2d ago

How to harden against SSH brute-forcing?

Recently, I discovered that SSH of my VPS server is constantly battered as follows.

    
Apr 06 11:15:14 abastro-personal-arm sshd[102702]: Unable to negotiate with 218.92.0.201 port 53768: no matching key exchange method found. Their offer: diffie>
Apr 06 11:30:29 abastro-personal-arm sshd[102786]: Unable to negotiate with 218.92.0.207 port 18464: no matching key exchange method found. Their offer: diffie>
Apr 06 11:45:36 abastro-personal-arm sshd[102881]: Unable to negotiate with 218.92.0.209 port 59634: no matching key exchange method found. Their offer: diffie>
Apr 06 12:01:02 abastro-personal-arm sshd[103019]: Unable to negotiate with 218.92.0.203 port 16976: no matching key exchange method found. Their offer: diffie>
Apr 06 12:05:49 abastro-personal-arm sshd[103066]: Unable to negotiate with 218.92.0.212 port 49130: no matching key exchange method found. Their offer: diffie>
Apr 06 12:07:09 abastro-personal-arm sshd[103077]: Connection closed by 162.142.125.122 port 56110 [preauth]
Apr 06 12:12:18 abastro-personal-arm sshd[103154]: Connection closed by 45.79.181.223 port 22064 [preauth]
Apr 06 12:12:19 abastro-personal-arm sshd[103156]: Connection closed by 45.79.181.223 port 22078 [preauth]
Apr 06 12:12:20 abastro-personal-arm sshd[103158]: Connection closed by 45.79.181.223 port 22112 [preauth]
Apr 06 12:21:26 abastro-personal-arm sshd[103253]: Connection closed by 118.25.174.89 port 36334 [preauth]
Apr 06 12:23:39 abastro-personal-arm sshd[103282]: Unable to negotiate with 218.92.0.252 port 59622: no matching key exchange method found. Their offer: diffie>
Apr 06 12:26:38 abastro-personal-arm sshd[103312]: Connection closed by 92.118.39.73 port 44400
Apr 06 12:32:22 abastro-personal-arm sshd[103373]: Unable to negotiate with 218.92.0.203 port 57092: no matching key exchange method found. Their offer: diffie>
Apr 06 12:49:48 abastro-personal-arm sshd[103556]: error: maximum authentication attempts exceeded for root from 98.22.89.155 port 53675 ssh2 [preauth]
Apr 06 12:49:48 abastro-personal-arm sshd[103556]: Disconnecting authenticating user root 98.22.89.155 port 53675: Too many authentication failures [preauth]
Apr 06 12:49:51 abastro-personal-arm sshd[103558]: error: maximum authentication attempts exceeded for root from 98.22.89.155 port 53775 ssh2 [preauth]
Apr 06 12:49:51 abastro-personal-arm sshd[103558]: Disconnecting authenticating user root 98.22.89.155 port 53775: Too many authentication failures [preauth]
Apr 06 12:49:53 abastro-personal-arm sshd[103561]: error: maximum authentication attempts exceeded for root from 98.22.89.155 port 53829 ssh2 [preauth]
Apr 06 12:49:53 abastro-personal-arm sshd[103561]: Disconnecting authenticating user root 98.22.89.155 port 53829: Too many authentication failures [preauth]
Apr 06 12:49:54 abastro-personal-arm sshd[103563]: Connection closed by 98.22.89.155 port 53862 [preauth]
Apr 06 12:50:41 abastro-personal-arm sshd[103576]: Invalid user  from 75.12.134.50 port 36312
Apr 06 12:54:26 abastro-personal-arm sshd[103621]: Connection closed by 165.140.237.71 port 54236
Apr 06 13:01:26 abastro-personal-arm sshd[103702]: Connection closed by 193.32.162.132 port 33380
Apr 06 13:03:40 abastro-personal-arm sshd[103724]: Unable to negotiate with 218.92.0.204 port 60446: no matching key exchange method found. Their offer: diffie>
Apr 06 13:11:49 abastro-personal-arm sshd[103815]: Received disconnect from 165.140.237.71 port 50952:11:  [preauth]
Apr 06 13:11:49 abastro-personal-arm sshd[103815]: Disconnected from authenticating user root 165.140.237.71 port 50952 [preauth]
Apr 06 13:19:08 abastro-personal-arm sshd[103897]: Unable to negotiate with 218.92.0.208 port 59274: no matching key exchange method found. Their offer: diffie>
Apr 06 13:33:36 abastro-personal-arm sshd[104066]: Received disconnect from 165.140.237.71 port 50738:11:  [preauth]
Apr 06 13:33:36 abastro-personal-arm sshd[104066]: Disconnected from authenticating user ubuntu 165.140.237.71 port 50738 [preauth]
Apr 06 13:34:50 abastro-personal-arm sshd[104079]: Unable to negotiate with 218.92.0.204 port 44816: no matching key exchange method found. Their offer: diffie>
Apr 06 13:50:32 abastro-personal-arm sshd[104249]: Unable to negotiate with 218.92.0.206 port 27286: no matching key exchange method found. Their offer: diffie>
Apr 06 13:51:58 abastro-personal-arm sshd[104261]: Received disconnect from 165.140.237.71 port 50528:11:  [preauth]
Apr 06 13:51:58 abastro-personal-arm sshd[104261]: Disconnected from authenticating user root 165.140.237.71 port 50528 [preauth]
Apr 06 14:01:25 abastro-personal-arm sshd[104351]: Invalid user  from 65.49.1.29 port 18519
Apr 06 14:01:28 abastro-personal-arm sshd[104351]: Connection closed by invalid user  65.49.1.29 port 18519 [preauth]

