A /31 point-to-point link has no broadcast address; both addresses in
the range are usable hosts. However, ovn-northd computed a traditional
subnet broadcast for the router port and included it in the priority-100
lr_in_ip_input L3 admission control drop flow (ip4.src == {...}). On a
/31, that "broadcast" address is the peer, so all traffic originating
from the /31 peer was dropped.
Skip the broadcast address in the admission control drop set for /31
networks so that /31 router ports work as expected.
Fixes: 936b640f4934 ("ovn: Implement basic logical L3 routing.")
Assisted-by: Claude Opus 4.8, Cursor
Signed-off-by: Han Zhou <[email protected]>
---
v1 -> v2: rebase on main
Documentation/ref/ovn-logical-flows.7.rst | 2 ++
NEWS | 4 ++++
northd/northd.c | 6 ++++-
tests/ovn-northd.at | 27 +++++++++++++++++++++++
4 files changed, 38 insertions(+), 1 deletion(-)
diff --git a/Documentation/ref/ovn-logical-flows.7.rst
b/Documentation/ref/ovn-logical-flows.7.rst
index 3d9936414f25..61a486a98301 100644
--- a/Documentation/ref/ovn-logical-flows.7.rst
+++ b/Documentation/ref/ovn-logical-flows.7.rst
@@ -2273,6 +2273,8 @@ contains the following flows to implement very basic IP
host functionality.
``REGBIT_EGRESS_LOOPBACK``.
- ``ip4.src`` is the broadcast address of any IP network known to the router.
+ Point-to-point (``/31``, RFC 3021) networks have no broadcast address and
+ are excluded, so that traffic from a ``/31`` peer is not dropped.
- A priority-100 flow parses DHCPv6 replies from IPv6 prefix delegation routers
(``udp.src == 547 && udp.dst == 546``). The ``handle_dhcpv6_reply`` is used
to
diff --git a/NEWS b/NEWS
index d1a6ad27fea8..360c32d4b1f5 100644
--- a/NEWS
+++ b/NEWS
@@ -66,6 +66,10 @@ Post v26.03.0
(type 11) and Parameter Problem (type 12) - generated by an external
router are un-NATed correctly. This makes Path MTU discovery and
traceroute work through stateless NAT.
+ - Logical router ports configured with a /31 (RFC 3021 point-to-point)
+ network are now supported. A /31 has no broadcast address, so the L3
+ admission control no longer drops traffic from the /31 peer as a
+ broadcast source.
OVN v26.03.0 - xxx xx xxxx
--------------------------
diff --git a/northd/northd.c b/northd/northd.c
index 62270893ab41..8f75fd860c5b 100644
--- a/northd/northd.c
+++ b/northd/northd.c
@@ -13146,7 +13146,11 @@ op_put_v4_networks(struct ds *ds, const struct
ovn_port *op, bool add_bcast)
ds_put_cstr(ds, "{");
for (int i = 0; i < op->lrp_networks.n_ipv4_addrs; i++) {
ds_put_format(ds, "%s, ", op->lrp_networks.ipv4_addrs[i].addr_s);
- if (add_bcast) {
+ /* A /31 point-to-point link (RFC 3021) has no broadcast address:
+ * both addresses in the range are usable hosts. Including the
+ * computed "broadcast" here would drop legitimate traffic from the
+ * /31 peer, so skip it for /31 networks. */
+ if (add_bcast && op->lrp_networks.ipv4_addrs[i].plen != 31) {
ds_put_format(ds, "%s, ", op->lrp_networks.ipv4_addrs[i].bcast_s);
}
}
diff --git a/tests/ovn-northd.at b/tests/ovn-northd.at
index f82902c61da5..420328e116d9 100644
--- a/tests/ovn-northd.at
+++ b/tests/ovn-northd.at
@@ -2476,6 +2476,33 @@ OVN_CLEANUP_NORTHD
AT_CLEANUP
])
+OVN_FOR_EACH_NORTHD_NO_HV([
+AT_SETUP([router LRP /31 L3 admission control])
+ovn_start
+
+# A /31 point-to-point link (RFC 3021) has no broadcast address; both
+# addresses of the /31 are usable hosts. The L3 admission control flow must
+# therefore not drop the computed "broadcast" address, otherwise traffic from
+# the /31 peer is dropped.
+check ovn-nbctl lr-add lr
+check ovn-nbctl lrp-add lr lrp31 00:00:00:00:00:01 10.0.0.0/31
+check ovn-nbctl lrp-add lr lrp24 00:00:00:00:00:03 10.0.2.1/24
+
+check ovn-nbctl --wait=sb sync
+ovn-sbctl dump-flows lr > lrflows
+AT_CAPTURE_FILE([lrflows])
+
+# The /31 peer (10.0.0.1) must not appear as a dropped broadcast source;
+# only the wider /24 network keeps its broadcast address.
+AT_CHECK([grep "lr_in_ip_input" lrflows | grep "priority=100" | grep "reg9" |
ovn_strip_lflows], [0], [dnl
+ table=??(lr_in_ip_input ), priority=100 , match=(ip4.src == {10.0.0.0}
&& reg9[[0]] == 0), action=(drop;)
+ table=??(lr_in_ip_input ), priority=100 , match=(ip4.src == {10.0.2.1,
10.0.2.255} && reg9[[0]] == 0), action=(drop;)
+])
+
+OVN_CLEANUP_NORTHD
+AT_CLEANUP
+])
+
# This test case tests that when a logical switch has load balancers associated
# (with VIPs configured), the below logical flow is added by ovn-northd.
# table=ls_out_pre_lb, priority=100, match=(ip), action=(reg0[[0]] = 1; next;)
--
2.38.1
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