Valid characters of a hostname

Question

What are the valid characters of a hostname  This would be something like a networked computer or a web domain   To put it in context  I am writing a PC game which connects to a remote server  so I have a field for hostname and a field for port  Obviously the port is a number in the Short range  but I need to know what all the possible hostname characters are  and any other pattern that might be required - does a hostname need to start with a letter     Examples of hostname include localhost or google com

User · Answer

A  name   Net  Host  Gateway  or Domain name  is a text string up to 24 characters drawn from the alphabet  A-Z   digits  0-9   minus sign  -   and period      Note that periods are only allowed when they serve to delimit components of  domain style names    See RFC-921   Domain Name System Implementation Schedule   for background   No blank or space characters are permitted as part of a name  No distinction is made between upper and lower case  The first character must be an alpha character  The last character must not be a minus sign or period  A host which serves as a GATEWAY should have  -GATEWAY  or  -GW  as part of its name  Hosts which do not serve as Internet gateways should not use  -GATEWAY  and  -GW  as part of their names  A host which is a TAC should have  -TAC  as the last part of its host name  if it is a DoD host  Single character names or nicknames are not allowed   This is provided in http   support microsoft com kb 149044

User · Answer

If you re registering a domain and the termination  ex  com  it is not IDN  as Aaron Hathaway said  Hostnames are composed of series of labels concatenated with dots  as are all domain names  For example  en wikipedia org is a hostname  Each label must be between 1 and 63 characters long  and the entire hostname  including the delimiting dots but not a trailing dot  has a maximum of 253 ASCII characters   The Internet standards  Requests for Comments  for protocols mandate that component hostname labels may contain only the ASCII letters a through z  in a case-insensitive manner   the digits 0 through 9  and the hyphen -  The original specification of hostnames in RFC 952  mandated that labels could not start with a digit or with a hyphen  and must not end with a hyphen  However  a subsequent specification  RFC 1123  permitted hostname labels to start with digits  No other symbols  punctuation characters  or white space are permitted   Later  Spain with it s  es   com es   org es   nom es   gob es and  edu es introduced IDN tlds  if your tld is one of  es or any other that supports it  any character can be used  but you can t combine alphabets like Latin  Greek or Cyril in one hostname  and that it respects the things that can t go at the start or at the end   If you re using non-registered tlds  just for local networking  like with local DNS or with hosts files  you can treat them all as IDN   Keep in mind some programs could not work well  especially old  outdated and unpopular ones

User · Answer

Checkout this wiki  specifically the section Restrictions on valid host names  Hostnames are composed of series of labels concatenated with dots  as are all domain names  For example   quot en wikipedia org quot  is a hostname  Each label must be between 1 and 63 characters long  and the entire hostname  including the delimiting dots but not a trailing dot  has a maximum of 253 ASCII characters  The Internet standards  Requests for Comments  for protocols mandate that component hostname labels may contain only the ASCII letters  a  through  z   in a case-insensitive manner   the digits  0  through  9   and the hyphen   -    The original specification of hostnames in RFC 952  mandated that labels could not start with a digit or with a hyphen  and must not end with a hyphen  However  a subsequent specification  RFC 1123  permitted hostname labels to start with digits  No other symbols  punctuation characters  or white space are permitted

User · Answer

It depends on whether you process IDNs before or after the IDN toASCII algorithm  that is  do you see the domain name pa  de    a d       in Greek or as xn--hxajbheg2az3al xn--jxalpdlp     In the latter case mdash where you are handling IDNs through the punycode mdash the old RFC 1123 rules apply      U 0041 through U 005A  A-Z   U 0061 through U 007A  a-z  case folded as each other  U 0030 through U 0039  0-9  and U 002D  -     and U 002E     of course  the rules for labels allow the others  with dots between labels   If you are seeing it in IDN form  the allowed characters are much varied  see http   unicode org reports tr36 idn-chars html for a handy chart of all valid characters   Chances are your network code will deal with the punycode  but your display code  or even just passing strings to and from other layers  with the more human-readable form as nobody running a server on the           domain wants to see their server listed as being on  xn--mgberp4a5d4ar

[networking] Valid characters of a hostname?

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