Get Peer IP Address and Port - c

Question

I manage to get the host information from particular requested network adapter using code below but i have no idea how to get peer ip address from host ip address.
struct sockaddr_in *sa = (struct sockaddr_in *)&item->ifr_addr;
ipAddr = ntohl(*((u_int32_t *)&sa->sin_addr));
if (pIpAddr != NULL)
{
*pIpAddr = ipAddr;
}
// Get the MAC address
if ( ioctl(s, SIOCGIFHWADDR, item) < 0 )
{
printf("_GetMacAddress : SIOCGIFHWADDR failed!\n");
return 0;
}
else
{
struct sockaddr *eth = (struct sockaddr *) &item->ifr_ifru.ifru_hwaddr;
unsigned long *low = (unsigned long *)&eth->sa_data[2];
unsigned short *high = (unsigned short*)&eth->sa_data[0];
//printf("%s : MAC = 0x%04x, 0x%08x", ntohs(*high), ntohl(*low));
printf("Interface %8s : IP %3d.%3d.%3d.%3d : MAC = %02x:%02x:%02x:%02x:%02x:%02x\n",
item->ifr_name,
((ipAddr >> 24)&0xff), ((ipAddr >> 16)&0xff), ((ipAddr >> 8)&0xff), (ipAddr&0xff),
((ntohs(*high)>> 8)&0x00ff), (ntohs(*high)&0x00ff),
((ntohl(*low)>> 24)&0x00ff), ((ntohl(*low)>> 16)&0x00ff), ((ntohl(*low)>> 8)&0x00ff), (ntohl(*low)&0x00ff));
if ((pMacHigh != NULL) && (pMacLow != NULL))
{
*pMacHigh = *high;
*pMacLow = *low;
}
}
The output is:
_GetMaxNetworkInterfaces 3
Interface lo : IP 127. 0. 0. 1 : MAC = 00:00:00:00:00:00
Interface enp4s0 : IP 192.168.128. 88 : MAC = f4:8e:38:ea:88:23
Interface wlp5s0 : IP 192.168. 53. 63 : MAC = b8:81:98:b7:71:90

Answer 1

If you are server listening for incoming connection, you can get peer address from accept() , in the second its argument, when a new socket is accepted
From man page of accept():
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
The argument addr is a pointer to a sockaddr structure. This structure is filled in with the address of the peer socket, as known to the communications layer. The exact format of the address returned addr is determined by the socket's address family (see socket(2) and the respective protocol man pages). When addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL.

Answer 2

I have no idea how to get peer ip address from host ip address.
You can't. The question doesn't make sense.
To get a peer address you have to have a peer, and to have a peer you have to have a connected socket, with which you can call getpeername().
Or, if you're a server, you can get it as a side-effect of accept() via the second and third arguments.

Answer 3

I did tried to implement arp-scan to find the connected client Ip address.
First we have to define the max device connected for my case will be 32. I manage to get following information from previous code
Interface lo : IP 127. 0. 0. 1 : MAC = 00:00:00:00:00:00
Interface enp4s0 : IP 192.168.128. 88 : MAC = f4:8e:38:ea:88:23
Interface wlp5s0 : IP 192.168. 53. 63 : MAC = b8:81:98:b7:71:90
The information that require by arp-scan is interface name and the device IP address. In order to get client ip address we have to remove the character after counting the third dot. The end result we will get is:
127.0.0.
192.168.128.
192.168.53.
the last step is to loop for max device
127.0.0.0~31
192.168.128.0~31
192.168.53.0~31
Then feed into arp-scan ping and we will get the response from client ip address. I am using this arp-scan reference.