Can domain name subdomains have an underscore in it

Question

Can subdomains  domain names  have underscore   in them

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A note on terminology  in furtherance to Bortzmeyer s answer  One should be clear about definitions  As used here    domain name is the identifier of a resource in a DNS database label is the part of a domain name in between dots  hostname is a special type of domain name which identifies Internet hosts   The hostname is subject to the restrictions of RFC 952 and the slight relaxation of RFC 1123  RFC 2181 makes clear that there is a difference between a domain name and a hostname           the fact that  any binary label can have an MX record does not imply that any binary name can be used as the host part of an e-mail address      So underscores in hostnames are a no-no  underscores in domain names are a-ok   In practice  one may well see hostnames with underscores  As the Robustness Principle says   Be conservative in what you send  liberal in what you accept    A note on encoding  In the 21st century  it turns out that hostnames as well as domain names may be internationalized  This means resorting to encodings in case of labels that contain characters that are outside the allowed set   In particular  it allows one to encode the   in hostnames  Update 2017-07  This is doubtful  see comments  The   still cannot be used in hostnames  Indeed  it cannot even be used in internationalized labels    The first RFC for internationalization was RFC 3490 of March 2003   Internationalizing Domain Names in Applications  IDNA    Today  we have    RFC 5890  IDNA  Definitions and Document Framework  RFC 5891  IDNA  Protocol  RFC 5892  The Unicode Code Points and IDNA  RFC 5893  Right-to-Left Scripts for IDNA  RFC 5894  IDNA  Background  Explanation  and Rationale  RFC 5895  Mapping Characters for IDNA 2008    You may also want to check the Wikipedia Entry  RFC 5890 introduces the term LDH  Letter-Digit-Hypen  label for labels used in hostnames and says      This is the classical label form used  albeit with some additional restrictions  in hostnames  RFC 952    Its syntax is identical to that described as the  preferred name syntax  in Section 3 5 of RFC 1034 as modified by RFC 1123  Briefly  it is a string consisting of ASCII letters  digits  and the hyphen with the further restriction that the hyphen cannot appear at the beginning or end of the string   Like all DNS labels  its total length must not exceed 63 octets    Going back to simpler times  this Internet draft is an early proposal for hostname internationalization  Hostnames with international characters may be encoded using  for example   RACE  encoding   The author of the  RACE encoding  proposal notes      According to RFC 1035  host parts must be case-insensitive  start and end with a letter or digit  and contain only letters  digits  and the hyphen character   -    This  of course  excludes any internationalized characters  as well as many other characters in the ASCII character repertoire  Further  domain name parts must be 63 octets or shorter in   length     All post-converted name parts that contain internationalized characters begin with the string  bq--         The string  bq--  was chosen because it is extremely unlikely   to exist in host parts before this specification was produced

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Most answers given here are false  It is perfectly legal to have an underscore in a domain name  Let me quote the standard  RFC 2181  section 11   Name syntax       The DNS itself places only one restriction on the particular labels   that can be used to identify resource records   That one   restriction relates to the length of the label and the full   name        Implementations of the DNS protocols must not place any   restrictions on the labels that can be used   In particular  DNS   servers must not refuse to serve a zone because it contains labels   that might not be acceptable to some DNS client programs    See also the original DNS specification  RFC 1034  section 3 5  Preferred name syntax  but read it carefully   Domains with underscores are very common in the wild  Check  jabber  tcp gmail com or  sip  udp apnic net   Other RFC mentioned here deal with different things  The original question was for domain names  If the question is for host names  or for URLs  which include a host name   then this is different  the relevant standard is RFC 1123  section 2 1  Host Names and Numbers  which limits host names to letters-digits-hyphen

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I followed the link to RFC1034 and read most of it and was surprised to see this   The labels must follow the rules for ARPANET host names   They must start with a letter  end with a letter or digit  and have as interior characters only letters  digits  and hyphen   There are also some restrictions on the length   Labels must be 63 characters or less   For clarification  a domain names are made up of labels which are separated by dots      This spec must be outdated because it doesn t mention the use of underscores  I can understand the confusion if anybody stumbles over this spec without knowing it is obsolete  It is obsolete  isn t it   I followed the link to RFC2181 and read some of it  Especially where it pertains to the issue of what is an authoritative  or canonical  name and the issue of what makes a valid DNS label   As posted earlier it states there s only a length restriction then to sum it up it reads    about names and valid labels   These are already adequately specified  however the specifications seem to be sometimes ignored  We seek to reinforce the existing specifications   Kind of leaves me wondering if  a length only restriction  is  adequate   Are we going to start seeing domain names like        soon  Isn t the internet screwed up enough

