---
title: Form an expression · Cloudflare Magic Firewall docs
description: Rules are written as using the Cloudflare Rules language - a domain-specific language (DSL) intended to mimic Wireshark semantics. For more information, refer to the Rules language documentation.
lastUpdated: 2024-08-13T19:56:56.000Z
source_url:
  html: https://a74404c8.preview.developers.cloudflare.com/magic-firewall/how-to/form-expressions/index.md
  md: https://a74404c8.preview.developers.cloudflare.com/magic-firewall/how-to/form-expressions/
---

Rules are written as using the Cloudflare Rules language - a domain-specific language (DSL) intended to mimic Wireshark semantics. For more information, refer to the [Rules language](https://developers.cloudflare.com/ruleset-engine/rules-language/) documentation.

To start with a simple case, review below how you would match a source IP:

```txt
ip.src == 192.0.2.0
```

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

```txt
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:

```txt
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:

```txt
(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.
