[lwip-devel] IP data packets are forwarded although it should never work in theory, but why?

[Benjamin Kalytta]

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:

Itf1:
IP: 192.168.4.1
Netmask: 255.255.255.0
Gateway: 0.0.0.0

Itf2:
IP: 172.17.1.1
Netmask: 255.255.255.0
Gateway: 0.0.0.0

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 192.168.4.2
Client 2 is connected to Itf2 and has ip 172.17.1.2

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

Now, client 1 is trying to connect to 172.17.1.1 which suprisingly succeedes 
which it shouldn't since Itf1 netmask restricts the ip range to 192.168.4.1 ... 
192.168.4.255 and therefore it should be never possible to connect to  
172.17.1.1.

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