When using raw sockets
to send TCP
data, it is allowed to leave the source ip address
zero, so that the kernel puts in the correct value. This is helpful, especially when several interfaces (with different IP
addresses) are in use.
My problem is now: To calculate to TCP
checksum, I would need to know, what the source IP address is going to be in the end. This seems to be impossible to me?
Is there anyway to determine t开发者_如何学编程he source IP
of my outgoing packets?
(An alternative would be, to specifically bind my raw socket to an address, but I'd rather not want to do that).
/edit: Using Linux
If you don't bind your socket, the kernel has to find the source address based on the destination address.
Basically a route lookup is done and a destination interface determined. After that, the IP is taken from that interface: the source for your packet.
So your question turns into performing a route lookup, the way ip route get
does.
EDIT
@nos mentioned using a different socket (UDP) and connecting it to that destination address. Getting it's locally-bound name using getsockname
should give you the source address that would be used for that destination.
精彩评论