Difference binding to INADDR_ANY and a specific IP

Question

I'm confused about the use and consequences of INADDR_ANY when binding a socket. Of course the INADDR_ANY listens to all the local interfaces. My question as about what consequences this does have.

I remember reading that binding to a specific IP address allows the kernel to handle the demultiplexing but can't find the reference any more.

Will the use of INADDR_ANY have consequences of this kind or will I simply just receive data from all my local ip's? What are the benifits and problems of using each kind of binding?

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EDIT: Found the reference. It's from Unix Network Programming (Stevens)

One advantage in binding a non-wildcard IP address is that the demultiplexing of a given destination IP address to a given server process is then done by the kernel.

What does this really mean?

Answer 1

Binding to specific interfaces is something to use only in very special circumstances, when the application needs to "know" the local IP addresses and the immediate network layout. A routing daemon program is perhaps the best example.

Another, more pedestrian example: if you have a multi-homed machine (i.e. a machine with more than one connection to the Internet, possibly different ISPs) you can bind to a specific interface to make sure that the connection goes through a given connection. Binding separately to each network interface, the application could detect link down etc.

Implementations of protocols that need to "know" the local IP address (infamous examples: FTP, SIP, UPnP) tend to bind sockets to specific interfaces. (I consider them to be bad protocols, since they violate the isolation between transport and application layers).

Save for these cases, it is normally over-engineering to bind to specific interfaces, because addresses and interfaces may change, and the program must detect these conditions to update the respective sockets.