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There is one additional thing you may need to know  If the host or subdomain part of the url contain an underscore  IE9  have not tested other versions  cannot write cookies   So be careful about that   -

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Not if you want it to resolve on the Internet    You cannot have  http   my subdomain example com is invalid   You can have  http   my-subdomain example com with a hyphen

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Recently the CAB-forum     decided that      All certificates containing an underscore character in any dNSName entry and having a validity period of more than 30 days MUST be revoked prior to January 15  2019  https   cabforum org 2018 11 12 ballot-sc-12-sunset-of-underscores-in-dnsnames    This means that you are no longer allowed to use underscores in domains that will have a ssl tls certificate       The Certification Authority Browser Forum  CA Browser Forum  is a voluntary gathering of leading Certificate Issuers  as defined in Section 2 1 a  1  and  2  below  and vendors of Internet browser software and other applications that use certificates  Certificate Consumers  as defined in Section 2 1 a  3  below

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Just created local project  with vagrant  and it was working perfectly when accessed over ip address  Then I added some name test to hosts file and tried accessing it that way  but I was getting  bad request - 400  all the time  Wasted hours until I figured out that just changing domain name to some-name test solves the problem  So at least locally on Mac OS it s not working

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Clarifying bortzmeyer and David Tonhofer  domain name and subdomain name labels can contain leading underscores  but nowhere else   As David Tonhofer wrote  labels are the in-between-the-periods parts and should follow the LDH rule except when specifying service labels and port labels to differentiate them from regular labels  Then they must occur at the beginning of the label which should be the  Short Names  from the Service Name and Port Number Registry  the port number with no leading 0s  or the protocol  ie  tcp  udp   These service labels are further limited to 15 characters    RFC2782 specifies prefixing service record subdomains with underscores  RFC6698 specifies prefixing port numbers with underscores in TLSA certificate records    Contrary to David Tonhofer s answer  IDN does not allows for encoding underscore      U 005F LOW LINE  or any other invalid ASCII character   From RFC5890          two new subsets of LDH labels are created by the   introduction of IDNA   These are called Reserved LDH labels  R-LDH   labels  and Non-Reserved LDH labels  NR-LDH labels   Reserved LDH   labels  known as  tagged domain names  in some other contexts  have   the property that they contain  --  in the third and fourth   characters but which otherwise conform to LDH label rules    Punycode encodes all ASCII codepoints as ASCII directly  including underscore  The resulting R-LDH would not conform the the LDH label rules  For example  S  com would be encoded as xn-- -zmb com which violates the rules  There may be a homographic codepoint which looks like an underscore that can be coded legally  perhaps     U FF3F fullwidth low line   but these kinds of codepoints would be categorized as DISALLOWED by RFC5892 under 2 3 IgnorableProperties as a Noncharacter Code Point   RACE  the other proposed IDN encoding scheme  was not accepted as a standard by IETF and should not be used

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Here my 2 cents from Java world   From a Spark Scala console  with Java 8   scala gt  new java net URI  spark   spark master   getHost res10  String   null  scala gt  new java net URI  spark   spark-master   getHost res11  String   spark-master  scala gt  new java net URI  spark   spark master google fr   getHost res12  String   null  scala gt  new java net URI  spark   spark master google fr   getHost res13  String   spark master google fr  scala gt  new java net URI  spark   spark-master google fr 3434   getHost res14  String   spark-master google fr  scala gt  new java net URI  spark   spark-master goo gle fr 3434   getHost res15  String   null   It s definitely a bad idea

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No  you can t use underscore in subdomain but hypen  dash   i e my-subdomain agahost com is acceptable and my subdomain agahost com would not be acceptable

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Individual TLD s can place their own rules  amp  restrictions on domains names as they see fit  such as to accomodate local languages   For example  according to the CIRA  Canada s  ca domain names are allowed         Letters a through z  and the following accented characters                                                   Note that Domain Names are not case sensitive  This means there will be no distinction made between upper case letters and lower case letters  A   a     The numbers 0123456789  and   The hyphen character   -   although it cannot be used to start or end a Domain Name        The maximum length is 63 characters  except each accented character reduces that limit by 4 characters    Source     Incidentally  this allows for around 4 Quadragintillion domain name possibilities  not counting sub-domains  for dot-ca domains

[dns] Can (domain name) subdomains have an underscore "_" in it?

A note on terminology, in furtherance to Bortzmeyer's answer

A note on encoding

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