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To prevent outsiders from initiating any access to your system, create a rule to block access by SYN packets from the outside using the state option with NEW Drop any new connections on the eth0 connection (assuming only eth0 is connected to the Internet or outside network)
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# prevent outside-initiated connections iptables -A INPUT -m state --state NEW -i eth0 -j DROP iptables -A FORWARD -m state --state NEW -i eth0 -j DROP
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To allow interaction by the internal network with the firewall, you allow input by all packets on the internal Ethernet connection, eth1 The valid internal network addresses are designated as the input source
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iptables -A INPUT -j ACCEPT -p all -i eth1 -s 19216800/24
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To implement masquerading, where systems on the private network can use the gateway s Internet address to connect to Internet hosts, you create a NAT table (-t nat) POSTROUTING rule with a MASQUERADE target
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iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
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PART V
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Controlling ICMP Packets
In addition, to allow ping and destination-reachable ICMP packets, you enter INPUT rules with the firewall as the destination To enable ping operations, you use both echo-reply and echo-request ICMP types, and for destination unreachable, you use the destinationunreachable type
iptables -A INPUT -j ACCEPT -p icmp -i eth0 --icmp-type \ echo-reply -d 10001 iptables -A INPUT -j ACCEPT -p icmp -i eth0 --icmp-type \ echo-request -d 10001 iptables -A INPUT -j ACCEPT -p icmp -i eth0 --icmp-type \ destination-unreachable -d 10001
Part V:
Security
At the end, IP forwarding is turned on again
echo 1 > /proc/sys/net/ipv4/ip_forward
Listing Rules
A listing of the iptables options shows the different rules for each option, as shown here:
# iptables -L Chain INPUT (policy DROP) target prot opt source LOG all -- !19216800/24 DROP all -- !19216800/24 DROP all -- 19216800/24 DROP all -- 127000/8 ACCEPT all -- anywhere ACCEPT tcp -- 10002 ACCEPT tcp -- 10002 ACCEPT !icmp DROP all ACCEPT all ACCEPT icmp ACCEPT icmp ACCEPT icmp Chain FORWARD target prot DROP all DROP all DROP all DROP all -- anywhere -- anywhere -- 19216800/24 -- anywhere -- anywhere -- anywhere (policy ACCEPT) opt source -- !19216800/24 -- 19216800/24 -- 127000/8 -- anywhere
destination anywhere anywhere anywhere anywhere anywhere anywhere 19216800/24 anywhere anywhere anywhere 10001 10001 10001 destination anywhere anywhere anywhere anywhere
LOG level warning
tcp dpt:http state RELATED,ESTABLISHED tcp spt:http state RELATED,ESTABLISHED state NEW icmp echo-reply icmp echo-request icmp destination-unreachable
state NEW
Chain OUTPUT (policy ACCEPT) target prot opt source DROP tcp -- anywhere
destination 19216800/24
state NEW tcp spt:http
# iptables -t nat -L Chain PREROUTING (policy ACCEPT) target prot opt source destination Chain POSTROUTING (policy ACCEPT) target prot opt source destination MASQUERADE all -- anywhere anywhere Chain OUTPUT (policy ACCEPT) target prot opt source destination
User-Defined Rules
For more complex rules, you may want to create your own chain to reduce repetition A common method is to define a user chain for both INPUT and FORWARD chains, so that you do not have to repeat DROP operations for each Instead, you have only one user chain that both FORWARD and INPUT chains feed into for DROP operations Keep in mind that both FORWARD and INPUT operations may have separate rules in addition to the ones they share
20:
Firewalls
In the next example, a user-defined chain called arriving is created The chain is defined with the -N option at the top of the script:
iptables -N arriving
A user chain has to be defined before it can be used as a target in other rules, so you have to first define and add all the rules for that chain, and then use it as a target The arriving chain is first defined and its rules added Then, at the end of the file, it is used as a target for both the INPUT and FORWARD chains The INPUT chain lists rules for accepting packets, whereas the FORWARD chain has an ACCEPT policy that will accept them by default
iptables -N arriving iptables -F arriving # IP spoofing, deny any packets on the internal network # that have an external source address iptables -A arriving -j LOG -i eth1 \! -s 19216800/24 iptables -A arriving -j DROP -i eth1 \! -s 19216800/24 iptables -A arriving -j DROP \! -i eth1 -s 19216800/24 # entries at end of script iptables -A INPUT -j arriving iptables -A FORWARD -j arriving
A listing of the corresponding rules is shown here:
Chain INPUT (policy DROP) target prot opt source arriving all -- 0000/0 Chain FORWARD (policy ACCEPT) target prot opt source arriving all -- 0000/0 Chain arriving (2 references) target prot opt source LOG all -- !19216800/24 DROP all -- !19216800/24 DROP all -- 19216800/24 destination 0000/0 destination 0000/0 destination 0000/0 0000/0 0000/0
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