From the pfSense WebGUI: Aliases act as placeholders for real hosts, networks or ports. They can be used to minimize the number of changes that have to be made if a host, network or port changes. The name of an alias can be entered instead of the IP address, network or port in all fields that have a red background. The alias will be resolved according to the list [on the Aliases page of the WebGUI]. If an alias cannot be resolved (e.g. because it has been deleted), the corresponding element (e.g. filter/NAT/shaper rule) will be considered invalid and skipped.
The best example is for blocking a list of hosts considered "bad". If a rule were added for each host to block individually, the rules list would grow quite large. By adding all of these hosts to an alias, only one firewall rule is necessary.
Say there are three web servers in a DMZ, and HTTP, HTTPS, and FTP traffic should be allowed to these servers. It can be accomplished with a single firewall rule and two aliases.
Now there is a single firewall rule that would have otherwise taken 9 separate rules to accomplish!
For Host and Network type aliases, a fully qualified domain name (FQDN) may be entered instead of an IP address. The FQDN will be resolved by DNS every 5 minutes (300 seconds) and updated internally. This can be useful for tracking dynamic DNS entries to identify sites or users that are unable to use a static IP.
The interval at which the resolution takes place may be adjusted under System > Advanced on the Firewall / NAT tab. Enter a new value in the Aliases Hostnames Resolve Interval field. Bear in mind that a lower interval will put a higher burden on the DNS server(s). With many hosts to resolve, the default is best. With only a few hosts, a lower value may be used such as 30 seconds.
A URL table alias is a URL that points to a plain text file containing IP and/or CIDR masked network addresses. The URL will be periodically downloaded and refreshed. The contents of the file would look like so:
192.0.2.0/24 172.22.59.49 192.168.0.128/26
Similar to a URL table in that the file format is the same. However, the content is only requested once and is immediately turned into a traditional alias.
On the main alias screen, click and the Bulk Import screen will be presented. A large text area on this page is used for IP address or CIDR entries. They may be entered/pasted here, one per line, to create a large alias quickly.