selftests: nft_nat: switch port shadow test cases to socat

There are now at least three distinct flavours of netcat/nc tool:
'original' version, one version ported from openbsd and nmap-ncat.

The script only works with original because it sets SOREUSEPORT option.

Other nc versions return 'port already in use' error and port shadow test fails:

PASS: inet IPv6 redirection for ns2-hMHcaRvx
nc: bind failed: Address already in use
ERROR: portshadow test default: got reply from "ROUTER", not CLIENT as intended

Switch to socat instead.

Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

tools/testing/selftests/netfilter/nft_nat.sh

···
 	local logmsg=""
 
 	# make shadow entry, from client (ns2), going to (ns1), port 41404, sport 1405.
-	echo "fake-entry" | ip netns exec "$ns2" nc -w 1 -p 1405 -u "$daddrc" 41404 > /dev/null
+	echo "fake-entry" | ip netns exec "$ns2" timeout 1 socat -u STDIN UDP:"$daddrc":41404,sourceport=1405
 
-	echo ROUTER | ip netns exec "$ns0" nc -w 5 -u -l -p 1405 >/dev/null 2>&1 &
-	nc_r=$!
+	echo ROUTER | ip netns exec "$ns0" timeout 5 socat -u STDIN UDP4-LISTEN:1405 &
+	sc_r=$!
 
-	echo CLIENT | ip netns exec "$ns2" nc -w 5 -u -l -p 1405 >/dev/null 2>&1 &
-	nc_c=$!
+	echo CLIENT | ip netns exec "$ns2" timeout 5 socat -u STDIN UDP4-LISTEN:1405,reuseport &
+	sc_c=$!
 
 	sleep 0.3
 
 	# ns1 tries to connect to ns0:1405.  With default settings this should connect
 	# to client, it matches the conntrack entry created above.
 
-	result=$(echo "" | ip netns exec "$ns1" nc -w 1 -p 41404 -u "$daddrs" 1405)
+	result=$(echo "data" | ip netns exec "$ns1" timeout 1 socat - UDP:"$daddrs":1405,sourceport=41404)
 
 	if [ "$result" = "$expect" ] ;then
 		echo "PASS: portshadow test $test: got reply from ${expect}${logmsg}"
···
 		ret=1
 	fi
 
-	kill $nc_r $nc_c 2>/dev/null
+	kill $sc_r $sc_c 2>/dev/null
 
 	# flush udp entries for next test round, if any
 	ip netns exec "$ns0" conntrack -F >/dev/null 2>&1
···
 test_port_shadowing()
 {
 	local family="ip"
+
+	conntrack -h >/dev/null 2>&1
+	if [ $? -ne 0 ];then
+		echo "SKIP: Could not run nat port shadowing test without conntrack tool"
+		return
+	fi
+
+	socat -h > /dev/null 2>&1
+	if [ $? -ne 0 ];then
+		echo "SKIP: Could not run nat port shadowing test without socat tool"
+		return
+	fi
 
 	ip netns exec "$ns0" sysctl net.ipv4.conf.veth0.forwarding=1 > /dev/null
 	ip netns exec "$ns0" sysctl net.ipv4.conf.veth1.forwarding=1 > /dev/null