dns - Why can I not ping addresses with leading or trailing underscores on linux -

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on windows:

  • accessing _.github.com in browser works
  • nslookup _.github.com works
  • ping _.github.com works

on linux (tested on 2 separate networks):

  • accessing _.github.com in browser works
  • host _.github.com works
  • ping _.github.com does not
  • python -c "import requests; requests.get('_.github.com')" does not

what going on here?

because linux (more specifically, component of such libresolv or ping itself) honoring rfcs. underscores not allowed in hostnames, , hostname looking when using ping. (underscores allowed in other types of dns records, example srv records, txt records such used dkim...)

see rfc 1123 section 2.1, , rfc 952. here other links discussion of topic:

stack overflow - can (hostname) subdomains have underscore “_” in it?

domainkey - underscores in dns

quora - why underscores not allowed in dns host names?

update: couple of people pointed out in comments, linux ping happy a_a.github.com. doing few more tests (centos 7 in case):

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 23.235.40.133: icmp_seq=1 ttl=59 time=29.7 ms

so seems linux ping not reject underscore completely, appear disallows being first or last character (or, consequence, character) in atom of hostname.


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