使用我自己的结构在套接字编程中填充 UDP 负载？

Question

I want to send a variable of custom struct over udp sockets using sendto(). This struct contains UDP payload. When I googled a bit, I found that it is possible to create raw socket in c using the flag, SOCK_RAW, while creating the socket. But I think, then i will have to populate ip header as well as udp header too. I want to avoid that.

But when I tried sending the custom structure over socket as mentioned above, I receive different data ( not the structure contents) when sniffed with wireshark. Please help.

I tried changing the struct to normal uint32_t variable and used bit shifting operations to populate this variable. I confirmed that this variable has exactly the contents that I intended to add to it.

But again, I am not able to send this too over sockets. Wireshark displays different content. Is it because in sendto(), we provide the address of the buffer and not its value?

uint32_t disc_req[1];

//disc_req is populated with bit shift operations in the following function
create_discovery_req(disc_req);

//now disc_req has exactly the required binary format - I confirmed this  

// But the following sendto() sends different data. Confirmed over wireshark.  
sendto(sockfd, &disc_req, sizeof(disc_req), 
    MSG_CONFIRM, (const struct sockaddr *) &servaddr, 
        sizeof(servaddr)); 

Expeced output is value of disc_req sent over socket and the same captured on wireshark. But the actual value of disc_req is something different.

Answer 1

The different content you are observing may be due to the sender and receiver having different endianess (host vs network). If so you will want to use hton...() and ntoh...() when working with multi-byte values.

In general, when you send an array or struct over UDP, treat it like a byte array.

Create a UDP socket, not a RAW socket:

sockfd = socket( AF_INET, SOCK_DGRAM, IPPROTO_UDP );

Setup your destination address using the method you prefer (ie gethostbyname() )

Then send the payload:

sendto( sockfd, ( char* ) &disc_req[0], sizeof( disc_req ), MSG_CONFIRM, (const struct sockaddr *) &servaddr, sizeof(servaddr));

To receive the payload:

recvfrom( sockfd, ( char* ) &disc_req[0], sizeof( disc_req ), 0, (const struct sockaddr *) &servaddr, sizeof(servaddr) );

The following is a complete example sending an NTP packet using UDP:

https://lettier.github.io/posts/2016-04-26-lets-make-a-ntp-client-in-c.html