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Common Match Criteria and Common Match Criteria conhnued

Common Match Criteria 




IP address or network
s192.168.0.0/24
d192.168.0.1

Network interface
i lo
0 eth1

Criteria can be inverted with
i etho  s 192.168.0.0/24

Most rules in the filter table involve allowing or denying packets based on their source or destination. Below are examples of options that can be used to create such rules.

A packet‘s source or destination can be specified with s or d, respectively. The option should be followed by an IP address or lP Net mask combination or hostname. Net masks can use CIDR (192.168.0.0/24) or

VLSN (192.168.0.0/255,255.255.0) notation. Using a hostname is not recommended because it will just be translated into an IP when the rule is stored anyway.

The following example would allow packets from any address on the 192.168.0.X network through the firewall.


Packets can also be matched based on the physical network interface they are arriving on or leaving through This is done with the i and -0 options, respectively. The following command would only allow packets destined for the local network to leave via etho (assuming all other packets are denied by default).

Another common interface based rule is the following, which allows all packets arriving on the system's loopback interface through the firewall.

Since only local processes have access to the loopback interface, traffic on Io is usually unfiltered. Thus, many firewall rulesets begin with a rule like the above.

Any match criterion may be negated by prep ending (with the quotes) to the value. While the quotes 3’9 not strictly necessary in these examples, they are considered best practice when dealing with a speclall

Character the bash shell might desire to expand. The following example would block all traffic except packets from 192.168.0.1



Common Match Criteria conhnued 


Transport protocol and port
p tcp dport 80
p udp sport 53
Port ranges can be specified with start:end

ICMP type

p icmp  icmp  type host unreachable

Packets can also be matched by their source or destination ports. Because port numbers are ambiguous unless associated with a transport protocol since tcp port 53 is distinct from udp port 53 references to ports must always specify a ayer  4 protocol with the p option. Destination ports are matched with
dport and source ports with sport. Ranges of ports can be listed as start port end port If end port is left out, it is assumed to be the highest possible port.

The following example would ailow tcp packets coming from port 123 of 19216801 to port 1024 or above of 19216802

ICMP packets, which include ping requests and responses, destination unreachable messages from routers, and many other types of network diagnostic messages, can be selectively filtered by specifying

icmp as the protocol and using the icmp type to match specific types. The following examples would explicitly deny ping requests and explicitly allow destination unreachable messages, respectively

While some networks choose to block pings requests, denying all ICMP packets is not recommended. Certain types, such as destination unreachabie messages represent important information that network clients should receive.


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