Date: Tue, 13 Nov 2012 23:06:04 -0800 From: Sean Chittenden <sean@chittenden.org> To: Joe Holden <lists@rewt.org.uk> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>, gnn@freebsd.org Subject: Re: 0.0.0.0/8 oddities... Message-ID: <7BE7E643-FB13-45DE-BA40-257B8ADFAA98@chittenden.org> In-Reply-To: <50A32FE7.2010206@rewt.org.uk> References: <DC8A0D79-8DF3-472F-9B1A-76BF8577A03C@chittenden.org> <50A20359.9080906@networx.ch> <7C614093-6408-49C6-8515-F6C09183453B@chittenden.org> <50A32FE7.2010206@rewt.org.uk>
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>>>> 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 =A73 of RFC5735, it = would appear that this network is valid: >>>>=20 >>>> https://tools.ietf.org/html/rfc5735#section-3 >>>>=20 >>>>> 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 =3D=3D 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. >>>=20 >>>> ?? 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. >>>=20 >>> Can you please check how other OS's (Linux, Windows) deal with it? >=20 > 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 & =A7? 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@chittenden.org
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