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Note In iptables' commands, chain names have to be entered in uppercase. This means that the chain names INPUT, OUTPUT, and FORWARD have to be written in uppercase. Packet-filtering rules are very similar to those used in ipchains with few exceptions. Rules are combined into different chains. The kernel uses chains to manage packets it receives and sends out. A chain is simply a checklist of rules. These rules specify what action to take for packets containing certain headers. The rules operate with an if-then-else structure. If a packet does not match the first rule, the next rule is then checked, and so on. If the packet does not match any rules, then the kernel consults chain policy. Usually, at this point the packet is rejected. If the packet does match a rule, it is passed to its target, which determines what to do with the packet. The standard targets are listed in Table 40-2. If a packet does not match any of the rules, it is passed to the chain's default target. Table 40-2: iptables Targets Target ACCEPT DROP REJECT QUEUE RETURN Function Allows packet to pass through the firewall Denies access by the packet (same as DENY in ipchains) Denies access and notifies the sender Sends packets to user space Jumps to the end of the chain and lets the default target process it
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A target could, in turn, be another chain of rules, even a chain of user-defined rules. A packet could be passed through several chains before finally reaching a target. In the case of userdefined chains, the default target is always the next rule in the chains from which it was called. This sets up a procedure or function call like flow of control found in programming languages. When a rule has a user-defined chain as its target, then, when activated, that userdefined chain is executed. If no rules are matched, execution returns to the next rule in the originating chain. The kernel uses three firewall chains: INPUT, OUTPUT, and FORWARD. When a packet is received through an interface, the INPUT chain is used to determine what to do with it. The kernel then uses its routing information to decide where to send it. If the kernel sends the packet to another host, the FORWARD chain is checked. Before the packet is actually sent, the OUTPUT chain is also checked. In addition, two NAT table chains, POSTROUTING and PREROUTING, are implemented to handle masquerading and packet address modifications. The built-in Netfilter chains are listed in Table 40-3.
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Chain INPUT OUTPUT FORWARD PREROUTING POSTROUTING
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Table 40-3: Netfilter Built-in Chains Descriptions Rules for incoming packets Rules for outgoing packets Rules for forwarded packets Rules for redirecting or modifying incoming packets, NAT table only Rules for redirecting or modifying outgoing packets, NAT table only
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You add and modify chain rules using the iptables commands. An iptables command consists of the keyword iptables, followed by an argument denoting the command to execute. For example, iptables -A is the command to add a new rule, whereas iptables -D is the command to delete a rule. The iptables commands are listed in Table 40-4. The following command simply lists the chains along with their rules currently defined for your system. The output shows the default values created by iptables commands. Table 40-4: iptables Commands Function Appends a rule to a chain Deletes matching rule from a chain Deletes rule rulenum (1 = first) from chain Inserts in chain as rulenum (default 1 = first) Replaces rule rulenum (1 = first) in chain Lists the rules in chain or all chains Renames a chain Deletes (flush) all rules in chain or all chains Replaces a rule; rules are numbered from 1 Zero counters in chain or all chains Create a new user-defined chain Deletes a user-defined chain Changes policy on chain to target
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Option -A chain -D chain -D chain rulenum -I chain [rulenum] -R chain rulenum -L [chain] -E [chain] -F [chain] -R chain -Z [chain] -N chain -X chain -P chain target
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iptables L -n Chain input (policy ACCEPT): Chain forward (policy ACCEPT): Chain output (policy ACCEPT):
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To add a new rule to a chain, you use -A. Use -D to remove it, and -R to replace it. Following the command, list the chain to which the rule applies, such as the INPUT, OUTPUT, or FORWARD chain, or else a user-defined chain. Next, you list different options that specify the actions you want taken. Most are the same as those used for ipchains, with a few exceptions. The -s option specifies the source address attached to the packet, -d specifies the destination address, and -j specifies the target. The ACCEPT target will allow a packet to pass. The -i option now indicates the input device and can only be used with the INPUT and
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FORWARD chains. The -o option indicates the output device and can only be used for OUTPUT and FORWARD chains. Table 40-5 lists several basic options. Many are similar to the ipchains options, but some, like -i, are different and others are missing (like y). Table 40-5: iptables Options Function Specify a protocol, such as TCP, UDP, ICMP, or ALL.
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