conntrack: Document all-zero IP SNAT behavior and add a test case.

Currently, conntrack in the kernel has an undocumented feature referred
to as all-zero IP address 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. In addition, a datapath feature flag is added for
the all-zero IP SNAT case. This will help applications on top of OVS,
like OVN, to determine this feature can be used.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Acked-by: Alin-Gabriel Serdean <aserdean@ovn.org>
Acked-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

NEWS

@@ -32,6 +32,9 @@ Post-v2.15.0
    - ovsdb-tool:
      * New option '--election-timer' to the 'create-cluster' command to set the
        leader election timer during cluster creation.
+   - OVS now reports the datapath capability 'ct_zero_snat', which reflects
+     whether the SNAT with all-zero IP address is supported.
+     See ovs-vswitchd.conf.db(5) for details.
 
 
 v2.15.0 - 15 Feb 2021

lib/ct-dpif.c

@@ -889,3 +889,11 @@ ct_dpif_get_timeout_policy_name(struct dpif *dpif, uint32_t tp_id,
                 dpif, tp_id, dl_type, nw_proto, tp_name, is_generic)
             : EOPNOTSUPP);
 }
+
+int
+ct_dpif_get_features(struct dpif *dpif, enum ct_features *features)
+{
+    return (dpif->dpif_class->ct_get_features
+            ? dpif->dpif_class->ct_get_features(dpif, features)
+            : EOPNOTSUPP);
+}

lib/ct-dpif.h

@@ -271,6 +271,11 @@ struct ct_dpif_timeout_policy {
                                                  * timeout attribute values */
 };
 
+/* Conntrack Features. */
+enum ct_features {
+    CONNTRACK_F_ZERO_SNAT = 1 << 0,  /* All-zero SNAT support. */
+};
+
 int ct_dpif_dump_start(struct dpif *, struct ct_dpif_dump_state **,
                        const uint16_t *zone, int *);
 int ct_dpif_dump_next(struct ct_dpif_dump_state *, struct ct_dpif_entry *);
@@ -325,5 +330,6 @@ int ct_dpif_timeout_policy_dump_done(struct dpif *dpif, void *state);
 int ct_dpif_get_timeout_policy_name(struct dpif *dpif, uint32_t tp_id,
                                     uint16_t dl_type, uint8_t nw_proto,
                                     char **tp_name, bool *is_generic);
+int ct_dpif_get_features(struct dpif *dpif, enum ct_features *features);
 
 #endif /* CT_DPIF_H */

lib/dpif-netdev.c

@@ -8577,6 +8577,7 @@ const struct dpif_class dpif_netdev_class = {
     NULL,                       /* ct_timeout_policy_dump_next */
     NULL,                       /* ct_timeout_policy_dump_done */
     dpif_netdev_ct_get_timeout_policy_name,
+    NULL,                       /* ct_get_features */
     dpif_netdev_ipf_set_enabled,
     dpif_netdev_ipf_set_min_frag,
     dpif_netdev_ipf_set_max_nfrags,

lib/dpif-netlink.c

@@ -3165,6 +3165,20 @@ dpif_netlink_ct_get_timeout_policy_name(struct dpif *dpif OVS_UNUSED,
     return 0;
 }
 
+static int
+dpif_netlink_ct_get_features(struct dpif *dpif OVS_UNUSED,
+                             enum ct_features *features)
+{
+    if (features != NULL) {
+#ifndef _WIN32
+        *features = CONNTRACK_F_ZERO_SNAT;
+#else
+        *features = 0;
+#endif
+    }
+    return 0;
+}
+
 #define CT_DPIF_NL_TP_TCP_MAPPINGS                              \
     CT_DPIF_NL_TP_MAPPING(TCP, TCP, SYN_SENT, SYN_SENT)         \
     CT_DPIF_NL_TP_MAPPING(TCP, TCP, SYN_RECV, SYN_RECV)         \
@@ -4007,6 +4021,7 @@ const struct dpif_class dpif_netlink_class = {
     dpif_netlink_ct_timeout_policy_dump_next,
     dpif_netlink_ct_timeout_policy_dump_done,
     dpif_netlink_ct_get_timeout_policy_name,
+    dpif_netlink_ct_get_features,
     NULL,                       /* ipf_set_enabled */
     NULL,                       /* ipf_set_min_frag */
     NULL,                       /* ipf_set_max_nfrags */

lib/dpif-provider.h

@@ -81,6 +81,7 @@ struct ct_dpif_dump_state;
 struct ct_dpif_entry;
 struct ct_dpif_tuple;
 struct ct_dpif_timeout_policy;
+enum ct_features;
 
 /* 'dpif_ipf_proto_status' and 'dpif_ipf_status' are presently in
  * sync with 'ipf_proto_status' and 'ipf_status', but more
@@ -562,6 +563,10 @@ struct dpif_class {
                                       uint16_t dl_type, uint8_t nw_proto,
                                       char **tp_name, bool *is_generic);
 
+    /* Stores the conntrack features supported by 'dpif' into features.
+     * The value is a bitmap of CONNTRACK_F_* bits. */
+    int (*ct_get_features)(struct dpif *, enum ct_features *features);
+
     /* IP Fragmentation. */
 
