Network traffic forwarding in Linux Kernel

When developing virtual network device drivers for Linux, sometimes there is a need of redirecting packets between the real device and a virtual one. Fortunately, the Linux kernel (version 4.14 in our case) has the right tools to do so, without any extra headache. Let’s look at redirecting incoming packets from a real network device. To do so, you don’t even have to change the sources of a real network driver itself. All you need to do is just use the mechanism of rx_handlers. To add your own handler, you need to use the following API call from your virtual driver:   int netdev_rx_handler_register (

struct net_device * dev,

 rx_handler_func_t * rx_handler,

 void * rx_handler_data);

The signature of a rx_handler is following: typedef rx_handler_result_t rx_handler_func_t(struct sk_buff **pskb);   Here is the list of possible return codes:   enum rx_handler_result { RX_HANDLER_CONSUMED, RX_HANDLER_ANOTHER, RX_HANDLER_EXACT, RX_HANDLER_PASS, };   This enum has a good amount of description in Linux sources, so we will stop in what we need – RX_HANDLER_ANOTHER. So, let’s summarize what we should do to redirect incoming packets to a different device:  
  1. Register rx_handler
  2. In rx_handler change dev (pointer to net_device struct) to the device we want to redirect packets to
  3. Return RX_HANDLER_ANOTHER to inform the kernel that we actually changed the destination device
Here is a simplified code of the handler:   static rx_handler_result_t handle_frame(struct sk_buff **pskb) {     struct sk_buff *skb = *pskb;      // virtnet_dev is a pointer to net_device struct // you can obtain the pointer with this call  // dev_get_by_name(&init_net,device_name_cstr); skb->dev = virtnet_dev; return RX_HANDLER_ANOTHER;     }   That’s it, we have a working solution for the problem. However, this type of solution comes with its own limitations. We cannot add a network device with a registered rx_handler to a virtual bridge. This is a known issue and it happens because bridge code uses the same API and tries to register rx_handler (but devices can have only one rx_handler registered!).   In our case, it wasn’t a problem, because in our topology only virtual devices were enslaved by a bridge. In the other case, it can be achieved by using protocol handlers and dev_forward_skb call, but this is a different subject to describe.   Another use case is redirecting all outgoing packets from the virtual device and sending them with a real one.    All network devices have their ndo_start_xmit callback. There we already have fully-formed packets, which are ready to be sent. Here is a simplified example of how to send all packets through a different network interface:   static netdev_tx_t virtnet_xmit(struct sk_buff *skb, struct net_device *dev) { // eth_dev is a pointer to net_device struct skb->dev = eth_dev; dev_queue_xmit(skb); return NETDEV_TX_OK; }   As you can see, it’s enough to change skb->dev and call dev_queue_xmit(skb)! You shouldn’t even call free() for skb, it’s already implemented in dev_queue_xmit logic.

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