NAT

Commandline Foo

Show all FORWARD rules

iptables -L  FORWARD

See all nat rules in live time

watch -n2 iptables -nvL -t nat --line-number

Remove a sepcific rule in a specific CHAIN with a certain LINE-NUMBER. CHAINNAME is e.g. [PREROUTING | INPUT | OUTPUT | POSTROUTING | ...] like in:

Chain POSTROUTING (policy ACCEPT 13551 packets, 823K bytes)  
num   pkts bytes target      prot opt in     out     source        destination        
1    13551  823K postmodules all  --  *      *       0.0.0.0/0     0.0.0.0/0        
2        0     0 SNAT        all  --  *      eth0   !192.168.1.4   0.0.0.0/0  to:192.168.1.4

#!/bin/bash
echo "Usage: nat_remove_line CHAINNAME LINE-NUMBER"
echo "view with"
echo "    iptables -nvL -t nat --line-number"
iptables -t nat -D $1 $2

Bug in Ubuntu for accessing virtual machines behind a bridge

On ubuntu 12.04 there is a Ubuntu bug #50093 (mentioned here) which prevents accessing a machine inside the bridges network:

> vim /etc/sysctl.conf
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0

Acitvate

sysctl -p /etc/sysctl.conf  

Make permanent

> vim /etc/rc.local
*** Sample rc.local file ***
/sbin/sysctl -p /etc/sysctl.conf
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS  --clamp-mss-to-pmtu
exit 0

Verify

tail /proc/sys/net/bridge/*
iptables -L  FORWARD

> brctl show
bridge name     bridge id               STP enabled     interfaces 
br1             8000.50e5492d616d       no              eth1 
                                                        vnet1
[...]

Accessing ports in servers behind a natted bridge and firewall

Be careful. By doing this, you open up ports to the outside world. If you are using pfSense in front of your host or another firewall you can simply restrict this by using VPN.

Access from internet to guest:

internet -> pfSense (WAN / host ip and port) 
         -> host port -> iptables -> nat bridge -> guest port

Access only via vpn to guest

internet -> pfSense (OpenVPN / host ip and port) 
         -> host port -> iptables -> nat bridge -> guest port

Use the next script ...

Bash script for setting ip tables

Thanks to the scripts mentioned here: http://wiki.libvirt.org/page/Networking#Forwarding_Incoming_Connections I am aware of duplicate code, feel free to reduce and inform me to my email on my site http://www.blue-it.org.

Beware of setting your firewall's ports correctly. They must forwarding for NAT pointing to the hosts (!!!) ip, not the one you mentioned in this script.

This example sets two

To add a rule

set_iptables add

To remove it

set_iptables remove

> vim ~/bin/set_iptables

#!/bin/bash

del_prerouting() {
	iptables -t nat -D PREROUTING -p tcp --dport ${1} -j DNAT --to ${2}:${3}
}

del_forward() {
     iptables -D FORWARD -d ${1}/32 -p tcp -m state --state NEW -m tcp --dport ${2} -j ACCEPT
}

del_output() {
    #- allows port forwarding from localhost but 
    #  only if you use the ip (e.g http://192.168.1.20:8888/)
	iptables -t nat -D OUTPUT -p tcp -o lo --dport ${1} -j DNAT --to ${2}:${1}
}

add_prerouting() {
	 iptables -t nat -A PREROUTING -p tcp --dport ${1} -j DNAT --to ${2}:${3}
}

add_forward() {
     iptables -I FORWARD -d ${1}/32 -p tcp -m state --state NEW -m tcp --dport ${2} -j ACCEPT
}

add_output() {
    #- allows port forwarding from localhost but 
    #  only if you use the ip (e.g http://192.168.1.20:8888/)
	iptables -t nat -I OUTPUT -p tcp -o lo --dport ${1} -j DNAT --to ${2}:${1}
}

###############################################
# ONLY EDIT HERE
Guest_ipaddr=192.168.11.2
Host_port=( '4444' '4445' '993' '587' '25' '465' '143' )
Guest_port=( '80' '443' '993' '587' '25' '465' '143' )
###############################################
 
length=$(( ${#Host_port[@]} - 1 ))
 
if [ "${1}" = "remove" ]; then
    for i in $(seq 0 $length); do
    	    del_prerouting ${Host_port[$i]}  ${Guest_ipaddr} ${Guest_port[$i]}
            del_forward ${Guest_ipaddr} ${Guest_port[$i]}
            #- allows port forwarding from localhost but 
			#  only if you use the ip (e.g http://192.168.1.20:8888/)
			del_output ${Host_port[$i]} ${Guest_ipaddr}
    done
fi

if [ "${1}" = "add" ]; then
    for i in `seq 0 $length`; do
    		add_prerouting ${Host_port[$i]} ${Guest_ipaddr} ${Guest_port[$i]}
    		add_forward ${Guest_ipaddr} ${Guest_port[$i]}
            #- allows port forwarding from localhost but 
			#  only if you use the ip (e.g http://192.168.1.20:8888/)
    		add_output ${Host_port[$i]} ${Guest_ipaddr}
    done
fi

###############################################
# ONLY EDIT HERE
Guest_ipaddr=192.168.33.2
Host_port=( '4444' '4445' '1814' )
Guest_port=( '80' '443' '1814' )
###############################################

length=$(( ${#Host_port[@]} - 1 ))

if [ "${1}" = "remove" ]; then
    for i in $(seq 0 $length); do
    	    del_prerouting ${Host_port[$i]}  ${Guest_ipaddr} ${Guest_port[$i]}
            del_forward ${Guest_ipaddr} ${Guest_port[$i]}
            #- allows port forwarding from localhost but 
			#  only if you use the ip (e.g http://192.168.1.20:8888/)
			del_output ${Host_port[$i]} ${Guest_ipaddr}
	done
fi

if [ "${1}" = "add" ]; then
    for i in `seq 0 $length`; do
    		add_prerouting ${Host_port[$i]} ${Guest_ipaddr} ${Guest_port[$i]}
    		add_forward ${Guest_ipaddr} ${Guest_port[$i]}
            #- allows port forwarding from localhost but 
			#  only if you use the ip (e.g http://192.168.1.20:8888/)
    		add_output ${Host_port[$i]} ${Guest_ipaddr}
    done
fi

# To server another guest simply add another block for another host ..
#
# Guest_ipaddr=another_ip 
# Host_port=( '123' '456' '789' )
# Guest_port=( '12' '45' '78' )
# length=$(( ${#Host_port[@]} - 1 ))
#
# and so on