Debian Stretch 9.6: openssh-server and old dropbear client don't work togheter

[Stuart Henderson]

On 2018/11/22 19:55, owl700 at gmail.com wrote:
> Hi, I have compatibility issues with the latest version of
> openssh-server and an old dropbear client, the dopbear client stops at
> preauth
> 
> ov 22 14:34:03  myhostname sshd[3905]: debug1: Client protocol version
> 2.0; client software version dropbear_0.46
> Nov 22 14:34:03 myhostname sshd[3905]: debug1: no match: dropbear_0.46
> Nov 22 14:34:03 myhostname sshd[3905]: debug1: Local version string
> SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u4
> Nov 22 14:34:03 myhostname sshd[3905]: debug1: Enabling compatibility
> mode for protocol 2.0
> Nov 22 14:34:03 myhostname sshd[3905]: debug2: fd 3 setting O_NONBLOCK
> Nov 22 14:34:03 myhostname sshd[3905]: debug2: Network child is on pid 3906
> Nov 22 14:34:03 myhostname sshd[3905]: debug3: preauth child monitor started
> Nov 22 14:34:03 myhostname sshd[3905]: debug3: privsep user:group
> 106:65534 [preauth]
> Nov 22 14:34:03 myhostname sshd[3905]: debug1: permanently_set_uid:
> 106/65534 [preauth]
> Nov 22 14:34:03 myhostname sshd[3905]: debug1: list_hostkey_types:
> ssh-rsa,rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256 [preauth]
> Nov 22 14:34:03 myhostname sshd[3905]: debug3: send packet: type 20 [preauth]
> Nov 22 14:34:03 myhostname sshd[3905]: debug1: SSH2_MSG_KEXINIT sent [preauth]
> 
> Can you help?

That ~13-year-old version of dbclient only has weak key exchange methods -
diffie-hellman-group1-sha1, "OpenSSH supports this method, but does not
enable it by default because is weak and within theoretical range of the
so-called Logjam attack" and diffie-hellman-group1-dss, disabled by default
in OpenSSH in 2015.

Also only weak CBC-mode ciphers, disabled by default in 2014.

The right answer is to run a newer client.

If there's no way to do that, least worst is probably to connect to
a jump host on the LAN (locked-down as much as possible), running
modern OpenSSH sshd but with weak kex/ciphers enabled, in this
case you could use something like

KexAlgorithms +diffie-hellman-group1-sha1
Ciphers +aes128-cbc

This is still not recommended, but at least you could keep the weak
crypto off the internet this way.