Add environment variable for disabling dnscache settings

2 files changed, 25 insertions, 14 deletions

diff --git a/README.md b/README.md
index ed265bc210..ed6714ef4b 100644
--- a/README.md
+++ b/README.md
@@ -369,6 +369,12 @@ echo "org.gradle.jvmargs=-Xmx4608M" >> ~/.gradle/gradle.properties
 * `TALPID_FORCE_USERSPACE_WIREGUARD` - Forces the daemon to use the userspace implementation of
    WireGuard on Linux.
 
+* `TALPID_DNS_CACHE_POLICY` - On Windows, this changes how DNS is configured:
+  * `1`: The default. This sets a global list of DNS servers that `dnscache` will use instead of
+         the servers specified on each interface.
+  * `0`: Only set DNS servers on the tunnel interface. This will misbehave if local custom DNS
+         servers are used.
+
 
 ## Building and running the desktop Electron GUI app
 
diff --git a/talpid-core/src/dns/windows/mod.rs b/talpid-core/src/dns/windows/mod.rs
index d8bd326786..17165c2364 100644
--- a/talpid-core/src/dns/windows/mod.rs
+++ b/talpid-core/src/dns/windows/mod.rs
@@ -1,7 +1,10 @@
 use crate::logging::windows::{log_sink, LogSink};
 
+use lazy_static::lazy_static;
 use log::{error, trace, warn};
-use std::{ffi::OsString, io, iter, mem, net::IpAddr, os::windows::ffi::OsStrExt, path::Path, ptr};
+use std::{
+    env, ffi::OsString, io, iter, mem, net::IpAddr, os::windows::ffi::OsStrExt, path::Path, ptr,
+};
 use talpid_types::ErrorExt;
 use widestring::WideCString;
 use winapi::um::{
@@ -21,6 +24,13 @@ use self::system_state::SystemStateWriter;
 const DNS_STATE_FILENAME: &'static str = "dns-state-backup";
 const DNS_CACHE_POLICY_GUID: &str = "{d57d2750-f971-408e-8e55-cfddb37e60ae}";
 
+lazy_static! {
+    /// Specifies whether to override per-interface DNS resolvers with a global DNS policy.
+    static ref GLOBAL_DNS_CACHE_POLICY: bool = env::var("TALPID_DNS_CACHE_POLICY")
+        .map(|v| v != "0")
+        .unwrap_or(true);
+}
+
 /// Errors that can happen when configuring DNS on Windows.
 #[derive(err_derive::Error, Debug)]
 pub enum Error {
@@ -49,15 +59,6 @@ impl super::DnsMonitorT for DnsMonitor {
     fn new(cache_dir: impl AsRef<Path>) -> Result<Self, Error> {
         unsafe { WinDns_Initialize(Some(log_sink), b"WinDns\0".as_ptr()).into_result()? };
 
-        if is_minimum_windows10() {
-            if let Err(error) = reset_dns_cache_policy() {
-                error!(
-                    "{}",
-                    error.display_chain_with_msg("Failed to reset DNS cache policy")
-                );
-            }
-        }
-
         let backup_writer = SystemStateWriter::new(
             cache_dir
                 .as_ref()
@@ -65,7 +66,11 @@ impl super::DnsMonitorT for DnsMonitor {
                 .into_boxed_path(),
         );
         let _ = backup_writer.remove_backup();
-        Ok(DnsMonitor {})
+
+        let mut monitor = DnsMonitor {};
+        monitor.reset()?;
+
+        Ok(monitor)
     }
 
     fn set(&mut self, interface: &str, servers: &[IpAddr]) -> Result<(), Error> {
@@ -103,7 +108,7 @@ impl super::DnsMonitorT for DnsMonitor {
             .into_result()
         }?;
 
-        if is_minimum_windows10() {
+        if *GLOBAL_DNS_CACHE_POLICY && is_minimum_windows10() {
             if let Err(error) = set_dns_cache_policy(servers) {
                 error!("{}", error.display_chain());
                 warn!("DNS resolution may be slowed down");
@@ -114,7 +119,7 @@ impl super::DnsMonitorT for DnsMonitor {
     }
 
     fn reset(&mut self) -> Result<(), Error> {
-        if is_minimum_windows10() {
+        if *GLOBAL_DNS_CACHE_POLICY && is_minimum_windows10() {
             reset_dns_cache_policy()
         } else {
             Ok(())
@@ -128,7 +133,7 @@ fn ip_to_widestring(ip: &IpAddr) -> WideCString {
 
 impl Drop for DnsMonitor {
     fn drop(&mut self) {
-        if is_minimum_windows10() {
+        if *GLOBAL_DNS_CACHE_POLICY && is_minimum_windows10() {
             if let Err(error) = reset_dns_cache_policy() {
                 warn!(
                     "{}",