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Asked: 2026-06-15T01:21:27+00:00

I am trying to build a packet that will be sent via UDP. However

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I am trying to build a packet that will be sent via UDP. However I am not getting the correct data on the receiving side.

In the packet I want to include an IP Header, UDP Header, and the data that needs to be sent. In this case I just want to send the word “Hello” along with random header information.

char *data = "Hello";
char *packet = (char *)malloc(sizeof(struct iphdr) + sizeof(struct udphdr) + strlen(data));
struct iphdr *ip = (struct iphdr*) packet;
struct udphdr *udp = (struct udphdr*) (packet + sizeof(struct iphdr));
char *send_buff = (char *) (packet + sizeof(struct iphdr) + sizeof(struct udphdr));
ip->saddr = inet_addr("1.2.3.4");
ip->daddr = inet_addr("5.6.7.8");
ip->ttl = 5;
udp->source = 5950;
udp->dest = 5950;
udp->len = sizeof(struct udphdr);
udp->check = 0;
strcpy(send_buff, data);

sendto(sock, packet, (sizeof(struct iphdr) + sizeof(struct udphdr) + strlen(data)), ROUTER_IP);

The problem I’m having is that the receiving end just gets random data so I’m assuming the number of bytes is incorrect somewhere.

On the receiving side I have it print out one of the fields of the IP header as a test, but it’s not correct.

char recv_buff[1000];
int recv_bytes = recvfrom(sock, recv_buff, sizeof(recv_buff));
struct iphdr *ip = (struct iphdr*) recv_buff;
cout << static_cast<int16_t>(ip->ttl) << endl;

Am I putting the packet together wrong or is there a problem on the receiving end?
I used this example http://www.winlab.rutgers.edu/~zhibinwu/html/c_prog.htm as a reference for putting together the packet.

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1 Answer

  1. Editorial Team

    You are creating the socket as socket(AF_INET, SOCK_DGRAM, 0); meaning that it’s a datagram (=UDP, typically) socket, so the network stack will automatically include IP header & UDP headers, etc.

    But since you are trying to create your own IP and UDP headers you must create a raw socket, then send the packet (and also calculate the checksum as your reference code is doing).

    To create a raw socket, use socket(AF_INET, SOCK_RAW, 0).

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