Msn web hosting - But how does this affect us? Take a
But how does this affect us? Take a look at our rule for port 20, the FTP-data port. The rule as we have it now assumes that the connection will be made by our client to the server. This will work if we use passive mode. But it is very difficult for us to configure a satisfactory rule to allow FTP active mode, because we may not know in advance what ports will be used. If we open up our firewall to allow incoming connections on any port, we are exposing our network to attack on all services that accept connections. The dilemna is most safely resolved by insisting that our users operate in passive mode. Most FTP servers and many FTP clients will operate this way. The popular ncftp client also supports passive mode, but it may require a small configuration change to make it default to passive mode. Many World Wide Web browsers such as the Netscape browser also support passive mode FTP, so it shouldn’t be too hard to find appropriate software to use. Alternatively, you can avoid the issue entirely by using an FTP proxy server that accepts a connection from the internal network and establishes connections to the outside network. In building your firewall, you will probably find a number of these sorts of problems. You should always give careful thought to how a service actually operates to be sure you have put in place an appropriate ruleset for it. A real firewall configuration can be quite complex. Summary of ipfwadm Arguments The ipfwadm has many different arguments that relate to IP firewall configuration. The general syntax is: ipfwadm category command parameters [options] Let’s take a look at each of these. Categories One and only one of the following must be supplied. The category tells the firewall what sort of firewall rule you are configuring: -I Input rule -O Output rule -F Forwarding rule Commands At least one of the following must be supplied and applies only to those rules that relate to the supplied category. The command tells the firewall what action to take. -a [policy] Append a new rule -i [policy] Insert a new rule -d [policy] Delete an existing rule -p policy Set the default policy -l List all existing rules -f Flush all existing rules The policies relevant to IP firewall and their meanings are: