CONFIG_IP_NF_MATCH_LIMIT - This module isn't exactly required but it's used in the example rc.firewall.txt. This option provides the LIMIT match, that adds the possibility to control how many packets per minute that are to be matched, governed by an appropriate rule. For example, -m limit --limit 3/minute would match a maximum of 3 packets per minute. This module can also be used to avoid certain Denial of Service attacks.

CONFIG_IP_NF_MATCH_MAC - This allows us to match packets based on MAC addresses. Every Ethernet adapter has its own MAC address. We could for instance block packets based on what MAC address is used and block a certain computer pretty well since the MAC address very seldom changes. We don't use this option in the rc.firewall.txt example or anywhere else.

CONFIG_IP_NF_MATCH_MARK - This allows us to use a MARK match. For example, if we use the target MARK we could mark a packet and then depending on if this packet is marked further on in the table, we can match based on this mark. This option is the actual match MARK, and further down we will describe the actual target MARK.

CONFIG_IP_NF_MATCH_MULTIPORT - This module allows us to match packets with a whole range of destination ports or source ports. Normally this wouldn't be possible, but with this match it is.

CONFIG_IP_NF_MATCH_TOS - With this match we can match packets based on their TOS field. TOS stands for Type Of Service. TOS can also be set by certain rules in the mangle table and via the ip/tc commands.

CONFIG_IP_NF_MATCH_TCPMSS - This option adds the possibility for us to match TCP packets based on their MSS field.

CONFIG_IP_NF_MATCH_STATE - This is one of the biggest news in comparison to ipchains. With this module we can do stateful matching on packets. For example, if we have already seen traffic in two directions in a TCP connection, this packet will be counted as ESTABLISHED. This module is used extensively in the rc.firewall.txt example.

CONFIG_IP_NF_MATCH_UNCLEAN - This module will add the possibility for us to match IP, TCP, UDP and ICMP packets that don't conform to type or are invalid. We could for example drop these packets, but we never know if they are legitimate or not. Note that this match is still experimental and might not work perfectly in all cases.

CONFIG_IP_NF_MATCH_OWNER - This option will add the possibility for us to do matching based on the owner of a socket. For example, we can allow only the user root to have Internet access. This module was originally just written as an example on what could be done with the new iptables. Note that this match is still experimental and might not work for everyone.

CONFIG_IP_NF_FILTER - This module will add the basic filter table which will enable you to do IP filtering at all. In the filter table you'll find the INPUT, FORWARD and OUTPUT chains. This module is required if you plan to do any kind of filtering on packets that you receive and send.

CONFIG_IP_NF_TARGET_REJECT - This target allows us to specify that an ICMP error message should be sent in reply to incoming packets, instead of plainly dropping them dead to the floor. Keep in mind that TCP connections, as opposed to ICMP and UDP, are always reset or refused with a TCP RST packet.

CONFIG_IP_NF_TARGET_MIRROR - This allows packets to be bounced back to the sender of the packet. For example, if we set up a MIRROR target on destination port HTTP on our INPUT chain and someone tries to access this port, we would bounce his packets back to him and finally he would probably see his own homepage.

Warning The MIRROR target is not to be used lightly. It was originally built as a test and example module, and will most probably be very dangerous to the person setting it up (resulting in serious DDoS if among other things).

CONFIG_IP_NF_NAT - This module allows network address translation, or NAT, in its different forms. This option gives us access to the nat table in iptables. This option is required if we want to do port forwarding, masquerading, etc. Note that this option is not required for firewalling and masquerading of a LAN, but you should have it present unless you are able to provide unique IP addresses for all hosts. Hence, this option is required for the example rc.firewall.txt script to work properly, and most definitely on your network if you do not have the ability to add unique IP addresses as specified above.

CONFIG_IP_NF_TARGET_MASQUERADE - This module adds the MASQUERADE target. For instance if we don't know what IP we have to the Internet this would be the preferred way of getting the IP instead of using DNAT or SNAT. In other words, if we use DHCP, PPP, SLIP or some other connection that assigns us an IP, we need to use this target instead of SNAT. Masquerading gives a slightly higher load on the computer than NAT, but will work without us knowing the IP address in advance.

CONFIG_IP_NF_TARGET_REDIRECT - This target is useful together with application proxies, for example. Instead of letting a packet pass right through, we remap them to go to our local box instead. In other words, we have the possibility to make a transparent proxy this way.

CONFIG_IP_NF_TARGET_LOG - This adds the LOG target and its functionality to iptables. We can use this module to log certain packets to syslogd and hence see what is happening to the packet. This is invaluable for security audits, forensics or debugging a script you are writing.

CONFIG_IP_NF_TARGET_TCPMSS - This option can be used to counter Internet Service Providers and servers who block ICMP Fragmentation Needed packets. This can result in web-pages not getting through, small mails getting through while larger mails don't, ssh works but scp dies after handshake, etc. We can then use the TCPMSS target to overcome this by clamping our MSS (Maximum Segment Size) to the PMTU (Path Maximum Transmit Unit).

CONFIG_IP_NF_COMPAT_IPCHAINS - Adds a compatibility mode with the obsolete ipchains. Do not look to this as any real long term solution for solving migration from Linux 2.2 kernels to 2.4 kernels, since it may well be gone with kernel 2.6.

CONFIG_IP_NF_COMPAT_IPFWADM - Compatibility mode with obsolescent ipfwadm. Definitely don't look to this as a real long term solution.

As you can see, there is a heap of options. I have briefly explained here what kind of extra behaviors you can expect from each module. These are only the options available in a vanilla Linux 2.4.9 kernel. If you would like to take a look at more options, I suggest you look at the patch-o-matic (POM) functions in Netfilter user-land which will add heaps of other options in the kernel. POM fixes are additions that are supposed to be added in the kernel in the future but have not quite reached the kernel yet. This may be for various reasons - such as the patch not being stable yet, to Linus Torvalds being unable to keep up, or not wanting to let the patch in to the mainstream kernel yet since it is still experimental.

You will need the following options compiled into your kernel, or as modules, for the rc.firewall.txt script to work. If you need help with the options that the other scripts need, look at the example firewall scripts section.

• CONFIG_PACKET

• CONFIG_NETFILTER

• CONFIG_IP_NF_CONNTRACK

• CONFIG_IP_NF_FTP

• CONFIG_IP_NF_IRC