diff --git a/NEWS b/NEWS
index 95cf922aa..2af3ff6d1 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,9 @@ Post-v2.15.0
    - Userspace datapath:
      * Auto load balancing of PMDs now partially supports cross-NUMA polling
        cases, e.g if all PMD threads are running on the same NUMA node.
+     * Add all-zero IP SNAT handling to conntrack. In case of collision,
+       using ct(src=0.0.0.0), the source port will be replaced with another
+       non-colliding port in the ephemeral range (1024, 65535).
    - ovs-ctl:
      * New option '--no-record-hostname' to disable hostname configuration
        in ovsdb on startup.
diff --git a/lib/conntrack.c b/lib/conntrack.c
index 99198a601..5f17729d4 100644
--- a/lib/conntrack.c
+++ b/lib/conntrack.c
@@ -108,8 +108,8 @@ static void set_label(struct dp_packet *, struct conn *,
 static void *clean_thread_main(void *f_);
 
 static bool
-nat_select_range_tuple(struct conntrack *ct, const struct conn *conn,
-                       struct conn *nat_conn);
+nat_get_unique_tuple(struct conntrack *ct, const struct conn *conn,
+                     struct conn *nat_conn);
 
 static uint8_t
 reverse_icmp_type(uint8_t type);
@@ -728,11 +728,11 @@ pat_packet(struct dp_packet *pkt, const struct conn *conn)
         }
     } else if (conn->nat_info->nat_action & NAT_ACTION_DST) {
         if (conn->key.nw_proto == IPPROTO_TCP) {
-            struct tcp_header *th = dp_packet_l4(pkt);
-            packet_set_tcp_port(pkt, th->tcp_src, conn->rev_key.src.port);
+            packet_set_tcp_port(pkt, conn->rev_key.dst.port,
+                                conn->rev_key.src.port);
         } else if (conn->key.nw_proto == IPPROTO_UDP) {
-            struct udp_header *uh = dp_packet_l4(pkt);
-            packet_set_udp_port(pkt, uh->udp_src, conn->rev_key.src.port);
+            packet_set_udp_port(pkt, conn->rev_key.dst.port,
+                                conn->rev_key.src.port);
         }
     }
 }
@@ -786,11 +786,9 @@ un_pat_packet(struct dp_packet *pkt, const struct conn *conn)
         }
     } else if (conn->nat_info->nat_action & NAT_ACTION_DST) {
         if (conn->key.nw_proto == IPPROTO_TCP) {
-            struct tcp_header *th = dp_packet_l4(pkt);
-            packet_set_tcp_port(pkt, conn->key.dst.port, th->tcp_dst);
+            packet_set_tcp_port(pkt, conn->key.dst.port, conn->key.src.port);
         } else if (conn->key.nw_proto == IPPROTO_UDP) {
-            struct udp_header *uh = dp_packet_l4(pkt);
-            packet_set_udp_port(pkt, conn->key.dst.port, uh->udp_dst);
+            packet_set_udp_port(pkt, conn->key.dst.port, conn->key.src.port);
         }
     }
 }
@@ -810,12 +808,10 @@ reverse_pat_packet(struct dp_packet *pkt, const struct conn *conn)
         }
     } else if (conn->nat_info->nat_action & NAT_ACTION_DST) {
         if (conn->key.nw_proto == IPPROTO_TCP) {
-            struct tcp_header *th_in = dp_packet_l4(pkt);
-            packet_set_tcp_port(pkt, th_in->tcp_src,
+            packet_set_tcp_port(pkt, conn->key.src.port,
                                 conn->key.dst.port);
         } else if (conn->key.nw_proto == IPPROTO_UDP) {
-            struct udp_header *uh_in = dp_packet_l4(pkt);
-            packet_set_udp_port(pkt, uh_in->udp_src,
+            packet_set_udp_port(pkt, conn->key.src.port,
                                 conn->key.dst.port);
         }
     }
@@ -1029,14 +1025,14 @@ conn_not_found(struct conntrack *ct, struct dp_packet *pkt,
                 }
             } else {
                 memcpy(nat_conn, nc, sizeof *nat_conn);
-                bool nat_res = nat_select_range_tuple(ct, nc, nat_conn);
+                bool nat_res = nat_get_unique_tuple(ct, nc, nat_conn);
 
                 if (!nat_res) {
                     goto nat_res_exhaustion;
                 }
 
                 /* Update nc with nat adjustments made to nat_conn by
-                 * nat_select_range_tuple(). */
+                 * nat_get_unique_tuple(). */
                 memcpy(nc, nat_conn, sizeof *nc);
             }
 
