How does configuring both `search` and `domain` in a `/etc/resolv.conf` effect DNS short name query sequence?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux (RHEL)
    • 8
    • 7

Issue

  • What will the DNS search sequence be if both search and domain directives are configured in a /etc/resolv.conf?

Resolution

  • According to the implementation of Glibc, if domain and search directives are both used, only the last instance will be used for DNS queries.

Diagnostic Steps

  • The instance, /etc/resolv.conf shows that the domain directive is configured to example.com and followed by the search directive which is configured to redhat.com:

    [root@localhost etc]# cat resolv.conf
    nameserver 8.8.8.8
    domain example.com
    search redhat.com
    
    [root@localhost resolv]# nslookup www
    Server:     8.8.8.8
    Address:    8.8.8.8#53
    
    Non-authoritative answer:
    www.redhat.com  canonical name = ds-www.redhat.com.edgekey.net.                 <----- Only the last instance, "search" is used for the DNS query.
    ds-www.redhat.com.edgekey.net   canonical name = ds-www.redhat.com.edgekey.net.globalredir.akadns.net.
    ds-www.redhat.com.edgekey.net.globalredir.akadns.net    canonical name = e3396.dscx.akamaiedge.net.
    Name:   www.example.com 
    Address: 184.xx.xx.xx
    Name:   www.example.com 
    Address: 00:00:00:00:00:aa
    Name:   www.example.com 
    Address: 00:00:00:00:00:aa
    
  • In another example, the search directive is configured and followed by the domain directive:

    [root@localhost etc]# cat resolv.conf
    nameserver 8.8.8.8
    search redhat.com
    domain example.com
    
    [root@localhost resolv]# nslookup www
    Server:     8.8.8.8
    Address:    8.8.8.8#53
    
    Non-authoritative answer:
    Name:   www.example.com                          <------- Only the last instance, "domain" is used for the DNS query.
    Address: xxx.xxx.xxx.xxx
    Name:   www.example.com
    Address: xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx
    

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