Question

Iptables mysteriously dropping packets

Asked by: beetos

I have an iptables firewall.  I have several networks connected together.  I'm trying to debug a packet which is getting lost.  

I have logging rules at the beginning and end of ALL chains.   I can trace other packets through these chains just fine, thus verifying the logging.  

The packet hits the last rule of the PREROUTING chain of the NAT table, which is the logging rule and is logged.   The default policy is ACCEPT, and as stated above, in all the other chains the logging rule is first.  However, the packet is lost - it never hits any other chain.

I would expect the packet to traverse the FORWARD chain of the MANGLE table next, but that does not happen.  Nor does it hit the INPUT chain of the MANGLE table.

What could be happening to my packet, and how can I prove it?

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Asked On
2009-09-30 at 18:29:21ID24775732
Tags

iptables routing debug debugging

Topics

Unix Networking

,

Linux Networking

Participating Experts
2
Points
500
Comments
6

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Answers

 

by: NopiusPosted on 2009-10-01 at 03:20:40ID: 25467229

According to iptables packet traversal chart, the packet may be dropped just after 'PREROUTING' chain if the routing process decides that there is no route.

http://www.shorewall.com.au/misc/netfilterflow.pdf

 

by: nociPosted on 2009-10-01 at 03:56:20ID: 25467394

- check your routing with netstat -rn

And verify that packets can go anywhere.

 

by: beetosPosted on 2009-10-01 at 10:10:42ID: 25470863

Nopius - thanks for the link to that chart.  That's an excellent tool for the discussion of iptables.

It definitely would appear that my packet is being dropped at the routing decision.  The question is why.  I have two other networks which communicate through the firewall to the network I'm trying to get to successfully.

So, to clarify, I have a vpn which connects to the firewall.  VPN -> FW -> NewNet works just fine.  I can see the packets come in, traverse the chains, and go on the the new network.  

I have other networks which talk through the firewall to the new network also:  HOSTA ->FW -> NewNet

The problem is machines behind the VPN server can't connect to the new network:  HOSTB -> FW -> Drop :(

The route from the firewall to the new network is proven by the other machines being able to connect through it.  I was assuming iptables rules in the firewall were causing the packet to be dropped, but via logging rules at the beginning and end of each chain i can see that the packet is being dropped between chains by the routing decision.  

But isn't the routing decision based on the Destination, and not the source?  If the route exists to the destination, why would it be dropped?


Thanks guys.

 

by: nociPosted on 2009-10-01 at 11:26:09ID: 25471637

Hm. VPN software has it's own filters..,
You might need to check what is allowed through tunnels.

 

by: beetosPosted on 2009-10-01 at 11:46:57ID: 25471910

thanks noci, but i'm actually making it past the vpn endpoint into the firewall.  Once the packet makes it into the firewall, and is passing through the iptables chains, shouldnt it be at the discretion of the firewall?

 

by: beetosPosted on 2009-10-01 at 12:34:41ID: 31635717

Nopius provided great clues, but Noci nailed it.  It was the vpn configuration on the fiewall side, which needed to add a rule place the network in a lookup table in the ip stack for the VPN.

20120131-EE-VQP-002

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