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Re: [lwip-users] Sending to a non-local network without default netif se

From: Fabian Koch
Subject: Re: [lwip-users] Sending to a non-local network without default netif set
Date: Mon, 17 Nov 2014 10:00:08 +0000

Hey Simon, Hey Erik,


we do have a setup with two netifs and we cannot determine what the customer will do with the device so we need to also check for „same subnet“ scenarios.

I already wrote and email to this (or devel?) list about that a while ago.


We also attacked the problem by including the src IP into the ip_route() function and I can provide a patch, but it just doesn’t feel right.


The reason this is needed is actually because LwIP uses ip_route() to find a netif when the local address is INADDR_ANY.


In udp_connect(), udp_sendto(), tcp_eff_send_mss(), snmp_send_trap() and etharp_add_static_entry().


In all those cases, ip_route() is actually not really a good function to find a matching netif. If we were to replace those instances with another function that is a bit more complicated and finds a fitting netif with more aspects including gateway settings and network reachability (if that’s even a word…) then it would make the final routing a bit easier.


Also, the default_netif construct is just too simple to make all this work correctly.


If you need a quick solution, you can make ip_route include the src, but then you also have to add that parameter to tcp_eff_send_mss().


Our middle part of ip_route() looks like this at the moment:


  /* iterate through netifs */

  for (netif = netif_list; netif != NULL; netif = netif->next) {

    if (netif_is_up(netif) && netif_is_link_up(netif)) {           //only consider interfaces which are up and have a link

      if(ip_addr_isany(src)) {    //when the source IP is INADDR_ANY, the socket is not bound to an interface => find the first match (netmask or is broadcast)

        if (ip_addr_netcmp(dest, &(netif->ip_addr), &(netif->netmask)) || ip_addr_cmp(IP_ADDR_BROADCAST, dest)) {

          /* return netif on which to forward IP packet (first matching netif when socket is not bound) */

          return netif;


      } else { // socket is bound to a specific IP so only match the right netif (matching IPs and either fits netmask or is broadcast)

        if(ip_addr_cmp((&netif->ip_addr), src) && (ip_addr_netcmp(dest, &(netif->ip_addr), &(netif->netmask)) || ip_addr_cmp(IP_ADDR_BROADCAST, dest))) {

          /* this socket is bound to a specific interface, so look for that */

          return netif;






This essentially makes sending on specific interfaces possible when they are bound to the IP. Unbound sockets just send on the best matching netif they can find.


For this to work, you also need to netif_set_link_up(&loop_netif) in netif.c if you’re using a loopback netif. (I wonder if it can be considered a bug that the loopif never gets a link up…)


I would still not like this in the main lwIP source tree, since it just doesn’t feel right to include the src IP here.






From: lwip-users-bounces+address@hidden [mailto:lwip-users-bounces+address@hidden On Behalf Of address@hidden
Sent: Freitag, 14. November 2014 21:29
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Sending to a non-local network without default netif set


HaaCee2 wrote:

I beg to differ....

I've added a task for this to the tracker:



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