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I purchased a new router (TP-Link ER6020), turned it on and connected my computer to it. The router is running in NAT mode, I did not change a setting (despite providing dsl authentication details). Then I ran a port scan using an online port scanner. It found one port (port 25 - SMTP), that was open (from WAN side).

I thought with NAT by default no port should be available from the WAN side (port 80 for config only from LAN). Am I wrong there? Any idea how I can and whether I need to fix this (I found no corresponding setting in the router)?

Edit: Using telnet I can connect to port 25 (from LAN!). But the router is not responding to the HELO command. And after a few seconds I get:

network error: software caused connection abort

and then

421 Cannot connect to SMTP server <ip-address> (<ip-address>:25), NB connect error 1460
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3 Answers 3

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The NAT function has nothing to do with blocking ports on the router - for that matter, it simply "hides" your internal computers addresses from the outside.

As for the open port you've found, try send Helo command to that address and port from telnet and see what response you get.

Also, though not very likely with this port, it might belong to some router that's between you and the scanner site.

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  • I can connect using telnet, but it does not respond to the HELO command. I checked with different port scanners: port 25 was always open. With NAT enabled and no port forwarding I was expecting to have no ports open. Apr 1, 2014 at 9:14
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There should be no reason to scan the WAN Port. It makes sense to scan you local computer or important devices for open ports that are vulnerable to attacks from the outside. After finding some ports that can make a threat block these ports on the router, on the firewall and the windows firewall.

Here is more information how to setup a network: https://www.youtube.com/watch?v=JFJq59WRwek&t=19s&ab_channel=SinisaMaricic

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    Jan 28 at 12:34
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What is the problem? If you dont want it open, close it. If you have no software using it, then it doesnt matter. Port 25 is normally used to send email by SMTP.

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