Why can't I ping addresses with leading or trailing underscores on linux

Question

In the windows:

• Access to _.github.comthe browser works
• nslookup _.github.com working
• ping _.github.com working

In linux (tested on two separate networks):

• Access to _.github.comthe browser works
• host _.github.com working
• ping _.github.com not
• python -c "import requests; requests.get('_.github.com')" not

What's going on here?

Answer 1

Because Linux (more precisely, some of its components, such as libresolv or even ping itself) is an honor to the RFC. Underscores are not allowed in host names, and the host name is what you look up when using ping. (Underscores are permitted in other types of DNS records, such as SRV records, TXT records, such as those used for DKIM ...)

. RFC 1123, 2.1, RFC 952. :

- (-) "_" ?

Domainkey - DNS

Quora - DNS?

. , Linux ping a_a.github.com. (CentOS 7 ):

ping: unknown host _.github.com
ping: unknown host a_.github.com
ping: unknown host _a.github.com

$ ping a_a.github.com
PING github.map.fastly.net (23.235.40.133) 56(84) bytes of data.
64 bytes from 23.235.40.133: icmp_seq=1 ttl=59 time=29.7 ms

, , ping Linux , , (, , ) .