Send SKB to transfer from kernel space

Question

I am currently writing a kernel module that modifies the package payload as a learning experience. I have package modifications, but now I want to send this new modified package after the original (I do not want to abandon the original). I can't seem to find a kernel function that sends an SKB for transmission. I tried dev_queue_xmit(nskb) , but it causes a kernel panic, I also tried skb->next = nskb , but it does nothing. Should I do SKB list processing? I'm not sure how to do this, as this article seems outdated .

EDIT:

So, I was able to fix the kernel panic when calling dev_queue_xmit (nskb), I accidentally did dev_queue_xmit (skb), which removes skb and causes panic from the network filter. The problem is that everything works, but I do not see sending repeated packets, there is no trace of the second packet being sent. TCPDump on the machine does not see anything, and TPCDump on the target does not see anything, the following is my code.

 unsigned int in_hook(void *priv, struct sk_buff *skb, const struct nf_hook_state *state) { struct sk_buff *nskb = skb_copy(skb, GFP_KERNEL); /* Various other variables not relevant to the problem */ __u32 saddr, daddr; saddr = ntohl(iph->saddr); if (saddr == ipToInt(10,0,2,12) || saddr == ipToInt(10,0,2,13)) { /*For loop that saves the payload contents into a variable */ /* Here is where the problem is, I have this if statement to prevent a feedback loop then if the ip matches, I call dev_queue_xmit(nskb) which is supposed to send out sk_buff's, but TCPDump doesn't show anything on any computer */ if (saddr == ipToInt(10,0,2,13)) { dev_queue_xmit(nskb); } /* Rest of the code that isn't relevant to sending packets */ } return NF_ACCEPT; } 

My network setup is as follows: 3 Ubuntu Server VM, all of them SSH'd from the host computer (macOS, if that matters, I don't know at the moment). A computer running the above kernel module tricks two other virtual machines. Then, the other two virtual machines communicate with each other through a netcat session. I hope that when I send one message from a virtual machine with ip 10.0.2.13, 10.0.2.12 will see two identical messages. I know that a confirmation error will confirm the connection, but I do not understand this. TCPDump on any of the three computers does not show anything except packets that should be sent.

I have so far tried dev_queue_xmit(nskb) as well as nskb->dev->netdev_ops->ndo_start_xmit(nskb, skb->dev) .

Answer 1

I realized skb_copy does not copy the ethernet skb header, so the packet sent never reaches its destination.

Answer 2

As far as I remember, dev_queue_xmit () is the right procedure to send. The question is, how did you prepare the skb you want to send? Also give us a prism from dmesg when a kernel panic occurred. Have you installed skb-> dev?