@@ -2210,130 +2206,222 @@ nat_range_hash(const struct conn *conn, uint32_t basis)
     return hash_finish(hash, 0);
 }
 
-static bool
-nat_select_range_tuple(struct conntrack *ct, const struct conn *conn,
-                       struct conn *nat_conn)
-{
-    enum { MIN_NAT_EPHEMERAL_PORT = 1024,
-           MAX_NAT_EPHEMERAL_PORT = 65535 };
-
-    uint16_t min_port;
-    uint16_t max_port;
-    uint16_t first_port;
-    uint32_t hash = nat_range_hash(conn, ct->hash_basis);
+/* Ports are stored in host byte order for convenience. */
+static void
+set_sport_range(struct nat_action_info_t *ni, const struct conn_key *k,
+                uint32_t hash, uint16_t *curr, uint16_t *min,
+                uint16_t *max)
+{
+    if (((ni->nat_action & NAT_ACTION_SNAT_ALL) == NAT_ACTION_SRC) ||
+        ((ni->nat_action & NAT_ACTION_DST))) {
+        *curr = ntohs(k->src.port);
+        *min = MIN_NAT_EPHEMERAL_PORT;
+        *max = MAX_NAT_EPHEMERAL_PORT;
+    } else {
+        *min = ni->min_port;
+        *max = ni->max_port;
+        *curr = *min + (hash % ((*max - *min) + 1));
+    }
+}
 
-    if ((conn->nat_info->nat_action & NAT_ACTION_SRC) &&
-        (!(conn->nat_info->nat_action & NAT_ACTION_SRC_PORT))) {
-        min_port = ntohs(conn->key.src.port);
-        max_port = ntohs(conn->key.src.port);
-        first_port = min_port;
-    } else if ((conn->nat_info->nat_action & NAT_ACTION_DST) &&
-               (!(conn->nat_info->nat_action & NAT_ACTION_DST_PORT))) {
-        min_port = ntohs(conn->key.dst.port);
-        max_port = ntohs(conn->key.dst.port);
-        first_port = min_port;
+static void
+set_dport_range(struct nat_action_info_t *ni, const struct conn_key *k,
+                uint32_t hash, uint16_t *curr, uint16_t *min,
+                uint16_t *max)
+{
+    if (ni->nat_action & NAT_ACTION_DST_PORT) {
+        *min = ni->min_port;
+        *max = ni->max_port;
+        *curr = *min + (hash % ((*max - *min) + 1));
     } else {
-        uint16_t deltap = conn->nat_info->max_port - conn->nat_info->min_port;
-        uint32_t port_index = hash % (deltap + 1);
-        first_port = conn->nat_info->min_port + port_index;
-        min_port = conn->nat_info->min_port;
-        max_port = conn->nat_info->max_port;
+        *curr = ntohs(k->dst.port);
+        *min = *max = *curr;
     }
+}
 
-    uint32_t deltaa = 0;
-    uint32_t address_index;
-    union ct_addr ct_addr;
-    memset(&ct_addr, 0, sizeof ct_addr);
-    union ct_addr max_ct_addr;
-    memset(&max_ct_addr, 0, sizeof max_ct_addr);
-    max_ct_addr = conn->nat_info->max_addr;
+/* Gets the initial in range address based on the hash.
+ * Addresses are kept in network order. */
+static void
+get_addr_in_range(union ct_addr *min, union ct_addr *max,
+                  union ct_addr *curr, uint32_t hash,
+                  bool ipv4)
+{
+    uint32_t offt, range;
 
