Mike's right. This is a very common false-positive event when people first setup ACLs or firewalls. Another common reason why DNS servers might be appearing to port-scan your network is due to misconfigured W2K boxes. Since W2K would rather use DNS than NetBIOS to register themselves, if you configure their primary DNS as your ISP's DNS servers, your W2K boxes will try to register themselves with the ISP's server. Most will not take the registration, rightly so, and will send back an NACK message. This message ends up banging against the inbound filter. Roger **************************************************************************** **** *Roger A. Grimes, Computer Security Consultant *CPA, MCSE (NT/2000), CNE (3/4), A+ *email: rogergat_private *cell: 757-615-3355 *Author of Malicious Mobile Code: Virus Protection for Windows by O'Reilly *http://www.oreilly.com/catalog/malmobcode **************************************************************************** ************* ----- Original Message ----- From: "Mike" <mikeat_private> To: "'Ronald Belchez'" <meukoneat_private>; <incidentsat_private> Sent: Friday, June 06, 2003 3:39 AM Subject: RE: strange traffic on UDP port 53 > After deploying a new mail server/internet gateway (behind a firewall) I > found a similar problem with packets being stopped by our firewall. > After performing an nslookup on the "offending" IP address I found it > belonged to our ISP. On querying them about this odd behavior the > explanation given (and other evidence seems to bear this out) was that > our mail server was performing DNS lookups for the delivery of mail and > on behalf of our internal network as it was configured as a forwarder > because it was behind a firewall. The IP address in question was merely > replying to DNS queries which had been forwarded to it by our ISPs' > primary DNS server and as the firewall would only allow DNS replies > through from certain IP addresses it was stopping any others. The > incrementing of the source ports you are seeing is due to the fact that > when the DNS reply is not acknowledged by the target system it tries > again on the next available port. > It is only usually a minor inconvenience (although the other day one > server filled my firewall log 4 times and I was alerted to possible port > scans a number of times during the day). If it bothers you too much try > filtering the logs to remove the offending entries or you can allow all > port 53 traffic in (unless like me you suffer from paranoid delusions > that everyone on the internet is out to get you). > > -----Original Message----- > From: Ronald Belchez [mailto:meukoneat_private] > Sent: 04 June 2003 22:14 > To: incidentsat_private > Subject: strange traffic on UDP port 53 > > > > Hi All, > > We don't have a firewall and is just relying on Access-list on our > border > > router. After i applied the new access-list I am continously receiving > > the logs showed below. The destination IP is our mail server (not > running > > any DNS service) while the source IP (unsolicited and using source port > > with some sort of incremental patterm, the denied packets logs is also > > continuous now for about 4 days) I am not aware of any trojan or worm > > using the below. I already tried searching google but cannot find the > > explanation or something that might help me understand the below.... > > Please advise. > > > > --logs starts here--- > > denied udp XX7.Y3.71.242(54067) -> XX3.Y1.246.66(53), 1 packet > > denied udp XX7.Y3.71.242(54070) -> XX3.Y1.246.66(53), 1 packet > > denied udp XX7.Y3.71.242(53967) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(53972) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(53979) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(53989) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(54003) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(53982) -> XX3.Y1.246.66(53), 34 packets > > denied udp XX7.Y3.71.242(54009) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(54027) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(54035) -> XX3.Y1.246.66(53), 2 packets > > denied udp XX7.Y3.71.242(54042) -> XX3.Y1.246.66(53), 2 packets > > ------------------------------------------------------------------------ > ---- > ------------------------------------------------------------------------ > ---- > > > ############################################################################ ######### > Note: > This message is for the named person's use only. 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