Re: [lwip-users] Variation in start-up IP address

[Luca Ceresoli]

Alex Brophy wrote:
> My application enables DHCP and auto-IP in lwipopts.h. My test 
> environment consists of a standalone board connected via ethernet cable 
> to a spare ethernet port on a PC. There is no DHCP server involved, so I 
> would expect the IP address to always come up as 169.254.254.169. Most 
> of the time it does, but occasionally, it comes up as 169.254.254.170 or 
> more rarely 169.254.254.171. Why does this happen? Is there a way to 
> make the IP address deterministic when DHCP is not available?

By design, AUTOIP-assigned addresses must not be deterministic.
If you're assuming the opposite, you'll have serious troubles as soon as
you'll connect your device to something more than your PC.

From RFC3927:
>1.5. Autoconfiguration Issues
>
>   Implementations of IPv4 Link-Local address autoconfiguration MUST
>   expect address conflicts, and MUST be prepared to handle them
>   gracefully by automatically selecting a new address whenever a
>   conflict is detected, as described in Section 2.  This requirement to

What you see may happen in the following scenario:
 1. The devices acquires IP address X
 2. The PC stores address X in the ARP cache
 3. The device is rebooted
 4. While the device probes for address X, the PC sends an ARP request
    for address X (it's legal for the PC to send it at any time)
 5. lwIP detects it as a conflict (autoip.c around line 500) and tries
    a new address.

Luca