Software-Defined Networking (SDN) is an emerging technology in the networking domain. This technology decouples the control plane and forwarding plane from the network elements such as routers and switches. The decoupling of two planes helps to develop more advanced and complex applications. Convention firewall fails to provide a strong protection against the attacks specifically against Distributed Denial-of-Service (DDoS) attacks on SDN. Existing firewall is dedicated hardware, more expensive, single point of failure, static and Access Control List (ACL) based firewall. All these features of existing firewall make it unsuitable for securing SDN. In our paper, we proposed distributed and dynamic Firewall system called SDN Firewall. SDN Firewall overcomes all the disadvantages of conventional firewall. SDN Firewall is implemented at POX Controller using 3 popular firewall techniques – Packet Filter Firewall, Stateful Inspection Firewall and Application Proxy. Analysis of results obtained from emulation of SDN Firewall in Mininet emulator shows that SDN Firewall is more secure than conventional firewall system. © Research India Publications.