vrf: Reset skb conntrack connection on VRF rcv

iptables -t nat -A POSTROUTING -p udp -m udp --dport 53 -j SNAT --to-source 2.2.2.2 -o vrf_1

To fix the "reverse-NAT" for replies.

When a packet is sent over a VRF, the POST_ROUTING hooks are called
twice: Once from the VRF interface, and once from the "actual" interface the packet will be sent from:
1) First SNAT: l3mdev_l3_out() -> vrf_l3_out() -> .. -> vrf_output_direct() This causes the POST_ROUTING hooks to run.
2) Second SNAT: 'ip_output()' calls POST_ROUTING hooks again.

Similarly for replies, first ip_rcv() calls PRE_ROUTING hooks, and second vrf_l3_rcv() calls them again.

As an example, consider the following SNAT rule:

Quoted 1 line

iptables -t nat -A POSTROUTING -p udp -m udp --dport 53 -j SNAT --to-source 2.2.2.2 -o vrf_1

In this case sending over a VRF will create 2 conntrack entries. The first is from the VRF interface, which performs the IP SNAT. The second will run the SNAT, but since the "expected reply" will remain the same, conntrack randomizes the source port of the packet:
e..g With a socket bound to 1.1.1.1:10000, sending to 3.3.3.3:53, the conntrack rules are:
udp 17 29 src=2.2.2.2 dst=3.3.3.3 sport=10000 dport=53 packets=1 bytes=68 [UNREPLIED] src=3.3.3.3 dst=2.2.2.2 sport=53 dport=61033 packets=0 bytes=0 mark=0 use=1
udp 17 29 src=1.1.1.1 dst=3.3.3.3 sport=10000 dport=53 packets=1 bytes=68 [UNREPLIED] src=3.3.3.3 dst=2.2.2.2 sport=53 dport=10000 packets=0 bytes=0 mark=0 use=1

i.e. First SNAT IP from 1.1.1.1 --> 2.2.2.2, and second the src port is SNAT-ed from 10000 --> 61033.

But when a reply is sent (3.3.3.3:53 -> 2.2.2.2:61033) only the later conntrack entry is matched:
udp 17 29 src=2.2.2.2 dst=3.3.3.3 sport=10000 dport=53 packets=1 bytes=68 src=3.3.3.3 dst=2.2.2.2 sport=53 dport=61033 packets=1 bytes=49 mark=0 use=1
udp 17 28 src=1.1.1.1 dst=3.3.3.3 sport=10000 dport=53 packets=1 bytes=68 [UNREPLIED] src=3.3.3.3 dst=2.2.2.2 sport=53 dport=10000 packets=0 bytes=0 mark=0 use=1

And a "port 61033 unreachable" ICMP packet is sent back.

The issue is that when PRE_ROUTING hooks are called from vrf_l3_rcv(), the skb already has a conntrack flow attached to it, which means nf_conntrack_in() will not resolve the flow again.

This means only the dest port is "reverse-NATed" (61033 -> 10000) but the dest IP remains 2.2.2.2, and since the socket is bound to 1.1.1.1 it's not received.
This can be verified by logging the 4-tuple of the packet in '__udp4_lib_rcv()'.

The fix is then to reset the flow when skb is received on a VRF, to let conntrack resolve the flow again (which now will hit the earlier flow).

To reproduce: (Without the fix "Got pkt_to_nat_port" will not be printed by
running 'bash ./repro'):

  $ cat run_in_A1.py
  import logging
  logging.getLogger("scapy.runtime").setLevel(logging.ERROR)
  from scapy.all import *
  import argparse

  def get_packet_to_send(udp_dst_port, msg_name):
      return Ether(src='11:22:33:44:55:66', dst=iface_mac)/ \
          IP(src='3.3.3.3', dst='2.2.2.2')/ \
          UDP(sport=53, dport=udp_dst_port)/ \
          Raw(f'{msg_name}\x0012345678901234567890')

parser = argparse.ArgumentParser() parser.add_argument('-iface_mac', dest="iface_mac", type=str, required=True,

                      help="From run_in_A3.py")

parser.add_argument('-socket_port', dest="socket_port", type=str,
required=True, help="From run_in_A3.py")
parser.add_argument('-v1_mac', dest="v1_mac", type=str, required=True,
help="From script")

args, _ = parser.parse_known_args()
iface_mac = args.iface_mac
socket_port = int(args.socket_port)
v1_mac = args.v1_mac

print(f'Source port before NAT: {socket_port}')

while True:
pkts = sniff(iface='_v0', store=True, count=1, timeout=10) if 0 == len(pkts):
print('Something failed, rerun the script :(', flush=True) break
pkt = pkts[0]
if not pkt.haslayer('UDP'):
continue

pkt_sport = pkt.getlayer('UDP').sport print(f'Source port after NAT: {pkt_sport}', flush=True)

pkt_to_send = get_packet_to_send(pkt_sport, 'pkt_to_nat_port') sendp(pkt_to_send, '_v0', verbose=False) # Will not be received

pkt_to_send = get_packet_to_send(socket_port, 'pkt_to_socket_port') sendp(pkt_to_send, '_v0', verbose=False)
break

  $ cat run_in_A2.py
  import socket
  import netifaces

print(f"{netifaces.ifaddresses('e00000')[netifaces.AF_LINK][0]['addr']}",
flush=True)
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE,

               str('vrf_1' + '\0').encode('utf-8'))

s.connect(('3.3.3.3', 53))
print(f'{s. getsockname()[1]}', flush=True)
s.settimeout(5)

while True:
try:
# Periodically send in order to keep the conntrack entry alive. s.send(b'a'*40)
resp = s.recvfrom(1024)
msg_name = resp[0].decode('utf-8').split('\0')[0] print(f"Got {msg_name}", flush=True)
except Exception as e:
pass

  $ cat repro.sh
  ip netns del A1 2> /dev/null
  ip netns del A2 2> /dev/null
  ip netns add A1
  ip netns add A2

ip -n A1 link add _v0 type veth peer name _v1 netns A2 ip -n A1 link set _v0 up

ip -n A2 link add e00000 type bond
ip -n A2 link add lo0 type dummy
ip -n A2 link add vrf_1 type vrf table 10001
ip -n A2 link set vrf_1 up
ip -n A2 link set e00000 master vrf_1

ip -n A2 addr add 1.1.1.1/24 dev e00000
ip -n A2 link set e00000 up
ip -n A2 link set _v1 master e00000
ip -n A2 link set _v1 up
ip -n A2 link set lo0 up
ip -n A2 addr add 2.2.2.2/32 dev lo0

ip -n A2 neigh add 1.1.1.10 lladdr 77:77:77:77:77:77 dev e00000 ip -n A2 route add 3.3.3.3/32 via 1.1.1.10 dev e00000 table 10001

  ip netns exec A2 iptables -t nat -A POSTROUTING -p udp -m udp --dport 53 -j \
	SNAT --to-source 2.2.2.2 -o vrf_1

sleep 5
ip netns exec A2 python3 run_in_A2.py > x &
XPID=$!
sleep 5

  IFACE_MAC=`sed -n 1p x`
  SOCKET_PORT=`sed -n 2p x`
  V1_MAC=`ip -n A2 link show _v1 | sed -n 2p | awk '{print $2'}`
  ip netns exec A1 python3 run_in_A1.py -iface_mac ${IFACE_MAC} -socket_port \
          ${SOCKET_PORT} -v1_mac ${SOCKET_PORT}
  sleep 5

kill -9 $XPID
wait $XPID 2> /dev/null
ip netns del A1
ip netns del A2
tail x -n 2
rm x
set +x

Signed-off-by Lahav Schlesinger <[email protected]>

---
 drivers/net/vrf.c | 4 ++++
 1 file changed, 4 insertions(+)

Quoted 1 line · Lahav Schlesinger

To fix the "reverse-NAT" for replies.

Thanks for the detailed explanation and use case.

Looks correct to me.

Reviewed-by David Ahern <[email protected]>

Quoted 7 lines · On Mon, 16 Aug 2021 12:08:00 -0600 David Ahern wrote:

Quoted 1 line · Lahav Schlesinger

To fix the "reverse-NAT" for replies.

Thanks for the detailed explanation and use case.

Looks correct to me.

Reviewed-by David Ahern <[email protected]>

I get a sense this is a fix.

Fixes 73e20b761acf ("net: vrf: Add support for PREROUTING rules on vrf device")

?

Or maybe it fixes commit a0f37efa8225 ("net: vrf: Fix NAT within a VRF")?

Quoted 4 lines

Quoted 2 lines

Looks correct to me.

Reviewed-by David Ahern <[email protected]>

Fixes 73e20b761acf ("net: vrf: Add support for PREROUTING rules on vrf device")

?

This one.

Quoted 2 lines

Or maybe it fixes commit a0f37efa8225 ("net: vrf: Fix NAT within a VRF")?