Question

On unix using C, my client is listening on port 68 with superuser mode. After sending DHCP discover message, when I try to receive, it blocks in recvfrom means there is no message received or is it like system has a process (DHCP client) listening on same port 68 which receives the message and thats my process are not able to receive the message. What is the problem?

I have set the socket option SO_REUSEADDR and SO_BROADCAST. I am sending to port 67.

struct dhcpmessage
{
    uint8_t op;
    uint8_t htype;
    uint8_t hlen;
    uint8_t hops;
    uint32_t xid;
    uint16_t secs;
    uint16_t flags;
    uint32_t ciaddr;
    uint32_t yiaddr;
    uint32_t siaddr;
    uint32_t giaddr;
    char chaddr[16];
    char sname[64];
    char file[128];
    char magic[4];
    char opt[3];
} __attribute__((__packed__));

#include<stdio.h>
#include<ctype.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<signal.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<errno.h>
#include<sys/file.h>
#include<sys/msg.h>
#include<sys/ipc.h>
#include<time.h>
#include"defs.h"
int main() {
int sockfd,listenfd,connfd;
const int on=1;
struct sockaddr_in servaddr,cliaddr,rservaddr;
if((sockfd=socket(AF_INET,SOCK_DGRAM,0)) < 0)
    die("socket");
if(setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&on,sizeof(on)) < 0)
    die("setsockopt");  

if(setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,&on,sizeof(on)) < 0)
    die("setsockopt");
bzero(&servaddr,sizeof(servaddr));
bzero(&cliaddr,sizeof(cliaddr));
cliaddr.sin_port = htons(68);
cliaddr.sin_family = AF_INET;
cliaddr.sin_addr.s_addr = htonl(INADDR_ANY);
if(bind(sockfd,(struct sockaddr*)&cliaddr,sizeof(cliaddr)) < 0)
    die("bind");

servaddr.sin_port = htons(67);
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = inet_addr("255.255.255.255");
struct dhcpmessage dhcpmsg;
bzero(&dhcpmsg,sizeof(dhcpmsg));
dhcpmsg.op = 1;
dhcpmsg.htype = 1;
dhcpmsg.hlen = 6;
dhcpmsg.hops = 0;
dhcpmsg.xid = htonl(1000);
dhcpmsg.secs = htons(0);
dhcpmsg.flags = htons(0x8000);
dhcpmsg.chaddr[0] = 0x00;
dhcpmsg.chaddr[1] = 0x1A;
dhcpmsg.chaddr[2] = 0x80;
dhcpmsg.chaddr[3] = 0x80;
dhcpmsg.chaddr[4] = 0x2C;
dhcpmsg.chaddr[5] = 0xC3;
dhcpmsg.magic[0]=99;
dhcpmsg.magic[1]=130;
dhcpmsg.magic[2]=83;
dhcpmsg.magic[3]=99;
dhcpmsg.opt[0]=53;
dhcpmsg.opt[1]=1;
dhcpmsg.opt[2]=1;
if(sendto(sockfd,&dhcpmsg,sizeof(dhcpmsg),0,(struct sockaddr*)&servaddr,sizeof(servaddr)) < 0)
    die("sendto");
struct dhcpmessage recvdhcpmsg;
socklen_t rservlen = sizeof(rservaddr);
if(recvfrom(sockfd,&recvdhcpmsg,sizeof(recvdhcpmsg),0,(struct sockaddr*)&rservaddr,&rservlen) < 0)
    die("recvfrom");
char *str = (char*)&recvdhcpmsg;
int i;
for(i=0;i<sizeof(recvdhcpmsg);i++)
    printf("%d_",str[i]);
printf("\n");
return 0;
}

Answer 1

The caveat is, you don’t have an IP address, you want to obtain one. – most OSs won’t allow you to bind/send UDP messages on a NIC w/o an IP address. dhcp clients typically use raw sockets for that purpose/and should set the src address to 0.0.0.0)

Such a raw socket will get all the packets, and your application will not if there’s a system dhcp client running