auth: send ScmCreds on FreeBSD and Dragonfly

rustbus/src/auth.rs

@@ -1,7 +1,9 @@
 //! Deals with authentication to the other side. You probably do not need this.
 
+use nix::sys::socket::{self, sendmsg};
 use nix::unistd::getuid;
-use std::io::{Read, Write};
+use std::io::{IoSlice, Read, Write};
+use std::os::fd::AsRawFd;
 use std::os::unix::net::UnixStream;
 
 fn write_message(msg: &str, stream: &mut UnixStream) -> std::io::Result<()> {
@@ -79,8 +81,21 @@ pub enum AuthResult {
 }
 
 pub fn do_auth(stream: &mut UnixStream) -> std::io::Result<AuthResult> {
+    // The D-Bus daemon expects an SCM_CREDS first message on FreeBSD and Dragonfly
+    #[cfg(any(target_os = "freebsd", target_os = "dragonfly"))]
+    let cmsgs = [socket::ControlMessage::ScmCreds];
+    #[cfg(not(any(target_os = "freebsd", target_os = "dragonfly")))]
+    let cmsgs = [];
+
     // send a null byte as the first thing
-    stream.write_all(&[0])?;
+    sendmsg::<()>(
+        stream.as_raw_fd(),
+        &[IoSlice::new(&[0])],
+        &cmsgs,
+        socket::MsgFlags::empty(),
+        None,
+    )?;
+
     write_message(&format!("AUTH EXTERNAL {}", get_uid_as_hex()), stream)?;
 
     let mut read_buf = Vec::new();