shadowsocks/shadowsocks-rust · error

`method` is required

Error message

`method` is required

What it means

Same branch as error 173: when SERVER_ADDR is provided, the code insists ENCRYPT_METHOD exists via .expect("`method` is required"). A per-server cipher is mandatory to construct ServerConfig, so a missing method aborts the process. clap's validation normally prevents this; the expect is a backstop.

Source

Thrown at src/service/local.rs:634

            }
            None => {
                logging::init_with_config("sslocal", &service_config.log);
            }
        }

        trace!("{:?}", service_config);

        let mut config = match config_path_opt {
            Some(cpath) => Config::load_from_file(&cpath, ConfigType::Local)
                .map_err(|err| ShadowsocksError::LoadConfigFailure(format!("loading config {cpath:?}, {err}")))?,
            None => Config::new(ConfigType::Local),
        };

        if let Some(svr_addr) = matches.get_one::<String>("SERVER_ADDR") {
            let method = matches
                .get_one::<String>("ENCRYPT_METHOD")
                .map(|x| x.parse::<CipherKind>().expect("method"))
                .expect("`method` is required");

            let password = match matches.get_one::<String>("PASSWORD") {
                Some(pwd) => read_variable_field_value(pwd).into(),
                None => {
                    // NOTE: svr_addr should have been checked by crate::vparser
                    if method.is_none() {
                        // If method doesn't need a key (none, plain), then we can leave it empty
                        String::new()
                    } else {
                        match crate::password::read_server_password(svr_addr) {
                            Ok(pwd) => pwd,
                            Err(..) => panic!("`password` is required for server {svr_addr}"),
                        }
                    }
                }
            };

            let svr_addr = svr_addr.parse::<ServerAddr>().expect("server-addr");

View on GitHub (pinned to 8eb0f0a65b)

Solutions

  1. Add --encrypt-method <cipher> (e.g. aes-256-gcm) whenever a SERVER_ADDR is specified.
  2. Use a config file (-c) that includes a server entry with its own "method" field.
  3. If invoking create programmatically, supply "ENCRYPT_METHOD" in the matches.

Example fix

// before
sslocal -b 127.0.0.1:1080 -s example.com:8388 -k secret

// after
sslocal -b 127.0.0.1:1080 -s example.com:8388 -k secret --encrypt-method aes-256-gcm
Defensive patterns

Strategy: validation

Validate before calling

if matches.get_one::<String>("SERVER_ADDR").is_some()
    && matches.get_one::<String>("ENCRYPT_METHOD").is_none()
{
    return Err("--encrypt-method is required when SERVER_ADDR is given".into());
}

Try / catch

let Some(s) = matches.get_one::<String>("ENCRYPT_METHOD") else {
    eprintln!("`method` is required");
    return ExitCode::FAILURE;
};

Prevention

When it happens

Trigger: Running sslocal with SERVER_ADDR set but no --encrypt-method flag, or calling local::create programmatically with ArgMatches lacking ENCRYPT_METHOD.

Common situations: Command lines copied from AEAD-free examples; scripts that drop the method flag when SERVER_ADDR is set; hand-built ArgMatches in tests.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of shadowsocks/shadowsocks-rust@8eb0f0a65b (2026-09-09). Data as JSON: /api/errors/d25e236bbc21d4c9. Report an issue: GitHub.