DNSの設定と名前解決

Welcome! Today we'll explore DNS (Domain Name System) in Linux. DNS is like the internet's phonebook, translating human-friendly domain names (like google.com) into machine-readable IP addresses (like 142.250.186.174).

Without DNS, you'd have to remember long numbers for every website or service you want to reach!

In Linux, the primary configuration file for how your system resolves domain names is /etc/resolv.conf. This file tells your computer which DNS servers to ask when it needs to find an IP address for a hostname.

Think of it as your local DNS directory.

The most important line in /etc/resolv.conf is nameserver. It specifies the IP address of a DNS server that your system should use.

• You can list multiple nameserver entries.
• Your system will try them in order until it gets a response.
• Common public DNS servers are Google's (8.8.8.8, 8.8.4.4) or Cloudflare's (1.1.1.1).

Let's see what DNS servers your system is currently configured to use. We can do this by displaying the contents of the /etc/resolv.conf file.

Try running the command below:

Besides nameserver, you might also see domain and search directives in /etc/resolv.conf:

• domain example.com: Sets the default domain name for the local host.
• search example.com mycorp.local: Specifies a list of domains to try appending to hostnames that aren't fully qualified.

For example, if you ping webserver and search is mycorp.local, it will try webserver.mycorp.local.

The host command is a simple utility for performing DNS lookups. It's great for quickly finding the IP address of a domain or the domain name for an IP address.

It provides concise answers, making it perfect for quick checks.

Let's use host to resolve a common domain. This command will query your configured DNS servers and return the associated IP address(es).

Run the example:

For more detailed DNS information, the dig (Domain Information Groper) command is your go-to tool. It provides a comprehensive output of DNS records, including authoritative nameservers, record types (A, MX, NS, etc.), and more.

It's invaluable for troubleshooting.

Execute dig to see a full DNS query in action. Notice the different sections of the output, like the ANSWER SECTION, which contains the IP address, and AUTHORITY SECTION, which lists the authoritative nameservers for the domain.

Try it out:

If you can't access a website by name but can by IP, it's likely a DNS issue. Here's a quick checklist:

• Check /etc/resolv.conf: Are the nameserver IPs correct and reachable?
• Test with ping 8.8.8.8: Can you reach an external DNS server by IP?
• Use dig google.com @8.8.8.8: Explicitly query a working DNS server to see if resolution works that way.
• Restart network service: Sometimes a network service restart can help apply changes.

You've learned about the key components of DNS client configuration in Linux.

Which of the following directives are commonly found in /etc/resolv.conf for configuring DNS client behavior?

Great job! You've learned how DNS works in Linux. You can now:

• Identify /etc/resolv.conf as the primary DNS client configuration file.
• Understand the role of nameserver, domain, and search directives.
• Use host for quick DNS lookups.
• Use dig for detailed DNS queries.
• Perform basic troubleshooting for name resolution issues.

Mastering these tools is key to managing network services effectively!