As you can see, it is happening quite frequently, and I am worried one might break in at some point. Since SSH access guards users with root-access, it can be quite serious once penetrated. How do I harden against these kind of attacks? Because this is VPS, disabling SSH is a no-go (SSH is my only entry of access). Are there ways to stop some of these attackers?

As always, thanks in advance!

126 comments

in addition to what others have said, disable root login

Use knockd and iptables, gone are the fucking bots.

change port + fail2ban + totp (google-authenticator)

Use key based auth only and then run ssh-audit.

Change the port.

I generally do a few things to protect SSH:
Disable password login and use keys only
Install and configure Fail2Ban
Disable root login via ssh altogether. Just change "permit root login" from "no password" to just "no". You can still become root via sudo or su after you're connected, but that would trigger an additional password request. I always connect as a normal user and then use sudo if/when I need it. I don't include NOPASSWD in my sudoers to make certain sudo prompts for a password. Doesn't do any good to force normal user login if sudo doesn't require a password.
If connecting via the same network or IPs, restrict the SSH open port to only the IPs you trust.
I don't have SSH internet visible. I have my own Wireguard server running on a separate raspberry pi and use that to access SSH when I'm away, but SSH itself is not open to the internet or forwarded in the router.
- I vote for wireguard here, I don't expose anything other than game servers to the internet

In addition to what others say, I also have ntfy notifications on successful login.
- That's genius. I'll do the same from now on.
  
  There is actually an example on their website.
  ntfy ssh login alert

The best way is to disable password login and use SSH keys only. Any further steps are not required, but you may additionally install fail2ban or sshguard.

Does SSH have to be your only way? Could you deploy something like Tailscale? Can you restrict the allowed IP ranges on SSH with a firewall rule?

Configure the firewall with a IP whitelist to only allow connections to ssh be made from your home IP.
Other then that, disable password logon for ssh and setup up key based authentication.
- Agreed, but be careful about the whitelist. If your home IP changes, you'll be locked out until you update it, so you should consider an IP range if that's a possibility for you. Likewise, if you'll be accessing it from multiple locations (say, a family member's house), then make sure to add those as well.

