Rope is a programming language that allows developers to write extensions to the Iptables

Iptables

iptables is a user space application program that allows a system administrator to configure the tables provided by the Linux kernel firewall and the chains and rules it stores...

/Netfilter components of Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 using a simple scripting language based on Reverse Polish notation

Reverse Polish notation

Reverse Polish notation is a mathematical notation wherein every operator follows all of its operands, in contrast to Polish notation, which puts the operator in the prefix position. It is also known as Postfix notation and is parenthesis-free as long as operator arities are fixed...

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It is a scriptable Iptables match module, used to identify whether IP packets passed to it match a particular set of criteria or not. Rope started life as a project to make the "string" match module of Iptables stronger and evolved fairly quickly into an open-ended scriptable packet matching mechanism.