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Old 06-04-2012, 02:05 PM   #1
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Distribution: Linux mint
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linux gre nat problem with traffic forwarding

[home net] -> (eth0)[centos router](eth1) -> [isp] -> office

isp network:

office net:

the centos router does nat for my home pcs and i wanted a nice gre tunnel to the office, so i ssh into both centos and office router, setup tun interfaces, assign ip addresses etc, ping test, both hosts(centos router and office router) can ping each other's gre tun interfaces.
my router's gre endpoint ip address is
office router's gre endpoint ip addr is

however, when i try to ping hosts at the office from within my [home net] i get timeouts, but if i try to ping the local gre endpoint it works.
user@ : ping -> times out. >
user@ : ping -> works \m/
user@ : ping -> phails

i'm not sure i understand why my centos box doesn't forward packets from my home net through the gre and back again.

err... yes, isp does natting and i do natting too. so home boxes are behind 2 nats but i dont care.

PS: i can't add a 'gre' tag to this thread lol

Last edited by seccentral; 06-04-2012 at 02:06 PM.
Old 06-05-2012, 12:40 AM   #2
Registered: Jan 2010
Location: Kurgan, Russia
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try to load the 'nf_conntrack_pptp' and 'nf_conntrack_proto_gre' module. maybe 'nf_nat_proto_gre'
Inspect all available gre and pptp modules with 'modprobe -l | grep pptp' anf 'modprobe -l | grep gre'
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Old 06-06-2012, 03:04 AM   #3
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[root@x-router ~]# modprobe -l | grep gre

[root@x-router ~]# modprobe -l | grep pptp

and on the remote office box:
[root@thor ~]# modprobe -l | grep gre
[root@thor ~]# modprobe -l | grep pptp
still no luck :/

Last edited by seccentral; 06-06-2012 at 03:06 AM.
Old 06-06-2012, 03:09 AM   #4
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Distribution: Linux mint
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oh lord, i had routes to the office internal network set up on my home nat box, but i forgot to put routes to my internal home network on the remote office router box.
now that i've placed them it's all good.

thanks for the module tip tho, it helped.


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