800
+++ nix3/src/libstore/build/local-derivation-goal.hh	2021-04-24 16:35:23.060968488 +0800
@@ -40,6 +40,12 @@
 
     Path chrootRootDir;
 
+    bool networked;
+    bool allowNetwork()
+    {
+        return derivationIsFixed(drv->type()) || networked;
+    }
+
     /* RAII object to delete the chroot directory. */
     std::shared_ptr<AutoDelete> autoDelChroot;