Question

SCP from Windows destroys SSH connections

Posted on November 27, 2023
DigitalOcean Droplets Ubuntu
Asked by d1b7968b962046a0b02da4e5573c8e

Hi, I’m trying to write a publish script that copies a bunch of files over from a Windows machine to a Linux droplet (Ubuntu). I can successfully SSH into the remote Linux machine and can also run small (single file) scp commands to copy files over.

When I run a recursive copy scp.exe -r [source dir] [remote dir] of a few megabytes, all SSH connections into my droplet stall. Eventually the original scp command times out with the error message:

ssh: connect to host IPADDRESS port 22: Connection timed out

I’m unable to reconnect to the droplet over ssh for about 5 minutes.

Some things I’ve noticed and tried:

Websites running on the droplet continue to be accessible during the freeze, so the droplet is still alive,.
I’ve tried changing the MTU on the remote droplet and it didn’t make a discernible difference.
I tried to look at the ssh logs on the remote server and noticed that the scp commands create connections to the server, but never close them. When I issue an ssh command, these connections are closed after use.
I have adjusted the MaxSessions and MaxStartups number in the /etc/ssh../sshd_config file (and restarted using sudo systemctl restart ssh.service)

Anyone know anything else i can try?

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Bobby Iliev

Site Moderator

• November 27, 2023

Accepted Answer

Hi there,

There are a few things that I could suggest here to try out:

Keep an Eye on Server Resources: Before initiating the transfer, use htop to monitor your Droplet’s performance in real-time. If you see CPU or memory usage spike during the transfer, consider upgrading your Droplet to have some extra buffer.
```
htop
```
This little tool gives you a dynamic overview of your Droplet’s health. If you don’t have it installed, you can get it with:
```
sudo apt install htop
```
Verbose SCP for Transparency: Use the -v flag with your SCP command to get a verbose output. This will give you a play-by-play of what SCP is doing behind the scenes.
```
scp -v -r /path/to/local/directory username@your_droplet_ip:/path/to/remote/directory
```
The verbose output can be quite extensive, but look for lines that begin with “debug1” or “debug2” — they often contain the clues you need.
Optimize SCP with Rsync: Rsync is a more robust tool for file transfers, especially for larger datasets. It only transfers what’s changed and can resume interrupted transfers without starting over.
```
rsync -avz --progress /path/to/local/directory username@your_droplet_ip:/path/to/remote/directory
```
The -a option preserves permissions and times, -v gives you verbose output, -z compresses file data during the transfer, and --progress shows you the transfer progress.

Let me know how it goes!

Best,

Bobby

alexdo

Site Moderator

• December 7, 2023

Heya, @d1b7968b962046a0b02da4e5573c8e

I would also recommend using rsync instead of scp. Bobby already mentioned why this would be beneficial for your setup.

On top of what’s already mentioned, you can add the following lines to your SSH client configuration file (usually ~/.ssh/config on the Windows machine) to send keep-alive packets, preventing the connection from timing out.

`Host *   ServerAliveInterval 60`

Adjust the ServerAliveInterval value as needed.

Regards

KFSys

Site Moderator

• November 27, 2023

Heya @d1b7968b962046a0b02da4e5573c8e,

It sounds to me like your Droplet just can’t handle the command as there might be a lot of files there.

Apart from what already is suggested, you can confirm this by being in the Droplet when you run the command and seeing if it’s responsive. I’m sure it won’t be.

To resolve the issue shortterm, you might want to add some SWAP:

https://www.digitalocean.com/community/tutorial-collections/how-to-add-swap-space

wap is a portion of hard drive storage that has been set aside for the operating system to temporarily store data that it can no longer hold in RAM. This lets you increase the amount of information that your server can keep in its working memory, with some caveats. The swap space on the hard drive will be used mainly when there is no longer sufficient space in RAM to hold in-use application data.

The information written to disk will be significantly slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a fallback for when your system’s RAM is depleted can be a good safety net against out-of-memory exceptions on systems with non-SSD storage available.