Obtaining the source IP and port of an INADDR_ANY client socket before the TCP three-way handshake?

问题

I'm on Windows 7, using bind before connect with SO_REUSEADDR, and setting the local address structure to IP address INADDR _ANY and port 0 (zero), in order to let the operating system select the source details for a client socket.

Firstly, I've read that it's not possible to get the source IP before connecting to the server, since it's being chosen at this point and several addresses can be valid. But the port is selected before the connection, so is there a way to get it? (getsockname() looks like to not work).

Secondly, about the source IP, is there a way to get it before a packet is sent to the server? I need the specific time between the moment the OS selected the source IP and the moment it starts the three-way handshake. The connect() function dominates the both.

回答1:

I'm on Windows 7, using bind before connect with SO_REUSEADDR

Why are you using SO_REUSEADDR in this situation? You don't need it, and it makes no sense for what you are attempting. SO_REUSEADDR should typically only be used for a listening socket, not a connecting socket.

setting the local address structure to IP address INADDR _ANY and port 0 (zero), in order to let the operating system select the source details for a client socket.

It is meaningless to bind() a client socket to INADDR_ANY:0. You can (and should) omit such a bind() completely and leave the socket unbound until connect() is called. The only time you should ever bind() a client socket is if you want to bind it to a specific local IP and/or Port. But you are not doing that in this situation, so get rid of it.

Firstly, I've read that it's not possible to get the source IP before connecting to the server, since it's being chosen at this point and several addresses can be valid.

Correct, unless you bind() to a specific source IP.

But the port is selected before the connection

Both source IP and source port are selected by connect() if the socket is unbound, or bound to source IP INADDR_ANY and/or source port 0. So you have no opportunity to query either value before connect() has selected them.

so is there a way to get it? (getsockname() looks like to not work).

getsockname() is exactly what you need. Just make sure you are not calling it until connect() has successfully connected to the server first. This is stated in the getsockname() documentation:

This call is especially useful when a connect call has been made without doing a bind first; the getsockname function provides the only way to determine the local association that has been set by the system.
...
The getsockname function does not always return information about the host address when the socket has been bound to an unspecified address, unless the socket has been connected with connect or accept (for example, using ADDR_ANY). A Windows Sockets application must not assume that the address will be specified unless the socket is connected. The address that will be used for the socket is unknown unless the socket is connected when used in a multihomed host. If the socket is using a connectionless protocol, the address may not be available until I/O occurs on the socket.

Secondly, about the source IP, is there a way to get it before a packet is sent to the server?

For TCP, you can retrieve the selected source IP using getsockname() immediately after connect() has successfully connected to the server. Not before.

I need the specific time between the moment the OS selected the source IP and the moment it starts the three-way handshake.

It is not possible to determine that detail. No socket application should ever need that detail. Why do you need it?

回答2:

I'm on Windows 7, using bind before connect with SO_REUSEADDR, and setting the local address structure to IP address INADDR _ANY and port 0 (zero), in order to let the operating system select the source details for a client socket.

Why? That's exactly what happens if you don't call bind() at all, during connect(). Binding a client socket to INADDR_ANY isn't correct in any case. Setting SO_REUSEADDR doesn't make sense either without specifying a non-zero port number. Just remove all this.

Firstly, I've read that it's not possible to get the source IP before connecting to the server, since it's being chosen at this point and several addresses can be valid.

Correct.

But the port is selected before the connection, so is there a way to get it?

Yes. getsockname().

(getsockname() looks like to not work)

Doesn't work how?

Secondly, about the source IP, is there a way to get it before a packet is sent to the server?

You can get it with getsockname() as soon as connect() has succeeded, but this involves sending packets to the server.

I need the specific time between the moment the OS selected the source IP and the moment it starts the three-way handshake. The connect() function dominates the both.

Bad luck.

来源：https://stackoverflow.com/questions/29331726/obtaining-the-source-ip-and-port-of-an-inaddr-any-client-socket-before-the-tcp-t