Add test and notes for 127/8

tests/linux-selftest.sh

@@ -50,8 +50,26 @@ pingtest 255.255.255.1 255.255.255.255 16 && result=1
 # But, even 255.255/16 is OK!
 pingtest 255.255.3.1 255.255.50.77 16 || result=1
 
+
 # Test support for zeroth host
 # pingtest 5.10.15.20 5.10.15.0 24  || result=1
+
+# Test support for not having all of 127 be loopback
+# pingtest 127.99.4.5 127.99.4.6 16 || result=1
+
+# Interestingly, the kernel is happy to assign these 127/8 addresses to veth
+# interfaces, but even though there's no explicit routing table entry for
+# them, the ping still gives "Invalid argument", like
+#
+# connect(5, {sa_family=AF_INET, sin_port=htons(1025), sin_addr=inet_addr("127.99.4.6")}, 16) = -1 EINVAL (Invalid argument)
+#
+# (a TCP socket connect yields the same error)  -- It's interesting because,
+# if you give a different destination address inside of the netns where
+# there's just no relevant route set, instead of EINVAL you'll get
+# ENETUNREACH.  So that confirms that 127/8 is being treated specially by
+# the kernel, seemingly outside of routing table/FIB lookups.
+
+
 # TODO: Let's also have some prefixes other than /24
 # TODO: It's slightly harder to have tests for TCP, not just ICMP, because
 #       the ping responder is in the kernel, while answering TCP via netcat