-    if (conn->key.dl_type == htons(ETH_TYPE_IP)) {
-        deltaa = ntohl(conn->nat_info->max_addr.ipv4) -
-                 ntohl(conn->nat_info->min_addr.ipv4);
-        address_index = hash % (deltaa + 1);
-        ct_addr.ipv4 = htonl(
-            ntohl(conn->nat_info->min_addr.ipv4) + address_index);
+    if (ipv4) {
+        range = (ntohl(max->ipv4) - ntohl(min->ipv4)) + 1;
+        offt = hash % range;
+        curr->ipv4 = htonl(ntohl(min->ipv4) + offt);
     } else {
-        deltaa = nat_ipv6_addrs_delta(&conn->nat_info->min_addr.ipv6,
-                                      &conn->nat_info->max_addr.ipv6);
-        /* deltaa must be within 32 bits for full hash coverage. A 64 or
+        range = nat_ipv6_addrs_delta(&min->ipv6,
+                                     &max->ipv6) + 1;
+        /* range must be within 32 bits for full hash coverage. A 64 or
          * 128 bit hash is unnecessary and hence not used here. Most code
          * is kept common with V4; nat_ipv6_addrs_delta() will do the
          * enforcement via max_ct_addr. */
-        max_ct_addr = conn->nat_info->min_addr;
-        nat_ipv6_addr_increment(&max_ct_addr.ipv6, deltaa);
-        address_index = hash % (deltaa + 1);
-        ct_addr.ipv6 = conn->nat_info->min_addr.ipv6;
-        nat_ipv6_addr_increment(&ct_addr.ipv6, address_index);
-    }
-
-    uint16_t port = first_port;
-    bool all_ports_tried = false;
-    /* For DNAT or for specified port ranges, we don't use ephemeral ports. */
-    bool ephemeral_ports_tried
-        = conn->nat_info->nat_action & NAT_ACTION_DST ||
-              conn->nat_info->nat_action & NAT_ACTION_SRC_PORT
-          ? true : false;
-    union ct_addr first_addr = ct_addr;
-    bool pat_enabled = conn->key.nw_proto == IPPROTO_TCP ||
-                       conn->key.nw_proto == IPPROTO_UDP;
-
-    while (true) {
+        offt = hash % range;
+        curr->ipv6 = min->ipv6;
+        nat_ipv6_addr_increment(&curr->ipv6, offt);
+    }
+}
+
+static void
+get_initial_addr(const struct conn *conn, union ct_addr *min,
+                 union ct_addr *max, union ct_addr *curr,
+                 uint32_t hash, bool ipv4)
+{
+    const union ct_addr zero_ip = {0};
+
+    /* all-zero CASE */
+    if (!memcmp(min, &zero_ip, sizeof(*min))) {
         if (conn->nat_info->nat_action & NAT_ACTION_SRC) {
-            nat_conn->rev_key.dst.addr = ct_addr;
-            if (pat_enabled) {
-                nat_conn->rev_key.dst.port = htons(port);
-            }
-        } else {
-            nat_conn->rev_key.src.addr = ct_addr;
-            if (pat_enabled) {
-                nat_conn->rev_key.src.port = htons(port);
-            }
+            *curr = conn->key.src.addr;
+        } else if (conn->nat_info->nat_action & NAT_ACTION_DST) {
+            *curr = conn->key.dst.addr;
+        }
+    } else {
+        get_addr_in_range(min, max, curr, hash, ipv4);
+    }
+}
+
+static void
+store_addr_to_key(union ct_addr *addr, struct conn_key *key,
+                  uint16_t action)
+{
+    if (action & NAT_ACTION_SRC) {
+        key->dst.addr = *addr;
+    } else {
+        key->src.addr = *addr;
+    }
+}
+
+static void
+next_addr_in_range(union ct_addr *curr, union ct_addr *min,
+                   union ct_addr *max, bool ipv4)
+{
+    if (ipv4) {
+        /* this check could be unified with IPv6, but let's avoid
+         * an unneeded memcmp() in case of IPv4. */
+        if (min->ipv4 == max->ipv4) {
+            return;
+        }
+
+        curr->ipv4 = (curr->ipv4 == max->ipv4) ?
+                      min->ipv4 :
+                      htonl(ntohl(curr->ipv4) + 1);
+    } else {
+        if (!memcmp(min, max, sizeof(*min))) {
+            return;
+        }
+
+        if (!memcmp(curr, max, sizeof(*curr))) {
+            *curr = *min;
+            return;
         }
 
