Add Unix daemon mode with PID file and privilege dropping

Implement core daemon infrastructure for running telemt as a background
  service on Unix platforms (Linux, FreeBSD, etc.):

  - Add src/daemon module with classic double-fork daemonization
  - Implement flock-based PID file management to prevent duplicate instances
  - Add privilege dropping (setuid/setgid) after socket binding
  - New CLI flags: --daemon, --foreground, --pid-file, --run-as-user,
    --run-as-group, --working-dir

  Daemonization occurs before tokio runtime starts to ensure clean fork.
  PID file uses exclusive locking to detect already-running instances.
  Privilege dropping happens after bind_listeners() to allow binding
  to privileged ports (< 1024) before switching to unprivileged user.

Signed-off-by: Vladimir Krivopalov <argenet@yandex.ru>

Cargo.toml

@@ -25,7 +25,7 @@ zeroize = { version = "1.8", features = ["derive"] }
 
 # Network
 socket2 = { version = "0.5", features = ["all"] }
-nix = { version = "0.28", default-features = false, features = ["net"] }
+nix = { version = "0.28", default-features = false, features = ["net", "user", "process", "fs", "signal"] }
 shadowsocks = { version = "1.24", features = ["aead-cipher-2022"] }
 
 # Serialization

src/cli.rs

@@ -1,10 +1,13 @@
-//! CLI commands: --init (fire-and-forget setup)
+//! CLI commands: --init (fire-and-forget setup), daemon options
 
 use std::fs;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 use rand::Rng;
 
+#[cfg(unix)]
+use crate::daemon::DaemonOptions;
+
 /// Options for the init command
 pub struct InitOptions {
     pub port: u16,
@@ -15,6 +18,64 @@ pub struct InitOptions {
     pub no_start: bool,
 }
 
+/// Parse daemon-related options from CLI args.
+#[cfg(unix)]
+pub fn parse_daemon_args(args: &[String]) -> DaemonOptions {
+    let mut opts = DaemonOptions::default();
+    let mut i = 0;
+
+    while i < args.len() {
+        match args[i].as_str() {
+            "--daemon" | "-d" => {
+                opts.daemonize = true;
+            }
+            "--foreground" | "-f" => {
+                opts.foreground = true;
+            }
+            "--pid-file" => {
+                i += 1;
+                if i < args.len() {
+                    opts.pid_file = Some(PathBuf::from(&args[i]));
+                }
+            }
+            s if s.starts_with("--pid-file=") => {
+                opts.pid_file = Some(PathBuf::from(s.trim_start_matches("--pid-file=")));
+            }
+            "--run-as-user" => {
+                i += 1;
+                if i < args.len() {
+                    opts.user = Some(args[i].clone());
+                }
+            }
+            s if s.starts_with("--run-as-user=") => {
+                opts.user = Some(s.trim_start_matches("--run-as-user=").to_string());
+            }
+            "--run-as-group" => {
+                i += 1;
+                if i < args.len() {
+                    opts.group = Some(args[i].clone());
+                }
+            }
+            s if s.starts_with("--run-as-group=") => {
+                opts.group = Some(s.trim_start_matches("--run-as-group=").to_string());
+            }
+            "--working-dir" => {
+                i += 1;
+                if i < args.len() {
+                    opts.working_dir = Some(PathBuf::from(&args[i]));
+                }
+            }
+            s if s.starts_with("--working-dir=") => {
+                opts.working_dir = Some(PathBuf::from(s.trim_start_matches("--working-dir=")));
+            }
+            _ => {}
+        }
+        i += 1;
+    }
+
+    opts
+}
+
 impl Default for InitOptions {
     fn default() -> Self {
         Self {

