Unleashing the Power of `curl`: Making DNS Queries and Resolving Domains for cURL requests

Sat May 5, 2023

Welcome to the world of curl, a command-line tool for transferring data with URLs. Today, we’ll explore how to use curl to resolve hostnames with custom DNS servers before running the desired cURL action.

Understanding DNS

What is DNS?

DNS, or Domain Name System, translates human-readable domain names into IP addresses, allowing browsers to load internet resources. DNS functions as the internet’s phonebook.

Importance of DNS in Internet Browsing

Without DNS, we would need to remember IP addresses for each website. DNS ensures a seamless browsing experience by resolving domain names quickly.

Getting Started with curl

Installation and Basic Usage

To start using curl, install it on your system. On most Unix-based systems, curl is pre-installed. Verify by running:

curl --version

If curl is not installed, install it using package managers like apt, yum, or brew depending on your OS:

sudo apt install curl   # For Debian-based systems
sudo yum install curl   # For Red Hat-based systems
brew install curl       # For macOS

Once installed, use curl to download files, test APIs, and make DNS queries.

Customizing your `curl` build with Nix

If you try the above and get the following error, then you will need to install a curl package that is built with these capabilities.

curl: option --dns-servers: the installed libcurl version doesn't support this

If you are using Nix with nixpkgs you can bring the right build into scope with the following nix-shell command:

nix-shell -p "((import <nixpkgs> {}).curl.override { c-aresSupport = true; })"

This will override the version of curl in your currently in scope nixpkgs package set to enable --dns-servers option as above.

If you want to override the version of curl package in a specific release of the nixpkgs package set you can run:

nix-shell -I nixpkgs=channel:nixos-23.05 \
  -p "((import <nixpkgs> {}).curl.override { c-aresSupport = true; })"

Making DNS Queries with curl

Command Breakdown

To make a DNS query with curl, use the --dns-servers option to specify the DNS server:

curl --dns-servers <DNS server IP> http://example.com

Examples and Use Cases

Replace <DNS server IP> with the IP address of the DNS server you wish to query:

curl --dns-servers 8.8.8.8 http://example.com

In this example, curl queries Google’s public DNS server (8.8.8.8) to resolve example.com.

Building curl with DNS Query Capabilities

If your current curl build does not support DNS querying, build a custom version. Using Nix, override the curl build to support DNS queries:

nix-shell -p "((import <nixpkgs> {}).curl.override { c-aresSupport = true; })"

To override the version of the curl package in a specific release of the nixpkgs package set:

nix-shell -I nixpkgs=channel:nixos-23.05 \
  -p "((import <nixpkgs> {}).curl.override { c-aresSupport = true; })"

These commands ensure curl is built with the necessary capabilities.

Using Environment-Specific DNS Servers

Pointing to Environment-Specific DNS Servers

In development, staging, and production environments, DNS configurations often differ. Developers can use environment-specific DNS servers to ensure their applications resolve domain names correctly in each environment.

Command Examples for Different Environments

Suppose you have different DNS servers for development, staging, and production environments. Use curl to point to these specific DNS servers:

Development Environment:
```
curl --dns-servers 192.168.1.100 http://dev.example.com
```
This command uses the development DNS server 192.168.1.100 to resolve dev.example.com.
Staging Environment:
```
curl --dns-servers 192.168.2.100 http://staging.example.com
```
This command uses the staging DNS server 192.168.2.100 to resolve staging.example.com.
Production Environment:
```
curl --dns-servers 192.168.3.100 http://example.com
```
This command uses the production DNS server 192.168.3.100 to resolve example.com.

Why Use of Different Resolvers

Using different resolvers for each environment ensures that DNS queries are handled according to the specific configurations and requirements of that environment. This practice helps in:

Consistent Testing: Ensuring that domain resolutions in the development and staging environments mirror those in production.
Accurate Troubleshooting: Isolating and diagnosing environment-specific DNS issues without affecting other environments.
Controlled Deployments: Managing DNS resolution changes and updates in a controlled manner, minimizing the risk of disruptions.

Advanced curl Configuration

Exploring DNS over HTTPS (DoH)

But wait, there’s more! curl also supports DNS over HTTPS (DoH), which enhances privacy and security during DNS resolution. By utilizing DoH, you can query DNS servers over encrypted HTTPS connections. Let’s unveil the power of DoH in action.

To make a DNS query using DoH, we can customize the DNS server by specifying a DoH endpoint URL. Let’s use google.dns as our DoH server for this example:

curl --doh-url https://dns.google/dns-query https://example.com

In this command:

--doh-url https://dns.google/dns-query: This option specifies the DoH endpoint URL. We use https://dns.google/dns-query, provided by Google, as an example. You can replace it with any other valid DoH endpoint URL.
https://example.com: This is the URL of the domain you want to resolve using DoH. Replace it with the desired domain name.

With this command, curl establishes an encrypted HTTPS connection to the DoH server and retrieves the IP address associated with the domain name, securing your DNS resolution.

Practical Applications

Real-World Scenarios and Practical Tips

Web Development: Test website responses using different DNS servers to ensure compatibility and performance.
Network Troubleshooting: Diagnose DNS-related issues by querying various DNS servers.
Enhanced Security: Use custom DNS servers for secure DNS queries to protect sensitive data.

Troubleshooting Common Issues

Common Errors and Solutions

Unsupported Options: Ensure your curl build supports the necessary options. Rebuild or update curl if needed.
Connection Issues: Verify network connectivity and correct DNS server addresses.

Security and Privacy Considerations

Ensuring Secure DNS Queries

Always use secure DNS servers to encrypt DNS queries, safeguarding against potential eavesdropping and data interception.

Privacy Benefits of Secure DNS Queries

Using secure DNS servers hides your DNS queries from third parties, ensuring that your browsing activity remains private.

Performance Optimization

Optimizing DNS Queries with curl

Use Reliable DNS Servers: Choose DNS servers known for speed and reliability.
Cache DNS Responses: Leverage DNS caching to reduce query times.

Performance Tips

Batch Requests: Combine multiple DNS queries to reduce overhead.
Monitor Performance: Regularly test and monitor DNS query performance.

Curl in Development Environments

Integrating curl in Various Development Environments

curl can be integrated into CI/CD pipelines to automate testing of DNS resolutions and ensure consistent performance across environments.

Case Studies

Examples of curl Usage in Industry

Web Services: Many companies use curl for automated testing of APIs, ensuring that DNS resolutions work as expected.
Security Firms: Use curl for secure communication and testing of security protocols.

Frequently Asked Questions (FAQs)

What is the purpose of using custom DNS servers with curl? Using custom DNS servers with curl ensures that DNS queries are handled according to the specific configurations and requirements of different environments, enhancing accuracy and reliability.

How can I specify a custom DNS server in curl? Use the --dns-servers option followed by the DNS server’s IP address.

What are the benefits of using secure DNS servers? Secure DNS servers encrypt DNS queries, preventing eavesdropping and enhancing user privacy.

Can I use curl for DNS queries without building a custom version? Yes, but if your curl build lacks support for DNS queries, you may need to build or install a version with the required capabilities.

How do I troubleshoot errors when using curl for DNS queries? Check for network connectivity, correct DNS server addresses, and ensure your curl build supports the necessary options.

Is secure DNS supported by all DNS servers? No, only DNS servers that explicitly offer secure DNS support can be used. Ensure the endpoint URL is correct.

Conclusion

curl is a powerful tool for making DNS queries using custom DNS servers. By customizing DNS servers, you can enhance security, troubleshoot effectively, and optimize performance. Embrace