23-2: Setting Initial Firewall Rules in .NET

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Listing 23-2: Setting Initial Firewall Rules
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bible:~ bible:~ bible:~ bible:~ bible:~ bible:~ # # # # # # iptables iptables iptables iptables iptables iptables -P -P -P -A -A -A INPUT DROP OUTPUT DROP FORWARD DROP INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT OUTPUT -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
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Here, you have set the default policy for all chains to DROP the packets. At this moment in time, all network connections, regardless of their originating address, will be dropped. To set or change the policy of a chain, you need to specify that this is a policy edit (-P), the chain (INPUT, OUTPUT, or FORWARD), and also what to do with the packet. It s a secure feeling knowing that any connection from the Internet that you do not need is dropped and the sender has to wait for a timeout before being notified. Imagine someone running a port scan of all 64,000 available ports on a TCP/IP machine. If they have to wait for a timeout on each port, it will take them quite a few hours to complete the full scan. It provides a kind of tar pit for any malicious users. This is also true for internal connection, too. If your users are interested in what they can and cannot connect to, without reading the network rules, then making them wait will, one hopes, deter them from pushing the network too hard. You have also configured the stateful firewall with the -m state declaration. This tells the firewall that you will allow any established or related connections on the INPUT chain. This may seem like quite a big security hole, but bear in mind that it will allow only a connection that has been established, not a new connection. For the stateful rules to kick in, you would have already had to allow a new connection through the chain.
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Depending on how paranoid you are about security, you may not want to allow all new connections from the firewall itself. However, when you wish to use the firewall machine as a server, or want to be able to bounce from the machine to other hosts without the burden of setting up new rules for every protocol or TCP port you wish to connect to, it is quite useful. At this point, your firewall is locked down with the exception of allowing outgoing connections. Now, suppose you want to allow an incoming SSH connection to the firewall.
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When you add a rule proper, you need to specify as much information as possible to have full control over the TCP/IP you are allowing into the trusted network. At a minimum, you need the chain, protocol, and destination port. With just this information, you do not have a very good rule because it does not specify the interface you are allowing the SSH connection to. Another option that can be set is the connection type: NEW This is a new connection; no other traffic is associated with this packet. ESTABLISHED This packet is from a machine you already have a connection to (remember, you both send and receive data when a connection exists). RELATED This packet is related to an existing connection. The FTP protocol, for example, makes a connection to the FTP server, and the FTP server actually makes a separate connection to the client. This separate connection from the server to the client is a RELATED connection.
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iptables A INPUT p tcp dport ssh i eth0 j ACCEPT
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In this example, you have told netfilter that you want to append (-A) a rule to the INPUT chain, specifying the TCP protocol (-p tcp), with a destination port (-dport) of ssh (port 22), incoming (-i) on the eth0 interface, and finally that you want to ACCEPT the packet (-j ACCEPT). The -j parameter means jump to a target. Remember that netfilter rules are in a chain, so you are saying, Stop processing this chain because you have a match and jump to the target. In this case, ACCEPT.
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Note The -dport parameter can take either a numerical port number or a service name that is specified in /etc/services.
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When setting up a rule for connections, you really need to know how the protocol works. In the case of SSH, it is well known that it is a TCP protocol, running on port 22. With this in mind, it is relatively easy to write a rule for it. It is up to you as to how you want to write the rule regarding the state of the connection, but because the initial INPUT state rule has allowed all ESTABLISHED and RELATED connections, you do not need to explicitly set the state to NEW because you have effectively allowed all connection types for SSH by not explicitly setting them.
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Caution When you do not specify something explicitly with an iptables rule, it is assumed that you want the default setting. For example, if you did not set the interface for the incoming connection, netfilter would have allowed an SSH connection on all network interfaces. This is indeed the same for the protocol type and the destination port. Be very careful how you write your rules, and make sure you explicitly set everything you wish to control; otherwise you will probably let in more than you think.
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