src/daemon/mod.rs

@@ -0,0 +1,523 @@
+//! Unix daemon support for telemt.
+//!
+//! Provides classic Unix daemonization (double-fork), PID file management,
+//! and privilege dropping for running telemt as a background service.
+
+use std::fs::{self, File, OpenOptions};
+use std::io::{self, Read, Write};
+use std::os::unix::fs::OpenOptionsExt;
+use std::path::{Path, PathBuf};
+
+use nix::fcntl::{Flock, FlockArg};
+use nix::unistd::{self, ForkResult, Gid, Pid, Uid, chdir, close, dup2, fork, getpid, setsid};
+use tracing::{debug, info, warn};
+
+/// Default PID file location.
+pub const DEFAULT_PID_FILE: &str = "/var/run/telemt.pid";
+
+/// Daemon configuration options parsed from CLI.
+#[derive(Debug, Clone, Default)]
+pub struct DaemonOptions {
+    /// Run as daemon (fork to background).
+    pub daemonize: bool,
+    /// Path to PID file.
+    pub pid_file: Option<PathBuf>,
+    /// User to run as after binding sockets.
+    pub user: Option<String>,
+    /// Group to run as after binding sockets.
+    pub group: Option<String>,
+    /// Working directory for the daemon.
+    pub working_dir: Option<PathBuf>,
+    /// Explicit foreground mode (for systemd Type=simple).
+    pub foreground: bool,
+}
+
+impl DaemonOptions {
+    /// Returns the effective PID file path.
+    pub fn pid_file_path(&self) -> &Path {
+        self.pid_file
+            .as_deref()
+            .unwrap_or(Path::new(DEFAULT_PID_FILE))
+    }
+
+    /// Returns true if we should actually daemonize.
+    /// Foreground flag takes precedence.
+    pub fn should_daemonize(&self) -> bool {
+        self.daemonize && !self.foreground
+    }
+}
+
+/// Error types for daemon operations.
+#[derive(Debug, thiserror::Error)]
+pub enum DaemonError {
+    #[error("fork failed: {0}")]
+    ForkFailed(#[source] nix::Error),
+
+    #[error("setsid failed: {0}")]
+    SetsidFailed(#[source] nix::Error),
+
+    #[error("chdir failed: {0}")]
+    ChdirFailed(#[source] nix::Error),
+
+    #[error("failed to open /dev/null: {0}")]
+    DevNullFailed(#[source] io::Error),
+
+    #[error("failed to redirect stdio: {0}")]
+    RedirectFailed(#[source] nix::Error),
+
+    #[error("PID file error: {0}")]
+    PidFile(String),
+
+    #[error("another instance is already running (pid {0})")]
+    AlreadyRunning(i32),
+
+    #[error("user '{0}' not found")]
+    UserNotFound(String),
+
+    #[error("group '{0}' not found")]
+    GroupNotFound(String),
+
+    #[error("failed to set uid/gid: {0}")]
+    PrivilegeDrop(#[source] nix::Error),
+
+    #[error("io error: {0}")]
+    Io(#[from] io::Error),
+}
+
+/// Result of a successful daemonize() call.
+#[derive(Debug)]
+pub enum DaemonizeResult {
+    /// We are the parent process and should exit.
+    Parent,
+    /// We are the daemon child process and should continue.
+    Child,
+}
+
+/// Performs classic Unix double-fork daemonization.
+///
+/// This detaches the process from the controlling terminal:
+/// 1. First fork - parent exits, child continues
+/// 2. setsid() - become session leader
+/// 3. Second fork - ensure we can never acquire a controlling terminal
+/// 4. chdir("/") - don't hold any directory open
+/// 5. Redirect stdin/stdout/stderr to /dev/null
+///
+/// Returns `DaemonizeResult::Parent` in the original parent (which should exit),
+/// or `DaemonizeResult::Child` in the final daemon child.
+pub fn daemonize(working_dir: Option<&Path>) -> Result<DaemonizeResult, DaemonError> {
+    // First fork
+    match unsafe { fork() } {
+        Ok(ForkResult::Parent { .. }) => {
+            // Parent exits
+            return Ok(DaemonizeResult::Parent);
+        }
+        Ok(ForkResult::Child) => {
+            // Child continues
+        }
+        Err(e) => return Err(DaemonError::ForkFailed(e)),
+    }
+
+    // Create new session, become session leader
+    setsid().map_err(DaemonError::SetsidFailed)?;
+
+    // Second fork to ensure we can never acquire a controlling terminal
+    match unsafe { fork() } {
+        Ok(ForkResult::Parent { .. }) => {
+            // Intermediate parent exits
+            std::process::exit(0);
+        }
+        Ok(ForkResult::Child) => {
