Re: Multiple connections from the same source address.

['Marcelo Ricardo Leitner' <marcelo.leitner@gmail.com>]

On Fri, Sep 08, 2017 at 03:01:22PM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 08 September 2017 15:50
> > On Fri, Sep 08, 2017 at 09:03:31AM +0000, David Laight wrote:
> > > Can anyone think how to create multiple outgoing connections
> > > from the same subset of local IP addresses and the same port?
> > >
> > > We bind() to the first IP address and port, then use
> > > SCTP_SOCKOPT_BINDX_ADD to add the second IP address.
> > > But even with IP_REUASADDR set you can't repeat that on
> > > a second socket.
> > 
> > When did you set IP_REUSEADDR? Checking the code, seems you have to set
> > it before any binding, because then the port entry will be created and
> > such flag will be initialized with that state.
> 
> Indeed, otherwise it has no effect.
> 

For both sockets, right? I'll debug this if yes.

> The point is that (IIRC) IP_REUSEADDR only lets you bind the same
> port number to 2 sockets - the IP addresses have to be different.

I'm not seeing where this could be happening. I see this:
                if (pp->fastreuse && sk->sk_reuse &&
                        sk->sk_state != SCTP_SS_LISTENING)
                        goto success;
Meaning that if the port allows reusing and so does the new socket, and
it's not in listening state, it would be allowed.

> Code that does this for TCP (like the old ftp data connection)
> only worked because the address it was binding was IN_ADDR_ANY
> and the existing socket had a local IP address.
> It always had to retry because two sockets couldn't be bound to
> the port and IN_ADDR_ANY either.
> 
> > > It is possible to bind a second socket by reversing the
> > > order of the IP addresses (this might be deemed to be a bug!)
> > 
> > Interesting. It does seem like a bug. Probably related to the above
> > behavior.
> 
> More likely the duplicate check is in some generic code that
> doesn't look at the extra addresses.

Can be.

  Marcelo