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Why are underscores not allowed in DNS host names?

April 26, 2017allowed DNS host names underscores

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Why are underscores not allowed in DNS host names?

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The BIND RFC states that underscores in host names are illegal. MultiNet did not enforce this rule until v4.0B. If you upgrade from v4.0A or prior and have a zone file that contains underscores or try to do a zone transfer from a zone with underscores you will get OPCOM errors and these zones will not be loaded correctly. It is recommended that all underscores be removed from host names. There is, however, a workaround. Underscores ARE allowed on CNAME records so you can change the original A record from an underscore to a hyphen, then create a CNAME record with the underscore in it and point it to the A record. For example, if you have the following A record: MARS_ALPHA.PROCESS.COM IN A 198.114.213.9 You could make the following entries instead: MARS-ALPHA.PROCESS.COM IN A 198.114.213.9 MARS_ALPHA. PROCESS.COM IN CNAME MARS-ALPHA. PROCESS.