dnscontrol/documentation/language-reference/top-level-functions/REV.md

name	parameters	parameter_types	ts_return
REV	address	address string	string

REV returns the reverse lookup domain for an IP network. For example REV("1.2.3.0/24") returns 3.2.1.in-addr.arpa. and REV("2001:db8:302::/48") returns 2.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa..

REV() is commonly used with the D() functions to create reverse DNS lookup zones.

These two are equivalent:

{% code title="dnsconfig.js" %}

D("3.2.1.in-addr.arpa", ...

{% endcode %}

{% code title="dnsconfig.js" %}

D(REV("1.2.3.0/24", ...

{% endcode %}

The latter is easier to type and less error-prone.

If the address does not include a "/" then REV() assumes /32 for IPv4 addresses and /128 for IPv6 addresses.

RFC compliance

REV() implements both RFC 2317 and the newer RFC 4183. The REVCOMPAT() function selects which mode is used. If REVCOMPAT() is not called, a default is selected for you. The default will change to RFC 4183 in DNSControl v5.0.

See REVCOMPAT() for details.

Host bits

v4.x: The host bits (the ones outside the netmask) must be zeros. They are not zeroed out automatically. Thus, REV("1.2.3.4/24") is an error.

v5.0 and later: The host bits (the ones outside the netmask) are ignored. Thus REV("1.2.3.4/24") and REV("1.2.3.0/24") are equivalent.

Examples

Here's an example reverse lookup domain:

{% code title="dnsconfig.js" %}

D(REV("1.2.3.0/24"), REGISTRAR, DnsProvider(BIND),
  PTR("1", "foo.example.com."),
  PTR("2", "bar.example.com."),
  PTR("3", "baz.example.com."),
  // If the first parameter is an IP address, DNSControl automatically calls REV() for you.
  PTR("1.2.3.10", "ten.example.com."),
);

D(REV("2001:db8:302::/48"), REGISTRAR, DnsProvider(BIND),
  PTR("1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0", "foo.example.com."),  // 2001:db8:302::1
  // If the first parameter is an IP address, DNSControl automatically calls REV() for you.
  PTR("2001:db8:302::2", "two.example.com."),                          // 2.0.0...
  PTR("2001:db8:302::3", "three.example.com."),                        // 3.0.0...
);

{% endcode %}

Automatic forward and reverse record generation

DNSControl does not automatically generate forward and reverse lookups. However it is possible to write a macro that does this. See PTR() for an example.