How I can choose network device in socket ()? - linux

Question

My plan is to write a user-based bandwidth control for Internet connection. To do this, I first want to write a network cable emulator.
My Linux box has three network devices:
Eth0 for normal connection to the network.
Eth1 and Eth2 are the endings of the emulated network cable.
So, what my program has to do is only to get every network packet from the input of eth1 and put it to the output ofeth2, and get every network packet from the input of eth2 and put it to the output of eth1.
With s = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL)), I can access every network packet. Hope that eth1and eth2 are in promiscuous mode.
But, how do I choose the network device? Later, I want to differentiate on the IP source-address that is in each packet, and how long this packet must wait, before I put it to the other network device.

Answer 1

With socket () you can not chose your network device. This is done with sendto () and recvfrom (). At argument 5 of recvfrom () and sendto () you put a sockaddr struct. There you have a field sockaddr.sll_ifindex where you choose your network card. In my environment 3 for enp3s0 and 4 for enp3s1.
See the cable-emulator code. I testet it 20 minutes and it worked without problems.
// network cable emulator
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
#include <netinet/ether.h>
#include <arpa/inet.h>
#include <linux/if_packet.h>
signed long long llmac (unsigned char* pmac)
{
signed long long sum= 0;
signed long long sumd;
for (signed long lauf= 0; lauf < 6; lauf++)
{
sumd= pmac[lauf];
sum= sum | (sumd << (40 - lauf*8));
}
return sum;
}
struct macliste
{
signed long long anz;
signed long long mac[100000];
macliste ();
void add (unsigned char* pmac);
signed long neu (unsigned char* pmac);
};
macliste::macliste ()
{
anz= 0;
}
void macliste::add (unsigned char* pmac)
{
if (neu (pmac))
{
mac[anz]= llmac (pmac);
anz++;
}
}
signed long macliste::neu (unsigned char* pmac)
{
signed long long smac;
smac= llmac (pmac);
for (signed long lauf= 0; lauf < anz; lauf++)
if (mac[lauf] == smac)
return 0;
return 1;
}
int main ()
{
int sockemp, socksenleft, socksenright;
signed long result;
unsigned char transferpuffer[10000];
struct sockaddr_ll sockaddrleft;
struct sockaddr_ll sockaddrright;
struct sockaddr_ll sockaddrboth;
signed long sockaddrsize= sizeof (sockaddrboth);
char ifnameleft[IFNAMSIZ] = "enp3s0";
char ifnameright[IFNAMSIZ] = "enp3s1";
struct ifreq ifindexleft;
struct ifreq ifindexright;
int indexleft;
int indexright;
// Socket öffnen
sockemp= socket (AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
printf ("sockemp: %5d\n", sockemp);
// Socket öffnen
socksenleft= socket (AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
printf ("socksenleft: %5d\n", socksenleft);
// Socket öffnen
socksenright= socket (AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
printf ("socksenright: %5d\n", socksenright);
printf ("\n");
/* den Index des Interfaces ermitteln */
printf ("Interfacename left: %s\n", ifnameleft);
printf ("Interfacename right: %s\n", ifnameright);
printf ("\n");
memset (&ifindexleft, 0, sizeof (struct ifreq));
memset (&ifindexright, 0, sizeof (struct ifreq));
strncpy (ifindexleft.ifr_name, ifnameleft, IFNAMSIZ);
strncpy (ifindexright.ifr_name, ifnameright, IFNAMSIZ);
result= ioctl (socksenleft, SIOCGIFINDEX, &ifindexleft);
result= ioctl (socksenright, SIOCGIFINDEX, &ifindexright);
indexleft= ifindexleft.ifr_ifindex;
indexright= ifindexright.ifr_ifindex;
printf ("interfaceresult: %5ld indexleft %5d\n", result, indexleft);
printf ("interfaceresult: %5ld indexright %5d\n", result, indexright);
printf ("\n");
/* Socketadresse vorbereiten */
memset (&sockaddrleft, 0, sizeof(struct sockaddr_ll));
sockaddrleft.sll_family = PF_PACKET; /* RAW communication */
sockaddrleft.sll_hatype = ARPHRD_ETHER; /* Ethernet */
sockaddrleft.sll_pkttype = PACKET_OTHERHOST; /* Ziel ist ein anderer Rechner */