+            // Final daemon child continues
+        }
+        Err(e) => return Err(DaemonError::ForkFailed(e)),
+    }
+
+    // Change working directory
+    let target_dir = working_dir.unwrap_or(Path::new("/"));
+    chdir(target_dir).map_err(DaemonError::ChdirFailed)?;
+
+    // Redirect stdin, stdout, stderr to /dev/null
+    redirect_stdio_to_devnull()?;
+
+    Ok(DaemonizeResult::Child)
+}
+
+/// Redirects stdin, stdout, and stderr to /dev/null.
+fn redirect_stdio_to_devnull() -> Result<(), DaemonError> {
+    let devnull = File::options()
+        .read(true)
+        .write(true)
+        .open("/dev/null")
+        .map_err(DaemonError::DevNullFailed)?;
+
+    let devnull_fd = std::os::unix::io::AsRawFd::as_raw_fd(&devnull);
+
+    // Redirect stdin (fd 0)
+    dup2(devnull_fd, 0).map_err(DaemonError::RedirectFailed)?;
+    // Redirect stdout (fd 1)
+    dup2(devnull_fd, 1).map_err(DaemonError::RedirectFailed)?;
+    // Redirect stderr (fd 2)
+    dup2(devnull_fd, 2).map_err(DaemonError::RedirectFailed)?;
+
+    // Close original devnull fd if it's not one of the standard fds
+    if devnull_fd > 2 {
+        let _ = close(devnull_fd);
+    }
+
+    Ok(())
+}
+
+/// PID file manager with flock-based locking.
+pub struct PidFile {
+    path: PathBuf,
+    file: Option<File>,
+    locked: bool,
+}
+
+impl PidFile {
+    /// Creates a new PID file manager for the given path.
+    pub fn new<P: AsRef<Path>>(path: P) -> Self {
+        Self {
+            path: path.as_ref().to_path_buf(),
+            file: None,
+            locked: false,
+        }
+    }
+
+    /// Checks if another instance is already running.
+    ///
+    /// Returns the PID of the running instance if one exists.
+    pub fn check_running(&self) -> Result<Option<i32>, DaemonError> {
+        if !self.path.exists() {
+            return Ok(None);
+        }
+
+        // Try to read existing PID
+        let mut contents = String::new();
+        File::open(&self.path)
+            .and_then(|mut f| f.read_to_string(&mut contents))
+            .map_err(|e| DaemonError::PidFile(format!("cannot read {}: {}", self.path.display(), e)))?;
+
+        let pid: i32 = contents
+            .trim()
+            .parse()
+            .map_err(|_| DaemonError::PidFile(format!("invalid PID in {}", self.path.display())))?;
+
+        // Check if process is still running
+        if is_process_running(pid) {
+            Ok(Some(pid))
+        } else {
+            // Stale PID file
+            debug!(pid, path = %self.path.display(), "Removing stale PID file");
+            let _ = fs::remove_file(&self.path);
+            Ok(None)
+        }
+    }
+
+    /// Acquires the PID file lock and writes the current PID.
+    ///
+    /// Fails if another instance is already running.
+    pub fn acquire(&mut self) -> Result<(), DaemonError> {
+        // Check for running instance first
+        if let Some(pid) = self.check_running()? {
+            return Err(DaemonError::AlreadyRunning(pid));
+        }
+
+        // Ensure parent directory exists
+        if let Some(parent) = self.path.parent() {
+            if !parent.exists() {
+                fs::create_dir_all(parent).map_err(|e| {
+                    DaemonError::PidFile(format!(
+                        "cannot create directory {}: {}",
+                        parent.display(),
+                        e
+                    ))
+                })?;
+            }
+        }
+
+        // Open/create PID file with exclusive lock
+        let file = OpenOptions::new()
+            .write(true)
+            .create(true)
+            .truncate(true)
+            .mode(0o644)
+            .open(&self.path)
+            .map_err(|e| {
+                DaemonError::PidFile(format!("cannot open {}: {}", self.path.display(), e))
+            })?;
+
+        // Try to acquire exclusive lock (non-blocking)
+        let flock = Flock::lock(file, FlockArg::LockExclusiveNonblock).map_err(|(_, errno)| {
+            // Check if another instance grabbed the lock
+            if let Some(pid) = self.check_running().ok().flatten() {
+                DaemonError::AlreadyRunning(pid)
