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


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.

 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
$ cat /sys/module/nf_conntrack/parameters/nf_conntrack_helper

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.


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"/>

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.

- Secure use of iptables and connection tracking helpers
- iptables connection tracking helpers
- Automatic Helper Assignment
- Netfilter Helpers


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