Debian Linix GRE protocol forwarding

I'm using tcpdump to see if gre protocol is forwarding properly from my Debian linux box.

Here is the command I'm using
tcpdump -i eth0 proto 47 > /root/dump.txt
But the log file is empty I'm thinking because the protocol isn't even coming in, right?
When I look for protocol 6 (tcp) it fills up as I try and pptp in.  Any ideas?  Should I be seeing the gre protocol coming in?
MainStaySolutionsAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

frashiiCommented:
You might want to check to see that you are monitoring the correct interface... When I ran the following command on eth0, I got no packets... but when I ran it on ppp0 I did.

Do an ifconfig and see if there are other interfaces.

firewall:/root # tcpdump -i ppp0 proto 47
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ppp0, link-type LINUX_SLL (Linux cooked), capture size 96 bytes
20:15:34.259140 IP xxxx.dyndns.org > xxxx.nwc.acsalaska.net: call 1039 ack 4294967295 no-payload
20:15:34.280656 IP xxx.dyndns.org > xxxx.nwc.acsalaska.net: call 1039 seq 0 gre-ppp-payload

2 packets captured
3 packets received by filter
0 packets dropped by kernel

firewall:/root # tcpdump -i eth0 proto 47
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes

0 packets captured
0 packets received by filter
0 packets dropped by kernel
0
frashiiCommented:
*ping* Did monitoring the ppp0 (or other encapsulated interface) show the GRE packets ?
0
MainStaySolutionsAuthor Commented:
No it didnt.
0
Ultimate Tool Kit for Technology Solution Provider

Broken down into practical pointers and step-by-step instructions, the IT Service Excellence Tool Kit delivers expert advice for technology solution providers. Get your free copy now.

http:// thevpn.guruCommented:
try

iptables -A OUPUT -p tcp --dport 47 -j LOG
iptables -A FORWARD -p tcp --dport 47 -j LOG
iptables -A INPUT -p tcp --dport 47 -j LOG

or UDP ..and then
tail -f /var/log/syslog and you will see the packets being logged if they are incoming/outgoing or being forwarded by the system
0
MainStaySolutionsAuthor Commented:
Thanks for the reply.  Interesting, I'm not seeing anything in the log files as I try to vpn through it.  I'm assuming that it means gre isn't passing through.
0
http:// thevpn.guruCommented:
Not necessarily in VPN is your traffic encrypted ?
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Linux Distributions

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.