+            } else {
+                DaemonError::PidFile(format!("cannot lock {}: {}", self.path.display(), errno))
+            }
+        })?;
+
+        // Write our PID
+        let pid = getpid();
+        let mut file = flock.unlock().map_err(|(_, errno)| {
+            DaemonError::PidFile(format!("unlock failed: {}", errno))
+        })?;
+
+        writeln!(file, "{}", pid).map_err(|e| {
+            DaemonError::PidFile(format!("cannot write PID to {}: {}", self.path.display(), e))
+        })?;
+
+        // Re-acquire lock and keep it
+        let flock = Flock::lock(file, FlockArg::LockExclusiveNonblock).map_err(|(_, errno)| {
+            DaemonError::PidFile(format!("cannot re-lock {}: {}", self.path.display(), errno))
+        })?;
+
+        self.file = Some(flock.unlock().map_err(|(_, errno)| {
+            DaemonError::PidFile(format!("unlock for storage failed: {}", errno))
+        })?);
+        self.locked = true;
+
+        info!(pid = pid.as_raw(), path = %self.path.display(), "PID file created");
+        Ok(())
+    }
+
+    /// Releases the PID file lock and removes the file.
+    pub fn release(&mut self) -> Result<(), DaemonError> {
+        if let Some(file) = self.file.take() {
+            drop(file);
+        }
+        self.locked = false;
+
+        if self.path.exists() {
+            fs::remove_file(&self.path).map_err(|e| {
+                DaemonError::PidFile(format!("cannot remove {}: {}", self.path.display(), e))
+            })?;
+            debug!(path = %self.path.display(), "PID file removed");
+        }
+
+        Ok(())
+    }
+
+    /// Returns the path to this PID file.
+    #[allow(dead_code)]
+    pub fn path(&self) -> &Path {
+        &self.path
+    }
+}
+
+impl Drop for PidFile {
+    fn drop(&mut self) {
+        if self.locked {
+            if let Err(e) = self.release() {
+                warn!(error = %e, "Failed to clean up PID file on drop");
+            }
+        }
+    }
+}
+
+/// Checks if a process with the given PID is running.
+fn is_process_running(pid: i32) -> bool {
+    // kill(pid, 0) checks if process exists without sending a signal
+    nix::sys::signal::kill(Pid::from_raw(pid), None).is_ok()
+}
+
+/// Drops privileges to the specified user and group.
+///
+/// This should be called after binding privileged ports but before
+/// entering the main event loop.
+pub fn drop_privileges(user: Option<&str>, group: Option<&str>) -> Result<(), DaemonError> {
+    // Look up group first (need to do this while still root)
+    let target_gid = if let Some(group_name) = group {
+        Some(lookup_group(group_name)?)
+    } else if let Some(user_name) = user {
+        // If no group specified but user is, use user's primary group
+        Some(lookup_user_primary_gid(user_name)?)
+    } else {
+        None
+    };
+
+    // Look up user
+    let target_uid = if let Some(user_name) = user {
+        Some(lookup_user(user_name)?)
+    } else {
+        None
+    };
+
+    // Drop privileges: set GID first, then UID
+    // (Setting UID first would prevent us from setting GID)
+    if let Some(gid) = target_gid {
+        unistd::setgid(gid).map_err(DaemonError::PrivilegeDrop)?;
+        // Also set supplementary groups to just this one
+        unistd::setgroups(&[gid]).map_err(DaemonError::PrivilegeDrop)?;
+        info!(gid = gid.as_raw(), "Dropped group privileges");
+    }
+
+    if let Some(uid) = target_uid {
+        unistd::setuid(uid).map_err(DaemonError::PrivilegeDrop)?;
+        info!(uid = uid.as_raw(), "Dropped user privileges");
+    }
+
+    Ok(())
+}
+
+/// Looks up a user by name and returns their UID.
+fn lookup_user(name: &str) -> Result<Uid, DaemonError> {
+    // Use libc getpwnam
+    let c_name = std::ffi::CString::new(name).map_err(|_| DaemonError::UserNotFound(name.to_string()))?;
+
+    unsafe {
+        let pwd = libc::getpwnam(c_name.as_ptr());
+        if pwd.is_null() {
+            Err(DaemonError::UserNotFound(name.to_string()))
+        } else {
+            Ok(Uid::from_raw((*pwd).pw_uid))
+        }
+    }
+}
+
+/// Looks up a user's primary GID by username.
