OpenSSH Always Hangs When Connecting to Remote

Darren Tucker dtucker at
Tue Sep 22 10:49:24 AEST 2015

On Mon, Sep 21, 2015 at 6:05 AM, Ángel González <keisial at> wrote:
> New versions tend to add more key exchanges, so yes. You can also use
> ssh -o KexAlgorithms=<algos use by dropbear> to test the hypothesis.

Note that this is not a definitive test because the server will still offer
its full list of key exchange and cipher methods, so depending on exactly
what and where the problem is, this could still potentially tickle MTU
blackhole problems.  You'd need to restrict the OpenSSH
server's KexAlgorithms, HostKeyAlgorithms, Ciphers and Compression settings
for an accurate test, and note that this configuration would be
significantly less safe than OpenSSH's defaults.

I'd try the MTU thing in the link I sent you.  Alternatively, if you have
access to both ends via some other means, find the ssh connection in the
output of "netstat" on both sides and check if the SendQ column stays
non-zero indicating that the network traffic never get acknowledged.

$ telnet openssh 22
Connected to localhost.
Escape character is '^]'.
t �?�� �P6���}:�
ssh-ed25519lchacha20-poly1305 at,aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm at,aes256-gcm at
,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at,
aes256-gcm at openssh.comumac-64-etm at,umac-128-etm at,
hmac-sha2-256-etm at,hmac-sha2-512-etm at,
hmac-sha1-etm at,umac-64 at,umac-128 at
,hmac-sha2-256,hmac-sha2-512,hmac-sha1�umac-64-etm at,
umac-128-etm at,hmac-sha2-256-etm at,
hmac-sha2-512-etm at,hmac-sha1-etm at,umac-64 at,
umac-128 at,hmac-sha2-256,hmac-sha2-512,hmac-sha1 none,
zlib at none,zlib at
telnet> quit
Connection closed

$ telnet dropear 22
Connected to rtr.
Escape character is '^]'.
�� �Hlm�O�)d��8� diffie-hellman-group1-sha1ssh-rs3des-cb3des-cbc
hmac-sha1,hmac-md5 hmac-sha1,hmac-md5 none none �h8u��!
telnet> quit
Connection closed

Darren Tucker (dtucker at
GPG key 8FF4FA69 / D9A3 86E9 7EEE AF4B B2D4  37C9 C982 80C7 8FF4 FA69
    Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.

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