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Strange Behaviour After Dynamically Changing Gateway Address

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Hi All,

I have encountered a weird problem when trying to change the gateway address on the fly. Currently, the embedded application waits and listens for a telnet session and response to commands and closes the telnet session fine - everything has been working well for a few years. Now the client wants to be able to set a gateway so that UDP data can be redirected to other subnets through a gateway.

Researching the documentation, I figured out I needed to change the code to add the gateway address to an ifconfig() call when a command in the telnet session is received to change the gateway, namely:

ifconfig(IF_ETH0, IFS_DOWN, IFS_END); //shut down TCP
ifconfig(IF_ETH0, IFS_IPADDR, aton (tcpAddr),
IFS_NETMASK, aton(subnet),
IFS_ROUTER_SET, aton(gatewayAddr),
IFS_UP, IFS_END); //startup on new address

Obviously, after invoking the this, the telnet session drops which is expected and simply starting up a new telnet session will re-establish comms with the embedded application.

This seems to work AS LONG AS I change to a valid gateway address that's active on the network. But what I found during my testing is that when I change to an inactive/invalid gateway address, it will work as long as the inactive/invalid address is always the same one. In other words, if I toggle between the valid and invalid gateway addresses, everything will work as expected.

As soon as I change the invalid gateway address to something other than the first invalid address, I can no longer telnet to the embedded application. I will get a "connection refused" response and can no longer get a telnet connection until a reboot (not good). Monitoring the ethernet card traffic (using Wireshark), I see that the TCP stack of the embedded app is responding to the telnet session request (SYN) with a restart (RST).

I tried doing the same sequence but with the code to set the gateway (IFS_ROUTER_SET) commented out and I never lost the telnet access. So I'm suspecting it's something to do with changing gateway address on th fly but what's weird is if I stick to the same valid and same invalid address, it's all fine.

Anyone done or encountered anthing like this? Is there more I need to do than just bringing the net interface down, updating the ip, subnet and gateway info and bringing the interface back up? And why does it work if I use the same valid/invalid address pairs?

Any insight on this would be greatly appreciated.

BTW, I'm using Dynamic C 9.21 on a BL2600 board.

Thanks for your consideration.
asked Apr 17, 2011 in Rabbit by diginorml New to the Community (0 points)

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