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Re: [lwip-users] Changing MAC address of network interface?

From: Noam Weissman
Subject: Re: [lwip-users] Changing MAC address of network interface?
Date: Thu, 7 Apr 2016 15:59:35 +0000


If you start networking with one MAC address and later change it you will 
have some packet lost. As the switch will try to send to the old MAC address.

Any way you should not connect TCP and during this connection change the MAC.

If you do not have any network activity, change MAC and start networking you 
should not have
any problems.

As I wrote before in all our devices we give the user (normally only 
technicians in our production)
the ability to change MAC address. This is done only once during software 
loading etc...


-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Freddie Chopin
Sent: Thursday, April 07, 2016 6:52 PM
To: address@hidden
Subject: Re: [lwip-users] Changing MAC address of network interface?

On czw, 2016-04-07 at 14:51 +0000, Noam Weissman wrote:
> If your TCP stack sends data before you change the MAC the older MAC 
> address is saved by the router/switch It is connected to.. also it 
> make sense that some packets are saved within the TCP stack buffering 
> with the older MAC address (before you change it).

That's the thing I'm asking about. If the router saves the MAC, will future 
connections (after I change the MAC) work correctly? How about currently active 
TCP connections?

> What I am saying that you can change MAC address on the fly but I 
> think it's better to do it before you connect the cable and start data 
> transfer.

I just want to know whether or not this can be easily done and whether this 
will work reliably. If it can be done, I would prefer to give the user such 
possibility. If it cannot be done, then I will write in the documentation that 
any change of MAC is effective only after hard reset of the device 
(de-initializing and re-initializing network interface is too disruptive).

Changing MAC will most likely be done extremely rarely, that's why I don't want 
to write a lot of code to actually perform such change. But if just changing 
hwaddr contents (and changing MAC in the EMAC driver of the chip) is enough, 
then why not?


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