-        bool found = conn_lookup(ct, &nat_conn->rev_key, time_msec(), NULL,
-                                 NULL);
-        if (!found) {
+        nat_ipv6_addr_increment(&curr->ipv6, 1);
+    }
+}
+
+static bool
+next_addr_in_range_guarded(union ct_addr *curr, union ct_addr *min,
+                           union ct_addr *max, union ct_addr *guard,
+                           bool ipv4)
+{
+    bool exhausted;
+
+    next_addr_in_range(curr, min, max, ipv4);
+
+    if (ipv4) {
+        exhausted = (curr->ipv4 == guard->ipv4);
+    } else {
+        exhausted = !memcmp(curr, guard, sizeof(*curr));
+    }
+
+    return exhausted;
+}
+
+/* This function tries to get a unique tuple.
+ * Every iteration checks that the reverse tuple doesn't
+ * collide with any existing one.
+ *
+ * in case of SNAT:
+ *    - for each src IP address in the range (if any)
+ *        - try to find a source port in range (if any)
+ *        - if no port range exists, use the whole
+ *          ephemeral range (after testing the port
+ *          used by the sender), otherwise use the
+ *          specified range
+ *
+ * in case of DNAT:
+ *    - for each dst IP address in the range (if any)
+ *        - for each dport in range (if any)
+ *             - try to find a source port in the ephemeral range
+ *               (after testing the port used by the sender)
+ *
+ * If none can be found, return exhaustion to the caller. */
+static bool
+nat_get_unique_tuple(struct conntrack *ct, const struct conn *conn,
+                     struct conn *nat_conn)
+{
+    union ct_addr min_addr = {0}, max_addr = {0}, curr_addr = {0},
+                  guard_addr = {0};
+    uint32_t hash = nat_range_hash(conn, ct->hash_basis);
+    bool pat_proto = conn->key.nw_proto == IPPROTO_TCP ||
+                     conn->key.nw_proto == IPPROTO_UDP;
+    uint16_t min_dport, max_dport, curr_dport;
+    uint16_t min_sport, max_sport, curr_sport;
+
+    min_addr = conn->nat_info->min_addr;
+    max_addr = conn->nat_info->max_addr;
+
+    get_initial_addr(conn, &min_addr, &max_addr, &curr_addr, hash,
+                     (conn->key.dl_type == htons(ETH_TYPE_IP)));
+
+    /* save the address we started from so that
+     * we can stop once we reach it. */
+    guard_addr = curr_addr;
+
+    set_sport_range(conn->nat_info, &conn->key, hash, &curr_sport,
+                    &min_sport, &max_sport);
+    set_dport_range(conn->nat_info, &conn->key, hash, &curr_dport,
+                    &min_dport, &max_dport);
+
+another_round:
+    store_addr_to_key(&curr_addr, &nat_conn->rev_key,
+                      conn->nat_info->nat_action);
+
+    if (!pat_proto) {
+        if (!conn_lookup(ct, &nat_conn->rev_key,
+                         time_msec(), NULL, NULL)) {
             return true;
-        } else if (pat_enabled && !all_ports_tried) {
-            if (min_port == max_port) {
-                all_ports_tried = true;
-            } else if (port == max_port) {
-                port = min_port;
-            } else {
-                port++;
-            }
-            if (port == first_port) {
-                all_ports_tried = true;
-            }
-        } else {
-            if (memcmp(&ct_addr, &max_ct_addr, sizeof ct_addr)) {
-                if (conn->key.dl_type == htons(ETH_TYPE_IP)) {
-                    ct_addr.ipv4 = htonl(ntohl(ct_addr.ipv4) + 1);
-                } else {
-                    nat_ipv6_addr_increment(&ct_addr.ipv6, 1);
-                }
-            } else {
-                ct_addr = conn->nat_info->min_addr;
-            }
-            if (!memcmp(&ct_addr, &first_addr, sizeof ct_addr)) {
-                if (pat_enabled && !ephemeral_ports_tried) {
-                    ephemeral_ports_tried = true;
-                    ct_addr = conn->nat_info->min_addr;
-                    first_addr = ct_addr;
-                    min_port = MIN_NAT_EPHEMERAL_PORT;
-                    max_port = MAX_NAT_EPHEMERAL_PORT;
-                } else {
-                    break;
-                }
+        }
+
+        goto next_addr;
+    }
+
+    int i, j, s_attempts, d_attempts;
+    FOR_EACH_PORT_IN_RANGE(i, d_attempts, curr_dport, min_dport, max_dport) {
+        nat_conn->rev_key.src.port = htons(curr_dport);
+        FOR_EACH_PORT_IN_RANGE(j, s_attempts, curr_sport, min_sport, max_sport) {
+            nat_conn->rev_key.dst.port = htons(curr_sport);
+            if (!conn_lookup(ct, &nat_conn->rev_key,
+                             time_msec(), NULL, NULL)) {
+                return true;
             }
-            first_port = min_port;
-            port = first_port;
-            all_ports_tried = false;
         }
     }
-    return false;
+
+    /* Check if next IP is in range and respin. Otherwise, notify
+     * exhaustion to the caller. */
+next_addr:
+    if (next_addr_in_range_guarded(&curr_addr, &min_addr,
+                                   &max_addr, &guard_addr,
+                                   conn->key.dl_type == htons(ETH_TYPE_IP))) {
+        return false;
+    }
+
+    goto another_round;
 }
 
