Form an expression

Rules are written as using the Cloudflare Rules language - a DSL intended to mimic Wireshark semantics. For more information, refer to the Rules language documentation.

To start with a simple case, here’s how you’d match a source IP:

ip.src == 192.0.2.0

Expressions can be more complex by joining multiple clauses via a logical operator:

ip.src == 192.0.2.1 && (tcp.flags.push || tcp.flags.reset)

​​ Capabilities

You can use Magic Firewall to skip or block packets based on source or destination IP, source or destination port, protocol, packet length, or bit field match.

​​ Restrictions

Wirefilter comparisons support CIDR notation, but only inside sets. For example:

ip.src == 192.0.2.0/24  # bad
ip.src in { 192.0.2.0/24 }  # good

Expressions have a complexity limit that is easily reached when many joined or nested clauses are in the expression. Here’s an example:

(tcp.dstport == 1000 || tcp.dstport == 1001) && (tcp.dstport == 1002 || tcp.dstport == 1003) && (tcp.dstport == 1004 || tcp.dstport == 1005) && (tcp.dstport == 1006 || tcp.dstport == 1007) && (tcp.dstport == 1008 || tcp.dstport == 1009) && (tcp.dstport == 1010 || tcp.dstport == 1011) && (tcp.dstport == 1012 || tcp.dstport == 1013) && (tcp.dstport == 1014 || tcp.dstport == 1015) && (tcp.dstport == 1016 || tcp.dstport == 1017)

If the limit is reached, the response will have a 400 status code and an error message of “ruleset exceeds complexity constraints.” Split the expression into multiple rules and try again.