A simple HTTP proxy server written in Go with the following features:

• Parsing a configuration file in YAML format: The proxy server reads a configuration file in YAML format to determine its behavior. The configuration file includes fields such as the port to bind to, a map of headers to add or modify in the forwarded request, and the target host and port to forward requests to.

• Validation of the configuration file using the validator package: The proxy server uses the validator package to validate the fields in the configuration file. If any errors are found, the proxy server will print a list of the errors and exit.

• An option to generate a default configuration file using the -g flag: The proxy server can generate a default configuration file named config.yaml by using the -g flag when starting the server.

• An option to run in verbose mode using the -v flag: The proxy server can be run in verbose mode by using the -v flag. In verbose mode, the server will print additional information about its behavior to the console.

• An option to specify a custom configuration file using the -c flag: The proxy server can use a custom configuration file by specifying the path to the file using the -c flag. The default configuration file is config.yaml.

• Reading the contents of an incoming HTTP request and forwarding it to a target host and port specified in the configuration file: When the proxy server receives an incoming HTTP request, it reads the contents of the request and forwards it to the target host and port specified in the configuration file.

• Modifying the headers of the forwarded request based on the header_map field in the configuration file: The proxy server can modify the headers of the forwarded request based on the header_map field in the configuration file. This field specifies a mapping of header names and values to add or modify in the forwarded request.

• Returning the response from the target host to the client: The proxy server receives the response from the target host and returns it to the client that made the original request.

The code consists of the following main components:

• The Config struct: This struct represents the configuration for the proxy server. It includes fields for the port to bind to, a map of headers to add or modify in the forwarded request, and the target host and port to forward requests to. The Config struct also includes validation tags for the fields, which are used by the validator package to perform validation.

• The Parse function: This function reads and parses the configuration file, performs validation using the validator package, and returns a pointer to a Config struct. If any errors are found during validation, the Parse function will print a list of the errors and exit the program.

• The main function: This is the entry point of the program. It handles command line flags, reads the configuration file, and starts an HTTP server to handle incoming requests.

To start the proxy server, run the following command:

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go run main.go

Use the following flags to customize the behavior of the proxy server:

• -g: Generate a default configuration file named config.yaml
• -v: Run in verbose mode
• -c: Specify a custom configuration file. The default is config.yaml.

For example, to generate a default configuration file and run the proxy server in verbose mode, use the following command:

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go run main.go -g -v

To specify a custom configuration file, use the -c flag followed by the path to the configuration file. For example:

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go run main.go -c /path/to/custom/config.yaml