[lwip-users] Communication between different subnets
hellish
Hi!
Recently I ported lwip 1.3.2 to my board running FreeRTOS and is now
doing some experiment within my school LAN.
Both my board and my PC are connected to school LAN via an 8-port switch
in my room. Their network interfaces are configured by DHCP. My board is
able to communicate with my PC(and any other PCs on the same subnet). But my
board can't connect to devices that reside on another subnet(for example,
the school FTP server) , while my PC can download files from that FTP just
fine.
I checked lwip's source code and it says:
* For unicast addresses, the packet is submitted to etharp_query(). In
* case the IP address is outside the local network, the IP address of
* the gateway is used.
So lwip modified destination IP address of the packets(sent from my
board) to that of the gateway? This confused me. Why change the destination
IP address? According to my network textbook, it is the destination MAC
address that should be changed to that of the gateway.
Could someone provide guidance as to how to communicate with devices on
another subnet using lwip? Thanks in advance.
Attached below is a snapshot of the network status:
My computer: /IP/ 172.20.123.144 /subnet mask/ 255.255.254.0 /default
gateway/ 172.20.122.1
My board running lwip: /IP/ 172.20.122.224 /subnet mask/ 255.255.254.0
/default gateway/ 172.20.122.1
school FTP server: IP 172.20.106.31
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Kieran Mansley
> * For unicast addresses, the packet is submitted to etharp_query().
> In
> * case the IP address is outside the local network, the IP address of
> * the gateway is used.
> So lwip modified destination IP address of the packets(sent from my
> board) to that of the gateway? This confused me. Why change the
> destination
> IP address? According to my network textbook, it is the destination
> MAC
> address that should be changed to that of the gateway.
Yes, your textbook is right. But to find the MAC address of the gateway
we have to use the IP address of the gateway in the lookup. The packet
itself will still be sent with the IP address of the FTP server.
It should just work if you've configured the netif with the gateway
address properly.
Can you get a packet capture to show what's going wrong. You've said it
doesn't work but we need more information about what goes wrong to debug
this problem.
Kieran
Kieran Mansley
> You computer Default Gateway address is wrong.
> >>My computer: /IP/ 172.20.123.144 /subnet mask/
> 255.255.254.0 /default
> gateway/ 172.20.122.1
> right?
Why? Looks OK to me. Perhaps you didn't spot his netmask setting?
Kieran
hellish
Thanks for clearing up the IP destination confusion! Sure, I'll use wireshark
to capture some packets tomorrow.
Then again, I remember doing this yesterday and wireshark didn't see any
packet (from my board to FTP server), hmm....
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Kieran Mansley
> Thanks for clearing up the IP destination confusion! Sure, I'll use wireshark
> to capture some packets tomorrow.
> Then again, I remember doing this yesterday and wireshark didn't see any
> packet (from my board to FTP server), hmm....
Where were you running wireshark? If on your PC you wouldn't expect it
to see packets as they're not going to or from it. If you can configure
your switch to do port mirroring you might be able to get them that way.
Alternatively get the packet capture by running it on something
mid-stream (e.g. the gateway). That probably isn't an option on your
live school network, so you might need to set up a little network of
your own to reproduce it.
If the traffic level is very low and you can get the console output from
your board you might have luck just using the debug statements in lwIP
to dump details of the packets sent and received. This is potentially
easy to try, though much less useful than a full packet capture.
hellish
Thanks for your reply!
After some serious debugging today I found that the real problem lies in my
FTP-related code. I thought something was wrong with lwip because of the
somewhat misleading comment in the lwip source code and the fact that
wireshark running on my PC didn't capture any packets.
Lesson learned:
1. When sending packets to non-local subnets, during the lookup stage lwip
changes the destination IP to that of the gateway in order to find gateway's
MAC address. The destination IP of the packet itself remains unchanged.
2. Wireshark can't capture packets that are neither sent or received by the
host.
Thanks again for your detailed suggestions, Kieran!
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