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[lwip-devel] IP data packets are forwarded although it should never work

From: Benjamin Kalytta
Subject: [lwip-devel] IP data packets are forwarded although it should never work in theory, but why?
Date: Tue, 6 Oct 2020 12:56:45 +0000

Hello LwIP users,

First let me describe some important confguration options: 

- TCP/IP is enabled, UDP, ICMP is enabled, and IP forwarding is disabled 
(#define IP_FORWARD 0) [lwipopts.h]
- There is no network default interface (netif_default == NULL) defined.
- There is no DNS server defined

There are two network interfaces on my board:



The board where LwIP (lwip-2.1.2) is installed, has running an http webserver 
(port 80) that accepts connections from any one.

Client 1 is connected to Itf1 and has ip
Client 2 is connected to Itf2 and has ip

Both, Client 1 and 2 are not connected to each other in any way.

Now, client 1 is trying to connect to which suprisingly succeedes 
which it shouldn't since Itf1 netmask restricts the ip range to ... and therefore it should be never possible to connect to

Is this an intended behaviour of LwIp stack or are there some options to 
prevent that? Or could this be some bug?

With Kind Regards,
Benjamin Kalytta

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