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Iptables and examples

This is just quick-reference for the kernel 2.4 "iptables" tool from the netfilter framework.
Current set of default tables:
filter (default table): Starts with built-in chains:
INPUT: Arriving.
FORWARD: Being routed.
OUTPUT: Locally generated

nat (traffic that creates new connections): Starts with built-in chains:
OUTPUT: Locally generated.

mangle (specialised packet alteration): Starts with built-in chains:
PREROUTING: Incoming, before routing.
OUTPUT: Locally generated.
INPUT: Arriving.
FORWARD: Being routed.

The admin can create/delete/rename additional chains for any target.
Each chain consists of a set of rules, consulted in order (thus the term "chain") until one's conditions match. If none match, the default policy applies, "-P" option. (Policies exist only for built-in chains.

Policy target may only be one of the four predefined rules.) Each rule has:
criterion: Which packets will be affected.
target: Which rule to consult next. (May optionally be one of the predefined rules ACCEPT, DROP, QUEUE=userspace-handled, or RETURN=policy.)
Each rule is assigned a rulenum, which can be used to refer to it in iptables commands.

Since rulesets live in RAM, one can preserve them to disk or reload them using iptables-save and iptables-restore, respectively.
Many of the more interesting features, such as stateful inspection, are via dynamically-loaded helper modules (option "-m").
$IPTABLES -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG 
--log-level DEBUG --log-prefix "IPT FORWARD packet died: "


## Create chain that blocks new connections, except if coming from inside.
# iptables -N block
# iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A block -m state --state NEW -i ! eth0 -j ACCEPT
# iptables -A block -j DROP
## Jump to that chain from INPUT and FORWARD chains.
# iptables -A INPUT -j block
# iptables -A FORWARD -j block

Type of Service (TOS) prioritisation: To maximize ssh response while maintaining maximum file data transfer over HTTP connections:
# /sbin/iptables -A PREROUTING -t mangle -p tcp --sport ssh -j TOS --set-tos Minimize-Delay
# /sbin/iptables -A PREROUTING -t mangle -p tcp --sport http -j TOS --set-tos Maximize-Throughput 

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