yum update

Loaded plugins: fastestmirror, langpacks

Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was

14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Name or service not known"

 One of the configured repositories failed (Unknown),

==============================

error ping..

nslookup yum.postgresql.org 

Name:   yonada.postgresql.org

Address: 174.143.35.196

--> OK

# ping yum.postgresql.org

ping: unknown host yum.postgresql.org

--> FAIL

이경우 dns의 문제가 아니다.. nssswitch 를 수정하라.

# vi /etc/nsswitch.conf 

hosts:      files

--> 아래로 변경

hosts:      files dns myhostname