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Debian Linix GRE protocol forwarding

Posted on 2007-03-18
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Last Modified: 2013-12-06
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?
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Question by:MainStaySolutions
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Expert Comment

by:frashii
ID: 18855625
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
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by:frashii
ID: 19006443
*ping* Did monitoring the ppp0 (or other encapsulated interface) show the GRE packets ?
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by:MainStaySolutions
ID: 20728161
No it didnt.
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Expert Comment

by:http:// thevpn.guru
ID: 20728231
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
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Author Comment

by:MainStaySolutions
ID: 20733771
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.
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http:// thevpn.guru earned 250 total points
ID: 20733797
Not necessarily in VPN is your traffic encrypted ?
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