Why Do I Get the Error "Name or service not known" When I Ping a Public Domain Name Configured for a Linux ECS?

Symptom

A public domain name configured for an ECS fails to be pinged, and the error message "Name or service not known" is displayed. However, the EIP of the ECS can be pinged.

Possible Cause

Generally, there are three possible causes:

• No or incorrect DNS server addresses are configured in /etc/resolv.conf.
• The DNS records are deleted from /etc/nsswitch.conf.
• The /lib64/libnss_dns.so.2 library file is lost.
  

  Run the following command to view all files used for resolving the domain name:

  strace -e trace=open ping www.baidu.com -c 1

  All files in the output affect domain name resolution.

Scenarios

The operations described in this section apply to ECSs running CentOS and EulerOS.

Solution

• No or incorrect DNS server addresses are configured in /etc/resolv.conf.

  nameserver is the most important item in /etc/resolv.conf. A nameserver entry defines the IP address of the DNS server used for domain name resolution.

  If there are no nameserver entries in /etc/resolv.conf, no DNS servers have been configured for domain name resolution. If there are multiple nameserver entries, the DNS servers are queried in the order listed in the file. The next DNS server is queried only when the previous one does not respond.

  Check the IP addresses of DNS servers configured in /etc/resolv.conf.

• The DNS records are deleted from /etc/nsswitch.conf.
  1. Check whether /etc/nsswitch.conf contains the DNS records.

     grep hosts /etc/nsswitch.conf

     If the output is as follows, the DNS option is not configured on the hosts line. As a result, the system does not read /etc/resolv.conf when resolving the domain name.

     #hosts:     db files nisplus nis dns
     hosts:      files myhostname

  2. Open /etc/nsswitch.conf, locate the hosts line, and add the DNS option.

     #hosts:     db files nisplus nis dns
     hosts:      files dns myhostname

     

     hosts lists the tools by priority that are used to search for IP addresses paired with domain names.

     file indicates the /etc/hosts file, and dns indicates DNS. By default, file is placed before dns. This means that the system first attempts to search for a domain name in /etc/hosts and then search for the domain name through DNS. If dns is not configured, DNS is not used.

• The /lib64/libnss_dns.so.2 library file is lost.
  1. /lib64/libnss_dns.so.2 is generated by the glibc package. Run the following command to check whether the package is modified.

     rpm -V glibc

     

     Generally, in Linux, running the rpm -qf /lib64/libnss_dns.so.2 command can generate the library file.

     If the output is as follows, /lib64/libnss_dns.so.2 is lost.

     missing     /lib64/libnss_dns.so.2

  2. Run the following command to create a soft link again:
     

     Run the ls -l /lib64/libnss_dns.so.2 command on a normal ECS. The command output will show that the source file of /lib64/libnss_dns.so.2 is /usr/lib64/libnss_dns-2.17.so.

     ln -s /usr/lib64/libnss_dns-2.17.so /usr/lib64/libnss_dns.so.2