How to detect internet connectivity on a non gateway interface linux - c++

Question

I have a Linux system with 2 NICs, one of which (eth0) is the default gateway. The other (eth1) is not a default gateway. Both have network connections with internet connectivity - so the link state is up.
I want to periodically check on eth1 to see if Internet traffic can be routed through it. Whilst I am checking eth1, I need to keep my Internet traffic routing through eth0 uninterrupted. If I can make a connection through eth1, I will then switch my default gateway (as that is my cheaper connection).
I am trying to check the connection by opening a socket, binding it to eth1 and connecting to a port. I have used 8.8.8.8:443 for my example.
bool canConnect(const string interface)
{
bool result = false;
sockaddr_in host;
timeval timeout;
int sock = ::socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0);
if (sock == -1)
return result;
struct ifreq ifr;
::strncpy(ifr.ifr_name, interface.c_str(), IFNAMSIZ);
int res = setsockopt(_sock, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr));
if (res < 0)
{
::close(sock);
return result;
}
int reuse = 1;
if (::setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(int)) < 0)
{
::close(sock);
return result;
}
::inet_pton(AF_INET, "8.8.8.8", host.sin_addr);
host.sin_family = AF_INET;
host.sin_port = htons(443);
if (::connect(sock, (struct sockaddr *)&host , sizeof(host)) < 0)
{
int error = errno;
if (error != EINPROGRESS) // socket is non-blocking
{
::close(sock);
return result;
}
}
fd_set writeSet;
fd_set readSet;
FD_ZERO(&readSet);
FD_ZERO(&writeSet);
FD_SET(sock, &readSet);
FD_SET(sock, &writeSet);
timeout.tv_sec = 5;
timeout.tv_usec = 0;
int res = ::select(sock + 1, &readSet, &writeSet, NULL, &timeout);
if (res < 0)
{
::close(sock);
return result;
}
else if (res == 0)
{
// function always exits here
::close(sock);
return result;
}
else if (FD_ISSET(sock, &writeSet) || FD_ISSET(sock, &readSet))
{
int val;
socklen_t len = sizeof(val);
::getsockopt(sock, SOL_SOCKET, SO_ERROR, &val, &len);
if (val == 0)
result = true;
}
::close(sock);
return result;
}
When I run the code, select() always times out. I never manage to make a connection to 8.8.8.8:443, even though eth1 has Internet connectivity (I have proven this by switching the default gateway to eth1 temporarily.
Am I missing some code, or do I need kernel options to make this work?
I have IP forwarding turn on.

Answer 1

If connect() is successful and waiting, you should be checking select() for writable only, not readable at all. This is stated in the connect() mangage:
EINPROGRESS
The socket is nonblocking and the connection cannot be
completed immediately. It is possible to select(2) or poll(2)
for completion by selecting the socket for writing. After
select(2) indicates writability, use getsockopt(2) to read the
SO_ERROR option at level SOL_SOCKET to determine whether
connect() completed successfully (SO_ERROR is zero) or
unsuccessfully (SO_ERROR is one of the usual error codes
listed here, explaining the reason for the failure).
Besides, a socket is readable only if there is inbound data to be read, but you are connecting to an HTTPS port, so there will never be any data to read in your example, as you are not sending an HTTPS handshake request.
Also, you should not be calling select() unless connect() actually fails with EINPROGRESS. If connect() returns 0, the connection was successful right way without blocking. So, you should move your select() call inside your if (connect < 0) block after checking errno.
Also, you are not passing the correct socket to setsockopt(SO_BINDTODEVICE).
Also, you are not passing the correct value to the dst parameter of inet_pton(). You are passing it an in_addr, but it expects an in_addr* instead.
Also, there is no need to use SO_REUSEADDR in this example, since you are not bind()'ing to a specific local IP/port pair before calling connect().
Try something more like this:
bool canConnect(const string &interface)
{
bool result = false;
int sock = ::socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0);
if (sock == -1)
return result;
struct ifreq ifr;
::strncpy(ifr.ifr_name, interface.c_str(), IFNAMSIZ);
int res = setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr));
if (res < 0)
{
::close(sock);
return result;
}
sockaddr_in host;
::inet_pton(AF_INET, "8.8.8.8", &(host.sin_addr));
host.sin_family = AF_INET;
host.sin_port = htons(443);
result = (::connect(sock, (struct sockaddr *)&host, sizeof(host)) == 0);
if (!result)
{
if (errno != EINPROGRESS) // socket is non-blocking
{
::close(sock);
return result;
}
fd_set writeSet;
timeval timeout;
FD_ZERO(&writeSet);
FD_SET(sock, &writeSet);
timeout.tv_sec = 5;
timeout.tv_usec = 0;
res = ::select(sock + 1, NULL, &writeSet, NULL, &timeout);
if (res <= 0)
{
::close(sock);
return result;
}
int val;
socklen_t len = sizeof(val);
::getsockopt(sock, SOL_SOCKET, SO_ERROR, &val, &len);
result = (val == 0);
}
::close(sock);
return result;
}
That being said, 8.8.8.8 is a DNS server, so why would you think it is running an HTTPS server as well? Make sure you are connecting to an actual HTTPS server (why HTTPS and not HTTP?). You might consider using getaddrinfo() instead to get the IP address for www.google.com (or any other commonly used webserver) instead of hard-coding 8.8.8.8.