Causes of SSH Error: kex_exchange_identification: Connection Closed by Remote Host

ssh

I setup a SSH server online that is publicly accessible by anyone. Therefore, I get a lot of connections from IPs all over the world. Weirdly, none actually try to authenticate to open a session.
I can myself connect and authenticate without any problem.

From time to time, I get the error: kex_exchange_identification: Connection closed by remote host in the server logs. What causes that?

Here is 30 minutes of SSH logs (public IPs have been redacted):

# journalctl SYSLOG_IDENTIFIER=sshd -S "03:30:00" -U "04:00:00"
-- Logs begin at Fri 2020-01-31 09:26:25 UTC, end at Mon 2020-04-20 08:01:15 UTC. --
Apr 20 03:39:48 myhostname sshd[18438]: Connection from x.x.x.207 port 39332 on 10.0.0.11 port 22 rdomain ""
Apr 20 03:39:48 myhostname sshd[18439]: Connection from x.x.x.207 port 39334 on 10.0.0.11 port 22 rdomain ""
Apr 20 03:39:48 myhostname sshd[18438]: Connection closed by x.x.x.207 port 39332 [preauth]
Apr 20 03:39:48 myhostname sshd[18439]: Connection closed by x.x.x.207 port 39334 [preauth]
Apr 20 03:59:36 myhostname sshd[22186]: Connection from x.x.x.83 port 34876 on 10.0.0.11 port 22 rdomain ""
Apr 20 03:59:36 myhostname sshd[22186]: error: kex_exchange_identification: Connection closed by remote host

And here is my SSH configuration:

# ssh -V
OpenSSH_8.2p1, OpenSSL 1.1.1d  10 Sep 2019
# cat /etc/ssh/sshd_config 
UsePAM yes
AddressFamily any
Port 22
X11Forwarding no
PermitRootLogin prohibit-password
GatewayPorts no
PasswordAuthentication no
ChallengeResponseAuthentication no
PrintMotd no # handled by pam_motd
AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2 /etc/ssh/authorized_keys.d/%u
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
LogLevel VERBOSE
UseDNS no
AllowUsers root
AuthenticationMethods publickey
MaxStartups 3:100:60

After searching the web, I have seen references to MaxStartups indicating that it could be the reason for this error but after changing the default value as shown in my sshd_config and attempting more than 3 connections, the server unambiguously indicates the probem

Apr 20 07:26:59 myhostname sshd[31468]: drop connection #3 from [x.x.x.226]:54986 on [10.0.0.11]:22 past MaxStartups

So, what causes error: kex_exchange_identification: Connection closed by remote host?

Best Answer

Weirdly, none actually try to authenticate to open a session.

Some spiders and services like Shodan scans public ipv4 addresses for open services, e.g. salt masters, ftp servers, RDPs, and also SSH services. These spiders usually only connect to the services without doing any valid authentication steps.

I get the error: kex_exchange_identification: Connection closed by remote host in the server logs. What causes that?

I haven't found conclusive answers about that, so... time to browse the source then.

In OpenSSH source code, kex_exchange_identification is a function to exchange server and client identification (duh), and the specified error happened if the socket connection between OpenSSH server and client is interrupted (see EPIPE), i.e. client already closed its connection.

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