INSTALL.userspace: Explain how and why to use iptables to drop packets.

Reported-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>

diff --git a/INSTALL.userspace b/INSTALL.userspace
index 6e6fcd4..10511b1 100644 (file)
--- a/INSTALL.userspace
+++ b/INSTALL.userspace
@@ -47,6 +47,19 @@ ovs-vswitchd will create a TAP device as the bridge's local interface,
 named the same as the bridge, as well as for each configured internal
 interface.
 
+Firewall Rules
+--------------
+
+On Linux, when a physical interface is in use by the userspace
+datapath, packets received on the interface still also pass into the
+kernel TCP/IP stack.  This can cause surprising and incorrect
+behavior.  You can use "iptables" to avoid this behavior, by using it
+to drop received packets.  For example, to drop packets received on
+eth0:
+
+    iptables -A INPUT -i eth0 -j DROP
+    iptables -A FORWARD -i eth0 -j DROP
+
 Bug Reporting
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