selftests/net: Add icmp.sh for testing ICMP dummy address responses

This adds a new icmp.sh selftest for testing that the kernel will respond
correctly with an ICMP unreachable message with the dummy (192.0.0.8)
source address when there are no IPv4 addresses configured to use as source
addresses.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/icmp.sh

+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+# Test for checking ICMP response with dummy address instead of 0.0.0.0.
+# Sets up two namespaces like:
+# +----------------------+                          +--------------------+
+# | ns1                  |    v4-via-v6 routes:     | ns2                |
+# |                      |                  '       |                    |
+# |             +--------+   -> 172.16.1.0/24 ->    +--------+           |
+# |             | veth0  +--------------------------+  veth0 |           |
+# |             +--------+   <- 172.16.0.0/24 <-    +--------+           |
+# |           172.16.0.1 |                          | 2001:db8:1::2/64   |
+# |     2001:db8:1::2/64 |                          |                    |
+# +----------------------+                          +--------------------+
+#
+# And then tries to ping 172.16.1.1 from ns1. This results in a "net
+# unreachable" message being sent from ns2, but there is no IPv4 address set in
+# that address space, so the kernel should substitute the dummy address
+# 192.0.0.8 defined in RFC7600.
+
+NS1=ns1
+NS2=ns2
+H1_IP=172.16.0.1/32
+H1_IP6=2001:db8:1::1
+RT1=172.16.1.0/24
+PINGADDR=172.16.1.1
+RT2=172.16.0.0/24
+H2_IP6=2001:db8:1::2
+
+TMPFILE=$(mktemp)
+
+cleanup()
+{
+    rm -f "$TMPFILE"
+    ip netns del $NS1
+    ip netns del $NS2
+}
+
+trap cleanup EXIT
+
+# Namespaces
+ip netns add $NS1
+ip netns add $NS2
+
+# Connectivity
+ip -netns $NS1 link add veth0 type veth peer name veth0 netns $NS2
+ip -netns $NS1 link set dev veth0 up
+ip -netns $NS2 link set dev veth0 up
+ip -netns $NS1 addr add $H1_IP dev veth0
+ip -netns $NS1 addr add $H1_IP6/64 dev veth0 nodad
+ip -netns $NS2 addr add $H2_IP6/64 dev veth0 nodad
+ip -netns $NS1 route add $RT1 via inet6 $H2_IP6
+ip -netns $NS2 route add $RT2 via inet6 $H1_IP6
+
+# Make sure ns2 will respond with ICMP unreachable
+ip netns exec $NS2 sysctl -qw net.ipv4.icmp_ratelimit=0 net.ipv4.ip_forward=1
+
+# Run the test - a ping runs in the background, and we capture ICMP responses
+# with tcpdump; -c 1 means it should exit on the first ping, but add a timeout
+# in case something goes wrong
+ip netns exec $NS1 ping -w 3 -i 0.5 $PINGADDR >/dev/null &
+ip netns exec $NS1 timeout 10 tcpdump -tpni veth0 -c 1 'icmp and icmp[icmptype] != icmp-echo' > $TMPFILE 2>/dev/null
+
+# Parse response and check for dummy address
+# tcpdump output looks like:
+# IP 192.0.0.8 > 172.16.0.1: ICMP net 172.16.1.1 unreachable, length 92
+RESP_IP=$(awk '{print $2}' < $TMPFILE)
+if [[ "$RESP_IP" != "192.0.0.8" ]]; then
+    echo "FAIL - got ICMP response from $RESP_IP, should be 192.0.0.8"
+    exit 1
+else
+    echo "OK"
+    exit 0
+fi