[lwip-devel] Using DHCP & AutoIP as connections change

[Bill Auerbach]

With DHCP how can I reuse IP addresses on link disconnects but not reuse addresses on physical connection changes?

If I call dhcp_stop when the cable is disconnected and dhcp_start when it’s connected, I get a different IP address each time this happens.  This is not good if the connection glitches.  The reason I’m doing this is that I also want to handle removing the cable from a DHCP-enabled connection to one which doesn’t have DHCP – in this case I want the device to get an AutoIP address.  If I don’t make these calls, the device will continue to work while using the DHCP-offered address which won’t be valid if the device has power cycled.

Is there a way to handle this case cleanly?  Too many customers change cabling without cycling power because they are spoiled by not having to do so with PCs.

Also, which I don’t think is right, calling dhcp_start and dhcp_stop also calls autoip_start and autoip_stop and doing so causes the netif to get the next IP address.  I believe the

  autoip->tried_llipaddr++;

in autoip_start should be removed or be used conditionally only when there are multiple netif’s.

I noticed that a DHCP server should try to give a device an IP address it already used prior. It can do so using DHCP_OPTION_REQUESTED_IP with DHCP_DISCOVER but lwIP doesn’t use this option.

In dhcp_discover(), we could have:

    if(!ip_addr_isany(&netif->ip_addr)) {

            dhcp_option(dhcp, DHCP_OPTION_REQUESTED_IP, 4);

            dhcp_option_long(dhcp, ntohl(netif->ip_addr.addr));

        }

to inform DHCP of a previous address.  This would fill in DHCP_OPTION_REQUESTED_IP if the lwIP device had a previous IP address.  Interestingly on a 5-year old Linksys router this option is ignored and I still get a new IP address with each dhcp stop and start call.

Thanks,

Bill