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//! Minimal TELNET (RFC 854) client.
//!
//! `telnet://host[:port]`. Optional input (from `-d`/`-T`/stdin) is sent after
//! connecting (with literal `0xFF` bytes doubled to `IAC IAC`, as RFC 854
//! requires); received data is returned with TELNET command sequences
//! stripped. Option negotiation is handled by refusing every option the server
//! offers or requests (answering `WILL` with `DONT` and `DO` with `WONT`), which
//! is enough for line-oriented banners and simple scripted exchanges. This is
//! not an interactive terminal.
use std::io::{Read, Write};
use crate::error::{Error, Result};
use crate::net::NetConfig;
use crate::url::Url;
const IAC: u8 = 255;
const DONT: u8 = 254;
const DO: u8 = 253;
const WONT: u8 = 252;
const WILL: u8 = 251;
const SB: u8 = 250;
const SE: u8 = 240;
/// Largest response we buffer (mirrors other protocols' 64 MiB cap).
const MAX_RESPONSE_BYTES: usize = 64 * 1024 * 1024;
/// Where the receive-side parser is inside the TELNET command stream. Kept
/// across reads, so a command sequence split over two TCP segments (or two
/// 8 KiB reads) is still recognised instead of leaking into the data.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum State {
/// Plain application data.
Data,
/// Saw `IAC`; the next byte is the command.
Iac,
/// Saw `IAC WILL|WONT|DO|DONT`; the next byte is the option.
Opt(u8),
/// Inside a subnegotiation (`IAC SB ...`), skipping until `IAC SE`.
Sb,
/// Saw `IAC` inside a subnegotiation.
SbIac,
}
/// Incremental TELNET receive parser.
struct Parser {
state: State,
}
impl Parser {
fn new() -> Self {
Parser { state: State::Data }
}
/// Consume `input`, appending application data to `out` and any
/// negotiation replies (refusals) to `replies`.
fn feed(&mut self, input: &[u8], out: &mut Vec<u8>, replies: &mut Vec<u8>) {
for &b in input {
self.state = match (self.state, b) {
(State::Data, IAC) => State::Iac,
(State::Data, _) => {
out.push(b);
State::Data
}
// `IAC IAC` is an escaped literal 0xFF data byte.
(State::Iac, IAC) => {
out.push(IAC);
State::Data
}
(State::Iac, WILL | WONT | DO | DONT) => State::Opt(b),
(State::Iac, SB) => State::Sb,
// Any other two-byte command (NOP, GA, ...): ignore.
(State::Iac, _) => State::Data,
(State::Opt(cmd), opt) => {
match cmd {
// Refuse whatever the server offers or asks for. WONT
// and DONT only confirm the default (disabled) state,
// so they need no answer (RFC 1143).
WILL => replies.extend_from_slice(&[IAC, DONT, opt]),
DO => replies.extend_from_slice(&[IAC, WONT, opt]),
_ => {}
}
State::Data
}
(State::Sb, IAC) => State::SbIac,
(State::Sb, _) => State::Sb,
(State::SbIac, SE) => State::Data,
// `IAC IAC` inside SB is escaped data; anything else, keep
// skipping.
(State::SbIac, _) => State::Sb,
};
}
}
}
/// Escape outgoing data: every literal `0xFF` must be sent as `IAC IAC`, or
/// the server would parse it as the start of a command.
fn escape_iac(input: &[u8]) -> Vec<u8> {
let mut out = Vec::with_capacity(input.len());
for &b in input {
out.push(b);
if b == IAC {
out.push(IAC);
}
}
out
}
/// Connect, send `input`, and return the received application data with TELNET
/// command bytes removed.
