how to generate barcode in visual basic 2010 s FIREWALLING YOUR HOSTS in Font

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CHAPTER 2 s FIREWALLING YOUR HOSTS
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Table 2-2. Incoming Traffic Flow
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eth0 eth0 eth0 eth1
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Any Any Any 192.168.0.0/24
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32768 to 61000 Any Any Any
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TCP TCP TCP TCP
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Destination Address
192.168.0.1 192.168.0.1 192.168.0.1 192.168.0.1
Destination Port
80 25 22 53
Of course, you can also conduct this same exercise for the outgoing traffic (see Table 2-3). Table 2-3. Outgoing Traffic Flow
Interface
eth0 eth0 eth0 eth1
Source Address
192.168.0.1 192.168.0.1 192.168.0.1 192.168.0.1
Source Port
80 25 22 25
Protocol
TCP TCP TCP TCP
Destination Address
Any Any Any 192.168.0.0/24
Destination Port
32768 to 61000 Any Any Any
You can model all the connections on your host this way to allow you to apply suitable iptables rules to your incoming and outgoing connections. You can then combine these lists of traffic into an overall test plan for your firewall rules. Then using a tool such as tcpdump, you can identify whether your rules cover all the incoming and outgoing traffic on your host.
The iptables Command
The iptables command principally controls adding and removing rules to your chains. You have already seen the -A flag, which adds rules to your firewall. When you use the -A flag to add a rule, it is appended to the end of the current rules in a chain. You can also add rules using the -I flag, which adds rules to the top of the chain of current rules. So why do you need the different types of flags to add rules to your firewall Well, the sequence of your rules is important. The rules in a chain are checked in sequence, in the order they are added, with the first rule added to the chain being checked first and the last rule added to the chain being checked last. With the -I flag you can also add a rule into a chain using a line number, which you can specify to place that rule exactly where you require in the chain. Let s look at the line numbers of rules. Line numbers are important because, as I have described, your rules are checked in a sequence in each chain. If you have a rule specifying all traffic is accepted into your host at line number 1 of the rules in a chain, then all traffic will be accepted by this rule and any following rules that may restrict traffic will be ignored. For example, let s look at the following two rules: puppy# iptables -I INPUT 1 -i eth0 -p tcp -j ACCEPT puppy# iptables -I INPUT 2 -i eth0 -p tcp --dport 143 -j DROP The first rule ACCEPTs all TCP traffic that enters the host from device eth0, and the number 1 after the chain indicates it is the first rule in the INPUT chain. The second rule DROPs all traffic that enters the host from device eth0 bound for port 143, or IMAP and the number 2 after the ,
CHAPTER 2 s FIREWALLING YOUR HOSTS
chain indicates it is the second rule in the INPUT chain. As the rules are checked in sequence, the second rule would be totally ignored because the first rule indicates all TCP traffic is to be accepted. So you should ensure your rules make logical sense and do not contradict each other. Each of your rules is assigned a line number in the chain to which they are assigned. You can see this line number and the details of the rules in a chain by using the -L flag to list your rules (see Listing 2-3). Listing 2-3. Listing Your Rules puppy# iptables -L INPUT -n --line-numbers Chain INPUT (policy DROP) num target prot opt source 1 ACCEPT tcp -- 0.0.0.0/0 2 ACCEPT tcp -- 0.0.0.0/0
destination 192.168.0.1 192.168.0.1
tcp dpt:80 tcp dpt:443
In Listing 2-3 I have listed all the rules in the INPUT chain. I have used two flags; the first -n tells iptables not to look up any IP addresses via DNS or port numbers via the /etc/services file but rather display the raw numerics. This makes the listing faster as it stops iptables waiting for DNS resolution and service lookups before displaying the rules. I have also specified the --line-numbers flag, which will show the rules with their line numbers. If I had omitted the chain name from the -L flag, it would have displayed all the rules from all chains. puppy# iptables -L -n --line-numbers Chain INPUT (policy DROP) num target prot opt source 1 ACCEPT tcp -- 0.0.0.0/0 2 ACCEPT tcp -- 0.0.0.0/0 Chain FORWARD (policy DROP) target prot opt source Chain OUTPUT (policy DROP) num target prot opt source 1 ACCEPT tcp -- 0.0.0.0/0 2 ACCEPT tcp -- 0.0.0.0/0
destination 192.168.0.1 192.168.0.1 destination destination 0.0.0.0/0 0.0.0.0/0
tcp dpt:80 tcp dpt:443
tcp spt:80 tcp spt:443
So now you want to add a rule in the INPUT chain at line 3. To do this you must use the -I flag with which you can specify the line number. The -A flag does not allow you to specify a line number. puppy# iptables -I INPUT 3 -i eth0 -p tcp --dport 22 -d 192.168.0.1 -j ACCEPT You can see, you have specified the required line number after the name of the chain in the -I flag. Now if you list the rules in the INPUT chain, you will see the new rule at line number 3 in Listing 2-4.
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