Why SFTP Fails on OpenWrt Default SSH

If Termark connects to SSH normally but the SFTP panel shows the following error on an OpenWrt device, it is not a Termark bug — OpenWrt uses Dropbear by default, which does not include SFTP:

Failed to load files: session not found: create SFTP client failed: error receiving version packet from server: server unexpectedly closed connection: unexpected EOF

Dropbear closes the channel when it receives an sftp subsystem request but cannot find the sftp-server binary, so the client never receives the version packet and reports unexpected EOF. Equivalent messages include subsystem request failed / sftp-server not found / unable to initialize SFTP.

Symptom

• SSH terminal works fine.
• SFTP panel fails to open, or file transfer reports unable to initialize SFTP / subsystem not found.
• The same Termark configuration works on Debian / Ubuntu / CentOS hosts.

Root Cause: Dropbear vs OpenSSH

	Dropbear (OpenWrt default)	OpenSSH
Size	~300 KB, for embedded devices	~6 MB
Purpose	Lightweight SSH shell + SCP	Full SSH suite
SFTP support	No — sftp-server binary is not bundled	Yes — /usr/lib/openssh/sftp-server included

OpenWrt chooses Dropbear to save flash space. SFTP is a separate subsystem (sftp-server executable) that Dropbear never ships. Without it, the SSH server has nothing to handle the client's sftp subsystem request, so it rejects the channel.

How to Verify

On the OpenWrt device:

# 1. Which SSH server is running?
ps | grep -E 'dropbear|sshd'

# 2. Is sftp-server present?
ls -l /usr/libexec/sftp-server /usr/lib/sftp-server 2>&1

# 3. Dropbear has no Subsystem config
cat /etc/config/dropbear 2>&1 | head -20

If sftp-server is missing and dropbear is running, that confirms the cause.

Fixes (pick one)

Option 1: Install openssh-sftp-server (recommended, ~1 MB)

Keeps Dropbear as the SSH server and only adds the SFTP subsystem:

opkg update
opkg install openssh-sftp-server

# Verify
ls -l /usr/libexec/sftp-server
# should show: -rwxr-xr-x 1 root root ... /usr/libexec/sftp-server

No reboot or config change is needed — Dropbear will automatically find sftp-server on the next SFTP connection. Reopen the SFTP panel in Termark to verify.

If Termark still fails, ensure Dropbear is recent enough (OpenWrt 21.02+). Older builds need a symlink:

mkdir -p /usr/libexec
ln -sf /usr/lib/sftp-server /usr/libexec/sftp-server 2>/dev/null

Option 2: Use SCP Instead

If flash space is extremely tight and you cannot install anything, use SCP for single-file transfers. Termark's SFTP panel requires SFTP, so this is only a workaround outside Termark:

scp file.bin root@openwrt:/tmp/

Option 3: Replace Dropbear with OpenSSH (heavier)

Only if you need full OpenSSH features (key restrictions, richer sshd_config):

opkg update
opkg install openssh-server openssh-sftp-server
/etc/init.d/dropbear disable
/etc/init.d/sshd enable
/etc/init.d/sshd start

Cost: ~5–6 MB more flash and higher RAM. Most users should prefer Option 1.

Still Not Working?

1. Installed but still fails — reconnect SSH after install; some Dropbear versions cache subsystem lookup per connection.
2. No space left — run df -h and opkg list-installed | wc -l to check flash usage; remove unused packages first.
3. Permission error after SFTP connects — SFTP now works, but the login user lacks read/write permission on the target path — check ls -ld /path.

Recommendation for Termark Users

For OpenWrt routers / NAS / soft-routers, Option 1 is the best balance: one opkg install (~1 MB) fixes SFTP for Termark and any other SFTP client, without replacing the lightweight SSH server.

Related pages

• SFTP CWD Tracking