Troubleshoot DNS Client Issues
General Information
Client perspective checking DNS configuration.
Lab Setup
The following virtual machines will be used:
- server1.example.com (192.168.1.150) → Perform all connectivity tests from here
- server2.example.com (192.168.1.151) → Install DNS caching here
Troubleshooting: Unbound
View (dump) the current contents of the unbound cache
unbound-control dump_cache
- This can be also redirected to a file to make edits and load back into cache
Purge the cache: Single record
unbound-control flush myhost1.example.com
Purge the cache: Entire Zone
unbound-control flush example.com
Load cache from a file (dumped there previously)
unbound-control load_cache < cache.txt
Troubleshoot DNS From a Client
Check to see what DNS servers are configured
cat /etc/resolv.conf
- Take note of whether the file is controlled via NetworkManager. If so, then the DNS servers are set via that service/from /etc/sysconfig/network-scripts/
Ensure a route exists to the DNS server
ip route sh
Ping the DNS server
ping -c4 <ip-of-dns>
Check to see if you can connect to port 53
telnet <ip-of-dns> 53 OR nc -v -w 3 <ip-of-dns> 53
Test a domain lookup against the DNS server directly
dig @<ip-of-dns> google.com
Dig returns a status indicator in the “Got Answer” section. Values can be:
- NOERROR - DNS resolve was successful
- NXDOMAIN - DNS info requested could not be found
- SERVFAIL - Error contacting a DNS server