+fn lookup_user_primary_gid(name: &str) -> Result<Gid, DaemonError> {
+    let c_name = std::ffi::CString::new(name).map_err(|_| DaemonError::UserNotFound(name.to_string()))?;
+
+    unsafe {
+        let pwd = libc::getpwnam(c_name.as_ptr());
+        if pwd.is_null() {
+            Err(DaemonError::UserNotFound(name.to_string()))
+        } else {
+            Ok(Gid::from_raw((*pwd).pw_gid))
+        }
+    }
+}
+
+/// Looks up a group by name and returns its GID.
+fn lookup_group(name: &str) -> Result<Gid, DaemonError> {
+    let c_name = std::ffi::CString::new(name).map_err(|_| DaemonError::GroupNotFound(name.to_string()))?;
+
+    unsafe {
+        let grp = libc::getgrnam(c_name.as_ptr());
+        if grp.is_null() {
+            Err(DaemonError::GroupNotFound(name.to_string()))
+        } else {
+            Ok(Gid::from_raw((*grp).gr_gid))
+        }
+    }
+}
+
+/// Reads PID from a PID file.
+#[allow(dead_code)]
+pub fn read_pid_file<P: AsRef<Path>>(path: P) -> Result<i32, DaemonError> {
+    let path = path.as_ref();
+    let mut contents = String::new();
+    File::open(path)
+        .and_then(|mut f| f.read_to_string(&mut contents))
+        .map_err(|e| DaemonError::PidFile(format!("cannot read {}: {}", path.display(), e)))?;
+
+    contents
+        .trim()
+        .parse()
+        .map_err(|_| DaemonError::PidFile(format!("invalid PID in {}", path.display())))
+}
+
+/// Sends a signal to the process specified in a PID file.
+#[allow(dead_code)]
+pub fn signal_pid_file<P: AsRef<Path>>(
+    path: P,
+    signal: nix::sys::signal::Signal,
+) -> Result<(), DaemonError> {
+    let pid = read_pid_file(&path)?;
+
+    if !is_process_running(pid) {
+        return Err(DaemonError::PidFile(format!(
+            "process {} from {} is not running",
+            pid,
+            path.as_ref().display()
+        )));
+    }
+
+    nix::sys::signal::kill(Pid::from_raw(pid), signal).map_err(|e| {
+        DaemonError::PidFile(format!("cannot signal process {}: {}", pid, e))
+    })?;
+
+    Ok(())
+}
+
+/// Returns the status of the daemon based on PID file.
+#[allow(dead_code)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum DaemonStatus {
+    /// Daemon is running with the given PID.
+    Running(i32),
+    /// PID file exists but process is not running.
+    Stale(i32),
+    /// No PID file exists.
+    NotRunning,
+}
+
+/// Checks the daemon status from a PID file.
+#[allow(dead_code)]
+pub fn check_status<P: AsRef<Path>>(path: P) -> DaemonStatus {
+    let path = path.as_ref();
+
+    if !path.exists() {
+        return DaemonStatus::NotRunning;
+    }
+
+    match read_pid_file(path) {
+        Ok(pid) => {
+            if is_process_running(pid) {
+                DaemonStatus::Running(pid)
+            } else {
+                DaemonStatus::Stale(pid)
+            }
+        }
+        Err(_) => DaemonStatus::NotRunning,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_daemon_options_default() {
+        let opts = DaemonOptions::default();
+        assert!(!opts.daemonize);
+        assert!(!opts.should_daemonize());
+        assert_eq!(opts.pid_file_path(), Path::new(DEFAULT_PID_FILE));
+    }
+
+    #[test]
+    fn test_daemon_options_foreground_overrides() {
+        let opts = DaemonOptions {
+            daemonize: true,
+            foreground: true,
+            ..Default::default()
+        };
+        assert!(!opts.should_daemonize());
+    }
+
+    #[test]
+    fn test_check_status_not_running() {
+        let path = "/tmp/telemt_test_nonexistent.pid";
+        assert_eq!(check_status(path), DaemonStatus::NotRunning);
+    }
+
+    #[test]
+    fn test_pid_file_basic() {
+        let path = "/tmp/telemt_test_pidfile.pid";
+        let _ = fs::remove_file(path);
+
+        let mut pf = PidFile::new(path);
+        assert!(pf.check_running().unwrap().is_none());
+
+        pf.acquire().unwrap();
+        assert!(Path::new(path).exists());
+
+        // Read it back
+        let pid = read_pid_file(path).unwrap();
+        assert_eq!(pid, std::process::id() as i32);
+
+        pf.release().unwrap();
+        assert!(!Path::new(path).exists());
+    }
+}

