openssh unix dev - Nov 2018 - Debian Stretch 9.6: openssh-server and old dropbear client don't work togheter

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?

Thanks

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.

Il giorno gio 22 nov 2018 alle ore 21:24 Stuart Henderson
<stu at spacehopper.org> ha scritto:
>
> 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.
Thanks Stuart

I have tried to use the right KexAlgorithm and Ciphers, but dropbear
client fail always

myhostname sshd[3905]: debug1: SSH2_MSG_KEXINIT sent [preauth]

There aren't other debug messages

Only for test purpose i have add all options i can in
/etc/sshd_config, but nothing, what am I doing wrong?

Ciphers
3des-cbc,blowfish-cbc,cast128-cbc,arcfour,arcfour128,arcfour256,aes128-cbc,aes192-cbc,aes256-cbc,rijndael-cbc
at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at
openssh.com,aes256-gcm at openssh.com,chacha20-poly1305 at openssh.com
HostbasedAcceptedKeyTypes
ssh-ed25519,ssh-ed25519-cert-v01 at
openssh.com,ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa-cert-v01
at openssh.com,ssh-dss-cert-v01 at openssh.com,ecdsa-sha2-nistp256-cert-v01 at
openssh.com,ecdsa-sha2-nistp384-cert-v01 at
openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com
HostKeyAlgorithms
ssh-ed25519,ssh-ed25519-cert-v01 at
openssh.com,ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa-cert-v01
at openssh.com,ssh-dss-cert-v01 at openssh.com,ecdsa-sha2-nistp256-cert-v01 at
openssh.com,ecdsa-sha2-nistp384-cert-v01 at
openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com
KexAlgorithms
diffie-hellman-group1-sha1,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group-exchange-sha256,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,curve25519-sha256,curve25519-sha256
at libssh.org
MACs
hmac-sha1,hmac-sha1-96,hmac-sha2-256,hmac-sha2-512,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-ripemd160
at openssh.com,umac-64 at openssh.com,umac-128 at openssh.com,hmac-sha1-etm at
openssh.com,hmac-sha1-96-etm at openssh.com,hmac-sha2-256-etm at
openssh.com,hmac-sha2-512-etm at openssh.com,hmac-md5-etm at
openssh.com,hmac-md5-96-etm at openssh.com,hmac-ripemd160-etm at
openssh.com,umac-64-etm at openssh.com,umac-128-etm at openssh.com
PubkeyAcceptedKeyTypes
ssh-ed25519,ssh-ed25519-cert-v01 at
openssh.com,ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa-cert-v01
at openssh.com,ssh-dss-cert-v01 at openssh.com,ecdsa-sha2-nistp256-cert-v01 at
openssh.com,ecdsa-sha2-nistp384-cert-v01 at
openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com