Move the ssh port to higher ranges, 30-60000. That alone will stop 99% of the attacks
Disable root logins, now usernames must be guessed too which will make success even lower
Then require SSH keys
At that point it's like being in a nuclear fallout nshelter behind a 3 meter thick steel door and you can hear some zombies scratching on the outside... I'm not worried about any of that shit
- Regarding SSH Keys, I was wondering how you keep your key safe and potentially usable from another client?
  
  Generate a unique key for each client or device. SSH keys identify devices, not people, so I do not recommend sharing the same key between two different devices.
  
  Be sure to use a passphrase
  
  You could get a hardware key (like a Yubikey) and authenticate with PIV or GPG.
- This is what I do. Changing the port to a higher number will prevent almost all bots.
  I understand that obscurity is not security but not getting probed is nice.
  Also ssh keys are a must.
  I do log in as root though.
  However, I block all IPs other than mine from connecting to this port in my host's firewall. I only need to log in from home, or my office, and in a crisis I can just log in to OVH and add whitelist my IP.
  
  I do log in as root though.
  Don't do that. You're one local piece of malware away from getting your server pwned. Logging in as an unprivileged user at least requires another exploit on the server to get root permissions.
- For added funs run an SSH tarpit to fuck with the attackers, something like endlessh.
  
  Well yeah, sure, but that doesn't really add to your security and it only costs you work and resources
- Changing the port is a total waste of time
  Changing the port is just like putting a picture of a window on your door. Harden SSH properly and don't waste time with security via obscurity
  
  That's not true.
  Security through obscurity isn't real security, sure, but it does a lot to reduce the noise in the logs so you can see the more real attacks. Hardening SSH properly is certainly more important, but changing the port also has value.
  
  I think the point behind it is to waste the sniffers time sniffing for ports that it could be using to be making attempts.
  Its not a security thing, it's just increasing the cost to snoop.

Disable passwordless login.
Disable password login.
Require SSH keys
Move SSH port to non-standard port
Reject connections to port 22
Install and enable fail2ban
About the best you can do.
- Don't reject connections to port 22, honeypot it and ban on connection attempt.
  
  I'd get myself banned this way. I forget the -p flag at least once per week.
  
  honeypot
  That's a lot more work.
- Using a nonstandard port doesn’t get you much, especially popular nonstandard ports like 2222.
  I used that port once and just as much junk traffic and ultimately regretted bothering.
  
  My experience running several ssh servers on uncommon nonstandard ports for over 10 years has been that it has eliminated all ssh brute forcing. I don't even bother with fail2ban. I probably should though, just in case.
  Also, PSA: if you use fail2ban, don't try tab completing rsync commands without using controlmaster or you will lock yourself out.
  
  My two cents: Using a nonstandard ssh port is good for dumping bots. True, you can easily do a port scan against a server and easily find all open ports nbd. But most off-the-shelf bots are looking for standard ports to penetrate. I know that when I format and reinstall the test server, as soon as I change the ssh port, bot noise goes down significantly. So, for a simple config edit and about 2 minutes of time, it seems worth the effort. It's just one layer tho. And yes, it goes without saying to pick a port other than 22, 222, 2222, etc.
  
  It gets rid of most of the login attempts for me. I don't use a popular port though. Pick a 5 digit port so they have to put in some effort to find it.

There's a dedicated tool named sshguard which works nicely.

harden sshd
use fail2ban or even better ~~CrowdStrike~~ CrowdSec
use a tool like the following to have a next-gen security solution: https://github.com/mrash/fwknop
- harden sshd
  More details:
  require keys to login
  don't allow login as root
  That should be plenty, but you could go a bit further and restrict the types of algorithms allowed (e.g. disallow RSA if you're worried about quantum attacks). For this, I recommend a subtractive config (e.g. HostbasedAcceptedAlgorithms=-rsa-*). This is way over the top since an attacker is unlikely to attack the cipher directly, but it could be part of an attack.
- Your answer to "how to harden SSH?" is "harden SSH"?
  I know your two other points gave concrete suggestions, but it's pretty funny you suggested to "harden sshd" when that is what OP is asking how to do.
  
  Yeah, I see your point. No use to repeat the same you can read in other comments or in those 274772 guides online. I was trying to imply to just generally harden ssh because then brute-force attempts should be no issue, unless you log everything and the disk space gets maxed out :D
- did you mean crowdsec instead of crowdstrike?
  
  Fml… yes, I meant CrowdSec. Thanks for the hint
  
  Vietnam stare

We can’t ever stop this kind of stuff, but with something like fail2ban you can set it up to block on too many failures.
Really though - ensuring your system is kept up to date and uses strong passwords or use a SSH keys is the best defence. Blocking doesn’t prevent them from trying a few times. Moving SSH to a non standard port will stop most of the automated attacks but it won’t stop someone who is dedicated.
- Move SSH to non-standard port, make endlessh use the default port. Only use SSH keys. Only allow correct users (so eg. your user and git/forgejo). Use fail2ban to aggressively ban (redirect to default port, so 22) and report to abuseipdb everything that fails to authenticate first try (wrong user, password instead of key), has non-compatible ciphers (generally, only allow TLS1.3 etc.), or fails in any other way. Just be sure that if you accidentally get banned yourself (eg. Ctrl+C-ing during authentication), you can use another IP (eg. force v4) for connecting.
  
  Nice list of suggestions, but implementing all of them feels a little over-the-top.
  
  Also, add 2FA
- Thanks, I will try fail2ban. I am using ED25519 for ssh keys, it seems like it's the best defense on the ssh side. Do you happen to know why this kind of attack is so prevalent?
  
  I'm not them, but among other reasons they are looking to build botnets (cryptomining, dosing, mass crawling), and they are searching for hosts with low security (or if you just made a mistake)

OP, here is what I do. It might seem overboard, and my way doesn't make it the best, or the most right, but it seems to work for me:
Fail2ban
UFW
Reverse Proxy
IPtraf (monitor)
Lynis (Audit)
OpenVas (Audit)
Nessus (Audit)
Non standard SSH port
CrowdSec + Appsec
No root logins
SSH keys
Tailscale
RKHunter
The auditing packages, like Lynis, will scour your server, and make suggestions as to how to further harden your server. Crowdsec is very handy in that it covers a lot of 'stuff'. It's not the only WAF around. There is Wazuh, Bunkerweb, etc. Lots of other great comments here with great suggestions. I tend to go overboard on security because I do not like mopping up the mess after a breach.
ETA: just looked up one of your attackers:
218.92.0.201 was found in our database! This IP was reported 64,044 times. Confidence of Abuse is 100%: ISP CHINANET jiangsu province network Usage Type Fixed Line ISP ASN AS4134 Domain Name chinatelecom.cn Country China City Shanghai, Shanghai
busy little cunts.
- It's absolutely overboard, and you can get 99% of the way there with this:
  WireGuard config (Tailscale in your case)
  Bind SSH to WireGuard IP only (so no public SSH port)
  SSH keys only, and disable root login over SSH
  That will require breaking WireGuard and openSSH's key-based authentication, which just isn't happening. The rest looks like mostly auditing. Even a firewall isn't necessary if no ports are accessible anyway (i.e. everything only accessible over Tailscale), and you can just configure iptables to block everything on the WAN IP and call it a day.
  
  sugar_in_your_tea @sh.itjust.works
  It's nice to be commented by someone famous.
  Open up the window, let some air into this room I think I'm almost chokin' from the smell of stale perfume And that cigarette you're smokin' 'bout scare me half to death Open up the window, sucker, let me catch my breath
- No Port-knocking? Amateurs! /s

In addition to other advice you could also use SSH over Wireguard. Wireguard basically makes the open port invisible. If you don't provide the proper key upfront you get no response. To an attacker the port might as well be closed.
Here's at least one article on the subject: https://rair.dev/wireguard-ssh/
- Exactly. No root login and no password login will do just fine as basic measures, but after that Wireguard is perfect tool for this, no weird rituals required and also quite useful for any other services you don't want and/or need to expose to the internet as well.
  
