Got it actually checking against a yaml file

src/main.rs

@@ -4,11 +4,11 @@ use std::net::Shutdown;
 use std::os::unix::fs::PermissionsExt;
 use std::os::unix::net::{UnixListener, UnixStream};
 use std::path::PathBuf;
-use std::thread;
+use std::{fs, thread};
 
 extern crate yaml_rust;
 
-use yaml_rust::{YamlEmitter, YamlLoader};
+use yaml_rust::{yaml::Hash, Yaml, YamlLoader};
 
 const RESPONSE_NO: [u8; 4] = [0x0, 0x2, b'N', b'O'];
 const RESPONSE_OK: [u8; 4] = [0x0, 0x2, b'O', b'K'];
@@ -80,20 +80,57 @@ fn write_string(s: &str, stream: &mut UnixStream) -> Result<(), Error> {
     Ok(())
 }
 
-fn check_auth(mut stream: UnixStream) -> Result<(), Error> {
+fn check_auth(userid: &str, password: &str) -> Result<bool, Error> {
+    let yaml = fs::read_to_string("test.yml")?;
+
+    let docs =
+        YamlLoader::load_from_str(&yaml)
+            .map_err(|_| Error::new(ErrorKind::Other, "Can't parse YAML"))?;
+
+    let doc = &docs[0];
+    let users = doc["users"]
+        .as_hash()
+        .ok_or(Error::new(ErrorKind::Other, "Can't find 'users' in YAML"))?;
+
+    let passwords = users[&Yaml::from_str(&userid)].as_vec();
+
+    if let Some(passwords) = passwords {
+        // Found the passwords for the user, look for any matches
+
+        for p in passwords {
+            if let Some(str) = p.as_str() {
+                if str == password {
+                    return Ok(true);
+                }
+            }
+        }
+    }
+
+    Ok(false)
+}
+
+
+fn communicate(mut stream: UnixStream) -> Result<(), Error> {
     let userid = read_string(&mut stream)?;
     let password = read_string(&mut stream)?;
     let service = read_string(&mut stream)?;
     let _realm = read_string(&mut stream)?;
 
-    stream.write_all(&RESPONSE_NO)?;
-    stream.shutdown(Shutdown::Both)?;
+    match check_auth(&userid, &password) {
+        Ok(true) => stream.write_all(&RESPONSE_OK)?,
+        Ok(false) => stream.write_all(&RESPONSE_NO)?,
+        Err(e) => {
+            stream.write_all(&RESPONSE_NO)?;
+            stream.shutdown(Shutdown::Both)?;
+            return Err(e)
+        }
+    }
 
     Ok(())
 }
 
 fn handle_client(stream: UnixStream) {
-    if let Err(error) = check_auth(stream) {
+    if let Err(error) = communicate(stream) {
         eprint!("{}", error);
     }
 }

test.yml

@@ -0,0 +1,3 @@
+users:
+  bp:
+    - some-rad-password