pub(crate) fn run(url: &Url, input: &[u8], cfg: &NetConfig) -> Result<Vec<u8>> {
if url.scheme != "telnet" {
return Err(Error::UnsupportedScheme(url.scheme.clone()));
}
let mut sock = cfg.connect(&url.host, url.port)?;
if !input.is_empty() {
sock.write_all(&escape_iac(input))?;
sock.flush()?;
}
let mut out: Vec<u8> = Vec::new();
let mut buf = [0u8; 8192];
let mut parser = Parser::new();
let mut replies: Vec<u8> = Vec::new();
loop {
let n = sock.read(&mut buf)?;
if n == 0 {
break;
}
replies.clear();
parser.feed(&buf[..n], &mut out, &mut replies);
if !replies.is_empty() {
sock.write_all(&replies)?;
sock.flush()?;
}
if out.len() > MAX_RESPONSE_BYTES {
return Err(Error::BadResponse(format!(
"telnet: response exceeds {MAX_RESPONSE_BYTES} bytes"
)));
}
}
Ok(out)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn iac_negotiation_is_stripped_and_refused() {
// "Hi" + IAC WILL ECHO(1) + "!" + IAC IAC (literal 0xFF) + IAC DO 24.
let data = [b'H', b'i', IAC, WILL, 1, b'!', IAC, IAC, IAC, DO, 24];
let (mut out, mut replies) = (Vec::new(), Vec::new());
Parser::new().feed(&data, &mut out, &mut replies);
assert_eq!(out, vec![b'H', b'i', b'!', IAC]);
assert_eq!(replies, vec![IAC, DONT, 1, IAC, WONT, 24]);
}
#[test]
fn sequences_split_at_every_byte_boundary_parse_identically() {
let data = [
b'a', IAC, WILL, 3, b'b', IAC, SB, 24, 1, IAC, IAC, IAC, SE, b'c', IAC, IAC, b'd',
];
let (mut want_out, mut want_rep) = (Vec::new(), Vec::new());
Parser::new().feed(&data, &mut want_out, &mut want_rep);
assert_eq!(want_out, vec![b'a', b'b', b'c', IAC, b'd']);
assert_eq!(want_rep, vec![IAC, DONT, 3]);
for split in 0..=data.len() {
let mut p = Parser::new();
let (mut out, mut rep) = (Vec::new(), Vec::new());
p.feed(&data[..split], &mut out, &mut rep);
p.feed(&data[split..], &mut out, &mut rep);
assert_eq!(
(out, rep),
(want_out.clone(), want_rep.clone()),
"split {split}"
);
}
}
#[test]
fn escape_iac_doubles_ff() {
assert_eq!(escape_iac(&[1, IAC, 2]), vec![1, IAC, IAC, 2]);
}
/// Drive the real `run()` against a server that deliberately splits IAC
/// sequences across TCP writes, and check the wire in both directions.
#[test]
fn run_handles_split_sequences_and_escapes_input() {
use std::net::TcpListener;
use std::time::Duration;
let l = TcpListener::bind("127.0.0.1:0").unwrap();
let port = l.local_addr().unwrap().port();
let h = std::thread::spawn(move || {
let (mut s, _) = l.accept().unwrap();
s.set_nodelay(true).unwrap();
let mut got = vec![0u8; 3];
s.read_exact(&mut got).unwrap();
for chunk in [
&[b'x', IAC][..],
&[WILL][..],
&[1, b'y', IAC, SB][..],
&[5, IAC][..],
&[SE, b'z'][..],
] {
s.write_all(chunk).unwrap();
std::thread::sleep(Duration::from_millis(30));
}
s.set_read_timeout(Some(Duration::from_secs(2))).unwrap();
let mut reply = [0u8; 3];
s.read_exact(&mut reply).unwrap();
(got, reply)
});
let url = Url::parse(&format!("telnet://127.0.0.1:{port}")).unwrap();
let out = run(&url, &[b'q', IAC], &NetConfig::default()).unwrap();
let (sent, reply) = h.join().unwrap();
assert_eq!(out, b"xyz");
assert_eq!(
sent,
vec![b'q', IAC, IAC],
"0xFF must be doubled on the wire"
);
assert_eq!(reply, [IAC, DONT, 1]);
}
}