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fix(server): offer only none SSH compression to avoid zlib@openssh.com desync (fixes #215) #217

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fix(server): offer only none SSH compression to avoid zlib@openssh.com desync (fixes #215) #217

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3 changes: 3 additions & 0 deletions CHANGELOG.md
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Expand Up @@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

## [Unreleased]

### Fixed
- **Advertise only `none` SSH compression so clients that negotiate `zlib@openssh.com` no longer drop mid-session** (#215). Cyberduck — and OpenSSH `sftp -C`, or any client that prefers delayed zlib — completed the SFTP handshake (`INIT` / `REALPATH` / `STAT` all succeeded) and then the connection died, with the server logging `SshEncoding: length invalid` on the next inbound packet. Root cause is russh's delayed-zlib (`zlib@openssh.com`) transport: the flate2 stream desyncs a few packets after compression activates post-auth, so russh decodes the following packet's length prefix out of corrupted plaintext. It reproduces on both russh 0.61.1 and 0.62.1, which rules out the channel-close path and pins it to compression rather than the SFTP layer (OpenSSH's default `sftp`, FileZilla, and paramiko all negotiate `none` and were unaffected). `build_russh_config` now sets `russh::Preferred { compression: Cow::Borrowed(&[compression::NONE]), ..DEFAULT }`, so the server advertises only `none` and every client falls back to the uncompressed transport — matching the Dropbear / OpenSSH `sftp-server` defaults used in Backend.AI kernel containers, where compression was never in play. Verified with `sftp -C` (now negotiates `compression: none` and lists/downloads cleanly) and a live Cyberduck 9.5 session. The underlying russh delayed-zlib desync is left to be fixed upstream.

## [2.2.3] - 2026-05-25

### Security
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15 changes: 15 additions & 0 deletions src/server/mod.rs
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Expand Up @@ -193,8 +193,23 @@ impl BsshServer {

tracing::info!(key_count = keys.len(), "Loaded host keys");

// russh's delayed zlib (`zlib@openssh.com`) compression desyncs and
// corrupts the channel stream after a few packets, so any client that
// negotiates compression — Cyberduck, `sftp -C` — fails mid-session
// with "SshEncoding: length invalid" (reproducible in russh 0.61.1 and
// 0.62.1). Until the upstream russh bug is fixed, advertise only "none"
// so clients fall back to the uncompressed transport, matching the
// Dropbear/OpenSSH sftp-server defaults used in Backend.AI containers.
// See https://github.com/lablup/bssh/issues/215.
const NO_COMPRESSION: &[russh::compression::Name] = &[russh::compression::NONE];
let preferred = russh::Preferred {
compression: std::borrow::Cow::Borrowed(NO_COMPRESSION),
..russh::Preferred::DEFAULT
};

Ok(russh::server::Config {
keys,
preferred,
auth_rejection_time: Duration::from_secs(3),
auth_rejection_time_initial: Some(Duration::from_secs(0)),
max_auth_attempts: self.config.max_auth_attempts as usize,
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