support setting IP_TRANSPARENT on sockets

[pepinns]

This allows you to set IP_TRANSPARENT on sockets for linux.

To setup for a local proxy you can use the following:
ref: https://powerdns.org/tproxydoc/tproxy.md.html

    export OUTBOUND_PORT=3100
    export PROXY_PORT=3000
    # move anything marked with a 1 --> table #100
    sudo ip rule add fwmark 1 lookup 100
    # table 100, treat all addrs as local loopback
    sudo ip route add local 0.0.0.0/0 dev lo table 100

    # all inbound traffic on $OUTBOUND_PORT should forward to localhost $PROXY_PORT transparently
    sudo iptables -t mangle -I POSTROUTING -p tcp --dport $OUTBOUND_PORT -j TPROXY --tproxy-mark 0x1/0x1 --on-port=$PROXY_PORT --on-ip=127.0.0.1

    # all output traffic for $OUTBOUND_PORT will get marked and sent to table #100 (loopback)
    sudo iptables -t mangle -A OUTPUT -p tcp -m tcp --dport $OUTBOUND_PORT -j MARK --set-mark 1

Once done, then newly accept()'ed sockets will have the local_addr() set to the originally requested address and not the currently bound listening address.

[Thomasdezeeuw]

I'm Ok with this change, but it needs to be behind the all feature (as it's not available on all OSs) and it needs to be moved and updated with some style changes. I've left some comments, but it shouldn't be too much work.

[Thomasdezeeuw]

There a typo in the docs: IP_TRANSPARENTF, otherwise LGTM.

[Thomasdezeeuw]

From the CI: can't use self.inner anymore, you need to use self.as_raw().

[pepinns]

Forgive me, happy to change it, but this confuses me and I'd like to understand what's going on.

I don't see as_raw in the source code for this project, is that from somewhere else? What does it do and why would my method need to be different than the others in the file? They appear to be using the same function(s).

Changing self.inner --> self.as_raw() still compiles, but I don't really understand why.
It looks like self.inner is a Socket which is actually a c_int which is a i32.
It also only seems to compile when used in this method, but put into other methods causes a no method named `as_raw` found for reference `&socket::Socket` in the current scope error.

Is there some implementation of as_raw for Socket or c_int somewhere?
my rust-analyzer is choking on this file so thats not really helping me either.

[pepinns]

Does it have to do with the feature directive somehow?

[Thomasdezeeuw]

This is due to #222 which I merged yesterday, e.g. see

socket2/src/socket.rs

Lines 694 to 697 in eaa5f8f

	unsafe {
	getsockopt::<c_int>(self.as_raw(), sys::SOL_SOCKET, sys::SO_BROADCAST)
	.map(|broadcast| broadcast != 0)
	}

It's the same as the old .inner, but we now use a TcpStream to take advantage of the niche representation of that type.

[pepinns]

Ok, rebased the branch against master and things look OK now.

[Thomasdezeeuw]

Thanks @pepinns!