Domain and Host Names

1
Domain and Host Names

• Mark Andrews
• Mark.Andrews_at_nominum.com

2
Domain name

• Characters 0x00 - 0xff
• 0-63 octets per label length octet
• 1-255 octets wire length
• Character set ASCII (0x00-0x7f)

3
Domain Name Presentation

• ASCII string of printable characters containing
  no white space
• Non printable are expressed as \DDD
• \ proceeding character means next character is
  literal
• . separates labels
• \ and .in labels are expressed as \\ and
  \.

4
Domain Name Uses

• Host names (http, ftp, telnet)
• Mail Boxes mark\.andrews.nominum.com
• Service location (SRV) _ldap._tcp.example.com
• Mail domains (MX)
• HESIOD databases (TXT)

5
Host name

• A subset of domain names
• Restricted character set
  A - Z, a - z, 0 - 9, - and .
• Restricted syntax
• hyphen internal to label
• periods can only be used to separate labels
• name must contain at least one alpha character

6
Mail Boxes

• A subset of domain names
• Used to encode email addresses
• First Label is the local part of the email
  address.
• Second and subsequent labels form a mail domain.

7
Mail Domains

• A subset of domain names.
• Identical syntax to host names.
• May or may not correspond to a host name.

8
Service Location

• Used to identify which machines provide a service
  for a given domain.
• First two labels identify service and protocol.
  Both of these labels have a leading underscore so
  as to not collide with a hostname.
• Third and subsequent labels are normally
  syntactically identical to a host name.

9
Embedded Host Names

• Most uses of domain names either are or contain
  hostnames (or mail domains).
• _ldap._tcp.example.com. SRV ...
• _at_ SOA host.example.com. mark\.andrews.example.co
  m. ...
• example.com. MX
• host.example.com. A ...

10
Relationships
All Domain Names
Mail Boxes
Host Names Mail Domains
Service Locations
11
International Domain Names

• Preserve a restricted character set in host
  names.
• Have labels that cannot be in a hostname.
• Need to be able to mix hostname labels with non
  hostname labels in the one domain name. i.e. SRV
  records should work with IDN