C中的“仅UDP数据报”套接字

Question

In Linux, Ubuntu 14.04: I'm writing a code that implements socket to send pure UDP datagrams which includes UDP header+payload, without any part of IP header.

I have created the socket

sokt_fd=socket(AF_INET, SOCK_RAW, IPPROTO_UDP)

Also, I have prepared the UDP header.

1. I want to leave the IP encapsulation process to the kernel.

2. I want to send the datagram over any available IP interface. (I do not want to specify the source IP, and also leave this task to the kernel).

3. Do I need to specify the destination IP address before sending the datagram.

4. I must use "sendto()" command to send the datagram; how I must fill the "sockaddr" data structure?

    #include <netinet/in.h> struct sockaddr { unsigned short sa_family;// address family, AF_xxx char sa_data[14];// 14 bytes of protocol address }; 

Answer 1

Don't use the sockaddr structure. Use sockaddr_in instead and cast it when you have to pass a sockaddr* to a function.

struct sockaddr_in myaddr;
int s;

myaddr.sin_family = AF_INET;
myaddr.sin_port = htons(3490);
inet_aton("63.161.169.137", &myaddr.sin_addr.s_addr);

s = socket(PF_INET, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)myaddr, sizeof(myaddr));

The socket API is designed for different addressing families, others are Infrared and Bluetooth. Since AF_INET is only one of the families the API functions use the general sockaddr type in the parameters.

Answer 2

There is a nice explanation of this in Chapter 3, "Sockets Introduction" in the well-known book Unix Network Programming, The Sockets Networking API (Volume 1) by Richard Stevens et. al. Let me quote:

Most socket functions require a pointer to a socket address structure as an argument. Each supported protocol suite defines its own socket address structure. The names of these structures begin with sockaddr_ and end with a unique suffix for each protocol suite.

For the IP (Internet protocol) suite, the structure is sockaddr_in so it follows that since your example is specifying the AF_INET address family when you created the socket that you would use the more specific sockaddr_in structure instead of the more generic sockaddr. The socket API, as a matter of efficiency uses the more generic sockaddr pointer in the signature prototype.

With regard to using send() versus sendto() , I have found that sendto() is used more commonly with UDP and send() with TCP sockets. Therefore, to answer your question in #3 above, with UDP you don't have to specify the destination address up front, but instead it is supplied as an argument to sendto() .

For a given udp_datagram and datagram_length , your code might look something like this:

uint32_t       address = inet_addr("1.2.3.4");   // can also provide hostname here
uint16_t       port    = 27890;
sockaddr_in_t  dest_addr;
memset(*dest_addr, 0, sizeof(dest_addr));

dest_addr.sin_family = AF_INET;
dest_addr.sin_port   = htons(port);
dest_addr.sin_addr.s_addr = htonl(address);

sendto(socket_fd, 
       (const char*)upd_datagram, 
       datagram_length,
       0,
       reinterpret_cast<sockaddr_t*>(&dest_addr),
       sizeof(dest_addr));

Answer 3

The address API really wanted to be object-oriented, but had to deal with the fact that C isn't an OO language. sockaddr can be seen as the "base class" and the parameter type that bind, connect, sendto, recvfrom, etc. use when they need an address. However, you must provide a "subclassed" address matching the socket domain that you're using. This is because Berkeley sockets can be used for a wide and extensible range of protocols. IPv4 and IPv6 are the most typical, but UNIX-based installs also support sockets as filesystem objects ("addressed" by path), and, for instance, a hypervisor driver can install support for special inter-VM or guest-to-host sockets. See man 7 socket for an overview.

If you use IPv4, you need to use sockaddr_in . If you use IPv6, you need to use sockaddr_in6 . In both cases, you need to cast your pointer to a sockaddr* .

To fill in a sockaddr_in , you need to do something like this:

struct sockaddr_in inet_addr;
inet_addr.sin_family = AF_INET;
inet_addr.sin_port = htons(port);
inet_addr.sin_addr.s_addr = htonl(ip_address_as_number);
struct sockaddr* addr = (struct sockaddr*)&inet_addr;

htons and htonl stand for "host to network (short)" and "host to network (long)", respectively. You need this because there was a time at which network drivers were too dumb to abstract away the machine's endianness and we can't go back in time to fix them. (The network byte order is big endian.)