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[root@serverA ~]# modprobe iptable_nat [root@serverA ~]# modprobe ip_nat_ftp
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With the necessary modules loaded, we define the default policies for all the chains For the INPUT, FORWARD, and OUTPUT chains in the filter table, we set the destination to be DROP, DROP, and ACCEPT, respectively For the POSTROUTING and PREROUTING chains, we set their default policies to ACCEPT This is necessary for NAT to work
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[root@serverA [root@serverA [root@serverA [root@serverA [root@serverA ~]# ~]# ~]# ~]# ~]# iptables iptables iptables iptables iptables -P -P -P -t -t INPUT DROP FORWARD DROP OUTPUT ACCEPT nat -P POSTROUTING ACCEPT nat -P PREROUTING ACCEPT
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With the default policies in place, we need to define the baseline firewall rule What we want to accomplish is simple: Let users on the inside network (eth1) make connections to the Internet, but don t let the Internet make connections back To accomplish this, we define a new chain called block that we use for grouping our state-tracking rules together The first rule in that chain simply states that any packet that is part of an established connection or that is related to an established connection is allowed through The second rule states that in order for a packet to create a new connection, it cannot originate from the eth0 (Internet-facing) interface If a packet does not match against either of these two rules, the final rule forces the packet to be dropped
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[root@serverA [root@serverA [root@serverA [root@serverA ~]# ~]# ~]# ~]# iptables iptables iptables iptables -N -A -A -A block block -m state --state ESTABLISHED,RELATED -j ACCEPT block -m state --state NEW -i ! eth0 -j ACCEPT block -j DROP
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With the blocking chain in place, we need to call on it from the INPUT and FORWARD chains We aren t worried about the OUTPUT chain, since only packets originating from the firewall itself come from there The INPUT and FORWARD chains, on the other hand, need to be checked Recall that when doing NAT, the INPUT chain will not be hit, so we need to have FORWARD do the check If a packet is destined to the firewall itself, we need the checks done from the INPUT chain
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[root@serverA ~]# iptables -A INPUT -j block [root@serverA ~]# iptables -A FORWARD -j block
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Finally, as the packet leaves the system, we perform the MASQUERADE function from the POSTROUTING chain in the NAT table All packets that leave from the eth0 interface go through this chain
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[root@serverA ~]# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
With all the packet checks and manipulation behind us, we enable IP forwarding (a must for NAT to work) and SYN cookie protection, plus we enable the switch that keeps the firewall from processing ICMP broadcast packets (Smurf attacks)
[root@serverA ~]# echo 1 > /proc/sys/net/ipv4/ip_forward [root@serverA ~]# echo 1 > /proc/sys/net/ipv4/tcp_syncookies [root@serverA ~]# echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
At this point, you have a working firewall for a simple environment If you don t run any servers, you can save this configuration and consider yourself done On the other hand, let s assume you have two applications that you want to make work through this firewall: a Linux system on the inside network that you need ssh access to from remote locations and a Windows system from which you want to run BitTorrent Let s start with the ssh case first To make a port available through the firewall, we need to define a rule that says, If any packet on the eth0 (Internet-facing) interface is TCP and has a destination port of 22, change its destination IP address to 1721613 This is accomplished by using the DNAT action on the PREROUTING chain, since we want to change the IP address of the packet before any of the other chains see it The second problem we need to solve is how to insert a rule on the FORWARD chain that allows any packet whose destination IP address is 1721613 and destination port is 22 to be allowed The key word is insert (-I) If we append the rule (-A) to the FORWARD chain, the packet will instead be directed through the block chain, because the rule iptables -A FORWARD -j block will apply first
[root@serverA ~]# iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 22 -j DNAT\ --to-destination 1721613 [root@serverA ~]# iptables -I FORWARD -p tcp -d 1721613 --dport 22 -j ACCEPT
We can apply a similar idea to make BitTorrent work Let s assume that the Windows machine that is going to use BitTorrent is 1721612 The BitTorrent protocol uses ports
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