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what would be the best strategy to intercept traffic from one machine to another

+1 vote
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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.

posted Aug 21, 2013 by Mandeep Sehgal

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1 Answer

+1 vote

Bridge with TPROXY, I guess.

answer Aug 24, 2013 by Dewang Chaudhary
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+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

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

0 votes

Consider the following example: you have a router between two networks, and you want to cut off the
router from the outside world using some iptables rules. However, all traffic that is forwarded by the router between the two networks basically is to be ignored by iptables (i.e., the router does not play firewall for any of the two networks).

Currently, if conntrack is loaded on the router, then conntrack -L on the router lists all the connections, not only those to and from the router, but also all connections between the two. Certainly, it takes some CPU cycles for the router to keep track of all the connections. Also, the number of connections that conntrack can take of is limited.

So is there a way to let Linux "bypass" conntrack and maybe other netfilter stuff when it comes to forwarded packets?

+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?


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