 static enum ct_update_res
diff --git a/lib/conntrack.h b/lib/conntrack.h
index 9553b188a..c68a83ccd 100644
--- a/lib/conntrack.h
+++ b/lib/conntrack.h
@@ -77,6 +77,14 @@ enum nat_action_e {
     NAT_ACTION_DST_PORT = 1 << 3,
 };
 
+#define NAT_ACTION_SNAT_ALL (NAT_ACTION_SRC | NAT_ACTION_SRC_PORT)
+#define NAT_ACTION_DNAT_ALL (NAT_ACTION_DST | NAT_ACTION_DST_PORT)
+
+enum {
+    MIN_NAT_EPHEMERAL_PORT = 1024,
+    MAX_NAT_EPHEMERAL_PORT = 65535
+};
+
 struct nat_action_info_t {
     union ct_addr min_addr;
     union ct_addr max_addr;
@@ -85,6 +93,28 @@ struct nat_action_info_t {
     uint16_t nat_action;
 };
 
+#define IN_RANGE(curr, min, max) \
+    (curr >= min && curr <= max)
+
+#define NEXT_PORT_IN_RANGE(curr, min, max) \
+    curr = (!IN_RANGE(curr, min, max) || curr == max) ? min : curr + 1
+
+/* if the current port is out of range increase the attempts by
+ * one so that in the worst case scenario the current out of
+ * range port plus all the in-range ports get tested.
+ * Note that curr can be an out of range port only in case of
+ * source port (SNAT with port range unspecified or DNAT),
+ * furthermore the source port in the packet has to be less than
+ * MIN_NAT_EPHEMERAL_PORT. */
+#define INIT_ATT(att, curr, min, max) \
+    att = (!IN_RANGE(curr, min, max)) ? (max - min) + 2 : (max - min) + 1
+
+/* loose in-range check, the first curr port can be any port out of
+ * the range. */
+#define FOR_EACH_PORT_IN_RANGE(idx, att, curr, min, max) \
+    for (idx = 0, INIT_ATT(att, curr, min, max); idx < att; idx++, \
+             NEXT_PORT_IN_RANGE(curr, min, max))
+
 struct conntrack *conntrack_init(void);
 void conntrack_destroy(struct conntrack *);
 
diff --git a/lib/ovs-actions.xml b/lib/ovs-actions.xml
index a0070e6c6..e815d5284 100644
--- a/lib/ovs-actions.xml
+++ b/lib/ovs-actions.xml
@@ -1839,8 +1839,7 @@ for <var>i</var> in [1,<var>n_members</var>]:
             <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.
+            is performed.
           </p>
 
           <p>
diff --git a/tests/system-userspace-macros.at b/tests/system-userspace-macros.at
index 71acc8618..a4bfe0c77 100644
--- a/tests/system-userspace-macros.at
+++ b/tests/system-userspace-macros.at
@@ -99,12 +99,9 @@ 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.
+# The userspace datapath always supports NULL SNAT, so no check is needed
 #
-m4_define([CHECK_CONNTRACK_NULL_SNAT],
-[
-    AT_SKIP_IF([:])
-])
+m4_define([CHECK_CONNTRACK_NULL_SNAT])
 
 # CHECK_CONNTRACK_TIMEOUT()
 #
