![]() ![]() # iptables -A FORWARD -p udp -s 192.168.1.18 -sport 53 -j ACCEPT If the DNS server is configured to respond to clients using UDP port 53, you can allow this traffic through with the following two rules: # iptables -A FORWARD -p udp -d 192.168.1.18 -dport 53 -j ACCEPT In this case, you can configure the DNS server either to use only TCP, or to use a UDP source port of 53 for any response that it sends back to clients that were using UDP to query the name server. This is because the policy relies on the use of state-tracking rules, and since UDP is a stateless protocol, there is no way to track it. Using a default deny policy makes it slightly more difficult to use UDP for DNS. ![]() Unlike the other services, DNS can use both TCP and UDP port 53. IMAP SSL # iptables -A FORWARD -m state -state NEW -p tcp \ -d 192.168.1.21 -dport 993 -j ACCEPTįinally, allow DNS access via port 53: # iptables -A FORWARD -m state -state NEW -p tcp \ -d 192.168.1.21 -dport 53 -j ACCEPT iptables also allows you to insert, append, delete, and replace rules in a chain. Using iptables, you can create new chains, delete chains, list the rules in a chain, flush chains (i.e., remove all rules from a chain), and set the default action for a chain. The iptables command makes changes to the Netfilter chains and rulesets. It is used whenever the system is acting as a router or gateway, and it applies to packets that are neither originating from nor destined for the local system. Finally, the FORWARD chain applies whenever a packet will be routed from one network interface to another through the system. The INPUT chain applies to packets that are received by and destined for the local system, and the OUTPUT chain applies to packets that are transmitted by the local system. The kernel defines three chains by default, but new chains of rules can be specified and linked to the predefined chains. Although Netfilter is the kernel component and iptables is the user-space configuration tool, these terms are often used interchangeably.Īn important concept in Netfilter is the chain, which consists of a list of rules that are applied to packets as they enter, leave, or traverse the system. The latest generation of filtering, called Netfilter, is manipulated with the iptables command and used exclusively with the 2.4.x and later series of kernels. It improved greatly on ipfw and is still in common use. The second generation of IP filtering was called IP chains. Since it was somewhat inflexible and inefficient for complex configurations, ipfw is rarely used now. The first generation of packet-filtering code, called ipfw (for “IP firewall”), provided basic filtering capability. Linux has long had the capability for filtering packets, and it has come a long way since the early days in terms of both power and flexibility. Protect your network with Linux’s powerful firewalling features. Finally, you’ll learn a few additional tricks to keep certain types of traffic from exiting your network. You’ll also see how to perform MAC address filtering and how to create a gateway that will authenticate machines based on login credentials. This chapter shows how to set up firewalls with Linux, FreeBSD, OpenBSD, and Windows, as well as how to test your firewall rulesets. In addition to the many firewall appliances that are available, Linux, BSD, and Windows all include some form of firewalling support. Additionally, firewall logs can be excellent tools to help you understand where the threats to your network originate.Ī variety of firewalls are available today. Doing so aids in preventing worm propagation and helps stop important confidential information from leaving an enterprise. You can use a firewall not only to limit what information flows into a network, but also to prevent the egress of information. The firewall is a key technology that is instrumental in enforcing these policies and can allow network administrators to delineate trust relationships between networks and hosts with a fine grain of detail.īy instituting a firewall, you can prevent unauthorized access to services at the network level before an attacker is given the chance to attempt to exploit them. When designing a network, it’s often desirable to define policies governing how and where certain vital network services can be accessed. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |