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I activated the Telnet client in Windows 7 and was instantly able to connect to servers although Windows Firewall is activated.

How does that work? Shouldn't the firewall block outgoing connections until I explicitly allow them?

Is a default firewall rule being created when you activate the telnet client in Win 7 and if so, which firewall rule should I look for?

I was unable to find any rule in my firewall for telnet.

Disclaimer: This is not really a problem I am trying to solve. I am just trying to understand how telnet is able to communicate without me opening the firewall for the program.

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Windows firewall follows the same principals as NAT firewalls. it allows virtually any outbound connection, but only allows inbound packets that are part of a connection initiated from the inside. – Frank Thomas Apr 9 '13 at 15:36
Huh, why the downvote? Just because I misunderstood the outbound default behaviour? – atticae Apr 9 '13 at 15:38
wasn't me. perhaps they didn't like your disclaimer, but since they didn't leave a comment, no way to know. – Frank Thomas Apr 9 '13 at 16:05
up vote 4 down vote accepted

Outbound communications are generally allowed by default. However, if you were running a Telnet Server, inbound connections would be blocked by default. The exceptions would be if the installer for the Telnet Server opened the ports as a part of the install.

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The standard firewall in Windows does not block outbound connections. If you want to block the telnet client (or anything else with outbound connections) you can use the Windows Firewall with Advanced Security to set up outbound rules.

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