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I just ran GRC's "Shields Up" on my connection, which has an Asus Wl600g ADSL modem/router serving as gatekeeper. All ports are reported as stealth, except port 23 (Telnet). Now, I can understand why Telnet would be open to the internal LAN (in fact sometimes I log into the router via Telnet), but I can see no reason why this port should be open to the whole Internet! Is this normal? (And if so, why?)
Jon Smark
Posted 2014-03-11T12:58:06.117
Reputation: 117
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This isn't normal.
Some routers have it open internally for management and some may even have it open to the outside workd but have an ACL (Access Control List) to only allow connections from certain IPs (mine allows connection from my work IP).
A fully open to everyone port is not the norm and shouldn't be like that. Did you perhaps purchase a refurbished or second hand router which wasn't properly wiped before you got it?
Is there a port forwarding on that port to an internal machine? someone could have setup a port forward and then subsequently opened the port if there is a machine on the inside of the network which can be administered through telnet.
Fazer87
Posted 2014-03-11T12:58:06.117
Reputation: 11 177
The router was purchased new. The only modification was a firmware upgrade (to a more recent ASUS official firmware). Also, there is no port forwarding to a Telnet port. Oh well, I'll try reflashing the firmware and see if the problem goes away. – Jon Smark – 2014-03-12T08:59:14.277