Re: [PATCH] nm-pptp-service: Grant proto GRE by firewalld

[Thomas Woerner <twoerner redhat com>]

Hello,

I am sorry for the late reply, I have been on vacation and am back at work 
since yesterday with still a huge amount of emails to take care of.

On 03/03/2017 10:24 AM, poma wrote:
> On 02.03.2017 20:32, Lubomir Rintel wrote:
> > On Wed, 2017-03-01 at 08:07 +0100, poma wrote:
> > > From 28b7713cda1deba1b54bd9e52b0d62716e356b66 Mon Sep 17 00:00:00 2001
> > > > From: poma <poma gmail com>
> > > Date: Wed, 1 Mar 2017 07:05:40 +0100
> > > Subject: [PATCH] nm-pptp-service: Grant proto GRE by firewalld.
> > >
> > > With recent kernels, the Poptop - The PPTP Server for Linux (pptpd) requires
> > > explicit load of nf_conntrack_pptp kernel module to achieve the operating state of the service itself.
> > > However this is not the case with the PPTP Client (pptp) on a Linux based platform.
> > > What is needed is to apply directly, rule within the firewalld, to grant proto gre,
> > > to achieve the operating state of the client itself.
> > >
> > > Ref.
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1187328
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1214643
> > > ---
> > >  src/nm-pptp-service.c | 16 ++++++++++------
> > >  1 file changed, 10 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/src/nm-pptp-service.c b/src/nm-pptp-service.c
> > > index 1710fd9..6a66386 100644
> > > --- a/src/nm-pptp-service.c
> > > +++ b/src/nm-pptp-service.c
> > > @@ -1113,7 +1113,7 @@ main (int argc, char *argv[])
> > > >         GMainLoop *main_loop;
> > > >         gboolean persist = FALSE;
> > > >         GOptionContext *opt_ctx = NULL;
> > > > -       char *conntrack_module[] = { "/sbin/modprobe", "nf_conntrack_pptp", NULL };
> > > > +       char *firewalld_grant_proto_gre[] = { "/bin/firewall-cmd", "--direct", "--add-rule", "ipv4", "filter", "INPUT", "0", "-p", 
> > > > "gre", "-j", "ACCEPT", NULL };
> > > > ...
> > > > +       if (!g_spawn_sync (NULL, firewalld_grant_proto_gre, NULL, 0, NULL, NULL, NULL, NULL, NULL, &error)) {
> > > > +               _LOGW ("granting proto gre by firewalld failed: %s", error->message);
> > > >                 g_error_free (error);
> > > >         }
> >
> > As Thomas Haller already suggested; we probably should not be removing
> > the module load. It doesn't seem to have anything to do with a missing
> > firewall rule.
>
> With only loaded modules it will not work if:
> $ sysctl net.netfilter.nf_conntrack_helper
> net.netfilter.nf_conntrack_helper = 0
> OR
> $ cat /sys/module/nf_conntrack/parameters/nf_conntrack_helper
> N
>
> The correct firewall(s) rule(s) should replace both.
>
> > I'm not sure either whether we should be punching holes in the firewall
> > automatically or if this is the proper way to do that. I'm especially
> > not sure if we should be calling firewall-cmd instead of talking D-Bus
> > and if it's all right that we don't clean up the rule when we tear down
> >  all PPTP connections.
> >
> > Adding Thomas Woerner to the loop; hopefully he'll be able to provide
> > some help.
> >
> > Lubo
>
> I wrote about firewalld because I use it.
> No one's stopping NM to talk via D-BUS with any implementation of a firewall that is used.
>
> So, what is your actual recommendation, in this particular case?

nf_conntrack_pptp is using the control port tcp/1723. The loading of the module 
might not be enough. With disabled automatic helper assignment, it is also 
needed to add rules in the raw table to actually use the helper:

iptables -t raw -A PREROUTING -p tcp -m tcp --dport 1723 -j CT --helper pptp

This should only be done in the zone only that is used for the connection.

firewalld is providing the pptp helper configuration file already:

<helper module="nf_conntrack_pptp" family="ipv4">
  <port protocol="tcp" port="1723"/>
</helper>

If the helper is used (in a service), then the module is loaded and the needed 
rules are applied to the used zone.

If you are interested we could work on an interface to load the pptp helper and 
apply the needed rule with firewalld for a pptp connection in the zone that 
will be used for the connection. I think it should be simple to do additional 
things like enabling the gre protocol also for this zone.

> Ref.
> - Secure use of iptables and connection tracking helpers
>   https://home.regit.org/netfilter-en/secure-use-of-helpers
> - iptables connection tracking helpers
>   http://www.odi.ch/weblog/posting.php?posting=663
> - Automatic Helper Assignment
>   http://www.firewalld.org/2016/10/automatic-helper-assignment
> - Netfilter Helpers
>   http://shorewall.org/Helpers.html

Regards,
Thomas