src/maestro/helpers.rs

@@ -10,7 +10,15 @@ use crate::transport::middle_proxy::{
     ProxyConfigData, fetch_proxy_config_with_raw, load_proxy_config_cache, save_proxy_config_cache,
 };
 
-pub(crate) fn parse_cli() -> (String, Option<PathBuf>, bool, Option<String>) {
+/// Parsed CLI arguments.
+pub(crate) struct CliArgs {
+    pub config_path: String,
+    pub data_path: Option<PathBuf>,
+    pub silent: bool,
+    pub log_level: Option<String>,
+}
+
+pub(crate) fn parse_cli() -> CliArgs {
     let mut config_path = "config.toml".to_string();
     let mut data_path: Option<PathBuf> = None;
     let mut silent = false;
@@ -55,34 +63,35 @@ pub(crate) fn parse_cli() -> (String, Option<PathBuf>, bool, Option<String>) {
                 log_level = Some(s.trim_start_matches("--log-level=").to_string());
             }
             "--help" | "-h" => {
-                eprintln!("Usage: telemt [config.toml] [OPTIONS]");
-                eprintln!();
-                eprintln!("Options:");
-                eprintln!("  --data-path <DIR>       Set data directory (absolute path; overrides config value)");
-                eprintln!("  --silent, -s            Suppress info logs");
-                eprintln!("  --log-level <LEVEL>     debug|verbose|normal|silent");
-                eprintln!("  --help, -h              Show this help");
-                eprintln!();
-                eprintln!("Setup (fire-and-forget):");
-                eprintln!(
-                    "  --init                  Generate config, install systemd service, start"
-                );
-                eprintln!("    --port <PORT>          Listen port (default: 443)");
-                eprintln!(
-                    "    --domain <DOMAIN>      TLS domain for masking (default: www.google.com)"
-                );
-                eprintln!(
-                    "    --secret <HEX>         32-char hex secret (auto-generated if omitted)"
-                );
-                eprintln!("    --user <NAME>          Username (default: user)");
-                eprintln!("    --config-dir <DIR>     Config directory (default: /etc/telemt)");
-                eprintln!("    --no-start             Don't start the service after install");
+                print_help();
                 std::process::exit(0);
             }
             "--version" | "-V" => {
                 println!("telemt {}", env!("CARGO_PKG_VERSION"));
                 std::process::exit(0);
             }
+            // Skip daemon-related flags (already parsed)
+            "--daemon" | "-d" | "--foreground" | "-f" => {}
+            s if s.starts_with("--pid-file") => {
+                if !s.contains('=') {
+                    i += 1; // skip value
+                }
+            }
+            s if s.starts_with("--run-as-user") => {
+                if !s.contains('=') {
+                    i += 1;
+                }
+            }
+            s if s.starts_with("--run-as-group") => {
+                if !s.contains('=') {
+                    i += 1;
+                }
+            }
+            s if s.starts_with("--working-dir") => {
+                if !s.contains('=') {
+                    i += 1;
+                }
+            }
             s if !s.starts_with('-') => {
                 config_path = s.to_string();
             }
@@ -93,7 +102,49 @@ pub(crate) fn parse_cli() -> (String, Option<PathBuf>, bool, Option<String>) {
         i += 1;
     }
 
