[libvirt] [PATCH] network: don't forward DNS requests from isolated networks

[Laine Stump]

This is in response to:

  https://bugzilla.redhat.com/show_bug.cgi?id=723862

which points out that a guest on an "isolated" network could
potentially exploit the DNS forwarding provided by dnsmasq to create a
communication channel to the outside.

This patch eliminates that possibility by adding the "--no-resolv"
argument to the dnsmasq commandline, which tells dnsmasq to not
forward on any requests that it can't resolv itself (by looking at its
own static hosts files and runtime lsit of dhcp clients), but to
instead return a failure for those requests.

This shouldn't cause any undesirable change from current
behavior, even in the case where a guest is currently configured with
multiple interfaces, one of them being connected to an isolated
network, and another to a network that does have connectivity to the
outside. If the isolated network's DNS server is queried for a name
it doesn't know, it will return "Refused" rather than "Unknown", which
indicates to the guest that it should query other servers, so it then
queries the connected DNS server, and gets the desired response.
---
 src/network/bridge_driver.c                     |   11 ++++++++---
 tests/networkxml2argvdata/isolated-network.argv |    3 ++-
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/network/bridge_driver.c b/src/network/bridge_driver.c
index b8c6c97..0a60bb8 100644
--- a/src/network/bridge_driver.c
+++ b/src/network/bridge_driver.c
@@ -531,10 +531,15 @@ networkBuildDnsmasqArgv(virNetworkObjPtr network,
 
     /* If this is an isolated network, set the default route option
      * (3) to be empty to avoid setting a default route that's
-     * guaranteed to not work.
+     * guaranteed to not work, and set --no-resolv so that no dns
+     * requests are forwarded on to the dns server listed in the
+     * host's /etc/resolv.conf (since this could be used as a channel
+     * to build a connection to the outside).
      */
-    if (network->def->forwardType == VIR_NETWORK_FORWARD_NONE)
-        virCommandAddArg(cmd, "--dhcp-option=3");
+    if (network->def->forwardType == VIR_NETWORK_FORWARD_NONE) {
+        virCommandAddArgList(cmd, "--dhcp-option=3",
+                             "--no-resolv", NULL);
+    }
 
     if (network->def->dns != NULL) {
         virNetworkDNSDefPtr dns = network->def->dns;
diff --git a/tests/networkxml2argvdata/isolated-network.argv b/tests/networkxml2argvdata/isolated-network.argv
index f801396..7ea2e94 100644
--- a/tests/networkxml2argvdata/isolated-network.argv
+++ b/tests/networkxml2argvdata/isolated-network.argv
@@ -1,5 +1,6 @@
 /usr/sbin/dnsmasq --strict-order --bind-interfaces --conf-file= \
---except-interface lo --dhcp-option=3 --listen-address 192.168.152.1 \
+--except-interface lo --dhcp-option=3 --no-resolv \
+--listen-address 192.168.152.1 \
 --dhcp-range 192.168.152.2,192.168.152.254 \
 --dhcp-leasefile=/var/lib/libvirt/dnsmasq/private.leases --dhcp-lease-max=253 \
 --dhcp-no-override\
-- 
1.7.3.4