Failed to connect to dropbear version 2020.81

[Ronny Meeus]

Hello

we have a dropbear version 2020,81 running in an old release of our SW
as our login server.
Currently we experience issues logging in on the system from the
remote system running Openssh (using dropbear also on the client side
we see a similar result).

At the end of the mail I pasted the output we get.
(after the trace "debug1: expecting SSH2_MSG_KEX_ECDH_REPLY", nothing
is seen anymore)

This issue is not seen persistently and seems to occur on random systems.
After a reboot the issue seems to be resolved, not clear whether it
will come back after some time or not.

For the moment it is impossible to collect local traces on the server
node since we cannot reach it anymore.
Doing a telnet to the port 2222 is actually showing the dropbear
version etc, meaning that the connection as such (at TCP level) seems
to be OK.

Is this a known issue and is there something we can do as a workaround
(or do we have means to collect more information about the root
cause)?

Thanks.

Best regards,
Ronny

~ # ssh -p 2222 root at 169.254.1.4 -vvvv
OpenSSH_8.4p1, OpenSSL 1.1.1j  16 Feb 2021
debug1: Reading configuration data /etc/ssh/ssh_config
debug2: resolve_canonicalize: hostname 169.254.1.4 is address
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' ->
'/root/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' ->
'/root/.ssh/known_hosts2'
debug1: Authenticator provider $SSH_SK_PROVIDER did not resolve; disabling
debug2: ssh_connect_direct
debug1: Connecting to 169.254.1.4 [169.254.1.4] port 2222.
debug1: Connection established.
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: identity file /root/.ssh/id_ecdsa type -1
debug1: identity file /root/.ssh/id_ecdsa-cert type -1
debug1: identity file /root/.ssh/id_ecdsa_sk type -1
debug1: identity file /root/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /root/.ssh/id_ed25519 type -1
debug1: identity file /root/.ssh/id_ed25519-cert type -1
debug1: identity file /root/.ssh/id_ed25519_sk type -1
debug1: identity file /root/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /root/.ssh/id_xmss type -1
debug1: identity file /root/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.4
debug1: Remote protocol version 2.0, remote software version dropbear_2020.81
debug1: no match: dropbear_2020.81
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to 169.254.1.4:2222 as 'root'
debug3: put_host_port: [169.254.1.4]:2222
debug3: hostkeys_foreach: reading file "/root/.ssh/known_hosts"
debug3: send packet: type 20
debug1: SSH2_MSG_KEXINIT sent
debug3: receive packet: type 20
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms:
curve25519-sha256,curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,ext-info-c
debug2: host key algorithms:
ecdsa-sha2-nistp256-cert-v01 at openssh.com,ecdsa-sha2-nistp384-cert-v01 at openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com,sk-ecdsa-sha2-nistp256-cert-v01 at openssh.com,ssh-ed25519-cert-v01 at openssh.com,sk-ssh-ed25519-cert-v01 at openssh.com,rsa-sha2-512-cert-v01 at openssh.com,rsa-sha2-256-cert-v01 at openssh.com,ssh-rsa-cert-v01 at openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ecdsa-sha2-nistp256 at openssh.com,ssh-ed25519,sk-ssh-ed25519 at openssh.com,rsa-sha2-512,rsa-sha2-256,ssh-rsa
debug2: ciphers ctos:
chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at openssh.com,aes256-gcm at openssh.com
debug2: ciphers stoc:
chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at openssh.com,aes256-gcm at openssh.com
debug2: MACs ctos:
umac-64-etm at openssh.com,umac-128-etm at openssh.com,hmac-sha2-256-etm at openssh.com,hmac-sha2-512-etm at openssh.com,hmac-sha1-etm at openssh.com,umac-64 at openssh.com,umac-128 at openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: MACs stoc:
umac-64-etm at openssh.com,umac-128-etm at openssh.com,hmac-sha2-256-etm at openssh.com,hmac-sha2-512-etm at openssh.com,hmac-sha1-etm at openssh.com,umac-64 at openssh.com,umac-128 at openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: compression ctos: none,zlib at openssh.com,zlib
debug2: compression stoc: none,zlib at openssh.com,zlib
debug2: languages ctos:
debug2: languages stoc:
debug2: first_kex_follows 0
debug2: reserved 0
debug2: peer server KEXINIT proposal
debug2: KEX algorithms:
curve25519-sha256,curve25519-sha256 at libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,kexguess2 at matt.ucc.asn.au
debug2: host key algorithms:
ssh-ed25519,ecdsa-sha2-nistp256,rsa-sha2-256,ssh-rsa
debug2: ciphers ctos: chacha20-poly1305 at openssh.com,aes128-ctr,aes256-ctr
debug2: ciphers stoc: chacha20-poly1305 at openssh.com,aes128-ctr,aes256-ctr
debug2: MACs ctos: hmac-sha1,hmac-sha2-256
debug2: MACs stoc: hmac-sha1,hmac-sha2-256
debug2: compression ctos: zlib at openssh.com,none
debug2: compression stoc: zlib at openssh.com,none
debug2: languages ctos:
debug2: languages stoc:
debug2: first_kex_follows 0
debug2: reserved 0
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305 at openssh.com MAC:
<implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305 at openssh.com MAC:
<implicit> compression: none
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY