Opening up a whole interface to incoming packets may not be restrictive enough and you may want more control as to what to allow and what to reject. Lets suppose we have a small network of computers that use the 192.168.0.x private subnet. We can open up our firewall to incoming packets from a single trusted IP address (for example, 192.168.0.4):
# Accept packets from trusted IP addresses iptables -A INPUT -s 192.168.0.4 -j ACCEPT # change the IP address as appropriate
Breaking this command down, we first append (-A) a rule to the INPUT chain for the source (-s) IP address 192.168.0.4 to ACCEPT all packets (also note how we can use the # symbol to add comments inline to document our script with anything after the # being ignored and treated as a comment).
Obviously if we want to allow incoming packets from a range of IP addresses, we could simply add a rule for each trusted IP address and that would work fine. But if we have a lot of them, it may be easier to add a range of IP addresses in one go. To do this, we can use a netmask or standard slash notation to specify a range of IP address. For example, if we wanted to open our firewall to all incoming packets from the complete 192.168.0.x (where x=1 to 254) range, we could use either of the following methods:
# Accept packets from trusted IP addresses iptables -A INPUT -s 192.168.0.0/24 -j ACCEPT # using standard slash notation iptables -A INPUT -s 192.168.0.0/255.255.255.0 -j ACCEPT # using a subnet mask
Finally, as well as filtering against a single IP address, we can also match against the MAC address for the given device. To do this, we need to load a module (the mac module) that allows filtering against mac addresses. Earlier we saw another example of using modules to extend the functionality of iptables when we used the state module to match for ESTABLISHED and RELATED packets. Here we use the mac module to check the mac address of the source of the packet in addition to it’s IP address:
# Accept packets from trusted IP addresses iptables -A INPUT -s 192.168.0.4 -m mac --mac-source 00:50:8D:FD:E6:32 -j ACCEPT
First we useÂ -mÂ macÂ to load the mac module and then we useÂ --mac-sourceÂ to specify the mac address of the source IP address (192.168.0.4). You will need to find out the mac address of each ethernet device you wish to filter against. RunningÂ ifconfigÂ (oriwconfigÂ for wireless devices) as root will provide you with the mac address.
This may be useful for preventing spoofing of the source IP address as it will allow any packets that genuinely originate from 192.168.0.4 (having the mac address 00:50:8D:FD:E6:32) but will block any packets that are spoofed to have come from that address. Note, mac address filtering won’t work across the internet but it certainly works fine on a LAN.
This Article has been referred fromÂ wiki.centos.org