Firewalls SYN, SYN+ACK, ACK packets at the start of the connection, but with UDP, there is no connection, so incoming and outgoing simply refer to the direction of the packet. Configuring the Red Hat Linux Firewall Configuring the Red Hat Linux firewall is very straightforward. Start the Security Level Configuration application by choosing Main Menu | System Settings | Security Level, or by typing redhat-config-securitylevel at the command prompt. If you are not logged on as root, and you haven’t recently supplied the root password to enable you to update system wide settings, you’ll be prompted to do that before the Security Level Configuration starts. When it does, you’ll see this window. Under the Security Level: drop-down list, the options are High, Medium, and No firewall: The High setting configures the firewall to reject all incoming TCP connections (by blocking TCP packets that have the SYN flag set, and ACK cleared), and reject all incoming UDP packets, with the exception of replies to DNS queries on port 53 that come from the name servers we told Red Hat Linux about when we configured the network settings. (If the DNS replies were blocked, then we would be unable to resolve any host names that aren’t defined in the local /etc/hosts file, which would make using the Internet very tedious.) Important Note that, since incoming connections are blocked, we can’t run a Web server or any other kind of server behind such a firewall. The Medium setting configures the firewall to reject all incoming TCP connections to ports in the range 0 to 1023, and also incoming UDP packets for these port numbers. NFS and X11 traffic are also blocked. The significance of the ports in the range 0 to 1023 is that these are “privileged” ports, and a program has to be running with root authority in order to open sockets on them. However, this does not mean that ports with numbers 1024 to 65536 can only have unprivileged programs opening sockets on them. It is quite possible to set up a Web server running with root authority listening on, 426

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