Score:0

Server

SSH login timeout before authentication

fvlasie

8/11/24, 5:31 AM

I have an OCI instance running Rocky Linux 9.2. It is running a production website but for some reason I have suddenly stopped being able to log in with SSH.

I have tested the connection to port 22 by using another OCI instance on the same subnet and security list.

I can confirm by checking /etc/secure.log via the web console that connections are reaching the server on port 22. There are messages of rejected root logins and I see an error of: "fatal: Timeout before authentication" that appears when I try to log in with an SSH client.

I know my password is correct (tested and working recently) and I have tested with valid ssh keys too (again recently working). I have tested from different locations around the globe just to be sure and all fail with the same message.

The SSH client gives me a message of: Operation timed out.

So I have ruled out firewall, security lists, sshd server running, and client error.

I have searched and searched the internet for the last couple of days but I could not find anyone with exactly these same symptoms. One of the closest things I found was a reference to changing the system limits that caused the loss of SSH for someone: Increase FD limitation then SSH not working?

I had been editing my /etc/sysctl.config but the problem persists even after undoing the few changes I made (and rebooting).

I am stumped and would appreciate any pointers to what I might try next.

Thank you!

145

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ssh

rocky-linux

AlexD

8/11/24, 8:58 AM

Use `ssh -v` on the client side to check negotiation and `tcpdump` to check for packet loss.

kab00m

8/11/24, 11:02 AM

Check MTU settings. Such a behavior often happen if starting packets goes through but first 1500-byte one does not. Change MTU on your client computer to 1280 (very safe for any network) and try again.

fvlasie

8/12/24, 2:51 AM

Thanks @AlexD! `ssh -vvv` does not reveal anything different from a working SSH connection other than not displaying any messages after `debug1: Connecting to example.com port 22.` until we see `ssh: connect to host example.com port 22: Operation timed out`

fvlasie

8/12/24, 2:52 AM

Thanks @kab00m! I tried a smaller packet size as suggested but the connection still fails.

fvlasie

8/12/24, 6:09 PM

`tcpdump ` show no dropped packets during a connection attempt...

kab00m

8/14/24, 10:34 AM

tcpdump will not show big packets if they were dropped somewhere on network. Maybe somehow you can set MTU 1280 on remote server and check again.

Score:0

Server

fvlasie

8/15/24, 4:54 PM

It turns out Cloudflare does not proxy port 22 which means that traffic gets dropped if going through Cloudflare. The solution turned out to be:

Use the IP address directly.
Create a subdomain in Cloudflare that is not proxied and connect through that.

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