-    (config_path, data_path, silent, log_level)
+    CliArgs {
+        config_path,
+        data_path,
+        silent,
+        log_level,
+    }
+}
+
+fn print_help() {
+    eprintln!("Usage: telemt [config.toml] [OPTIONS]");
+    eprintln!();
+    eprintln!("Options:");
+    eprintln!("  --data-path <DIR>       Set data directory (absolute path; overrides config value)");
+    eprintln!("  --silent, -s            Suppress info logs");
+    eprintln!("  --log-level <LEVEL>     debug|verbose|normal|silent");
+    eprintln!("  --help, -h              Show this help");
+    eprintln!("  --version, -V           Show version");
+    eprintln!();
+    #[cfg(unix)]
+    {
+        eprintln!("Daemon options (Unix only):");
+        eprintln!("  --daemon, -d            Fork to background (daemonize)");
+        eprintln!("  --foreground, -f        Explicit foreground mode (for systemd)");
+        eprintln!("  --pid-file <PATH>       PID file path (default: /var/run/telemt.pid)");
+        eprintln!("  --run-as-user <USER>    Drop privileges to this user after binding");
+        eprintln!("  --run-as-group <GROUP>  Drop privileges to this group after binding");
+        eprintln!("  --working-dir <DIR>     Working directory for daemon mode");
+        eprintln!();
+    }
+    eprintln!("Setup (fire-and-forget):");
+    eprintln!(
+        "  --init                  Generate config, install systemd service, start"
+    );
+    eprintln!("    --port <PORT>          Listen port (default: 443)");
+    eprintln!(
+        "    --domain <DOMAIN>      TLS domain for masking (default: www.google.com)"
+    );
+    eprintln!(
+        "    --secret <HEX>         32-char hex secret (auto-generated if omitted)"
+    );
+    eprintln!("    --user <NAME>          Username (default: user)");
+    eprintln!("    --config-dir <DIR>     Config directory (default: /etc/telemt)");
+    eprintln!("    --no-start             Don't start the service after install");
 }
 
 pub(crate) fn print_proxy_links(host: &str, port: u16, config: &ProxyConfig) {

src/maestro/mod.rs

@@ -47,8 +47,56 @@ use crate::transport::middle_proxy::MePool;
 use crate::transport::UpstreamManager;
 use helpers::parse_cli;
 
