SSH / SFTP DIAGNOSTIC JOURNEY

SSH & SFTP Troubleshooting

Start with the stage where the connection actually fails. These five guides separate pre-auth transport, authentication, subsystem startup, chroot enforcement, and later SFTP session termination so you do not fix the wrong layer.

Choose the earliest stage that matches your evidence

1 · Before authenticationkex_exchange_identification: Connection Closed by Remote Host — pre-auth connection pressure, blocking, sshd, or network path.

2 · During authenticationToo Many Authentication Failures — offered identities and key selection before MaxAuthTries.

3 · Subsystem will not startSubsystem Request Failed on Channel 0 — requested SFTP subsystem and effective sshd configuration.

4 · Chroot explicitly rejectedBad Ownership or Modes for Chroot Directory — repair the exact path component without weakening the jail.

5 · SFTP ends with EOFUnexpected End-of-File from SFTP Server — locate startup/access versus later-session termination.

SSH Permission Denied (publickey)

Trace target, user, available key, offered key and server authorization before generating another identity.

SSH · AUTH

kex_exchange_identification: Connection Closed by Remote Host

Pre-auth: diagnose connection pressure, blocking, sshd, and network path.

PRE-AUTH

Too Many Authentication Failures

Authentication: prove which keys or methods consume attempts before changing limits.

AUTH

Subsystem Request Failed on Channel 0

Subsystem startup: separate configuration, execution, and account restrictions.

SUBSYSTEM

Fatal: Bad Ownership or Modes for Chroot Directory

Chroot: preserve the root-owned boundary while creating writable space inside it.

CHROOT

Received Unexpected End-of-File from SFTP Server

Session/EOF: identify the failure boundary before editing sshd_config.

SESSION / EOF