  Just remember that you'll only be able to SSH in w/ a device that's already configured for WireGuard. So if you're at a friend's house and haven't set up your phone to do it yet, you'll be forced to use the VPS console to get in. Make sure this is what you want before you do it.

https://media.ccc.de/v/38c3-attack-mining-how-to-use-distributed-sensors-to-identify-and-take-down-adversaries

For security disable password authentication - use public key instead, disable root login via ssh - use sudo or su from another user.
To reduce the number of attempts of others trying to get in change the ssh port and/or set-up fail2ban.
You could also set a firewall rule to only allow ssh from your IP address, if you have a static address at home and only need access from there, or have a way to VPN into your home network. Make sure you have a static address if you do this though, you don't want your IP to change and be left locked out of your server.
- You could also set a firewall rule to only allow ssh from your IP address
  You can also broaden this to a region. You may still want to access SSH from various places around your country (e.g. when visiting family or friends), but likely won't ever need to from most of the rest of the world, so block everything except IPs from your region (or regions you care about, e.g. any VPSs you have).
  
  No if you're doing that, use a VPN through your firewall. Local traffic is a fair exception as this can only ever be a device on your network, but that depends on your threat model (as those local devices could be compromised). Opening to "your region's" IP range opens you to a lot more than LAN access..

Disable passwords and use public private keys and don't worry about it
- This is the only answer you need to read. It's a non-problem if you just do this, and there's no reason not to do it.
  
  Agreed!

You don't. This is normal. Ensure key-only auth, ensure you do not login directly as root, maybe install fail2ban and you're good. Some people move the port to a nonstandard one, but that only helps with automated scanners not determined attackers.
You could look into port-knocking if you want it really safe.
- Some people move the port to a nonstandard one, but that only helps with automated scanners not determined attackers.
  While true, cleaning up your logs such that you can actually see a determined attacker rather than it just getting buried in the noise is still worthwhile.

You can look up for:
Setting up max authentication attemps per connection -> slows up a lot brute force attacks. If your password is strong enough, that's already a big step to secure your server.
Generate SSH Keys and disable password authentication -> do this only if you're connecting through the same devices, because you won't be able to connect from any device that has not being set up. Personally I don't use this because I want to be able to access my server even if I'm not home and without my laptop
Set up Crowdsec -> it's a local service which scans logs and will block access to any suspicious IPs. It also relies on a crowdsourced list of IPs that are identified as threat and will preventively block them
- Is this an alternative/replacement to fail2ban or something you would use along with f2b?
  
  You could technically still use it alongside f2b, but in my experience Crowd-Sec seems to do a better job and can do the same things.

For a general guide on how to make ssh more secure I stick to https://www.sshaudit.com/
You can check your config and they also provide step by step guides for several distros...

A few replies here give the correct advice. Others are just way off.
To those of you who wrote anything else than "disable passwords, use key based login only and you're good" - please spend more time learning the subject before offering up advice to others.
(fail2ban is nice to run in addition, I do so myself, but it's more for to stop wasting resources than having to do with security since no one is bruteforcing keys)
- Eh, while I agree that some recommendations are dodgy at best, I'll argue that Wireguard is not only adding to security, it also makes Fail2Ban obsolete. Due to the way it works, you'll completely hide the fact that you're even running a SSH server at all, and this includes even Wireguard itself. More importantly though, it's pretty much impossible to set up Wireguard in an insecure way, whereas SSH provides you with plenty of footguns. You're not risking locking yourself out either.
  Also, security comes in layers.
  
  You’re not risking locking yourself out either.
  In a VPS, you should always be able to fall back to the web console. So locking yourself out shouldn't be a major concern.
- There's more to it than that.
  I recommend geoip blocking anything outside of your expected operating regions in addition to using key-based logins. iptables operates at a lower level in the network stack than SSH, so the vulnerability surface is a lot lower, and blocking before something actually looks at the packets cleans up the logs. This is huge because it makes it a lot more obvious when there's a legitimate attack.
  Cover yourself with layers:
  block obviously bad packets at the firewall level
  eliminate insecure modes of login (only allow key-based login)
  something like fail2ban to ban the few who make it through 1 & 2
  use a secure root password so if someone does get in, they're less likely to get root access
  have your services run as non-privileged users to limit issues if something gets compromised
  If you only do one thing, it should be only allowing key-based logins. If you do two, run SSH on a non-standard port or set up geoip blocking (second is more work, but a lot more effective).
  
