add dns config flag

src/chrome/browser/thorium_flag_entries.h

@@ -29,10 +29,15 @@
      "Disable Thorium Top Bar Icons",
      "Disables the custom colored top bar icons in Thorium, and restores the default grey Chromium icon colors.",
      kOsDesktop, SINGLE_VALUE_TYPE("disable-thorium-icons")},
+    {"disable-thorium-dns-config",
+     "Disable Thorium Custom DNS Config",
+     "Disables the custom DNS configuration used by default in Thorium. Useful when this config breaks something, "
+     "due to external apps or a non-standard system DNS config setting.",
+     kOsDesktop, SINGLE_VALUE_TYPE("disable-thorium-dns-config")},
     {"force-high-contrast",
      "Enable High Contrast Mode",
      "Enables high contrast mode for all Thorium instances.",
-     kOsAll, SINGLE_VALUE_TYPE(switches::kForceHighContrast)},
+     kOsDesktop, SINGLE_VALUE_TYPE(switches::kForceHighContrast)},
 
 #if BUILDFLAG(IS_WIN)
     {"disable-aero",

src/net/dns/dns_client.cc

@@ -9,7 +9,9 @@
 #include <string>
 #include <utility>
 
+#include "base/command_line.h"
 #include "base/functional/bind.h"
+#include "base/logging.h"
 #include "base/memory/raw_ptr.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/rand_util.h"
@@ -243,16 +245,28 @@ class DnsClientImpl : public DnsClient {
  private:
   std::optional<DnsConfig> BuildEffectiveConfig() const {
     DnsConfig config;
-    // in Bromite it is sufficient to have secure DoH enabled to give the overrides priority
-    if (config_overrides_.dns_over_https_config && config_overrides_.secure_dns_mode) {
-      config = config_overrides_.ApplyOverrides(DnsConfig());
+    if (!base::CommandLine::ForCurrentProcess()->HasSwitch("disable-thorium-dns-config")) {
+      // in Bromite it is sufficient to have secure DoH enabled to give the overrides priority
+      if (config_overrides_.dns_over_https_config && config_overrides_.secure_dns_mode) {
+        config = config_overrides_.ApplyOverrides(DnsConfig());
+      } else {
+        if (!system_config_) {
+          LOG(WARNING) << "BuildEffectiveConfig(): no system configuration";
+          return std::nullopt;
+        }
+
+        config = config_overrides_.ApplyOverrides(system_config_.value());
+      }
     } else {
-      if (!system_config_) {
-        LOG(WARNING) << "BuildEffectiveConfig(): no system configuration";
-        return std::nullopt;
+      if (config_overrides_.OverridesEverything()) {
+        config = config_overrides_.ApplyOverrides(DnsConfig());
+      } else {
+        if (!system_config_)
+          LOG(WARNING) << "BuildEffectiveConfig(): system configuration not set";
+          return std::nullopt;
+
+        config = config_overrides_.ApplyOverrides(system_config_.value());
       }
-
-      config = config_overrides_.ApplyOverrides(system_config_.value());
     }
 
     UpdateConfigForDohUpgrade(&config);