diff -Naur nix-2.3.10.orig/src/libstore/build.cc nix-2.3.10/src/libstore/build.cc
--- nix-2.3.10.orig/src/libstore/build.cc	1970-01-01 08:00:01.000000000 +0800
+++ nix-2.3.10/src/libstore/build.cc	2021-04-24 16:17:08.778875340 +0800
@@ -840,9 +840,16 @@
     /* Whether this is a fixed-output derivation. */
     bool fixedOutput;
 
+    bool networked;
+
     /* Whether to run the build in a private network namespace. */
     bool privateNetwork = false;
 
+    bool allowNetwork()
+    {
+        return fixedOutput || networked;
+    }
+
     typedef void (DerivationGoal::*GoalState)();
     GoalState state;
 
@@ -1181,6 +1188,8 @@
 {
     trace("have derivation");
 
+    fixedOutput = drv->isFixedOutput();
+
     retrySubstitution = false;
 
     for (auto & i : drv->outputs)
@@ -1197,6 +1206,8 @@
 
     parsedDrv = std::make_unique<ParsedDerivation>(drvPath, *drv);
 
+    networked = parsedDrv->getBoolAttr("__networked");
+
     /* We are first going to try to create the invalid output paths
        through substitutes.  If that doesn't work, we'll build
        them. */
@@ -1932,7 +1943,7 @@
         else if (settings.sandboxMode == smDisabled)
             useChroot = false;
         else if (settings.sandboxMode == smRelaxed)
-            useChroot = !fixedOutput && !noChroot;
+            useChroot = !allowNetwork() && !noChroot;
     }
 
     if (worker.store.storeDir != worker.store.realStoreDir) {
@@ -2109,7 +2120,7 @@
                 "nogroup:x:65534:\n") % sandboxGid).str());
 
         /* Create /etc/hosts with localhost entry. */
-        if (!fixedOutput)
+        if (!allowNetwork())
             writeFile(chrootRootDir + "/etc/hosts", "127.0.0.1 localhost\n::1 localhost\n");
 
         /* Make the closure of the inputs available in the chroot,
@@ -2323,7 +2334,7 @@
            us.
         */
 
-        if (!fixedOutput)
+        if (!allowNetwork())
             privateNetwork = true;
 
         userNamespaceSync.create();
@@ -2534,7 +2545,7 @@
        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 (fixedOutput) {
+    if (allowNetwork()) {
         for (auto & i : parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings()))
             env[i] = getEnv(i);
     }
@@ -2823,7 +2834,7 @@
             /* Fixed-output derivations typically need to access the
                network, so give them access to /etc/resolv.conf and so
                on. */
-            if (fixedOutput) {
+            if (allowNetwork()) {
                 ss.push_back("/etc/resolv.conf");
 
                 // Only use nss functions to resolve hosts and