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All

I have a Cisco 877 router and a Linode VPS running OpenBSD 5.9 with a GRE tunnel running in between, which works great and I can ping from either side. I have set up a static route in the Cisco router to route traffic to WhatsMyIP.org (so I can see if it's working) but, try as I might, I can't get OpenBSD's PF to apply NAT to traffic from the GRE tunnel. The configuration parses, the traffic routes, but I don't get any states being generated.

Is what I'm trying to achieve even possible? My topology and /etc.pf.conf are below. (NOTE: Updated as per Bink's answer)

#       $OpenBSD: pf.conf,v 1.54 2014/08/23 05:49:42 deraadt Exp $
#
# See pf.conf(5) and /etc/examples/pf.conf

set skip on lo

block return    # block stateless traffic

ext_if = "em0"
int_if = "gre0"
int_net = "192.168.2.0/24"

pass out on $ext_if from $int_net to any nat-to ($ext_if)
pass           # establish keep-state

# By default, do not permit remote connections to X11
block return in on ! lo0 proto tcp to port 6000:6010

pass quick on gre proto gre no state

Topology: GRE Tunnel Topology

ifconfig output (IPs redacted):

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 32768
        priority: 0
        groups: lo
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3
        inet 127.0.0.1 netmask 0xff000000
em0: flags=18843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,MPSAFE> mtu 1500
        lladdr f2:3c:91:0a:5b:a9
        priority: 0
        groups: egress
        media: Ethernet autoselect (1000baseT full-duplex)
        status: active
        inet E.F.G.H netmask 0xffffff00 broadcast E.F.G.255
enc0: flags=0<>
        priority: 0
        groups: enc
        status: active
pflog0: flags=141<UP,RUNNING,PROMISC> mtu 33144
        priority: 0
        groups: pflog
gre0: flags=9011<UP,POINTOPOINT,LINK0,MULTICAST> mtu 1476
        priority: 0
        groups: gre
        tunnel: inet A.B.C.D -> E.F.G.H
        inet 172.16.56.1 --> 172.16.56.2 netmask 0xffffff00
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It’s not clear to me why you need a static route on your router, but, regardless, you have no rule in your pf.conf that relates to NAT. Try something like the following after your macros in pf.conf:

match out on egress from ! ( egress ) nat-to ( egress )

Or:

match out on $int_if from ! ( $int_if ) nat-to ( $int_if )

If those don’t work, post the output of ifconfig.

  • Sorry, code blindness, didn't even notice that I'd left the nat-to off the pass rule. I've fixed this and even though I'm now getting states, I'm still not getting any response. I'll update my original post with my ifconfig shortly. – Aaron Mason Jun 24 '16 at 1:01
  • Also, the static route is so that only certain traffic goes over the tunnel, not all traffic. – Aaron Mason Jun 24 '16 at 1:04
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It seems that:

pass out on $ext_if from $int_net to any nat-to $ext_if

... doesn't work, it has to be this:

match out on $ext_if from $int_net to any nat-to $ext_if
pass      on $ext_if from $int_net to any

Also, it helps to make sure net.inet.ip.forwarding is set to 1.

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