Re: [lwip-users] DHCP and changing the MAC address

[Leon Woestenberg]

Tom,

> Gratuitous ARP: Up through 0.7.2, I show no code in netif_set_ipaddr
> that uses etharp_query(netif, ipaddr, NULL). Outside of the cmp for 
>
It seems I added the code to the CVS main trunk only (i.e. HEAD) in-line
with the concept of not adding new features to the stable trunk (0.7.x).

So, only when you do a CVS checkout will you see this. However, your
solution works perfectly for a Ethernet-only lwIP host:

> Implemented in dhcp at the end of dhcp_bind, 
>
>   /* netif is now bound to DHCP leased address */
>   dhcp_set_state(dhcp, DHCP_BOUND);
>
>   etharp_query(netif, &dhcp->offered_ip_addr, NULL);
Yup, nice place to put it. I want to forward this change into lwIP, but
first I have to remove the dependency on Ethernet.

> The arp entry on my PC client _does_ get updated! 
> Following is a dump ( which performs 4 calls ) is this correct?
>
> No. Time        Source           Destination      Protocol Info
> 51  9.375649  Qualstar_ff:ff:07  Broadcast        ARP      Who has 
> 216.101.63.200?  Tell 0.0.0.0
> 52  9.805944  Qualstar_ff:ff:07  Broadcast        ARP      Who has 
> 216.101.63.200?  Tell 0.0.0.0
> 53  10.314237 Qualstar_ff:ff:07  Broadcast        ARP      Who has 
> 216.101.63.200?  Tell 0.0.0.0
> 55  10.822746 Qualstar_ff:ff:07  Broadcast        ARP      Who has 
> 216.101.63.200?  Tell 216.101.63.200
Seems fine. Some explanation:

Packets #51 through #53 probably(*) is the lwIP host, that has just 
received an DHCP_ACK for a REQUESTED address 216.101.63.200 OFFERED 
earlier by the DHCP server. It asks around if anyone else has this 
address in use. See dhcp_check().

After a second and a half, after nobody has replied on the offered 
address (which is good), dhcp_bind() kicks in, taking the address.

Your modification (see above) now emits a gratuitous ARP message,
with ARP/IP source pair (Qualstar_ff:ff:07 / 216.101.63.200) in packet 
#55. This should update all local network hosts' ARP caches (and it does).

(*)You can see the relationship between DHCP and ARP simply by using
"dhcp || arp" as the Ethereal filter (IIRC).

> I have also added etharp_query(netif, &dhcp->offered_ip_addr, NULL) to my
> change-MAC-on-a-static-IP routine also and that works as well.
Great!

> Let me know if there is any other testing I might be able to do.
Nope, but I am planning some changes in this area which might 
destabilize this a bit. However, staying on 0.7 is the safe path
for a production quality stack.

Upcoming changes in CVS:

interface up/down capability
interface link type knowledge
making arp link layer independent

Regards,

Leon.