conntrack: document NULL SNAT behavior and add a test case

Currently, conntrack in the kernel has an undocumented feature referred
to as NULL SNAT. Basically, when a source port collision is detected
during the commit, the source port will be translated to an ephemeral
port. If there is no collision, no SNAT is performed.

This patchset documents this behavior and adds a self-test to verify
it's not changing.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: 0-day Robot <robot@bytheb.org>

lib/ovs-actions.xml

@@ -1833,6 +1833,16 @@ for <var>i</var> in [1,<var>n_members</var>]:
             connection, will behave the same as a bare <code>nat</code>.
           </p>
 
+          <p>
+            For SNAT, there is a special case when the <code>src</code> IP
+            address is configured as all 0's, i.e.,
+            <code>nat(src=0.0.0.0)</code>. In this case, when a source port
+            collision is detected during the commit, the source port will be
+            translated to an ephemeral port. If there is no collision, no SNAT
+            is performed. Note that this is currently only implemented in the
+            Linux kernel datapath.
+          </p>
+
           <p>
             Open vSwitch 2.6 introduced <code>nat</code>.  Linux 4.6 was the
             earliest upstream kernel that implemented <code>ct</code> support for

tests/system-kmod-macros.at

@@ -99,6 +99,13 @@ m4_define([CHECK_CONNTRACK_FRAG_OVERLAP],
 #
 m4_define([CHECK_CONNTRACK_NAT])
 
+# CHECK_CONNTRACK_NULL_SNAT()
+#
+# Perform requirements checks for running conntrack SNAT NULL tests.
+# The kernel always supports NULL SNAT, so no check is needed.
+#
+m4_define([CHECK_CONNTRACK_NULL_SNAT])
+
 # CHECK_CONNTRACK_TIMEOUT()
 #
 # Perform requirements checks for running conntrack customized timeout tests.

tests/system-traffic.at

@@ -4433,6 +4433,52 @@ tcp,orig=(src=10.1.1.1,dst=10.1.1.2,sport=<cleared>,dport=<cleared>),reply=(src=
 OVS_TRAFFIC_VSWITCHD_STOP
 AT_CLEANUP
 
+
+AT_SETUP([conntrack - NULL SNAT])
+AT_SKIP_IF([test $HAVE_NC = no])
+CHECK_CONNTRACK()
+CHECK_CONNTRACK_NULL_SNAT()
+OVS_TRAFFIC_VSWITCHD_START()
+
+ADD_NAMESPACES(at_ns0, at_ns1)
+ADD_VETH(p0, at_ns0, br0, "10.1.1.1/24")
+ADD_VETH(p1, at_ns1, br0, "10.1.1.2/24")
+NS_CHECK_EXEC([at_ns0], [ip route add 172.1.1.0/24 via 10.1.1.2])
+
+OVS_START_L7([at_ns1], [http])
+
+AT_DATA([flows.txt], [dnl
+table=0,priority=30,ct_state=-trk,ip,action=ct(table=0)
+table=0,priority=20,ct_state=-rpl,ip,nw_dst=10.1.1.0/24,actions=ct(commit,nat(src=0.0.0.0),table=10)
+table=0,priority=20,ct_state=+rpl,ip,nw_dst=10.1.1.0/24,actions=resubmit(,10)
+table=0,priority=20,ip,nw_dst=172.1.1.2,actions=ct(commit,nat(dst=10.1.1.2),table=10)
+table=0,priority=10,arp,action=normal
+table=0,priority=1,action=drop
+table=10,priority=20,ct_state=+rpl,ip,nw_dst=10.1.1.0/24 actions=ct(table=20,nat)
+table=10,priority=10,ip,nw_dst=10.1.1.0/24 actions=resubmit(,20)
+table=20,priority=10,ip,nw_dst=10.1.1.1,action=1
+table=20,priority=10,ip,nw_dst=10.1.1.2,action=2
+])
+AT_CHECK([ovs-ofctl --bundle add-flows br0 flows.txt])
+
+dnl - Test to make sure src nat is NOT done when not needed
+NS_CHECK_EXEC([at_ns0], [echo "TEST" | nc -p 30000 10.1.1.2 80 > nc-1.log])
+AT_CHECK([ovs-appctl dpctl/dump-conntrack | grep "orig=.src=10\.1\.1\.1,"], [0], [dnl
+tcp,orig=(src=10.1.1.1,dst=10.1.1.2,sport=30000,dport=80),reply=(src=10.1.1.2,dst=10.1.1.1,sport=80,dport=30000),protoinfo=(state=TIME_WAIT)
+])
+
+dnl - Test to make sure src nat is done when needed
+NS_CHECK_EXEC([at_ns0], [echo "TEST2" | nc -p 30001 172.1.1.2 80 > nc-2.log])
+NS_CHECK_EXEC([at_ns0], [echo "TEST3" | nc -p 30001 10.1.1.2 80 > nc-3.log])
+AT_CHECK([ovs-appctl dpctl/dump-conntrack | grep 30001 | grep "orig=.src=10\.1\.1\.1," | sed -e 's/port=30001/port=<clnt_s_port>/g' -e 's/sport=80,dport=[[0-9]]\+/sport=80,dport=<rnd_port>/g' | sort], [0], [dnl
+tcp,orig=(src=10.1.1.1,dst=10.1.1.2,sport=<clnt_s_port>,dport=80),reply=(src=10.1.1.2,dst=10.1.1.1,sport=80,dport=<rnd_port>),protoinfo=(state=TIME_WAIT)
+tcp,orig=(src=10.1.1.1,dst=172.1.1.2,sport=<clnt_s_port>,dport=80),reply=(src=10.1.1.2,dst=10.1.1.1,sport=80,dport=<clnt_s_port>),protoinfo=(state=TIME_WAIT)
+])
+
+OVS_TRAFFIC_VSWITCHD_STOP
+AT_CLEANUP
+
+
 AT_SETUP([conntrack - simple DNAT])
 CHECK_CONNTRACK()
 CHECK_CONNTRACK_NAT()

tests/system-userspace-macros.at

@@ -96,6 +96,16 @@ m4_define([CHECK_CONNTRACK_FRAG_OVERLAP])
 #
 m4_define([CHECK_CONNTRACK_NAT])
 
+# CHECK_CONNTRACK_NULL_SNAT()
+#
+# Perform requirements checks for running conntrack SNAT NULL tests.
+# The userspace datapath does not support NULL SNAT.
+#
+m4_define([CHECK_CONNTRACK_NULL_SNAT],
+[
+    AT_SKIP_IF([:])
+])
+
 # CHECK_CONNTRACK_TIMEOUT()
 #
 # Perform requirements checks for running conntrack customized timeout tests.