this post was submitted on 06 Apr 2025
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140
How to harden against SSH brute-forcing? (sh.itjust.works)
submitted 3 weeks ago by [email protected] to c/[email protected]
134 comments fedilink hide all child comments
 

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!

(page 2) 50 comments
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[–] [email protected] 18 points 3 weeks ago (1 children)

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.

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[–] [email protected] 16 points 3 weeks ago

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

[–] [email protected] 20 points 3 weeks ago* (last edited 3 weeks ago) (4 children)
[–] [email protected] 13 points 3 weeks ago (2 children)

did you mean crowdsec instead of crowdstrike?

[–] [email protected] 1 points 3 weeks ago

Vietnam stare

[–] [email protected] 4 points 3 weeks ago

Fml… yes, I meant CrowdSec. Thanks for the hint

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[–] [email protected] 42 points 3 weeks ago (11 children)

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

[–] [email protected] 5 points 3 weeks ago (1 children)

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.

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[–] [email protected] 13 points 3 weeks ago (1 children)

For added funs run an SSH tarpit to fuck with the attackers, something like endlessh.

[–] [email protected] 9 points 3 weeks ago (1 children)

Well yeah, sure, but that doesn't really add to your security and it only costs you work and resources

[–] [email protected] 5 points 3 weeks ago

100% agree, that is a "totally for fun" exercise

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[–] [email protected] 21 points 3 weeks ago* (last edited 3 weeks ago) (4 children)

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.

[–] [email protected] 6 points 3 weeks ago

No Port-knocking? Amateurs! /s

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[–] [email protected] 58 points 3 weeks ago (2 children)
  1. Disable passwordless login.
  2. Disable password login.
  3. Require SSH keys
  4. Move SSH port to non-standard port
  5. Reject connections to port 22
  6. Install and enable fail2ban

About the best you can do.

[–] [email protected] 11 points 3 weeks ago (2 children)

Don't reject connections to port 22, honeypot it and ban on connection attempt.

[–] [email protected] 3 points 3 weeks ago (3 children)

honeypot

That's a lot more work.

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[–] [email protected] 7 points 3 weeks ago (8 children)

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.

[–] [email protected] 5 points 3 weeks ago (2 children)

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.

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[–] [email protected] 19 points 3 weeks ago

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.

[–] [email protected] 4 points 3 weeks ago

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.

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[–] [email protected] -4 points 3 weeks ago

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.

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