mailing list archives
Re: Nmap says Host down when actually host is up.
From: "Kris Katterjohn" <katterjohn () gmail com>
Date: Fri, 26 Oct 2007 18:20:25 -0500
On 10/26/07, kx <kxmail () gmail com> wrote:
I can't say for ICMP, but I have definitely written a generic UDP
server on a Solaris box before that had multiple IP addresses, that
was listening on all IPs, and when the server would reply to a UDP
packet, the kernel behavior would be to reply from the IP addresses on
the Solaris box that was closest to the source, not necessarily from
the IP address it received the packet on.
Now, in this case it made sense, I would send a packet from a subnet
connected to the Solaris box, but I would send it to the IP address
not on the subnet. The response would come back from the IP address on
Solaris has IP 10.10.1.5 and 10.10.100.5
I am IP 10.10.1.6
10.10.1.6 -- UDP --> 10.10.100.5
10.10.1.6 <-- UDP -- 10.10.1.5
Not as clear as is what is going on below, and as Kris stated, it
shouldn't happen with ICMP, but just throwing it out for
Maybe the host sending the ICMP echo reply from the wrong address
misinterpreted the RFC. RFC 1122 says it's OK for the transport
layers to support this behavior (ICMP is on the same layer as IP, thus
this isn't applicable for it):
126.96.36.199 Choosing a Source Address
When it sends an initial connection request (e.g., a
TCP "SYN" segment) or a datagram service request (e.g.,
a UDP-based query), the transport layer on a multihomed
host needs to know which source address to use. If the
application does not specify it, the transport layer
must ask the IP layer to perform the conceptual
GET_SRCADDR(remote IP addr, TOS)
-> local IP address
Here TOS is the Type-of-Service value (see Section
188.8.131.52), and the result is the desired source address.
The following rules are suggested for implementing this
(a) If the remote Internet address lies on one of the
(sub-) nets to which the host is directly
connected, a corresponding source address may be
chosen, unless the corresponding interface is
known to be down.
(b) The route cache may be consulted, to see if there
is an active route to the specified destination
network through any network interface; if so, a
local IP address corresponding to that interface
may be chosen.
(c) The table of static routes, if any (see Section
184.108.40.206) may be similarly consulted.
(d) The default gateways may be consulted. If these
gateways are assigned to different interfaces, the
interface corresponding to the gateway with the
highest preference may be chosen.
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- Re: Nmap says Host down when actually host is up., (continued)