ZA, ShieldUp and port detection
For me, the known problem at 113…
The IDENT protocol’s port 113 is quite problematical and tricky to stealth. If the user’s port 113 is completely stealthed, connections to some remote Internet servers such as eMail, Internet Relay Chat (IRC), and others, may be delayed or denied altogether. For this reason, many NAT routers and personal firewalls do not attempt to stealth port 113, they settle for leaving it closed. One of the first things that caught my eye about the ZoneAlarm personal firewall was that it was clever about handling port 113: It “adaptively stealthed” the port.
Even after many years, the (free) ZoneAlarm personal firewall from Zone Labs is the only personal firewall to “adaptively” stealth port 113. Unlike any other firewall or NAT router (any of which could also do the same) this allows port 113 to be stealthed to any passing Internet scanners or probes, but “unstealthed” for any valid IDENT connection attempts originating from remote servers with which the user’s computer is attempting to connect. (Since this could easily be done by any personal firewall or even NAT routers, I am hopeful that this feature might yet appear in other products.)
“Adaptive Stealthing” means that when a TCP SYN packet arrives to request a connection to your machine’s port 113, ZoneAlarm checks, on the fly, to see whether your machine currently has any sort of “relationship” with the remote machine (such as a pending outgoing connection attempt). If so, the remote machine is considered to be “friendly” and its IDENT request packet is allowed to pass through ZoneAlarm’s firewall. But if the IDENT originating machine is not known to ZoneAlarm as a “friendly” machine, the connection requesting packet is dropped and discarded, rendering port 113 stealth to all unknown port scanners. It’s very slick.
IDENT, ZoneAlarm, and ShieldsUP!
Even though your computer’s web browser already has a relationship with the web server at GRC, our tests originate from a different “foreign” IP address. ZoneAlarm therefore drops incoming packets to port 113 from this different probing IP address and ZoneAlarm users see that port 113 is stealthed to passing Internet scans.
To demonstrate how ZoneAlarm (and perhaps someday other firewalls or NAT routers) selectively “unstealth” port 113 — but only for known “friendly” machines — we simply initiate a connection from your web browser to the ShieldsUP! scanning IP. Even though the connection attempt will ultimately fail (since there’s no web server at the probing address), ZoneAlarm will note the outgoing attempt and will unstealth port 113 for subsequent probes: http://grc.com/x/ne.dll?rh1dkyd2
More info: http://www.grc.com/port_113.htm
