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The iptables command is the key to configuring the Netfilter system A quick glance at its online help with the iptables -h command shows an impressive number of configuration options In this section, we will walk through some of those options and learn how to use them At the heart of the command is the ability to define individual rules that are made a part of a rule chain Each individual rule has a packet-matching criterion and a corresponding action As a packet traverses a system, it will traverse the appropriate chains, as we saw in Figure 13-3 earlier in the chapter Within each chain, each rule will be executed on the packet in order When a rule matches a packet, the specified action is taken on the packet These individual actions are referred to as targets
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Managing Chains
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The format of the command varies by the desired action on the chain These are the possible actions: iptables -t table -A chain rule-spec [ options ] iptables -t table -D chain rule-spec iptables -t table -I chain [ rulenum ] rule-spec [ options ] iptables -t table -R chain rulenum rule-spec [ options ] iptables -t table -L chain [ options ] iptables -t table -F chain [ options ] iptables -t table -Z chain [ options ] iptables -t table -N chain iptables -t table -X [ chain ] iptables -t table target -P chain iptables -t table -E chain [new-chain] Append rule-spec to chain Delete rule-spec from chain Insert rule-spec at rulenum If no rule number is specified, the rule is inserted at the top of the chain Replace rulenum with rule-spec on chain List the rules on chain Flush (remove all) the rules on chain Zero all the counters on chain Define a new chain called chain Delete chain If no chain is specified, all nonstandard chains are deleted Define the default policy for a chain If no rules are matched for a given chain, the default policy sends the packet to target Rename chain to new-chain
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Recall that there are several built-in tables (NAT, filter, mangle, and raw) and five built-in chains (PREROUTING, POSTROUTING, INPUT, FORWARD, and OUTPUT) Recall that Figure 13-4 shows their relationships However, as rules become more complex, it is sometimes necessary to break them up into smaller groups Netfilter lets you do this by defining your own chain and placing it within the appropriate table When traversing the standard chains, a matching rule can trigger a jump to another chain in the same table For example, let s create a chain called to_net10 that handles all the packets destined to the 10000/8 network that is going through the FORWARD chain
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[root@serverA ~]# iptables -t filter -N to_net10 [root@serverA ~]# iptables -t filter -A FORWARD -d 10000/8 -j to_net10 [root@serverA ~]# iptables -t filter -A to_net10 -j RETURN
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In this example, the to_net10 chain doesn t do anything but return control back to the FORWARD chain To create a sample table named to_net10 for the IPv6 firewall, we would use
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[root@serverA ~]# ip6tables -t filter -N to_net10
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TIP Every chain should have a default policy That is, it must have a default action to take in the event a packet fails to meet any of the rules When designing a firewall, the safe approach is to set the default policy (using the -P option in iptables) for each chain to be DROP and then explicitly insert ALLOW rules for the network traffic that you do want to allow
TIP The filter table is the default table used whenever a table name is not explicitly specified with the iptables command Therefore the rule: # iptables -t filter -N example_chain can also be written as: # iptables -N example_chain
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