This doesn't have anything to do with reverse DNS. The message is generated by the IP masquerading code in Linux 2.2 kernels during the "reverse" or de-masquerade step when processing inbound ICMP packets: http://lxr.linux.no/source/net/ipv4/ip_masq.c?v=2.2.19#L1749 Generally ICMP messages like this are in response to an outbound IP packet from one of your masqueraded hosts. For some reason the ICMP replies are getting mangled. I'd say that this might be due to corruption of the packets (network problems?) though it's certainly possible this is some kind of ICMP spoofing attack or probe. I'm not sure if throwing a mangled ICMP packet describing activity in the range of ports reserved by Linux for masqueraded traffic would generate this kind of message or not. But in any event, this should put you on the right track. Good luck. David -----Original Message----- From: mstevensonat_private [mailto:mstevensonat_private] Sent: Thursday, November 01, 2001 11:12 AM To: incidentsat_private Subject: Strange kernel happenings I keep getting the same kernel messages from a few of my linux servers EVERY DAY: Kernel Messages: 1,7c1 < ksum from 63.94.31.225! < IP_MASQ:reverse ICMP: failed checksum from 63.94.31.225! < IP_MASQ:reverse ICMP: failed checksum from 141.198.38.114! < IP_MASQ:reverse ICMP: failed checksum from 63.94.31.225! < IP_MASQ:reverse ICMP: failed checksum from 63.94.31.225! < IP_MASQ:reverse ICMP: failed checksum from 63.94.31.225! < IP_MASQ:reverse ICMP: failed checksum from 65.205.2.1! the IP's however, are not consistent. Usually different IP's every day. I've tried to look this up, but am having a hard time finding information on what this means. Kinda looks like someone from the outside world is spoofing IP's, sending ICMP traffic to the server, but when the server tries to verify with a reverse lookup it flags and says "I don't like ICMP traffic from this address because it looks suspicious!" Any ideas anyone? Miles Stevenson QuickHire Network Support Specialist ---------------------------------------------------------------------------- This list is provided by the SecurityFocus ARIS analyzer service. For more information on this free incident handling, management and tracking system please see: http://aris.securityfocus.com ---------------------------------------------------------------------------- This list is provided by the SecurityFocus ARIS analyzer service. For more information on this free incident handling, management and tracking system please see: http://aris.securityfocus.com
This archive was generated by hypermail 2b30 : Thu Nov 01 2001 - 09:58:28 PST