[lwip-users] Multicast and ICMP-Echo

[Thomas Ubensee]

I was trying to ping a multicast address on a LWIP-Stack (using the master code) with LWIP_MULTICAST_PING enabled.

I could see in Wireshark that lwip send an ICMP-echo which linux ping did not recognize. It seems to me that the problem is that LWIP responses the ICMP-echo with the multicast-address as source and NOT with the unicast address of the interface where the ICMP-Request came from.

When changing the code inside of icmp.c around line 190 from:

...

    /* At this point, all checks are OK. */

    /* We generate an answer by switching the dest and src ip addresses,

     * setting the icmp type to ECHO_RESPONSE and updating the checksum. */

    iecho = (struct icmp_echo_hdr *)p->payload;

    ip_addr_copy(iphdr->src, *ip_current_dest_addr());

    ip_addr_copy(iphdr->dest, *ip_current_src_addr());

    ICMPH_TYPE_SET(iecho, ICMP_ER);

#if CHECKSUM_GEN_ICMP

...

to:

...

    /* At this point, all checks are OK. */

    /* We generate an answer by switching the dest and src ip addresses,

     * setting the icmp type to ECHO_RESPONSE and updating the checksum. */

    iecho = (struct icmp_echo_hdr *)p->payload;

#if LWIP_MULTICAST_PING

    

    if (ip_addr_ismulticast(ip_current_dest_addr())) {

        /*Replace Multicast address with interface ip address*/

        ip_addr_copy(iphdr->src, inp->ip_addr);

    }

    else {

        ip_addr_copy(iphdr->src, *ip_current_dest_addr());

    }

#else

    ip_addr_copy(iphdr->src, *ip_current_dest_addr());

#endif

    ip_addr_copy(iphdr->dest, *ip_current_src_addr());

    ICMPH_TYPE_SET(iecho, ICMP_ER);

#if CHECKSUM_GEN_ICMP

...

Linux ping works fine.

Regards,

Thomas