Hello everyone,
I am trying to figure out how to receive broadcast messages in VPP (vpp
version 19.08 in case that matters).
This is in the context of some changes we are considering in the VPP
NAT HA functionality. That code in e.g. plugins/nat/nat_ha.c uses UDP
messages to communicate information about NAT sessions between
different VPP servers. It is currently using unicast messages, but we
are considering the possibility of using broadcast messages instead,
hoping that could be more efficient in case there are more than two
servers involved. For example, when a new NAT session has been created,
we could send a broadcast message about the new session, that would
reach several other VPP servers, without need to send a separate
unicast message to each server.
The code in plugins/nat/nat_ha.c calls udp_register_dst_port() to
register that it wants to receive UDP traffic, like this:
udp_register_dst_port (ha->vlib_main, port,
nat_ha_handoff_node.index, 1);
This works fine for unicast messages; when such packets arrive at the
given port, they get handled by the nat_ha_handoff_node as desired.
However, if broadcast packets arrive, those packets are dropped
instead, they do not arrive at the nat_ha_handoff_node.
For example, if the IP address of the relevant interface on the
receiving side is 10.10.50.1/24 then unicast UDP messages with
destination 10.10.50.1 are handled fine. However, if the destination is
10.10.50.255 (the broadcast address for that /24 subnet) then the
packets are dropped. Here is an example of a packet trace when such a
packet is received from 10.10.50.2:
02:41:19:250212: rdma-input
rdma: Interface101 (3) next-node bond-input
02:41:19:250214: bond-input
src 02:fe:ff:76:e4:5d, dst ff:ff:ff:ff:ff:ff, Interface101 ->
BondEthernet0
02:41:19:250214: ethernet-input
IP4: 02:fe:ff:76:e4:5d -> ff:ff:ff:ff:ff:ff 802.1q vlan 1015
02:41:19:250215: ip4-input
UDP: 10.10.50.2 -> 10.10.50.255
tos 0x80, ttl 254, length 92, checksum 0x02fa
fragment id 0x0002, flags DONT_FRAGMENT
UDP: 1234 -> 2345
length 72, checksum 0x0000
02:41:19:250216: ip4-lookup
fib 0 dpo-idx 0 flow hash: 0x00000000
UDP: 10.10.50.2 -> 10.10.50.255
tos 0x80, ttl 254, length 92, checksum 0x02fa
fragment id 0x0002, flags DONT_FRAGMENT
UDP: 1234 -> 2345
length 72, checksum 0x0000
02:41:19:250217: ip4-drop
UDP: 10.10.50.2 -> 10.10.50.255
tos 0x80, ttl 254, length 92, checksum 0x02fa
fragment id 0x0002, flags DONT_FRAGMENT
UDP: 1234 -> 2345
length 72, checksum 0x0000
02:41:19:250217: error-drop
rx:BondEthernet0.1015
02:41:19:250217: drop
ethernet-input: no error
So the packet ends up at ip4-drop when I would have liked it to come to
nat_ha_handoff_node.
Does anyone have a suggestion about how to make this work?
Is some special configuration of the receiving interface needed to tell
VPP that we want it to receive broadcast packets?
Best regards,
Elias
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