Re: "wrong" ifindex on received VLAN tagged packet?

[David Ahern <dsahern@gmail.com>]

On 3/6/18 3:02 PM, Lawrence Kreeger wrote:
> Hello,
> 
> I'm trying to run mstpd on a per VLAN basis using one traditional
> linux bridge per VLAN.  I'm running it on kernel version 4.12.4.  It
> works fine for untagged frames, but I'm having a problem with VLAN
> tagged BPDUs arriving on the socket with the ifindex of the bridge
> itself, and not the VLAN tagged interface.  For example, I have a
> tagged interface eth0.100 connected to the bridge "vlan100".  When
> packets arrive, they have the ifindex of vlan100, which mstpd doesn't
> recognize as a valid spanning tree interface, so it drops them.  Is
> there something needed to be set in the kernel to get the ifindex of
> eth0.100 instead?  This is how mstpd opens the raw socket:
> 
> 
> /* Berkeley Packet filter code to filter out spanning tree packets.
>    from tcpdump -s 1152 -dd stp
>  */
> static struct sock_filter stp_filter[] = {
>     { 0x28, 0, 0, 0x0000000c },
>     { 0x25, 3, 0, 0x000005dc },
>     { 0x30, 0, 0, 0x0000000e },
>     { 0x15, 0, 1, 0x00000042 },
>     { 0x6, 0, 0, 0x00000480 },
>     { 0x6, 0, 0, 0x00000000 },
> };
> 
> /*
>  * Open up a raw packet socket to catch all 802.2 packets.
>  * and install a packet filter to only see STP (SAP 42)
>  *
>  * Since any bridged devices are already in promiscious mode
>  * no need to add multicast address.
>  */
> int packet_sock_init(void)
> {
>     int s;
>     struct sock_fprog prog =
>     {
>         .len = sizeof(stp_filter) / sizeof(stp_filter[0]),
>         .filter = stp_filter,
>     };
> 
>     s = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_802_2));

try ETH_P_ALL

>     if(s < 0)
>     {
>         ERROR("socket failed: %m");
>         return -1;
>     }
> 
>     if(setsockopt(s, SOL_SOCKET, SO_ATTACH_FILTER, &prog, sizeof(prog)) < 0)
>         ERROR("setsockopt packet filter failed: %m");
>     else if(fcntl(s, F_SETFL, O_NONBLOCK) < 0)
>         ERROR("fcntl set nonblock failed: %m");
>     else
>     {
>         packet_event.fd = s;
>         packet_event.handler = packet_rcv;

And then packet_rcv using recvfrom:
                struct sockaddr_ll sll;
                char buf[4096];
                socklen_t alen;
                int len;

                alen = sizeof(sll);
                len = recvfrom(sd, buf, sizeof(buf), 0,
                                (struct sockaddr *)&sll, &alen);

And sll.sll_ifindex will show vlan device indices.


> 
>         if(0 == add_epoll(&packet_event))
>             return 0;
>     }
> 
>     close(s);
>     return -1;
> }
> 
> Thanks, Larry
>