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Re: [lwip-users] fixed IP and DCHP on the same net interface?

From: Piero 74
Subject: Re: [lwip-users] fixed IP and DCHP on the same net interface?
Date: Tue, 16 Dec 2008 17:08:51 +0100

i found a solution... but i "forced" stack implementation guidelines...

I did a post to lwip_dev with this idea... because i need a feedback... here a copy.

Hi all.

I found a solution for my problem, but i need community feedback.

The problem is:

I have my ip board, and i need to simplify the installer work.
So, in a business LAN with a DHCP server, the idea is to have FW which starts, as default, with DHCP client enabled, get an IP to himself, and the installer can read the IP using
'ping <nomeHOST>'  (i used code found in forum for NBNS.. very good code!!! Thanks Alain!)
After this, a professional installer could decide tho disable DHCP client and use a fixed IP, through our SW which  changes IP board setting.

Ok... but the problem happens if the installer put the board in a small LAN without DHCP or try to connect it directly to his pc using eth cable.
In this case i need a fixed IP.

I tried to use a net interface with fixed IP, and after start DHCP client... in my company LAN DHCP server doesn't answer (or routers block broadcast packet with src IP different than

So, my solution, is:

- create TWO interfaces on the same HW, one with fixed IP and one with DHCP on and ip =
- i changed code in ethernetif scheleton, in function ethernetif_input() in this way:

  switch (htons(ethhdr->type)) {

    /* PPPoE packet? */
    // not implemented additional code
    // not implemented additional code
#endif /* PPPOE_SUPPORT */

    /* IP or ARP packet? */
/* test*/
    // create a copy of pbuf for each netif. ONLY for ARP packet
    for( netifCurr = netifLocal; netifCurr != NULL; netifCurr = netifCurr->next )
      ptemp = pbuf_alloc(PBUF_RAW, p->tot_len, PBUF_POOL);
      if (ptemp != NULL)

          pbuf_header(ptemp, -ETH_PAD_SIZE); // drop the padding word

        pbuf_copy(ptemp, p);
        // full packet send to tcpip_thread to process
        if (netifCurr->input(ptemp, netifCurr)!=ERR_OK)
          LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_input: ARP input error\n"));
          ptemp = NULL;
    pbuf_free(p); // free original pbuf
  case ETHTYPE_IP:
    /* full packet send to tcpip_thread to process */
    // if there are multiple interface on the same emac hw, send to first interface
    // ip layer will route packets to correct netif
    if (netifLocal->input(p, netifLocal)!=ERR_OK)
      LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_input: IP input error\n"));
      p = NULL;
    ret = TRUE;
    p = NULL;

so.. now i duplicate ARP packets and send a copy of each interface, and use always the first interface for IP packet, because IP layer route packet between interfaces

In this way, until board gets IP from DHCP, it answers using fixed IP, after it answers using dynamic IP

Any comment??

2008/12/16 Alain M. <address@hidden>
The real question is: do you really want to do that? what if you connect 2 cameras, what happens? plus the problem on another message...

IMHO, this would work better:
try DHCP for some time, withou any ip, then use autoip...


Piero 74 escreveu:

Hi all

I tested a network camera, which seems work in this way:

it has a DHCP client inside, which starts on power on. But until an IP from DHCP server is available, it answers on fixed ip (i.e.

Can i do the same using lwip?

I tried to bring up an interface with fixed IP and after start DHCP, but it doesn't work: i didn't receive a new ip address from DHCP server.

I saw DHCP traffic using wireshark: in normal situation, DHCP client sends packet from to broadcast, and the server answers, in my situation i have DHCP request from to broadcast...
i suppose this could be the problem

Any idea??


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