如何在C中将数据包写入TAP接口？

Question

Does any one know how to write a packet into a TAP interface in C? Or any other language?

I have constructed an Ethernet datagram by myself, and I want to write it to a specific tap interface.

Thanks! :)

About TAP: http://en.wikipedia.org/wiki/TUN/TAP

Answer 1

Assuming you've created a TAP interface already (using ip tuntap add ), you can use a Packet Socket to write data into it (try man 7 packet for more info).

To start with, use int sockfd = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)) to create a packet socket.

The second step is to find the interface index ( ifindex ) of the TAP interface. Just run the command ip link and note down the number at the beginning of each line. For instance,

[nav@blumarine Test02]$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: p12p1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT qlen 1000
    link/ether 00:26:b9:24:82:16 brd ff:ff:ff:ff:ff:ff
3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DORMANT qlen 1000
    link/ether c4:17:fe:8c:f7:c8 brd ff:ff:ff:ff:ff:ff

the ifindex of lo will be 1 and that of eth0 will be 3.

The third step is to fill in a sockaddr_ll structure to pass to the sendto() api. This structure is used to hold the info about your packet's destination address. Fill it exactly this way:

struct sockaddr_ll SendSockAddr;
SendSockAddr.sll_family   = AF_PACKET;
SendSockAddr.sll_halen    = ETH_ALEN;
SendSockAddr.sll_ifindex  = ifindex;   // The number we just found earlier..
SendSockAddr.sll_protocol = htons(ETH_P_ALL);
SendSockAddr.sll_hatype   = 0;
SendSockAddr.sll_pkttype  = 0;

And finally, use sendto() to send your packet out. Use man 3 sendto for more info on sendto() and man 7 packet for more insight into struct sockaddr_ll .

Good luck :)