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[Discuss] Limit the number of ip addresses which can connect to a port

Write a small server that does the ACL management you want, and
proxies the user request to the real server.  Run the actual server on
a different port that's firewalled off to the outside world and/or
only binds on localhost.

Derek D. Martin   GPG Key ID: 0xDFBEAD02
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On Wed, Nov 01, 2017 at 12:31:43PM -0400, Tom Luo wrote:
> Hi, Derek,
> Thanks for your suggestions. However, this is not exactly what I want.
> The service is running in the server. Users can only connect to the server
> using an assigned TCP port and a password.
> This service using port/password to identify a user. Different users will
> get different ports and their own unique password.
> For example, user A will connect to port 8001 using password "testpassword".
> User B will connect to port 8002 using password "testpassword2".
> Every user has to pay a fee to use the service.
> This works very well if no user shares port/password with other people.
> To reduce this passowrd/port sharing issue, I propose to limit the number
> of ip addresses connected to a port.
> If a user A (assigned port 8010) shares his/her password with a person C,
> user A can connect to port 8010 and use the service.
> If at the same time the person C tries to connect to port 8010 from another
> ip address. the firewall should decline the new connection.
> I also check the connections and I see one user can have many connections
> to the assigned port at the same time. So, I cannot use the number of
> active connections on a port to solve this issue.
> The only thing I can think about is using IP address. I think video service
> providers like hulu and netflix face the same issue. But, I don't know how
> they deal with the password sharing issue.
> I hope I explained the issue clearly. BTW, I don't have the source code of
> the service, so I cannot change the service itself.
> Thanks a lot!
> Tom
> On Wed, Nov 1, 2017 at 10:54 AM, Derek Atkins <warlord at> wrote:
> > Tom,
> >
> > Tom Luo <mariolzx at> writes:
> >
> > > Yes. I want only one IP gets access to the service. However, I don't own
> > > this application and I don't have the source code. That is why I can only
> > > using firewall to handle it.
> > > If there is no software capable to handle this, I am thinking about
> > writing
> > > a shell script to do it myself.
> >
> > Just so I understand:   You have a service running on a server which
> > *anyone* can use.  But once *someone* is using it, further connections
> > can only come from that single IP address.   And then, once all
> > connections drop again (i.e., nobody is using the service), then it
> > opens up to anyone on any IP address again until someone else connects?
> >
> > Do I have this right?
> >
> > If so, I'm honestly not sure how to do this outside the application
> > itself.  You MIGHT be able to do it with tcp_wrappers with some state on
> > the machine for the number of open connections.
> >
> > Another option is that you MIGHT be able to do this with something like
> > fail2ban + firewalld.  Every time there is a first-time connection then
> > you add a firewall rule that limits access to only that IP address, and
> > then once the user "logs out" you remove that restriction.  Of course
> > you would need to ensure that the connection/disconnection get logged
> > properly, and you'd need to write the fail2ban scripts.
> >
> > If you cannot modify the application itself then this might be
> > challenging to get all the connect/disconnect messages to properly line
> > up.
> >
> > > Thanks,
> >
> > -derek
> >
> > --
> >        Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
> >        Member, MIT Student Information Processing Board  (SIPB)
> >        URL:    PP-ASEL-IA     N1NWH
> >        warlord at MIT.EDU                        PGP key available
> >
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