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android: Avoid DNS leak due to blocking TUN device without DNS servers 

It looks like Android 9 incorrectly continues to use the regular DNS
servers after the blocking TUN device is replaced with the actual
interface.  Setting DNS servers prevents that (since all traffic is
blocked, which ones doesn't really matter but local/loopback addresses
are rejected).
Interestingly, if the VPN server later does not assign any DNS servers, there
is no fallback to the non-VPN DNS servers for some reason (that's definitely
not as documented).  This could potentially be a problem as we don't
offer an option to configure DNS servers in the VPN profile.

Neither issue is seen on older Android versions (only tested on 7.1.1).
This commit is contained in:
Tobias Brunner 2018-09-21 15:07:44 +02:00
parent e89cfe4854
commit 84ee9577e1
1 changed files with 3 additions and 0 deletions
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3
src/frontends/android/app/src/main/java/org/strongswan/android/logic/CharonVpnService.java
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@ -930,6 +930,9 @@ public class CharonVpnService extends VpnService implements Runnable, VpnStateSe
mCache.addAddress("fd00::fd02:1", 128);
mCache.addRoute("0.0.0.0", 0);
mCache.addRoute("::", 0);
/* set DNS servers to avoid DNS leak later */
mBuilder.addDnsServer("8.8.8.8");
mBuilder.addDnsServer("2001:4860:4860::8888");
/* use blocking mode to simplify packet dropping */
mBuilder.setBlocking(true);
ParcelFileDescriptor fd = establishIntern();