0.0.0.0/8 oddities...

[Sean Chittenden]

Hello. I ran in to an interesting situation in what appears to be an exotic situation. Specifically, after reviewing RFC5735 again and searching for a datacenter-local or rack-local IP range (i.e trying to provide services that are guaranteed to be provided in the same rack as the server), I settled on the 0.0.0.0/8 network. Per §3 of RFC5735, it would appear that this network is valid:

https://tools.ietf.org/html/rfc5735#section-3

>    0.0.0.0/8 - Addresses in this block refer to source hosts on "this"
>    network.  Address 0.0.0.0/32 may be used as a source address for this
>    host on this network; other addresses within 0.0.0.0/8 may be used to
>    refer to specified hosts on this network ([RFC1122], Section 3.2.1.3).

And this works as expected, with regards to TCP services. But ICMP? Not so much. Is there a reason that ICMP would fail, but TCP (e.g. ssh) works? For example, I pulled 0.42.123.10 and 0.42.123.20 as IP addresses to use for NTP servers, but much to my surprise, I could ssh between the hosts, but I couldn't ping. Is this intentional? I understand that 0.0.0.0/32 == INADDR_ANY for source addresses, but it doesn't appear that there should be a restriction of inbound echoreq packets. According to tcpdump(1), the host is receiving echoreq packets, however no echorep packets are generated. As a work around, I threw things in to a more traditional RFC1918 network and things immediately worked for both SSH and ICMP. 

?? Any thoughts as to why? It doesn't appear that the current behavior abides by RFC5735.

-sc


--
Sean Chittenden
sean at chittenden.org