sockaddrleft.sll_ifindex = indexleft; /* Interface-Index */
sockaddrleft.sll_halen = ETH_ALEN; /* Address length*/
/* Socketadresse vorbereiten */
memset (&sockaddrright, 0, sizeof(struct sockaddr_ll));
sockaddrright.sll_family = PF_PACKET; /* RAW communication */
sockaddrright.sll_hatype = ARPHRD_ETHER; /* Ethernet */
sockaddrright.sll_pkttype = PACKET_OTHERHOST; /* Ziel ist ein anderer Rechner */
sockaddrright.sll_ifindex = indexright; /* Interface-Index */
sockaddrright.sll_halen = ETH_ALEN; /* Address length*/
/* Socketadresse vorbereiten */
memset (&sockaddrboth, 0, sizeof(struct sockaddr_ll));
sockaddrboth.sll_family = PF_PACKET; /* RAW communication */
sockaddrboth.sll_hatype = ARPHRD_ETHER; /* Ethernet */
sockaddrboth.sll_pkttype = PACKET_OTHERHOST; /* Ziel ist ein anderer Rechner */
sockaddrboth.sll_halen = ETH_ALEN; /* Address length*/
// Schnittstellen in den Promiskuitätsmodus schalten
struct ifreq ifr;
int raw_socket;
char deviceleft[100]= "enp3s0";
char deviceright[100]= "enp3s1";
memset (&ifr, 0, sizeof (struct ifreq));
// Open A Raw Socket Deviceleft
if ((raw_socket = socket (PF_PACKET, SOCK_RAW, htons (ETH_P_ALL))) < 1)
{
printf ("ERROR: Could not open socket, Got #?\n");
exit (1);
}
/* Set the device to use */
strcpy (ifr.ifr_name, deviceleft);
/* Get the current flags that the device might have */
if (ioctl (raw_socket, SIOCGIFFLAGS, &ifr) == -1)
{
perror ("Error: Could not retrive the flags from the device.\n");
exit (1);
}
/* Set the old flags plus the IFF_PROMISC flag */
ifr.ifr_flags |= IFF_PROMISC;
if (ioctl (raw_socket, SIOCSIFFLAGS, &ifr) == -1)
{
perror ("Error: Could not set flag IFF_PROMISC");
exit (1);
}
printf ("Entering promiscuous mode\n");
/* Configure the device */
if (ioctl (raw_socket, SIOCGIFINDEX, &ifr) < 0)
{
perror ("Error: Error getting the device index.\n");
exit (1);
}
// Open A Raw Socket Deviceright
if ((raw_socket = socket (PF_PACKET, SOCK_RAW, htons (ETH_P_ALL))) < 1)
{
printf ("ERROR: Could not open socket, Got #?\n");
exit (1);
}
/* Set the device to use */
strcpy (ifr.ifr_name, deviceright);
/* Get the current flags that the device might have */
if (ioctl (raw_socket, SIOCGIFFLAGS, &ifr) == -1)
{
perror ("Error: Could not retrive the flags from the device.\n");
exit (1);
}
/* Set the old flags plus the IFF_PROMISC flag */
ifr.ifr_flags |= IFF_PROMISC;
if (ioctl (raw_socket, SIOCSIFFLAGS, &ifr) == -1)
{
perror ("Error: Could not set flag IFF_PROMISC");
exit (1);
}
printf ("Entering promiscuous mode\n");
/* Configure the device */
if (ioctl (raw_socket, SIOCGIFINDEX, &ifr) < 0)
{
perror ("Error: Error getting the device index.\n");
exit (1);
}
// Pakete transferieren
getchar ();
signed long packetcounter= 0;
macliste emplisteleft;
macliste emplisteright;
while (1)
{
result= recvfrom (sockemp, transferpuffer, 10000, 0, (struct sockaddr*) (&sockaddrboth), (socklen_t*)&sockaddrsize);
if ((sockaddrboth.sll_ifindex != indexleft) && (sockaddrboth.sll_ifindex != indexright))
continue;
packetcounter++;
if ((sockaddrboth.sll_ifindex == indexleft) && (emplisteright.neu (transferpuffer + 6)))
{
emplisteleft.add (transferpuffer + 6); // Ethernet II Paket Quelladresse
result= sendto (socksenright, transferpuffer, result, 0, (struct sockaddr*) &sockaddrright, sizeof (struct sockaddr_ll));
printf ("sendresult: %5ld %2d\n", result, indexright);
}
if ((sockaddrboth.sll_ifindex == indexright) && (emplisteleft.neu (transferpuffer + 6)))
{
emplisteright.add (transferpuffer + 6); // Ethernet II Paket Quelladresse
result= sendto (socksenleft, transferpuffer, result, 0, (struct sockaddr*) &sockaddrleft, sizeof (struct sockaddr_ll));
printf ("sendresult: %5ld %2d\n", result, indexleft);
}
printf ("Packet %10ld empresult: %5ld %2d %20llx %10lld %10lld\n", packetcounter, result, sockaddrboth.sll_ifindex, llmac (transferpuffer + 6), emplisteleft.anz, emplisteright.anz);
}
return 0;
}
compile with gcc 9.3.0 under ubuntu 20.04 server:
g++ transferraw.cc -std=c++17 -Wall -Wextra -Wconversion -pedantic-errors -O2 -o transferraw