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.
>>> The check to drop ICMP replies to a source of 0.0.0.0/8 was added
>>> in r120958 as part of a fix for link local addresses.  It was only
>>> applied to ICMP which is inconsistent as you've found out.
>>> 
>>>> ?? Any thoughts as to why? It doesn't appear that the current behavior abides by RFC5735.
>>> Reading this section and RFC1122 it is not entirely clear to me
>>> what the allowed scope of 0.0.0.0/8 is.  I do agree though that
>>> blocking it only in ICMP is not useful if it is allowed in the
>>> normal IP input path.
>>> 
>>> Can you please check how other OS's (Linux, Windows) deal with it?
> 
> 0/8 is not supposed to be used, as per the rfc.  As such it doesn't work on most systems (Linux, network appliance vendors included) so this working *should* be a bug, IMO.

Where does it say that it shouldn't be used? Which RFC & §? There are plenty of RFCs and I haven't exhaustively read things, so I reserve the right to be wrong & corrected, but I haven't seen anything that says, "do not use 0.0.0.0/8."  0.0.0.0/32, yes, that's a reserved and special IP address, but the remainder of the /8? It's a stretch to argue that it can't be used.

-sc

--
Sean Chittenden
sean at chittenden.org