Snort mailing list archives
Re: Re; loopback traffic
From: Frank Knobbe <frank () knobbe us>
Date: Wed, 19 May 2004 20:17:13 -0500
On Wed, 2004-05-19 at 17:49, Richard Bejtlich wrote:
I've checked firewall logs, and the kernel, of course, is spitting out "martian source" errors.. because packets from 127.0.0.1 should never be on the wire, right? -- I'm seeing these packets as well. They began appearing just after midnight GMT on 17 May 04 at one of my sites, but not others. A new alert just arrived a few minutes ago. The source port is 80 TCP and the destination ranges from 1012 to 1992 TCP. They are all RST ACKs.
They just now started? Or are you just now starting to watch for loop back traffic? I've seen these since October last year. This appears to get asked every couple months on this list, so here is again the usual reply (see below). Regards, Frank ---8<---[forwarded without permission]--->8--- From: Dan Hanson <dhanson () securityfocus com> To: incidents () securityfocus com Subject: Administrivia: Are you seeing portscans from source 127.0.0.1 source port 80? Date: Tue, 28 Oct 2003 08:59:56 -0700 (MST) I am posting this in the hopes of dulling the 5-6 messages I get every day that are reporting port scans to their network all of which have a source IP of 127.0.0.1 and source port 80. It is likely Blaster (check your favourite AV site for a writeup, I won't summarize here). The reason that people are seeing this has to do with some very bad advice that was given early in the blaster outbreak. The advice basically was that to protect the Internet from the DoS attack that was to hit windowsupdate.com, all DNS servers should return 127.0.0.1 for queries to windowsupdate.com. Essentially these suggestions were suggesting that hosts should commit suicide to protect the Internet. The problem is that the DoS routine spoofs the source address, so when windowsupdate.com resolves to 127.0.0.1 the following happens. Infected host picks address as source address and sends Syn packet to 127.0.0.1 port 80. (Sends it to itself) (This never makes it on the wire, you will not see this part) TCP/IP stack receives packet, responds with reset (if there is nothing listening on that port), sending the reset to the host with the spoofed source address (this is what people are seeing and mistaking for portscans) Result: It looks like a host is port scanning ephemeral posts using packets with source address:port of 127.0.0.1:80 Solution: track back the packets by MAC address to find hte infected machine. Turn of NS resolution of windowsupdate.com to 127.0.0.1. Hope that helps D
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Current thread:
- Re; loopback traffic Richard Bejtlich (May 19)
- Re: Re; loopback traffic Frank Knobbe (May 19)