+#[cfg(unix)]
+use crate::daemon::{DaemonOptions, PidFile, drop_privileges};
+
 /// Runs the full telemt runtime startup pipeline and blocks until shutdown.
+///
+/// On Unix, daemon options should be handled before calling this function
+/// (daemonization must happen before tokio runtime starts).
+#[cfg(unix)]
+pub async fn run_with_daemon(
+    daemon_opts: DaemonOptions,
+) -> std::result::Result<(), Box<dyn std::error::Error>> {
+    run_inner(daemon_opts).await
+}
+
+/// Runs the full telemt runtime startup pipeline and blocks until shutdown.
+///
+/// This is the main entry point for non-daemon mode or when called as a library.
+#[allow(dead_code)]
 pub async fn run() -> std::result::Result<(), Box<dyn std::error::Error>> {
+    #[cfg(unix)]
+    {
+        // Parse CLI to get daemon options even in simple run() path
+        let args: Vec<String> = std::env::args().skip(1).collect();
+        let daemon_opts = crate::cli::parse_daemon_args(&args);
+        run_inner(daemon_opts).await
+    }
+    #[cfg(not(unix))]
+    {
+        run_inner().await
+    }
+}
+
+#[cfg(unix)]
+async fn run_inner(
+    daemon_opts: DaemonOptions,
+) -> std::result::Result<(), Box<dyn std::error::Error>> {
+
+    // Acquire PID file if daemonizing or if explicitly requested
+    // Keep it alive until shutdown (underscore prefix = intentionally kept for RAII cleanup)
+    let _pid_file = if daemon_opts.daemonize || daemon_opts.pid_file.is_some() {
+        let mut pf = PidFile::new(daemon_opts.pid_file_path());
+        if let Err(e) = pf.acquire() {
+            eprintln!("[telemt] {}", e);
+            std::process::exit(1);
+        }
+        Some(pf)
+    } else {
+        None
+    };
+
     let process_started_at = Instant::now();
     let process_started_at_epoch_secs = SystemTime::now()
         .duration_since(UNIX_EPOCH)
@@ -58,7 +106,11 @@ pub async fn run() -> std::result::Result<(), Box<dyn std::error::Error>> {
     startup_tracker
         .start_component(COMPONENT_CONFIG_LOAD, Some("load and validate config".to_string()))
         .await;
-    let (config_path, data_path, cli_silent, cli_log_level) = parse_cli();
+    let cli_args = parse_cli();
+    let config_path = cli_args.config_path;
+    let data_path = cli_args.data_path;
+    let cli_silent = cli_args.silent;
+    let cli_log_level = cli_args.log_level;
 
     let mut config = match ProxyConfig::load(&config_path) {
         Ok(c) => c,
@@ -555,6 +607,17 @@ pub async fn run() -> std::result::Result<(), Box<dyn std::error::Error>> {
         std::process::exit(1);
     }
 
+    // Drop privileges after binding sockets (which may require root for port < 1024)
+    if daemon_opts.user.is_some() || daemon_opts.group.is_some() {
+        if let Err(e) = drop_privileges(
+            daemon_opts.user.as_deref(),
+            daemon_opts.group.as_deref(),
+        ) {
+            error!(error = %e, "Failed to drop privileges");
+            std::process::exit(1);
+        }
+    }
+
     runtime_tasks::apply_runtime_log_filter(
         has_rust_log,
         &effective_log_level,

src/main.rs

@@ -4,6 +4,8 @@ mod api;
 mod cli;
 mod config;
 mod crypto;
+#[cfg(unix)]
+mod daemon;
 mod error;
 mod ip_tracker;
 #[cfg(test)]
@@ -20,7 +22,42 @@ mod tls_front;
 mod transport;
 mod util;
 
-#[tokio::main]
-async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
-    maestro::run().await
+fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
+    // On Unix, handle daemonization before starting tokio runtime
+    #[cfg(unix)]
+    {
+        let args: Vec<String> = std::env::args().skip(1).collect();
+        let daemon_opts = cli::parse_daemon_args(&args);
+
+        // Daemonize if requested (must happen before tokio runtime starts)
+        if daemon_opts.should_daemonize() {
+            match daemon::daemonize(daemon_opts.working_dir.as_deref()) {
+                Ok(daemon::DaemonizeResult::Parent) => {
+                    // Parent process exits successfully
+                    std::process::exit(0);
+                }
+                Ok(daemon::DaemonizeResult::Child) => {
+                    // Continue as daemon child
+                }
+                Err(e) => {
+                    eprintln!("[telemt] Daemonization failed: {}", e);
+                    std::process::exit(1);
+                }
+            }
+        }
+
+        // Now start tokio runtime and run the server
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()?
+            .block_on(maestro::run_with_daemon(daemon_opts))
+    }
+
+    #[cfg(not(unix))]
+    {
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()?
+            .block_on(maestro::run())
+    }
 }