     /* Disables or enables conntrack fragment reassembly.  The default

lib/ovs-actions.xml

@@ -2132,6 +2132,16 @@ 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

ofproto/ofproto-dpif.c

@@ -1389,6 +1389,24 @@ check_ct_timeout_policy(struct dpif_backer *backer)
     return !error;
 }
 
+/* Tests whether 'backer''s datapath supports the all-zero SNAT case. */
+static bool
+dpif_supports_ct_zero_snat(struct dpif_backer *backer)
+{
+    enum ct_features features;
+    bool supported = false;
+
+    if (!ct_dpif_get_features(backer->dpif, &features)) {
+        if (features & CONNTRACK_F_ZERO_SNAT) {
+            supported = true;
+        }
+    }
+    VLOG_INFO("%s: Datapath %s ct_zero_snat",
+              dpif_name(backer->dpif), (supported) ? "supports"
+                                                   : "does not support");
+    return supported;
+}
+
 /* Tests whether 'backer''s datapath supports the
  * OVS_ACTION_ATTR_CHECK_PKT_LEN action. */
 static bool
@@ -1590,6 +1608,7 @@ check_support(struct dpif_backer *backer)
         dpif_supports_explicit_drop_action(backer->dpif);
     backer->rt_support.lb_output_action =
         dpif_supports_lb_output_action(backer->dpif);
+    backer->rt_support.ct_zero_snat = dpif_supports_ct_zero_snat(backer);
 
     /* Flow fields. */
     backer->rt_support.odp.ct_state = check_ct_state(backer);
@@ -5605,6 +5624,7 @@ get_datapath_cap(const char *datapath_type, struct smap *cap)
     smap_add(cap, "explicit_drop_action",
              s.explicit_drop_action ? "true" :"false");
     smap_add(cap, "lb_output_action", s.lb_output_action ? "true" : "false");
+    smap_add(cap, "ct_zero_snat", s.ct_zero_snat ? "true" : "false");
 }
 
 /* Gets timeout policy name in 'backer' based on 'zone', 'dl_type' and

ofproto/ofproto-dpif.h

@@ -204,7 +204,10 @@ struct group_dpif *group_dpif_lookup(struct ofproto_dpif *,
     DPIF_SUPPORT_FIELD(bool, explicit_drop_action, "Explicit Drop action")  \
                                                                             \
     /* True if the datapath supports balance_tcp optimization */            \
-    DPIF_SUPPORT_FIELD(bool, lb_output_action, "Optimized Balance TCP mode")
+    DPIF_SUPPORT_FIELD(bool, lb_output_action, "Optimized Balance TCP mode")\
+                                                                            \
+    /* True if the datapath supports all-zero IP SNAT. */                   \
+    DPIF_SUPPORT_FIELD(bool, ct_zero_snat, "Conntrack all-zero IP SNAT")
 
 
 /* Stores the various features which the corresponding backer supports. */

tests/system-kmod-macros.at

@@ -99,6 +99,17 @@ m4_define([CHECK_CONNTRACK_FRAG_OVERLAP],
 #
 m4_define([CHECK_CONNTRACK_NAT])
 
+# CHECK_CONNTRACK_ZEROIP_SNAT()
+#
+# Perform requirements checks for running conntrack all-zero IP SNAT tests.
+# The kernel always supports all-zero IP SNAT, so no check is needed.
+# However, the Windows datapath using the same netlink interface does not.
+#
+m4_define([CHECK_CONNTRACK_ZEROIP_SNAT],
+[
+    AT_SKIP_IF([test "$IS_WIN32" = "yes"])
+])
+
 # CHECK_CONNTRACK_TIMEOUT()
 #
 # Perform requirements checks for running conntrack customized timeout tests.

tests/system-traffic.at

@@ -4487,6 +4487,52 @@ 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 - all-zero IP SNAT])
+AT_SKIP_IF([test $HAVE_NC = no])
+CHECK_CONNTRACK()
+CHECK_CONNTRACK_ZEROIP_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_ZEROIP_SNAT()
+#
+# Perform requirements checks for running conntrack all-zero IP SNAT tests.
+# The userspace datapath does not support all-zero IP SNAT.
+#
+m4_define([CHECK_CONNTRACK_ZEROIP_SNAT],
+[
+    AT_SKIP_IF([:])
+])
+
 # CHECK_CONNTRACK_TIMEOUT()
 #
 # Perform requirements checks for running conntrack customized timeout tests.

vswitchd/vswitch.xml

@@ -6181,6 +6181,15 @@ ovs-vsctl add-port br0 p0 -- set Interface p0 type=patch options:peer=p1 \
         True if the datapath supports OVS_ACTION_ATTR_DROP.  If false,
         explicit drop action will not be sent to the datapath.
       </column>
+      <column name="capabilities" key="ct_zero_snat"
+              type='{"type": "boolean"}'>
+        True if the datapath supports all-zero SNAT. This is a special case
+        if 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.
+      </column>
     </group>
 
     <group title="Common Columns">