CODEWITHSUNDEEP
windowssocketsnetwork-programmingtcpwireshark
H42
H42

Reputation: 825

Windows 10 - SYN/ACK packet not sent in response to SYN packets

I tried to run a simple 1-page site with Flask in Python on port 5000 of my computer 192.168.0.113, which is running Windows 10. On the same computer, I can view the site by using localhost:5000 in a web browser.

I tried to use another computer 192.168.0.134 on the same LAN to view the site. However, 192.168.0.113 never replied with a SYN/ACK packet to the SYN packet from 192.168.0.134, even if the firewall of 192.168.0.113 is completely turned off. This image is the captured flow of the packets:

image

If I did it reversely (i.e. 192.168.0.134 as the server to host the site, and 192.168.0.113 tried to connect as the client), then 192.168.0.113 is able to connect.

I noted from some previous posts (especially this one) that the problem can be solved by disabling TCP window scaling and TCP timestamps. I checked in netsh of 192.168.0.113 and noted that TCP window scaling had already been disabled and TCP timestamps was normal. I tried to disable both of them, and also tried to disable just one of them. However, all combinations failed and 192.168.0.113 still wonn't sent SYN/ACK packets in response to SYN packets from 192.168.0.134.

How can I get this fixed?

Upvotes: 1

Views: 2530

Answers (1)

Liam Kelly
Liam Kelly

Reputation: 3724

More broadly you are only showing traffic going in one direction in the Wireshark capture.

You probably are not listening on an external IP addresses. The Flask application takes an IP address argument to listen on, if it is localhost or 127.0.0.1 it will not respond to external connection. Set this to 0.0.0.0 to listen on all IP addresses. Verify this by running the command netstat -a -b to see which IP address you application is listening on.

The Flask Quick start page goes through this (see Externally Visible Server)

flask run --host=0.0.0.0

Upvotes: 1

Related Questions