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Netfilter is essentially a framework for packet management that can check packets for particular network protocols and notify parts of the kernel listening for them Built on the Netfilter framework is the packet selection system implemented by IPtables With IPtables, different tables of rules can be set up to select packets according to differing criteria Netfilter currently supports three tables: filter, nat, and mangle Packet filtering is implemented using a filter table that holds rules for dropping or accepting packets Network address translation operations such as IP masquerading are implemented using the NAT table that holds IP masquerading rules The mangle table is used for specialized packet changes Changes can be made to packets before they are sent out, when they are received, or as they are being forwarded This structure is extensible in that new modules can define their own tables with their own rules It also greatly improves efficiency Instead of all packets checking one large table, they access only the table of rules they need to IP table rules are managed using the iptables command For this command, you will need to specify the table you want to manage The default is the filter table, which need not be specified You can list the rules you have added at any time with the -L and -n options, as shown here The -n option says to use only numeric output for both IP addresses and ports, avoiding a DNS lookup for hostnames You could, however, just use the -L option to see the port labels and hostnames:
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iptables -L -n
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NOTE In IPtables commands, chain names have to be entered in uppercase, as with the chain names
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INPUT, OUTPUT, and FORWARD
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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
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Target ACCEPT DROP REJECT QUEUE RETURN TABLE 20-2 IPtables Targets
Function Allow packet to pass through the firewall Deny access by the packet Deny access and notify the sender Send packets to user space Jump to the end of the chain and let the default target process it
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, 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 20-2 If a packet does not match any of the rules, it is passed to the chain s default target
Targets
A target can, 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, when activated, that userdefined chain is executed If no rules are matched, execution returns to the next rule in the originating chain
TIP Specialized targets and options can be added by means of kernel patches provided by the
Netfilter site For example, the SAME patch returns the same address for all connections A patch-o-matic option for the Netfilter make file will patch your kernel source code, adding support for the new target and options You can then rebuild and install your kernel
Firewall and NAT Chains
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 20-3
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