Valdis.Kletnieks@vt.edu wrote:
On Thu, 12 May 2005 12:23:19 CDT, John Kristoff said:
I think there always has been some justification. Here is a very small sample of real traffic that I can assure is not Slammer traffic, but it is being filtered nonetheless (IP addresses removed):
May 12 09:15:30.598 CDT[...] denied udp removed(53) -> removed(1434), 1 packet May 12 09:26:30.210 CDT[...] denied tcp removed(80) -> removed(1434), 1 packet May 12 09:32:23.122 CDT[...] denied tcp removed(80) -> removed(1434), 1 packet May 12 09:42:38.558 CDT[...] denied udp removed(123) -> removed(123), 1 packet May 12 10:12:50.422 CDT[...] denied udp removed(53) -> removed(1434), 1 packet
Looks like a good justification to *NOT* filter. Somebody nuked the reply packets for 2 DNS lookups and 2 hits to web pages just because the user's machine picked 1434 as the ephemeral port. Oh, and one machine that got slapped across the face for having the temerity to ask what time it was. ;)
For TCP, you can filter it statefully, don't allow connections inbound to 1433/1434, 135-139, etc. For UDP, you could risk allowing source 53/123/etc either "period", or "to >1023" or "to 1434" depending on the your taste, or just tolerate the collateral damage. (And yes, there's always the wise-arse using nmap -g53 or -g123 etc) Jeff