  Still no. Here's the reasoning: A well known SSHd is the most secure codebase you'll find out there. With key-based login only, it's not possible to brute force entry. Thus, changing port or running fail2ban doesn't add anything to the security of your system, it just gets rid of bot login log entries and some - very minimal - resource usage.
  If there's a public SSHd exploit out, attackers will portscan and and find your SSHd anyway. If there's a 0-day out it's the same.
  (your points 4 and 5 are outside the scope of the SSH discussion)

What VPS are you using?
You should be able to setup a firewall, blocking all access to the SSH port. Then setup a VPN so that only you can access via SSH after making your VPN connection.
If you connect via a static IP, you can also create an ACL for the VPN connection just in case. You can set an ACL for the SSH port forward rule directly as well, but I don’t like that personally. I prefer keeping things behind the VPN.
- Exactly, this I what alI do!
- This is the correct answer. Never expose your SSH port on the public web, always use a VPN. Tailscale, Netmaker or Netbird make it piss easy to connect to your VPS securely, and because they all use NAT traversal you don't have to open any ports in your firewall.
  Combine this with configuring UFW on the server (in addition to the firewall from the VPS provider - layered defence is king) and Fail2Ban. SSH keys are also a good idea. And of course disable root SSH just in case.
  With a multi-layered defence like this you will be functionally impervious to brute force attacks. And while each layer of protection may have an undiscovered exploit, it will be unlikely that there are exploits to bypass every layer simultaneously (Note for the pendants; I said "unlikely", not "impossible". No defence is perfect).
  
  This is not "the correct answer". There's absolutely nothing wrong with "exposing" SSH.

Like others said, disable password auth and setup auth keys instead.
Bonus points for moving the ssh port, using fail2ban and also setting up a tarpit with something like endlessh.
If you wanna go extreme use Wireguard to connect to your server and only allow ssh over wireguard in your firewall.

fail2ban is mandatory equipment for any ssh server accessible to the public especially on its default port. It's highly configurable, but the default settings will do fine at making it statistically impossible for any user or password to be brute forced.
- I don’t really get the love for fail2ban. Sure, it helps keep your logs clean, but with a solid SSH setup (root disabled, SSH keys enforced), I’m not bothered by the login attempts.
  
  You should be. Most of it's noise, but if there's a serious attack, you'll appreciate clean logs.
  I think fail2ban is nice as like a third or fourth layer of defense. In order of my priorities:
  key-only login, root login completely disabled
  solid root password, and user privilege separation (have each service use its own user)
  geoip bans - if you never plan to support clients in a given region, block it at the firewall level (or better yet, whitelist the handful of regions you care about); I do this by port, so SSH gets a much more restricted set of allowed regions than HTTPS
  fail2ban - especially if you have a relatively large whitelist
  only access SSH over a Wireguard VPN - Wireguard doesn't show up in port scans, and SSH can bind to the VPN host instead of 0.0.0.0, so the ability to login via SSH will be completely hidden
  If you're not going to do 3-5, at least change the default SSH port to cut down on log noise.

https://www.crowdsec.net/
Take the concept of Fail2Ban and add in a community blocklist of thousands of IPs so that you’re blocking not only IPs that have attacked you, but others as well.
It’s neat because they have a number of collections you can download from the community that include readymade parsers for other kinds of logs, and other attack scenarios you can guard against. For example, if you run Nginx or Caddy as webservers on that machine, you can download associated collections for each that can parse your web access log files and ban IPs based on IPs probing your web server for unprotected admin panels, or abusive AI crawlers.
You can even write your own scenarios. I wrote one that immediately blocks you after just one attempt to log in using an account like root, admin,adm,administrator, etc.
- +1 for Crowdsec

If you can use another method, disabling SSH entirely would do it. ;)
This is how Talos Linux achieves best-in-class security properties.
https://www.siderolabs.com/blog/how-to-ssh-into-talos-linux/

If you use keys or strong passwords, it really shouldn't be practical for someone to brute-force.
You can make it more-obnoxious via all sorts of security-through-obscurity routes like portknocking or fail2ban or whatever, or disable direct root login via PermitRootLogin, but those aren't very effective compared to just using strong credentials.

