Virtual IPs

When using additional addresses for features like NAT or binding services to different interfaces, you can add extra addresses to already defined interfaces using Virtual IPs.


Virtual IPs also play a vital role in high availability setups

Types and their usage

OPNsense supports different types of virtual addresses all with their specific purposes, which we will explain below.

IP Alias

A standard extra address, which you can use to bind services to or use in NAT rules.

The address will act like a normal interface address, which means it will respond to ICMP ping requests and will generate ARP traffic (OSI layer 2).

Additionally you can add an alias into an existing CARP group (by setting its VHID).

Usually the subnet mask should match the interfaces or be defined as a single address (/32 or /128).


Specifies an address for use in a high availability cluster, acts like a regular address when the node is in MASTER state.

Internally a custom mac address is generated needed for the protocol. More information about CARP can be found in our high availability section.


The virtual MAC address of a CARP interface is 00:00:5e:00:01:XX, where the last two digits will be populated by its vhid.


CARP uses IP protocol number 112 (0x70), to detect priority it will send out advertisements using or FF02::12.

Proxy ARP

Does not add a real address to an interface, instead it will use choparp to reply to arp requests on the network. This can sometimes be practical in situations where clients should be let to believe an address is local.


The other type won’t respond to ICMP ping messages or reply to ARP requests, it merely is a definition of an address (or range) which can be used in NAT rules. This is convenient when the firewall has a public IP block routed to its WAN IP address, IP Alias, or a CARP VIP.


The interface should validate suitable combinations of settings, below you will find a detailed explanation for everyone of them.


The type of address, as defined in Types.


The interface this address belongs to.


Either Network or Single address, only has affect when creating NAT rules, where Proxy ARP and Other combined with Expansion will generate separate addresses for all items in the netmask.


When applicable, expand netmask to separate addresses.


The address and netmask to assign, when assigning multiple addresses in the same network, the masks usually should match.


Only applies to IP Alias types, usually this field should be empty, except some tunnel devices (ppp/pppoe/tun) expect the gateway address to be defined.

Virtual IP Password

The password used to encrypt CARP packets over the network, should be the same on preferred master and backup node(s).

VHID Group

The Virtual Host ID. This is a unique number that is used to identify the redundancy group to other nodes in the group, and to distinguish between groups on the same network. Acceptable values are from 1 to 255. This must be the same on all members of the group.

Advertising Frequency

Defines how often is advertised that this interface is part of a group (Base defined in seconds) and how much to skew when sending advertisements. A higher skew means less preferred.


User friendly description of this VIP


The status page shows all configured carp VHID groups and their active status. Our status screen also offers some buttons to disable carp or force a node into maintenance mode.

All different statuses are detailed below.


Usually this indicates there is an issue with the interface, often this relates to not disconnected interfaces or other technical problems.


In backup state this interface is part of a cluster and listening to advertisements. If for some reason it won’t receive advertisements for a short period of time, it will transition to master.


Marks the active node, while listening to advertisements seen on the network. If another node is seen with a better advertisement it might transition to backup (depending on preempt setting, found on the System ‣ High Availability ‣ Settings page).


Displayed when Temporarily Disable CARP is clicked on this page.