The reqwest crate is a popular HTTP client for Rust.
By default, it doesn't send a user agent at all.
This can be confirmed by compiling the following
use reqwest::Client;
use std::collections::HashMap;
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
let _res = Client::new().get("http://192.168.3.5/foobla").send().await?;
Ok(())
}
Which results in the following access log line on the target server
192.168.3.20 - - [28/Jan/2023:00:53:54 +0000] "GET /foobla HTTP/1.1" 404 292 "-" "-"
Developers are able to set a user-agent