Source filter APIs enable an application to filter the datagrams that it receives based on the source address. There are two categories of source filter APIs: Basic and advanced. Both categories enable multicast receiver applications to designate the unicast addresses (source addresses) and the multicast group (destination address).

Basic (delta-based) APIs

Some applications need the simplicity of a delta-based API in which each function call specifies a single source address to be added to or removed from the filter. Such applications typically fall into the following types:

Any-source multicast

By default, all source addresses are accepted. Individual source addresses can be turned off and back on as needed. This type is also known as the exclude mode, because the source filter contains a list of excluded sources. The following SETSOCKOPT options are included.

Address family	SETSOCKOPT options
IPv4	IP_ADD_MEMBERSHIP
	IP_BLOCK_SOURCE
	IP_DROP_MEMBERSHIP
	IP_UNBLOCK_SOURCE
Protocol independent	MCAST_BLOCK_SOURCE
	MCAST_JOIN_GROUP
	MCAST_LEAVE_GROUP
	MCAST_UNBLOCK_SOURCE

Source-specific multicast

Only the source addresses that are specified in a list are accepted. The list is initially empty; IP addresses can be added to or deleted from the list one at a time. This filter type also is known as the include mode, because the source filter contains a list of included sources. The following SETSOCKOPT options are included.

Address family	SETSOCKOPT options
IPv4	IP_ADD_SOURCE_MEMBERSHIP
	IP_DROP_MEMBERSHIP
	IP_DROP_SOURCE_MEMBERSHIP
Protocol independent	MCAST_JOIN_SOURCE_GROUP
	MCAST_LEAVE_GROUP
	MCAST_LEAVE_SOURCE_GROUP

Advanced (full-state) APIs

These APIs enable an application to use a source filter that is comprised of zero or more source addresses. The application can retrieve the current filter or replace it with a new filter. The following IOCTL commands are included:

• SIOCGIPMSFILTER
• SIOCGMSFILTER
• SIOCSIPMSFILTER
• SIOCSMSFILTER

Consider the following points about multicast source filters:

• Sockets cannot mix IPv4 group-membership APIs with protocol-independent group-membership APIs. Within each API type, delta-based options cannot be mixed with full-state options. Mixing these calls for the same socket results in an EINVAL error.
• The number of groups that can be joined depends on the socket type; there is a maximum of 20 groups for UDP sockets and a maximum of 256 groups for RAW sockets. Exceeding this limit results in an ETOOMANYREFS error.
• Within each group or group and interface pair, an application can use calls for only one type of basic API, either any-source multicast or source-specific multicast. Mixing options will result in an EINVAL error. However, an application can use different methods for different sockets.
• If the filter mode is set to include and the source list is empty, then the entry corresponding to the requested interface and multicast address is deleted, if present. If no such entry is present, then the request is ignored.
• For each socket, you can specify a maximum of 64 source filters for each multicast address and interface address pair. If the call causes the number of filters to exceed this maximum, an ENOBUFS error is returned.