The maximum length of a domain is 255 including the final dot.

[drahoslavzan]

Thanks for the useful library.

I think there should be a check if the domain name has a dot at the end, if not then the length should not go over 254.

https://github.com/chmike/domain/blob/f6a50c133799ad801336e9049a55bc9b90ce7d7e/check.go#L16

[chmike]

Thank you very much for your interest and contribution to this package.

According to domain name specifications RFC 1035:

2.3.4. Size limits

names 255 octets or less

According to the SMTP RFC 5321:

4.5.3.1.2. Domain

The maximum total length of a domain name or number is 255 octets.

I’m not sure if a domain with a final dot is a valid domain. It doesn’t seam so considering the "preferred syntax" described in section 2.3.1 of RFC 1035. It might be a DNS internal specific thing.

Do you have any references to share to help clarify this ?

[drahoslavzan]

domain name ending with a dot is a valid domain, absolute domain name I think it is even mentioned in the RFC you are referring to.

but I just found an article that explains domain name length constraints with references. There is also a leading byte for each label, thus the maximum name is only 253 octets (hidden leading byte for the first label and null label or the dot at the end).

https://www.directedignorance.com/blog/maximum-length-of-domain-name

[chmike]

For now I will stick to the 255 octet upper limit. The reason to do so are

• the RFCs are all very clear and consistent on this upper 255 octet limit
• the trailing 0 byte, trailing dot, or length bytes are application and programming language specific and not talked about in the RFCs
• rejecting lengths above 253 or 254 can easily be added after a successful check with the current code, the opposite is not easy to do

I leave the issue open as further inputs are welcome.

[chmike]

Here is a new relevant input on this matter. It is a superuser answer. In summary:

• the domain name is 253 ASCII character long
• it may be 254 when it has a trailing dot
• some constrains on ASCII letters are for host names, not the DNS protocol and thus domain names. For instance _minecraft._tcp.example.com is a valid domain name, not a valid host name registered as A/AAAA record.