Can I bind a client socket to an ip not belongs to any interfaces?

Question

For a client socket, I can use bind() to bind it to a specific source Ip address to select a specific interface. Or I can use connect() directly then it will pick the source ip based on routing table.

I wonder can I bind a client socket to an ip not belongs to any interfaces ? E.g.: I have two interfaces: eth0 : ip0 eth1 : ip1

(1) If I bind the client socket to ip2. Is this feasible ? (2) If (1) is feasible, assuming client socket sent packets thru eth0. Then I configure the iptables in this client host, to forward all incoming packets to ip0 (eth0). In this case, if there are packets sent back from server side with destination ip address is ip2 (assuming this packet will reach my client host). Will my client socket receive the packet ?

Thanks in advance.

Answer 1

I don't really understand your question, but here goes:

For client sockets, you typically want the the OS and its routing table to pick the best interface for you using any available port. In which case, you bind to INADDR_ANY (0) and port 0. Or don't explicitly call bind at at all. Just call connect() and it will do the right thing.

If you need the client connection to occur through a specific interface, then bind the socket to a specific IP address. And then the OS will attempt to use that interface for the subsequent connect call and all traffic after that.

Attempting to bind the socket to an IP that doesn't belong to a local interface is surely going to result in an error.

Not sure what you mean about the iptables stuff. Sounds dicey.

Answer 2

Please have a look at:

https://www.rsyslog.com/doc/v8-stable/configuration/modules/omfwd.html#ipfreebind

MAN:

https://man7.org/linux/man-pages/man7/ip.7.html

IP_FREEBIND (since Linux 2.4) If enabled, this boolean option allows binding to an IP address that is nonlocal or does not (yet) exist. This permits listening on a socket, without requiring the underlying network interface or the specified dynamic IP address to be up at the time that the application is trying to bind to it. This option is the per-socket equivalent of the ip_nonlocal_bind /proc interface described below.