Uly.me

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Troubleshoot Nameservers with Dig

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Here’s how to troubleshoot nameservers with dig.

dig hostname
dig domain.com

To test a specific nameserver.

dig @nameserver hostname
dig @nameserver ip-address

Other options.

dig hostname.domain.com +short
dig hostname.domain.com +noall +answer
dig @nameserver MX domain.com
dig @nameserver AAAA domain.com

Use Curl on Load Balancers

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You can use the curl command to test if your load balancer is working as expected. The curl command will alternately and randomly access several instances in your load balancer. If you have 3 instances behind your load balancer, it will be cycled across all 3. The -m1 command means max time is set to 1 second.

while true; do curl -m1 [IP_ADDRESS]; done

Testing a Network Port Connection

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You can test a network port connection using the “nc” command on Linux and “telnet” on Windows.

Format:
$ nc -zv domain.com port
C:\> telnet domain.com port

$ nc -zv domain.com 80
Connection to domain.com 80 port [tcp/http] succeeded!
C:\> telnet -zv domain.com 80
Connecting To domain.com...Could not open connection to the host, on port 80: Connect failed

For Telnet, if a connection is successful, it returns no message.

Check DB Connection on AWS

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When setting up your cloud infrastructure, you can check if your instances have access to the database by performing this command.

$ nc -zv 10.0.0.45 3306
$ nc -zv domain.com 3306
$ nc -zv endpoint.amazonaws.com 3306

If connection has succeeded, you’ll get a message like this in Linux and MacOS …

Connection to endpoint.amazonaws.com 3306 port [tcp/mysql] succeeded!

On Windows, you can use Telnet to test your DB connection.

C:\>telnet endpoint.amazonaws.com 3306

If connection has succeeded, the result is “no message.” If there’s a problem, you’ll get this ….

Connecting To endpoint.amazonaws.com...Could not open 
connection to the host, on port 3306: Connect failed