By default, firewall
rules are saved in the /etc/sysconfig/iptables or /etc/sysconfig/ip6tables files.
The iptables service starts before any DNS-related
services when a Linux system is booted. This means that firewall rules can only
reference numeric IP addresses (for example, 192.168.0.1). Domain names (for example,
host.example.com) in such rules produce errors.
Regardless of their destination,
when packets match a particular rule in one of the tables, a target or action is applied to them. If the
rule specifies anACCEPT target for a matching packet, the packet
skips the rest of the rule checks and is allowed to continue to its
destination. If a rule specifies a DROPtarget,
that packet is refused access to the system and nothing is sent back to the
host that sent the packet. If a rule specifies a QUEUE target, the packet is passed to user-space.
If a rule specifies the optional REJECT target, the packet is dropped, but an
error packet is sent to the packet's originator.
Every chain has a default policy to ACCEPT, DROP, REJECT,
or QUEUE.
If none of the rules in the chain apply to the packet, then the packet is dealt
with in accordance with the default policy.
The iptables command configures these tables, as
well as sets up new tables if necessary.
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