libslirp, darwin.linux-builder: fix DNS resolution using libslirp on MacOS (#398952)

nixos/modules/profiles/nix-builder-vm.nix

@@ -111,14 +111,6 @@ in
       };
     };
 
-    # DNS fails for QEMU user networking (SLiRP) on macOS.  See:
-    #
-    # https://github.com/utmapp/UTM/issues/2353
-    #
-    # This works around that by using a public DNS server other than the DNS
-    # server that QEMU provides (normally 10.0.2.3)
-    networking.nameservers = [ "8.8.8.8" ];
-
     # The linux builder is a lightweight VM for remote building; not evaluation.
     nix.channel.enable = false;
 

pkgs/by-name/li/libslirp/fix-dns-for-darwin.patch

@@ -0,0 +1,46 @@
+From 735904142f95d0500c0eae6bf763e4ad24b6b9fd Mon Sep 17 00:00:00 2001
+From: Samuel Thibault <samuel.thibault@ens-lyon.org>
+Date: Wed, 26 Mar 2025 08:42:35 +0100
+Subject: [PATCH] apple: Fix getting IPv4 DNS server address when IPv4 and IPv4
+ are interleaved
+
+When getting an IPv4 DNS server address, if libresolv returns
+
+IPv4
+IPv6
+IPv4
+IPv6
+
+(or just IPv4 and IPv6)
+
+we would still have found == 1 on the second iteration and thus take the
+IPv6 even if it's not the proper af. We can as well just completely ignore
+the non-matching af entries.
+
+Fixes #85
+---
+ src/slirp.c | 7 +++++--
+ 1 file changed, 5 insertions(+), 2 deletions(-)
+
+diff --git a/src/slirp.c b/src/slirp.c
+index bccee53..62a018a 100644
+--- a/src/slirp.c
++++ b/src/slirp.c
+@@ -289,9 +289,12 @@ static int get_dns_addr_libresolv(int af, void *pdns_addr, void *cached_addr,
+     found = 0;
+     DEBUG_MISC("IP address of your DNS(s):");
+     for (int i = 0; i < count; i++) {
+-        if (af == servers[i].sin.sin_family) {
+-            found++;
++        if (af != servers[i].sin.sin_family) {
++            continue;
+         }
++
++        found++;
++
+         if (af == AF_INET) {
+             addr = &servers[i].sin.sin_addr;
+         } else { // af == AF_INET6
+-- 
+GitLab
+

pkgs/by-name/li/libslirp/package.nix

@@ -20,6 +20,12 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-Eqdw6epFkLv4Dnw/s1pcKW0P70ApZwx/J2VkCwn50Ew=";
   };
 
+  patches = [
+    # https://gitlab.freedesktop.org/slirp/libslirp/-/commit/735904142f95d0500c0eae6bf763e4ad24b6b9fd
+    # Vendorized due to frequent instability of the upstream repository.
+    ./fix-dns-for-darwin.patch
+  ];
+
   separateDebugInfo = true;
 
   nativeBuildInputs = [