After changing internet provider "ssh_exchange_identification: read: Connection reset by peer"

Question

After a change of Internet provider, hence of modem/router and IP address, I cannot log in from home to a server via ssh as I used to do with the previous provider. The change of circumstances is at the client/host/home side.
If I ask ssh what happened with ssh -YC guest@server.com -v, the result is:

OpenSSH_7.2p2 Ubuntu-4ubuntu2.8, OpenSSL 1.0.2g 1 Mar 2016
debug1: Reading configuration data /home/user/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to server.com [aaa.bbb.ccc.ddd] port 22.
debug1: Connection established.
debug1: identity file /home/user/.ssh/id_rsa type 1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.8
ssh_exchange_identification: read: Connection reset by peer

What are the steps to restore the previous safe situation?

The client/localhost is a desktop system running on Ubuntu 16, although this is reproducible with other Ubuntu releases. The server is the login node of a computer cluster at the workplace; this runs on Debian Buster. The new provider is reputable and is not blacklisted. The factors that ostensibly changed between before and afterwards are those linked to the new service: hardware-wise the router, software-wise the IP address.

Answer 1

If you interact with a real ssh server, when you run nc SERVERIP 22 you must receive after a few seconds 2/3 of the welcome message of SSH like one of these examples:

SSH-2.0-OpenSSH_5.3
SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u7
SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u1 
SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3

Usually, when you run ssh -v you must have somewhere two lines:

debug1: Local version string SSH-2.0-OpenSSH_8.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9p1 Debian-10+deb10u1

My perception is that you have a firewall that blocks the ssh connection, so it can be:

• on your side (router)
• on your ISP
• on the destination server (can be either a firewall or /etc/hosts.deny or /etc/hosts.allow)

You must test:

• a ssh connection on port 22 from your current location to some other location (ask a friend):

  IF this ssh session is working, that means the problem is not on your side

• a ssh connection to your server, port 22, but from another location

  IF this connection is not working, you can think you have a IP filtering on the server side