What Does 0.0.0.0/0 Mean in Networking?

bindchronycidrdomain-name-systemnetworking

For example in an 'allow' statement in BIND or in Chrony config files.

Does allow 0.0.0.0/0 mean allowing any clients or allowing no clients?

My question isn't about the specific programs, but just generally what is the correct interpretation of this CIDR notation.

Best Answer

It literally means all or any hosts - 99.99% of the time.

The only exception I can provide at hand is the whitelist of Cisco routers, where such "zero whitelist" means negating all hosts. It is a pretty specific case, though.

Related Solutions

Windows BIND DNS – How to Set Up a BIND DNS Server to Respond with a Single IP Address

Try this:

In named.conf:

options {
    recursion yes;
}

zone "." IN {
    type master;
    file "named.root";
};

In "named.root":

$ORIGIN .
$TTL 1D
@    IN     SOA  @ none. ( 0 1D 1H 1W 3H );
     IN     NS   @
*    IN     A    a.b.c.d

Web-server – How to use DNS/Hostnames or Other ways to resolve to a specific IP:Port

You cannot use the DNS to point to a port (unless the client supports SRV records, most don't).

Websites and Protocols with Host Headers

You will have to put some front-end method in place to do this. Typically you would use a front end web server or a dedicated proxy software to forward the connection from port 80 to port !80 based on the name of the server being requested in the header. Some firewalls can also forward based on the host header too.

SRV Records

Some clients support lookups of SRV records which indicate hostname and port number of server for the specified service (ie the user specifies "example.com", the client looks up a SRV record and gets "server101.example.com" on port "255"; then connects to that). Some clients also implement this where it is not required (my last smartphone would lookup the SRV records when setting up a new e-mail account for example).

Unfortunately support for SRV records is highly uncommon. Only a few notable protocols mandate it's support (Jabber/XMPP, Kerberos, LDAP, SIP) and not every client supports it even when mandated.