TCP connections stuck at syn_sent

RouterOS General
1

Hello!

I have a very strange problem here with a RB450G board (v5.26, firmware 3.07).
Configuration has just been reset, only a few port forwarding rules have been added.

It starts with a failing TCP connection establishment to Amazon servers (AWS). I recognize this through a custom program running on my 192.168.88.2 machine. It requests a file from a webserver on a micro instance every 10 seconds. It does this by WinAPI WinHttpSendRequest & Co. which seems to keep 3 connections open for that.
Sometimes, windows seems to re-establish these connections.
But sometimes this fails because the connections are suddenly stuck at syn_sent (as I can see by Winbox (Firewall - Connections window)).

Okay, so no new connections are possible to AWS at this time. But pinging still works. Also, already established connections still work fine (RDP for example).
I’m also not able to browse to amazon.de anymore, the IP is different but it also gets stuck at syn_sent.
(After some time, these problems suddenly disappear)

But the really strange thing is that (at the same time!) all these problems are not existent on my other machine, 192.168.88.4.
So if I browse amazon.de from that machine for example, or the website running on the micro instance, it just works and I see the established connection on Winbox (192.168.88.4 → amazon: established, but 192.168.88.2 → amazon: syn_sent…).

So my conclusion is that it must be a problem of the router because the outside sees only my public IP so I think Amazon ACKs to 192.168.88.2 but the router doesn’t recognize it for some reason.
Is there some limitation of SYNs, or can the route table get confused somehow or… I don’t know..

Thank you in advance!

2

Okay I realized that this doesn’t have to do with AWS.
I stopped the program, but still, sometimes I cannot browse to amazon.de.
At the same time, amazon.com works.
And again, at the same time, browsing amazon.de from another computer in LAN works too!

After over 15 years of computer experience, I’m still wracking my brain about what this could be.

I’m still coming to the conclusion that it must be a bug in the router system.
It can’t be my system because a syn is actually sent, and it usually works again after a few minutes without doing anything special.
It can’t be amazon because it is browseable from another LAN IP.

What could I try to find out more?

3

Hi, sorry to dig up that old thread but I’m experiencing the same exact problem on 6.38.5 and 6.39.1. Http connections stuck AT syn_sent. Did you find out what was causing this issue ?

Thanks