Domain Name System(DNS) Syntax Rules
What is a domain name, Why IANA cares?
Domain names are used to access machines over local network or internet with human readable syntax instead of IPs. www.example.com is the global example domain name that is reserved for educational purposes by IANA. IANA(International Assigned Numbers Association) manages DNS root zone and many other critical protocols and standards of internet. At first glance, i will highlight the right most part of domain name "com", our example domain name. "com" is defined by IANA as a top level domain name (TLD) and there are 312 TLDs and six kind of top level domain names presented in IANA DNS Root Zone:
tlds-alpha-by-domain.txt
Details of DNS Jargon(Syntax)
www.example.com is a standard domain name and following list is showing the aspects and rules of a domain name. I have picked rules and create this list with wiki and referenced RFCs
Final Touch, Regular Expression Pattern for Domain Name Syntax
The initial motivation of searching all these information is creating a unique pattern to validate domain names whilst user interactions or in any side of applications. However, without reducing the complexity of 312 tlds and their combinations, it is almost impossible to write a regex pattern to meet these requirements. But pattern creation tradeoff must be well analyzed and shouldn' t be copy-pasted from any resource on the internet. It must be flexible enough and meet the requirements of both above rules and you. I prefer this pattern:
Console
Domain names are used to access machines over local network or internet with human readable syntax instead of IPs. www.example.com is the global example domain name that is reserved for educational purposes by IANA. IANA(International Assigned Numbers Association) manages DNS root zone and many other critical protocols and standards of internet. At first glance, i will highlight the right most part of domain name "com", our example domain name. "com" is defined by IANA as a top level domain name (TLD) and there are 312 TLDs and six kind of top level domain names presented in IANA DNS Root Zone:
- country-code(tr, ru, us, uk...),
- sponsored(xxx, jobs, travel),
- infrastructure(arpa),
- generic-restricted(biz, name, pro),
- generic(com, info, net),
- test(.భారత్)
tlds-alpha-by-domain.txt
# Version 2011121300, Last Updated Tue Dec 13 15:07:02 2011 UTC AC AD AE AERO AF AG AI AL AM AN AO AQ AR ARPA AS ASIA AT AU AW AX AZ BA BB BD BE BF BG BH BI BIZ BJ BM BN BO BR BS BT BV BW BY BZ CA CAT CC CD CF CG CH CI CK CL CM CN CO COM COOP CR CU CV CW CX CY CZ DE DJ DK DM DO DZ EC EDU EE EG ER ES ET EU FI FJ FK FM FO FR GA GB GD GE GF GG GH GI GL GM GN GOV GP GQ GR GS GT GU GW GY HK HM HN HR HT HU ID IE IL IM IN INFO INT IO IQ IR IS IT JE JM JO JOBS JP KE KG KH KI KM KN KP KR KW KY KZ LA LB LC LI LK LR LS LT LU LV LY MA MC MD ME MG MH MIL MK ML MM MN MO MOBI MP MQ MR MS MT MU MUSEUM MV MW MX MY MZ NA NAME NC NE NET NF NG NI NL NO NP NR NU NZ OM ORG PA PE PF PG PH PK PL PM PN PR PRO PS PT PW PY QA RE RO RS RU RW SA SB SC SD SE SG SH SI SJ SK SL SM SN SO SR ST SU SV SX SY SZ TC TD TEL TF TG TH TJ TK TL TM TN TO TP TR TRAVEL TT TV TW TZ UA UG UK US UY UZ VA VC VE VG VI VN VU WF WS XN--0ZWM56D XN--11B5BS3A9AJ6G XN--3E0B707E XN--45BRJ9C XN--80AKHBYKNJ4F XN--90A3AC XN--9T4B11YI5A XN--CLCHC0EA0B2G2A9GCD XN--DEBA0AD XN--FIQS8S XN--FIQZ9S XN--FPCRJ9C3D XN--FZC2C9E2C XN--G6W251D XN--GECRJ9C XN--H2BRJ9C XN--HGBK6AJ7F53BBA XN--HLCJ6AYA9ESC7A XN--J6W193G XN--JXALPDLP XN--KGBECHTV XN--KPRW13D XN--KPRY57D XN--LGBBAT1AD8J XN--MGBAAM7A8H XN--MGBAYH7GPA XN--MGBBH1A71E XN--MGBC0A9AZCG XN--MGBERP4A5D4AR XN--O3CW4H XN--OGBPF8FL XN--P1AI XN--PGBS0DH XN--S9BRJ9C XN--WGBH1C XN--WGBL6A XN--XKC2AL3HYE2A XN--XKC2DL3A5EE0H XN--YFRO4I67O XN--YGBI2AMMX XN--ZCKZAH XXX YE YT ZA ZM ZW
Details of DNS Jargon(Syntax)
www.example.com is a standard domain name and following list is showing the aspects and rules of a domain name. I have picked rules and create this list with wiki and referenced RFCs
- www, example and com are labels [wiki]
- each label can be maximum 63 characters long [RFC1035, page 10] [RFC1123, page 13]
- dots(.) are seperators [wiki]
- com is top level domain (TLD)
- www and example are subdomains [wiki]
- a domain can have maximum 127 subdomains [wiki]
- domain name has maximum 255 characters [RFC1035, page 9][RFC1123, page 13]
- the hierarchy of domains descends from right to left [wiki]
- LDH Rule: labels must contain only Letters, Digits and Hypen (not start or end with hypen) [RFC3696, page 3]
- top level domain names not be all numeric [RFC3696, page 3]
- domain names are not case sensitive [RFC3696, page 3]
Final Touch, Regular Expression Pattern for Domain Name Syntax
The initial motivation of searching all these information is creating a unique pattern to validate domain names whilst user interactions or in any side of applications. However, without reducing the complexity of 312 tlds and their combinations, it is almost impossible to write a regex pattern to meet these requirements. But pattern creation tradeoff must be well analyzed and shouldn' t be copy-pasted from any resource on the internet. It must be flexible enough and meet the requirements of both above rules and you. I prefer this pattern:
Console
true true true true true false false
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