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I have a PI 4 with ethernet connection and I am unable to connect to the pi from any external connections (SSH, VNC, or Nginx). All external connection results in connection refused but I can access these services just fine within the Pi.

I did a nmap scan on which returned this:

Starting Nmap 7.92 ( https://nmap.org ) at 2021-12-02 22:28 EST
Nmap scan report for dhcp-234-248.resnet.purdue.edu (128.211.234.248)
Host is up (0.033s latency).
Not shown: 96 closed tcp ports (reset)
PORT     STATE    SERVICE
22/tcp   open     ssh
25/tcp   filtered smtp
80/tcp   open     http
5900/tcp open     vnc

Nmap done: 1 IP address (1 host up) scanned in 1.48 seconds

So IP address is accessible but I can't access the services?

Also, there are no iptables or ufw installed.

Anyone know what could be causing this? Thanks.

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  • is 128.211.234.248 the IP address of the Pi? Unusual for a pi to be directly connected to the internet like that, but, who knows with .edu
    – Bravo
    Dec 3 '21 at 4:39
  • Yes, it is, our school's internet is weird. All devices connected by the ethernet are automatically exposed to the internet, with all the ports forwarded with their own unique IP address (I think).
    – sqz
    Dec 3 '21 at 5:25
  • (I think) - important that you know for sure - seems like you may be wrong :p but, ip a on the pi shows that address?
    – Bravo
    Dec 3 '21 at 5:54
  • Yes it indeed shows the same IP address
    – sqz
    Dec 3 '21 at 14:53
  • Also, Connecting the ethernet cable to my laptop allow RDP connections from the internet.
    – sqz
    Dec 3 '21 at 15:13
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Ok, I figured it out. As it turns out, when I installed Docker on my machine, it automatically added the following records to the host file.

# Added by Docker Desktop
128.211.195.246 host.docker.internal
128.211.195.246 gateway.docker.internal
# To allow the same kube context to work on the host and the container:
127.0.0.1 kubernetes.docker.internal
# End of section

and when I tried to ssh into the Pi via IP, ssh will resolve to host.docker.internal, resulting in a connection refused error. Removing these records from hosts file allowed me to connect to the PI.

The reason why that issue did not show up during nmap is that I had to use my cloud-hosted server to run the scan which has a perfectly fine hosts file.

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  • I am not sure if this is a docker's feature or not. If not, should I file a bug report to docker?
    – sqz
    Dec 3 '21 at 21:27
  • does "on my machine" refer to the pi?
    – Bravo
    Dec 3 '21 at 22:32
  • It’s referring to my laptop which I was trying to connect to the PI from.
    – sqz
    Dec 3 '21 at 23:44
  • 1
    so, it was an issue on the laptop, not on the raspberry pi
    – Bravo
    Dec 4 '21 at 0:48
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    Yes. The docker installation on the laptop lead to incorrect resolution of PI’s IP address.
    – sqz
    Dec 4 '21 at 18:19

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