Send IP packet from Linux Kernel without destination mac address

arp, c, ethernet, ip, linux-kernel

Solution

If you still want to use `ip_local_out` or `ip_queue_xmit`, then you can create your flowinfo like this.

struct flowi4 fl4 = {
    .flowi4_oif = skb->dev->ifindex,
    .daddr = iph->daddr,
    .saddr = iph->saddr,
};

then create dst(rtable) with

ip_route_output_key

set to skb_buff with

skb_dst_set

Problem

I want to transmit an skb that contains a valid tcp and ip header in the linux kernel. It should go out a specific interface without being routed. My problem is, that I cannot use `dev_queue_xmit` because I dont know the destination mac-address. My attempts to find out the mac-address with `arp_find` failed: ``` ... mh = (struct ethhdr *) skb_push(skb, sizeof(struct ethhdr)); ... arp_find(mh->h_dest, skb); //this or the next line val = dev_queue_xmit(skb); //crashes kernel ``` Experiments with `ip_local_out` also failed. I set the ip header information and call `ip_local_out` which also results in a kernel crash. I could not use `ip_queue_xmit` because I was not able to find out, what data to provide in the `struct flowi *fl` field. So my questions: - How can I send out an skb on a device with ip information and no knowledge about lower levels? - If there is no answer to the first question: How can I find out the destination mac/trigger an arp request for the destination ip? Remark: - I would like to avoid using (raw) sockets. - I have successfully transmitted self-made skbs via `dev_queue_xmit` if I had the destination mac address. So the code building the skb is not an issue.

Original source