Review your favorite Linux distribution.
Go Back > Forums > Linux Forums > Linux - Server
User Name
Linux - Server This forum is for the discussion of Linux Software used in a server related context.


  Search this Thread
Old 06-04-2012, 01:05 PM   #1
LQ Newbie
Registered: Mar 2011
Distribution: Linux mint
Posts: 26

Rep: Reputation: 0
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 01:06 PM.
Old 06-04-2012, 11:40 PM   #2
Registered: Jan 2010
Distribution: Debian, Centos, Ubuntu, Slackware
Posts: 358
Blog Entries: 3

Rep: Reputation: 47
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'
1 members found this post helpful.
Old 06-06-2012, 02:04 AM   #3
LQ Newbie
Registered: Mar 2011
Distribution: Linux mint
Posts: 26

Original Poster
Rep: Reputation: 0
[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 02:06 AM.
Old 06-06-2012, 02:09 AM   #4
LQ Newbie
Registered: Mar 2011
Distribution: Linux mint
Posts: 26

Original Poster
Rep: Reputation: 0
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.


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Forwarding mail from linux nat sever to exchange server martino Linux - Newbie 1 09-05-2008 09:53 AM
Problem with port forwarding (NAT) on FC 5 heberrdacruz Linux - Networking 49 08-20-2006 12:32 AM
GRE keepalives. How can a SUSE 9.0 Linux box send GRE keepalives? dlef Linux - Networking 1 06-28-2005 12:00 PM
iptables + NAT + Port forwarding problem SirGertrude Linux - Networking 9 05-14-2004 04:02 AM
bizarre nat forwarding problem ignorantliwong Linux - Networking 1 09-04-2003 10:12 PM > Forums > Linux Forums > Linux - Server

All times are GMT -5. The time now is 09:39 AM.

Main Menu
Write for LQ is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Facebook: linuxquestions Google+: linuxquestions
Open Source Consulting | Domain Registration