preloadNSS: rework the dns query workaround

backport of NixOS#5384 to nix 2.4

src/libmain/shared.cc

@@ -15,9 +15,14 @@
 #include <sys/stat.h>
 #include <unistd.h>
 #include <signal.h>
-#include <sys/types.h>
-#include <sys/socket.h>
-#include <netdb.h>
+#ifdef __linux__
+#include <features.h>
+#endif
+#ifdef __GLIBC__
+#include <gnu/lib-names.h>
+#include <nss.h>
+#include <dlfcn.h>
+#endif
 
 #include <openssl/crypto.h>
 
@@ -121,21 +126,30 @@ static void preloadNSS() {
        been loaded in the parent. So we force a lookup of an invalid domain to force the NSS machinery to
        load its lookup libraries in the parent before any child gets a chance to. */
     std::call_once(dns_resolve_flag, []() {
-        struct addrinfo *res = NULL;
-
-        /* nss will only force the "local" (not through nscd) dns resolution if its on the LOCALDOMAIN.
-           We need the resolution to be done locally, as nscd socket will not be accessible in the
-           sandbox. */
-        char * previous_env = getenv("LOCALDOMAIN");
-        setenv("LOCALDOMAIN", "invalid", 1);
-        if (getaddrinfo("this.pre-initializes.the.dns.resolvers.invalid.", "http", NULL, &res) == 0) {
-            if (res) freeaddrinfo(res);
-        }
-        if (previous_env) {
-             setenv("LOCALDOMAIN", previous_env, 1);
-        } else {
-             unsetenv("LOCALDOMAIN");
+#ifdef __GLIBC__
+        /* On linux, glibc will run every lookup through the nss layer.
+         * That means every lookup goes, by default, through nscd, which acts as a local
+         * cache.
+         * Because we run builds in a sandbox, we also remove access to nscd otherwise
+         * lookups would leak into the sandbox.
+         *
+         * But now we have a new problem, we need to make sure the nss_dns backend that
+         * does the dns lookups when nscd is not available is loaded or available.
+         *
+         * We can't make it available without leaking nix's environment, so instead we'll
+         * load the backend, and configure nss so it does not try to run dns lookups
+         * through nscd.
+         *
+         * This is technically only used for builtins:fetch* functions so we only care
+         * about dns.
+         *
+         * All other platforms are unaffected.
+         */
+        if (dlopen (LIBNSS_DNS_SO, RTLD_NOW) == NULL) {
+            printMsg(Verbosity::lvlWarn, fmt("Unable to load nss_dns backend"));
         }
+        __nss_configure_lookup ("hosts", "dns");
+#endif
     });
 }