Using a specific network interface for a socket in windows - windows

Question

Is there a reliable way in Windows, apart from changing the routing table, to force a newly created socket to use a specific network interface? I understand that bind() to the interface's IP address does not guarantee this.

Answer 1

(Ok second time lucky..)
FYI there's another question here perform connect() on specific network adapter along the same lines...
According to The Cable Guy
Windows XP and Windows Server® 2003
use the weak host model for sends and
receives for all IPv4 interfaces and
the strong host model for sends and
receives for all IPv6 interfaces. You
cannot configure this behavior. The
Next Generation TCP/IP stack in
Windows Vista and Windows Server 2008
supports strong host sends and
receives for both IPv4 and IPv6 by
default on all interfaces except the
Teredo tunneling interface for a
Teredo host-specific relay.
So to answer your question (properly, this time) in Windows XP and Windows Server 2003 IP4 no, but for IP6 yes. And for Windows Vista and Windows 2008 yes (except for certain circumstances).
Also from http://www.codeguru.com/forum/showthread.php?t=487139
On Windows, a call to bind() affects
card selection only incoming traffic,
not outgoing traffic. Thus, on a
client running in a multi-homed system
(i.e., more than one interface card),
it's the network stack that selects
the card to use, and it makes its
selection based solely on the
destination IP, which in turn is based
on the routing table. A call to bind()
will not affect the choice of the card
in any way.
It's got something to do with
something called a "Weak End System"
("Weak E/S") model. Vista changed to a
strong E/S model, so the issue might
not arise under Vista. But all prior
versions of Windows used the weak E/S
model.
With a weak E/S model, it's the
routing table that decides which card
is used for outgoing traffic in a
multihomed system.
See if these threads offer some
insight:
"Local socket binding on multihomed
host in Windows XP does not work" at
http://www.codeguru.com/forum/showthread.php?t=452337
"How to connect a port to a specified
Networkcard?" at
http://www.codeguru.com/forum/showthread.php?t=451117.
This thread mentions the
CreateIpForwardEntry() function, which
(I think) can be used to create an
entry in the routing table so that all
outgoing IP traffic with a specified
server is routed via a specified
adapter.
"Working with 2 Ethernet cards" at
http://www.codeguru.com/forum/showthread.php?t=448863
"Strange bind behavior on multihomed
system" at
http://www.codeguru.com/forum/showthread.php?t=452368
Hope that helps!

Answer 2

I'm not sure why you say bind is not working reliably. Granted I have not done exhaustive testing, but the following solution worked for me (Win10, Visual Studio 2019). I needed to send a broadcast message via a particular NIC, where multiple NICs might be present on a computer. In the snippet below, I want the broadcast message to go out on the NIC with IP of .202.106.
In summary:
create a socket
create a sockaddr_in address with the IP address of the NIC you want to send FROM
bind the socket to that FROM sockaddr_in
create another sockaddr_in with the IP of your broadcast address (255.255.255.255)
do a sendto, passing the socket created is step 1, and the sockaddr of the broadcast address.
`
static WSADATA wsaData;
static int ServoSendPort = 8888;
static char ServoSendNetwork[] = "192.168.202.106";
static char ServoSendBroadcast[] = "192.168.255.255";
`
... < snip >
if ( WSAStartup(MAKEWORD(2,2), &wsaData) != NO_ERROR )
return false;
// Make a UDP socket
SOCKET ServoSendSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
int iOptVal = TRUE;
int iOptLen = sizeof(int);
int RetVal = setsockopt(ServoSendSocket, SOL_SOCKET, SO_BROADCAST, (char*)&iOptVal, iOptLen);
// Bind it to a particular interface
sockaddr_in ServoBindAddr={0};
ServoBindAddr.sin_family = AF_INET;
ServoBindAddr.sin_addr.s_addr = inet_addr( ServoSendNetwork ); // target NIC
ServoBindAddr.sin_port = htons( ServoSendPort );
int bindRetVal = bind( ServoSendSocket, (sockaddr*) &ServoBindAddr, sizeof(ServoBindAddr) );
if (bindRetVal == SOCKET_ERROR )
{
int ErrorCode = WSAGetLastError();
CString errMsg;
errMsg.Format ( _T("rats! bind() didn't work! Error code %d\n"), ErrorCode );
OutputDebugString( errMsg );
}
// now create the address to send to...
sockaddr_in ServoSendAddr={0};
ServoSendAddr.sin_family = AF_INET;
ServoSendAddr.sin_addr.s_addr = inet_addr( ServoSendBroadcast ); //
ServoSendAddr.sin_port = htons( ServoSendPort );
...
#define NUM_BYTES_SERVO_SEND 20
unsigned char sendBuf[NUM_BYTES_SERVO_SEND];
int BufLen = NUM_BYTES_SERVO_SEND;
ServoSocketStatus = sendto(ServoSendSocket, (char*)sendBuf, BufLen, 0, (SOCKADDR *) &ServoSendAddr, sizeof(ServoSendAddr));
if(ServoSocketStatus == SOCKET_ERROR)
{
ServoUdpSendBytes = WSAGetLastError();
CString message;
message.Format(_T("Error transmitting UDP message to Servo Controller: %d."), ServoSocketStatus);
OutputDebugString(message);
return false;
}