trivia: extracting length from UDP packets

thelordmule · 01-22-2007, 12:53 AM

Hi *,
I am trying to find out if it is possible to obtain the header of an incoming UDP packet via its header. I am using the sendto/recvfrom functions but am curious as to whether it can be done. I am able to use the return value of recvfrom, but suppose that the buffer was not big enough then I don't mind the discarding of excess, but I still need to know how many bytes were in the original incoming packet.

here is the minimal server you should worry about.

Code:

   
   int sockfd,n;
   struct sockaddr_in servaddr,cliaddr;
   socklen_t len;
   int s_port = 9000;

   sockfd=socket(AF_INET,SOCK_DGRAM,0);

   bzero(&servaddr,sizeof(servaddr));
   servaddr.sin_family = AF_INET;
   servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
   servaddr.sin_port=htons(s_port);
   bind(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));

   printf("Port: %d\n", s_port);

   char buf[5000];
   n = recvfrom(sockfd, buf, 5000, 0, (struct sockaddr *)&cliaddr, &len);
   if (n < 0)
      return -1;

   // do more stuff

oh and I am not willing to recompile kernel or use external programs to feed this info, this is for high performance and should be accessible via standard *nix software API.

cheers

95se · 01-22-2007, 07:30 PM

You can try passing MSG_TRUNC for the flags argument. This will make recvfrom return the length of the packet received, even if it was truncated (ie. the size of the packet, not the amount of data that was stored in the buffer). If it's larger than the buffer length, then you can figure out how much was cut off by subtracting the buffer length from the return value.

thelordmule · 01-23-2007, 10:46 PM

I was planning to use MSG_PEEK to find out the actual amount of data readable from the recv buffer, but I found that I cannot use this in winsock since it is strongly discouraged. I would still like to know the amount that was readable and the amount delivered. thanks for the help though.