DNS

December 8, 2014 ยท View on GitHub

Domain Name System.

Part of the application layer.

Standard IANA port: 53/UDP

Protocol that convert strings into IPs, for example:

http://www.google.com -> 173.194.34.34

Before before using an address such as www.google.com, any program such as a browser must first resolve the hostname www.google.com into an IP by asking that from a server.

Linux systems usually offer man resolver C library interface, which any program can use to resolve DNS names. The resolver library may cache results across applications that have already been resolved.

DNS on WAN

On the Internet, hostnames are resolved to IPs by DNS servers.

You must pay to reserve hostnames so they can be resolved to the IP of your choice.

TODO how to DNS servers find out all the hostnames in the world?

DNS on LAN

DNS can also be done for local networks:

computer2 -> 192.168.0.3

In which case the DNS server normally resides on the router.

Client computers on the network are informed that it is a DNS server via DHCP.

On your LAN, people can use the host name to communicate between computers

For example, John is running an Apache server on the usual port 80. He has hostname john.

Mary is on the same network. Therefore, she can refer to john simply as john. For example:

ping john
firefox john

TODO if many people set up the same hostname, then what?

Wildcard DNS

It is possible to redirect all subdomains that don't match any other rule to a single IP by using a wildcard DNS record:

This can be used to implement functionality like GitHub pages on a website, allowing users to have their own subdomains for potentially non safe HTML content, while avoiding CSRF attacks.

host utility

Does DNS and rDNS

DNS:

host www.google.com

Sample output:

www.google.com has address 74.125.206.147
www.google.com has address 74.125.206.106
www.google.com has address 74.125.206.104
www.google.com has address 74.125.206.105
www.google.com has address 74.125.206.103
www.google.com has address 74.125.206.99
www.google.com has IPv6 address 2a00:1450:400c:c0a::93

rDNS:

host 173.194.40.194

Sample output:

194.40.194.173.in-addr.arpa domain name pointer par10s12-in-f2.1e100.net

TODO understand that output. Why is google.com nowhere to be seen?

TODO: why does host 74.125.206.147 (one of the IPs for www.google.com) give:

Host 147.206.125.74.in-addr.arpa. not found: 3(NXDOMAIN)

resolv.conf

cat /etc/resolv.conf

Lists DNS servers.

This file may be automatically generated by utilities.

On Ubuntu 12.04, you should never edit that file manually. By default it contains:

nameserver 127.0.1.1

which is localhost, and is used indirectly by the NetworkManger system, which you should use instead.

TODO does it specify the config file location? resolv.conf

getaddrinfo

POSIX function to resolve hostnames:

http://pubs.opengroup.org/onlinepubs/009695399/functions/freeaddrinfo.html

hostname utility

Print currently desired hostname:

echo $HOSTNAME
hostname

In the default bash PS1 line for Ubuntu and many systems you see: ciro@ciro-Thinkpad-T430, then the hostname is ciro-Thinkpad-T430.

Change hostname for cur session:

h=
sudo hostname "$h"

prompt PS1 is not changed immediately.

Change hostname permanently

h=
echo "$h" | sudo tee /etc/hostname

Set hostname in Windows

Host is referred to as "computer name". Good name choice, that is exactly what host is.

wmic computersystem where name="%COMPUTERNAME%" call rename name="NEW-NAME"

Zone file

When you register for a domain of your own, you will start thinking about this: it is the main setting on your registrar interface.

http://en.wikipedia.org/wiki/Zone_file

apex domain

@ in the zone file means the domain you own without any subdomain.

E.g., if you own cirosantilli.com, @ means cirosantilli.com itself, while www means www.cirosantilli.com.

Apex domains are more restrictive than subdomains, and certain hosting services advise against it, such as GitHub Pages.

The main problem is that in services such as GitHub pages you don't get an actual IP, so you can't point the Apex to an IP (which is simple), and the CNAME "workaround" is not good enough in that case.

naked domain

The apex domain is sometimes called naked domain, since it has no subdomain.

CNAME record

TODO File that tells DNS to redirect to another domain name, creating an alias.

http://en.wikipedia.org/wiki/CNAME_record

A

Points a domain to an IP. The final part of the resolution.

rDNS

Reverse DNS

http://en.wikipedia.org/wiki/Reverse_DNS_lookup

Protocol that transforms an IP into a hostname.

Not always supported on all DNS servers.

DDNS

Dynamic DNS.

A way to update DNS as IPs change.

Useful for example if you want to give a hostname for your home network, in which the IP is dynamic for most ISPs. A DDNS service like http://www.noip.com can give you a persistent hostname anyways.

TODO what is it exactly? How does it work? A protocol? Part of DNS?

Tools

  • dig