3 ### Initialization and finishing touches for firewall scripts
5 ### (c) 2008 Mark Wooding
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25 ###--------------------------------------------------------------------------
26 ### Address classification.
28 ### The objective of address classification is to work out what kind of
29 ### networks a packet is travelling between, in order to make filtering
32 ### Address classification is done in the mangle table, by attaching
33 ### appropriate marks to the packet. We split the Internet into a number of
34 ### address classes, and make forwarding decisions based on the classes of
35 ### the source and destination addresses.
37 ### The mark word is split into three fields: the FROM and TO fields simply
38 ### record the source and destination classes numerically; the MASK field is
39 ### used to determine whether forwarding should occur. There is a mask bit
40 ### for each address class. Source classification sets mask bits according
41 ### to the forwarding policy for the source address class. Destination
42 ### classification clears all of the mask bits except for the one
43 ### corresponding to the actual destination class. Therefore, forwarding is
44 ### permitted if and only if the mask bits are not all zero.
46 ### The mangle chains are arranged as follows.
48 ### The PREROUTING hook simply invokes in-classify and out-classify chains as
49 ### subroutines. These will tail-call appropriate classification chains.
51 ### The in-classify chain is responsible for both source address
52 ### classification and verifying that the packet arrived from the correct
53 ### interface. It does an initial dispatch on the source interface, to
54 ### in-IFACE. The in-IFACE chain dispatches to mark-from-CLASS when it
55 ### recognizes an address belonging to the CLASS; if no matches succeed, it
56 ### goes to bad-source-address, which logs a message and drops the packet.
57 ### The default interface is special. If no explicit matches are found, it
58 ### dispatches to in-default which forbids a few obviously evil things and
59 ### finally dispatches to mark-from-untrusted.
61 ### The out-classify is simpler because it doesn't care about the interface.
62 ### It simply checks each network range in turn, dispatching to mark-to-CLASS
63 ### on a match or mark-to-DEFAULT (probably untrusted) if there is no match.
65 clearchain mangle:in-classify mangle:in-default mangle:out-classify
66 clearchain mangle:local-source
68 ## Packets over the loopback interface are automatically trusted. All manner
69 ## of weird stuff happens on lo, and it's best not to second-guess it.
70 run iptables -t mangle -A in-classify -i lo -j ACCEPT
72 ## Local bootp packets have bizarre addresses. Don't block them just because
74 run iptables -t mangle -A in-classify -j RETURN \
75 -s 0.0.0.0 -d 255.255.255.255 \
76 -p udp --source-port $port_bootpc --destination-port $port_bootps
78 ## Since packets with source and destination addresses both local will go
79 ## over the loopback interface, I shouldn't see a packet from me over any
80 ## other interface. Except that I will if I sent a broadcast or multicast.
81 ## Allow the broadcasts, and remember not to trust them.
82 run iptables -t mangle -A local-source -j RETURN \
83 -m addrtype --dst-type BROADCAST
84 run iptables -t mangle -A local-source -j RETURN \
85 -m addrtype --dst-type MULTICAST
86 run iptables -t mangle -A local-source -g bad-source-address
87 run iptables -t mangle -A in-classify -j local-source \
88 -m addrtype --src-type LOCAL
91 ## Define the important networks.
98 ## Mark addresses reachable on non-default interfaces as not reachable on the
100 trace "nets = $allnets"
101 for net in $allnets; do
106 run iptables -t mangle -A in-$defaultiface \
107 -s ${net#*:} -g bad-source-address
112 ## Fill in the black holes in the network.
114 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 \
115 127.0.0.0/8 192.0.2.0/24
117 run iptables -t mangle -A in-default -s $addr -g bad-source-address
121 ## Put the final default decision on the in-default chain, and attach the
122 ## classification chains to the PREROUTING hook.
123 run iptables -t mangle -A in-$defaultiface -g mark-from-$defaultclass
124 run iptables -t mangle -A PREROUTING -j in-classify
125 run iptables -t mangle -A PREROUTING -j out-classify
127 ## Now it's safe to let stuff through.
128 for i in PREROUTING INPUT FORWARD OUTPUT POSTROUTING; do
129 run iptables -t mangle -P $i ACCEPT
133 ###----- That's all, folks --------------------------------------------------