DNS Client and Cache

Document revision:	1.3 (November 28, 2007, 10:44 GMT)
Applies to:	V3.0

General Information

Summary

DNS cache is used to minimize DNS requests to an external DNS server as well as to minimize DNS resolution time. This is a simple recursive DNS server with local items.

Specifications

Packages required: system
License required: Level1
Submenu level: /ip dns
Standards and Technologies: DNS
Hardware usage: Not significant

Description

A MikroTik router with DNS feature enabled can be set as a DNS server for any DNS-compliant client. Moreover, MikroTik router can be specified as a primary DNS server under its dhcp-server settings. When the remote requests are enabled, the MikroTik router responds to TCP and UDP DNS requests on port 53.

Additional Resources

DNS Cache Setup

Submenu level: /ip dns

Description

DNS facility is used to provide domain name resolution for router itself as well as for the clients connected to it.

Property Description

allow-remote-requests (yes | no; default: no) - specifies whether to allow network requests

cache-max-ttl (time; default: 1w) - specifies maximum time-to-live for cache records. In other words, cache records will expire unconditionally after cache-max-ttl time. Shorter TTL received from DNS servers are respected

cache-size (integer: 512..10240; default: 2048KiB) - specifies the size of DNS cache in KiB

cache-used (read-only: integer) - displays the current cache size in KiB

primary-dns (IP address; default: 0.0.0.0) - primary DNS server

secondary-dns (IP address; default: 0.0.0.0) - secondary DNS server

Notes

If the property use-peer-dns under /ip dhcp-client is set to yes then primary-dns under /ip dns will change to a DNS address given by DHCP Server.

Example

To set 159.148.60.2 as the primary DNS server and allow the router to be used as a DNS server, do the following:

[admin@MikroTik] ip dns> set primary-dns=159.148.60.2 \
\... allow-remote-requests=yes
[admin@MikroTik] ip dns> print
            primary-dns: 159.148.60.2
          secondary-dns: 0.0.0.0
  allow-remote-requests: yes
             cache-size: 2048KiB
          cache-max-ttl: 1w
             cache-used: 7KiB
[admin@MikroTik] ip dns>

Cache Monitoring

Submenu level: /ip dns cache

Description

This menu provides a list with all address (DNS type "A") records stored on the server

Property Description

address (read-only: IP address) - IP address of the host

name (read-only: name) - DNS name of the host

ttl (read-only: time) - remaining time-to-live for the record

All DNS Entries

Submenu level: /ip dns cache all

Description

This menu provides a complete list with all DNS records stored on the server

Property Description

data (read-only: text) - DNS data field. IP address for type "A" records. Other record types may have different contents of the data field (like hostname or arbitrary text)

name (read-only: name) - DNS name of the host

ttl (read-only: time) - remaining time-to-live for the record

type (read-only: text) - DNS record type

Static DNS Entries

Submenu level: /ip dns static

Description

The MikroTik RouterOS has an embedded DNS server feature in DNS cache. It allows you to link the particular domain names with the respective IP addresses and advertize these links to the DNS clients using the router as their DNS server. This feature can also be used to provide fake DNS information to your network clients. For example, resolving any DNS request for a certain set of domains (or for the whole Internet) to your own page.

The server is capable of resolving DNS requests based on POSIX basic regular expressions, so that multiple requets can be matched with the same entry. In case an entry does not conform with DNS naming standards, it is considered a regular expression and marked with ‘R’ flag. The list is ordered and is checked from top to bottom. Regular expressions are checked first, then the plain records.

Property Description

address (IP address) - IP address to resolve domain name with

name (text) - DNS name to be resolved to a given IP address. May be a regular expression

ttl (time) - time-to-live of the DNS record

Notes

Reverse DNS lookup (Address to Name) of the regular expression entries is not possible. You can, however, add an additional plain record with the same IP address and specify some name for it.

Remember that the meaning of a dot (.) in regular expressions is any character, so the expression should be escaped properly. For example, if you need to match anything within example.com domain but not all the domains that just end with example.com, like www.another-example.com, use name=".*\\.example\\.com"

Regular expression matching is significantly slower than of the plain entries, so it is advised to minimize the number of regular expression rules and optimize the expressions themselves.

Example

To add a static DNS entry for www.example.com to be resolved to 10.0.0.1 IP address:

[admin@MikroTik] ip dns static> add name www.example.com address=10.0.0.1
[admin@MikroTik] ip dns static> print
Flags: D - dynamic, X - disabled, R - regexp
 #     NAME               ADDRESS                                 TTL
 0     www.example.com    10.0.0.1                                1d
[admin@MikroTik] ip dns static>

Flushing DNS cache

Command name: /ip dns cache flush

Command Description

flush - clears internal DNS cache

Example

[admin@MikroTik] ip dns> cache flush
[admin@MikroTik] ip dns> print
              primary-dns: 159.148.60.2
            secondary-dns: 0.0.0.0
    allow-remote-requests: yes
               cache-size: 2048 KiB
            cache-max-ttl: 1w
               cache-used: 10 KiB
[admin@MikroTik] ip dns>