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How to realize if packet has traversed from Netfilter

+1 vote
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I know that packet traverses through the Net Filter hooks but how to practically realize that, any suggestions...

posted Dec 2, 2013 by Majula Joshi

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It's not really clear what you are asking. Are you just looking for general documentation?
http://netfilter.org/

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+1 vote

Is there a way to find out if there any iptables rules set on a machine ?

There are some indirect ways which will not always work; for example, I know that on most hosts, iptables -S will return the following output (when no iptable rules are set)
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT

So you can check whether or not the number of output lines is greater than 3 (as an indication of whether or not iptables rules are set). But there are hosts on which there are more chains then these 3; these chains are set by application/services, even without any iptable rules which are set. And after running iptables -F on these machines, iptables -S will still show more than 3 chains, even that there are no iptables rules set in these chains.

So the question is - is there a way to know whether or not netfilter rules are set on a host, regardless of the number of chains ?

0 votes

Is it possible to bind multiple address families in netfilter queue? I see IPv4 show up in my queue, but not ARP. With error code removed, here is how I'm calling nfq_bind:

netfilterqueue_handle = nfq_open();
netfilterqueue_queue = nfq_create_queue( netfilterqueue_handle, 0,

nfq_bind_pf( netfilterqueue_handle, AF_INET );
nfq_bind_pf( netfilterqueue_handle, NF_ARP );

I'm thinking the more likely possibility is the iptable rules I'm using to send traffic to the queue are too restrictive. Here are the rules I have:

# Generated by iptables-save v1.4.21 on Sat Feb 14 10:40:46 2015
*nat
:PREROUTING ACCEPT [161:14105]
:INPUT ACCEPT [56:4995]
:OUTPUT ACCEPT [56:4496]
:POSTROUTING ACCEPT [56:4496]
-A POSTROUTING -s 10.0.1.0/24 -o eth0 -j MASQUERADE
COMMIT
# Completed on Sat Feb 14 10:40:46 2015
# Generated by iptables-save v1.4.21 on Sat Feb 14 10:40:46 2015
*filter
:INPUT ACCEPT [1017:217421]
:FORWARD DROP [53:2307]
:OUTPUT ACCEPT [934:211104]
:MYRA - [0:0]
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j MYRA
-A FORWARD -s 10.0.1.0/24 -o eth0 -m conntrack --ctstate NEW -j MYRA
-A MYRA -j NFQUEUE --queue-num 0 --queue-bypass
COMMIT
# Completed on Sat Feb 14 10:40:46 2015

Do I have to add another FORWARD line to get ARP to jump to MYRA? What would it look like?

+1 vote

According to your experience what would be the best strategy to intercept traffic from one machine to another and process some (not all) request in a transparent way.

I explain, i have two machines:

192.168.1.1/24  192.168.1.2/24

All I want to do is to intercept traffic from a specific port(s), i.e. 4000/tcp and process it in a 'machine in the middle'.

192.168.1.1/24  machine-in-the-middle  192.168.1.2/24

The idea is that when 192.168.1.1 connects to 192.168.1.2:4000 then the machine in the middle will answer those requests, but the remaining traffic from 192.168.1.1 to 192.168.1.2 keep forwarding as is, and the same for the opposite direction.

+5 votes

I want to forward all http traffic coming in from a perticular IP at local port 8080, to 2 particular IP Addresses (port 80). Is it enough to prepend the following in my IPtable

-A PREROUTING -s xx.xx.xx.xx/24 -p tcp --dport 8080 -j DNAT --to-destination xxx.xxx.xxx.xxx:80
-A PREROUTING -s xx.xx.xx.xx/24 -p tcp --dport 8080 -j DNAT --to-destination yyy.yyy.yyy.yyy:80

+1 vote

How do I add a multiport rule

 nft add rule ip filter output tcp dports 99,200 ip daddr 1.1.1.1
 counter meta oif eth0 fails.

I see that the range works,

 nft add rule ip filter output tcp dport 99-105 ip daddr 1.1.1.1
 counter meta oif eth0
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