How does kernel assign ip address and nic to sockets

Question

When you write something like:

   s=socket(AF_INET,SOCK_STREAM)
   s.connect((siteIp,80))
   s.send(...

How does the linux kernel know from which nic it is supposed to send the packets when no information about binding the socket is provided.

How can i for example replace this default nic with my own tun/tap interface?

Also where can i get more in depth information about this topic?

Remy Lebeau · Answer 1 · 2014-05-24T15:47:21.470

0

The OS uses its internal routing tables to make a best guess about which NIC has the best chance of reaching the IP you are connecting to. Routes are associated with NICs and priorities. If an outbound packet has a destination IP that matches a given route, the associated NIC is used. Otherwise, a route with an appropriate priority (such as a gateway or VPN) is used, since the packet IP may be on another network reached through that route.

If you want to specify which NIC is used, you have to bind() before you connect().

edited May 24 '14 at 15:47

answered May 24 '14 at 03:55

Remy Lebeau

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but how does a vpn route all trafiic through itself? – sia May 24 '14 at 06:53
A VPN has an entry in the OS routing tables, just like any other NIC/adapter. – Remy Lebeau May 24 '14 at 15:44

How does kernel assign ip address and nic to sockets

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