Always use the default network DNS servers if a VPN does not set any.nougat-dev

When a VPN provides no DNS servers, DNS lookups are usually sent
to the default network's DNS servers. However, if a DNS lookup
is explicitly made on the VPN (e.g., via Network#openConnection),
then it just fails.

This breaks system proxies which perform network traffic on VPNs
on behalf of other apps, e.g., the download manager.

Fix this by doing the query to the default DNS servers (via the
default network) instead. This is consistent with what we do with
DNS queries that do not specify a network. While this is a change
in behaviour, it shouldn't cause much breakage because the query
would previously just fail.

Bug: 29498052
Change-Id: Ie4002c9835bb1ff6d3d92c00c9c04e634fc3cda4

1 files changed, 13 insertions, 1 deletions

diff --git a/server/NetworkController.cpp b/server/NetworkController.cpp
index 3364577c..014d9267 100644
--- a/server/NetworkController.cpp
+++ b/server/NetworkController.cpp
@@ -191,8 +191,18 @@ uint32_t NetworkController::getNetworkForDns(unsigned* netId, uid_t uid) const {
     if (checkUserNetworkAccessLocked(uid, *netId) == 0) {
         // If a non-zero NetId was explicitly specified, and the user has permission for that
         // network, use that network's DNS servers. Do not fall through to the default network even
-        // if the explicitly selected network is a split tunnel VPN or a VPN without DNS servers.
+        // if the explicitly selected network is a split tunnel VPN: the explicitlySelected bit
+        // ensures that the VPN fallthrough rule does not match.
         fwmark.explicitlySelected = true;
+
+        // If the network is a VPN and it doesn't have DNS servers, use the default network's DNS
+        // servers (through the default network). Otherwise, the query is guaranteed to fail.
+        // http://b/29498052
+        Network *network = getNetworkLocked(*netId);
+        if (network && network->getType() == Network::VIRTUAL &&
+                !static_cast<VirtualNetwork *>(network)->getHasDns()) {
+            *netId = mDefaultNetId;
+        }
     } else {
         // If the user is subject to a VPN and the VPN provides DNS servers, use those servers
         // (possibly falling through to the default network if the VPN doesn't provide a route to
@@ -201,6 +211,8 @@ uint32_t NetworkController::getNetworkForDns(unsigned* netId, uid_t uid) const {
         if (virtualNetwork && virtualNetwork->getHasDns()) {
             *netId = virtualNetwork->getNetId();
         } else {
+            // TODO: return an error instead of silently doing the DNS lookup on the wrong network.
+            // http://b/27560555
             *netId = mDefaultNetId;
         }
     }