When you do not use the -f option to keep connections open and rebind on the connections, the tool can exhaust its available ports, causing the tool to crash. You can work around this problem on test systems by changing TCP settings on the system. For example, on Linux systems, set the following parameters in the /etc/sysctl.conf file: net.ipv4.tcp_fin_timeout = 30 net.ipv4.tcp_tw_recycle = 1 net.ipv4.tcp_tw_reuse = 1 The parameter net.ipv4.tcp_fin_timeout sets the length of time in seconds to wait for a final FIN packet before forcing a close of the socket. The default is 60 (seconds). The parameter net.ipv4.tcp_tw_recycle enables fast recycling of TIME_WAIT sockets. The default is 0 (false). Enabling this can cause Network Address Translation (NAT) issues. The parameter net.ipv4.tcp_tw_reuse enables reuse of TIME_WAIT sockets for new connections. The default is 0 (false). These settings are recommended only for testing, and not for production systems. After making the changes to /etc/sysctl.conf, reload the configuration with the sysctl command: # sysctl -p