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When creating a rule to match packets that always use the same port numbers, everything is easy. Of course, this isn t always the case. For example, a user who connects to a web server will always connect to that web server on port 80, but the packets sent back from the web
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CHAPTER 5 CONFIGURING YOUR SERVER FOR SECURITY
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server use a randomly chosen port number above 1024. You could create a rule in which outgoing packets on all ports above 1024 are opened, but that s not ideal for security reasons. A smart way of dealing with this problem is by using stateful packet filters. A stateful packet filter analyzes if a packet that goes out is part of an already established connection, and, if it is, it allows the answer to go out. Stateful packet filters are useful for replies that are sent by web servers and for FTP servers as well because, in the case of an FTP server, the connection is established on port 21, and, once the session is established, data is sent over port 20 to the client. By using the --state option you can indicate what state a rule should look at. This functionality, however, is not a part of the core Netfilter modules, and an additional module has to be loaded to allow for state checking. Therefore, in every rule that wants to look at the state that a packet is in, the -m state option is used first, followed by the exact state the rule is looking at. For example, -m state --state RELATED,ESTABLISHED would look at packets that are part of related packets that are already allowed, or packets that are a part of an established session. The state module isn t the only module that can be used, and many other modules are available for more advanced configurations. For example, the nth module allows you to have a look at every nth packet (such as every third for example). Further discussion of modules is out of the scope of this book, so check the documentation page of the Netfilter web site at www.netfilter.org/documentation for more in-depth information.
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Creating the Rules
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Based on this information, you should be able to create some basic rules. Let s assume that you have a server that has only one NIC. On this network card, you want to allow requests to the web server to come in and replies from it to go out. Also, you want to allow SSH traffic. For the rest, no other services are needed. Like any other Netfilter configuration, you would start this configuration by creating some policies. Every chain needs its own policy. The following commands make sure that no packet comes in or out of your server by setting the policy for each chain to DROP: iptables -P FORWARD DROP iptables -P INPUT DROP iptables -P OUTPUT DROP Now that everything is blocked, you can start by allowing some packets to go in and out. First and foremost, you have to enable the loopback interface because the policies that you ve just defined also disable all traffic on the loopback interface and that s not good (because many services rely on the loopback interface). Without loopback interface, for example, you have no way to start the graphical environment on your machine, and many other services will fail as well. Imagine that the login process queries an LDAP server that runs on the localhost. Now open the loopback interface using the following two rules: iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT
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CHAPTER 5 CONFIGURING YOUR SERVER FOR SECURITY
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In these two rules, the -A option is used to refer to the chain the rules have to be added to. You are using -A, and so the rule is just appended to the INPUT and the OUTPUT chains. This would make the rule the last rule that is added to the chain, just before the policy that is always the last rule in a chain that is evaluated. Next, -i lo and -o lo are used to indicate that this rule matches to everything that happens on the loopback interface. As the third and last part of these two rules, the target is specified by using the -j option (which is short for jump to target ). In this case, the target is to accept all matching packets. So, now you have a server that allows nothing on the external network interfaces, but the loopback interface is completely open. Next, it s time to do what you want to do on your server: allow incoming SSH and HTTP traffic and allow replies to the allowed incoming traffic to be returned. Note that these two requirements consist of two parts: a part that is configured in the INPUT chain and a part that is configured in the OUTPUT chain. Let s start with some nice rules that define the input chain: iptables iptables iptables iptables -A -A -A -A INPUT INPUT INPUT INPUT -m -p -p -j state --state ESTABLISHED,RELATED -j ACCEPT tcp --dport 22 -m state --state NEW -j ACCEPT tcp --dport 80 -m state --state NEW -j ACCEPT LOG --log-prefix "Dropped illegal incoming packet: "
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The first rule in this INPUT chain tells Netfilter that all packets that are part of an already established or related session are allowed in. Next, for packets coming in on SSH port 22 that have a state NEW, the second rule indicates that they are allowed as well. Thirdly, packets that are sent to TCP destination port 80 (notice the combination between -p tcp and --dport 80 in this rule) and have a state NEW are accepted as well. The last rule finally makes sure that all packets that didn t match any of the earlier rules are logged before they are dropped by the policy at the end of the rule. Note that logging all dropped packets as a default may cause big problems.
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Caution Use logging only if you need to troubleshoot your firewall. It s generally a bad idea to switch on
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logging by default, because, if not done properly, it can cause huge amounts of information to be written to your log files.
Now that you have defined the INPUT chain, let s do the OUTPUT chain as well. No specific services have to be allowed out, with the exception of the replies to incoming packets that were allowed, and so creating the OUTPUT chain is rather simple and consists of just two rules: iptables -A OUTPUT -m state RELATED,ESTABLISHED -j ACCEPT iptables -A OUTPUT -j LOG --log-prefix "Dropped illegal outgoing packet: " The use of these two rules should be clear from the explanation earlier in this section. Note that it is a good idea to turn on logging for the OUTPUT rule (unlike for the INPUT rule).
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