One of the simplest is geoip blocks. Here's an article using iptables, and there may be a nicer way w/ whatever firewall you're using.
For reference, here are the areas I see in your logs (using this service):
218.92.0.201 - China
162.142.125.122 - US (Michigan)
45.79.181.223 - US (New Jersey)
118.25.174.89 - China
92.118.39.73 - Romania
98.22.89.155 - US (Nebraska)
75.12.134.50 - US (Tennessee)
165.140.237.71 - US (Washington)
65.49.1.29 - US (California)
If you don't expect valid users to come from those areas, block them. A lot of those in the US are probably from VPN users, so be careful if people are using a VPN to connect to your services.
If you can do it w/ iptables, it'll be a lot more efficient than doing it at the application layer. I also recommend using something like fail2ban to block individual IPs within regions you care about to get any stragglers that make it through the first tier of blocks. Since this is a VPS, you can also check what firewall settings your provider has and see if you can configure it there so it doesn't make it to your instance in the first place.
- Thanks a lot! Geoblocking makes a lot of sense, will try!
  
  I highly recommend using key-based SSH authentication exclusively for all users on your server, and disallow root login as well.
  Geoblocking mostly cuts down on the spam, but also constrains where an actual attack can come from. If there's some kind of zero-day attack on SSH, this will dramatically reduce the risk you're hit.

I assume you have root login denied in your ssh config, other things would be having fail2ban and some geofencing (blocking IPs from countries you know you are never going to log in from).

If you can, employ tailscale ssh and prevent ssh from outside tailscale network. Tailscale have a good guide for this

I use fail2ban and a non-standard SSH port. 99% of this junk disappears if you run sshd on port 9.
Also, disable password for login - Only use keys.

~~Bad~~ eh security advice: use an alternative ssh port. Lots of actors try port 22 and other common alternatives. Much fewer will do a full port scan looking for an ssh server then try brute forcing.

I think VPN is the proper way to go about this, but another method is to do port knocking with fkwnop so your SSH port won't respond until the host receives a magic packet.

Change the default port and 99% of the bots will be gone

Do you want to prevent brute forcing or do you want to prevent the attack getting in?
If you want to prevent brute forcing then software like fail2ban helps a little, but this is only a IP based block, so with IPv6 this is not really helpfull against a real attack, since rotating IP addresses is trivial. But still can slow down the attacker. Also limiting the amount of sessions and auth tries does significantly slow down the attacker.
If you just want to not worry about it set strong passwords, and when it is a multi user system where other ppl might access it, configure Public Key Auth so you can be sure the other users have strong passwords (or keys in this case) to authenticate.
With strong passwords or keys it is basically impossible to brute force your way in with ssh.

Welcome to the internet! Your system will get probed. Make sure you run as little as possible services on open ports and only high quality ones such as OpenSSH. Don't freak out because of your logs. You're fine as long as your system is up to date and password login disabled! Don't listen to the fail2ban or VPN crowd. Those are only snake oil.
A VPN is probably just as (in)secure as OpenSSH. There is no gain in complicating things. OpenSSH is probably one of the most well tested code for security around.

Setup fail2ban

Does it need to be exposed to the internet? Putting it behind a vpn would be best.
Besides that, just make sure only the users you need to have access to ssh logins, and use keys for extra hardening. Keep your system updated. Limit that system’s access to other systems on your network, so if it is compromised, they can’t use it as a pivot point for the rest of your setup.
The other commenter’s suggestion of fail2ban is also solid.

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