# Tricks around SSH

12 January 2016

ssh sure is a powerful tool for securely transferring files over a network.
For my own reference, I referenced two tricks than happen to be useful on a daily basis.

# Transfer numerous files over SSH

If you transfer a big number of files over ssh, one connection is opened per file. Issues may arise as your connection may be interrupted by the server policy.

You can use tar to make a single file of a directory, compress it if need be, and send it through ssh before piping it to a file or to the converse tar command to decompress it. Creating a file and then sending it may be an option which can be tedious if your disk quota on the server is limited.

When you send files to a server:

tar cvzf - dir_to_copy/ | ssh user@host "cat > /path/to/archive.tar.gz"
tar cvzf - dir_to_copy/ | ssh user@host "tar xvzf - -C /path/to/dir"


When you send files from a server:

ssh user@host "tar cvzf - dir_to_copy/" | cat > /path/to/archive.tar.gz
ssh user@host "tar cvzf - dir_to_copy/" | tar xvzf - -C /path/to/dir


Note the -C option to tar that would decompress the file to the proper directory.
As for the single -, it causes tar to read/write from stdin/stdout.

# Connection through a proxy/gateway and .ssh/config

When I connect to my account on some university I am related with, I need to create a tty terminal on a gateway before connecting another server (say student01) where I have a proper account. Only the gateway is visible from outside.

The recommended way to connect is:

ssh -t gateway.university.edu student01


This is quite inconvenient, esp. when I want to transfer files with scp. The following configuration can be put in a .ssh/config if you happen to need to deal with such configuration:

Host gateway
Hostname gateway.university.edu
User olive
#ForwardX11 yes

Host university
Hostname student01
User olive
ProxyCommand ssh gateway nc %h %p
#ForwardX11 yes


scp and also ssh-copy-id work properly now, and I only have one password left to type (on gateway) when I connect there.