Efficiently Managing Multiple Linux Hosts: SSH, File Transfers, and Automation

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Managing operations across multiple Linux hosts, especially when some are inaccessible from the public internet, can be challenging. This requires efficient SSH workflows, file transfers, and automation techniques to simplify the work while ensuring security and reliability. Whether you're working with remote servers, embedded devices, or air-gapped systems, this guide will help streamline your workflow.

1. Establishing Efficient SSH Workflows

Using SSH Agent for Seamless Authentication

Typing passwords or passphrases repeatedly can slow down operations. Use the SSH agent to cache credentials securely:

eval $(ssh-agent)
ssh-add ~/.ssh/id_rsa  # Add your private key

This allows seamless authentication across multiple hops.

SSH Config for Quick Access

Instead of remembering long SSH commands, configure ~/.ssh/config:

Host server1
    HostName 192.168.1.100
    User myuser
    IdentityFile ~/.ssh/id_rsa
    ProxyJump jumphost

Now, you can simply connect using:

ssh server1

The ProxyJump directive simplifies multi-hop SSH connections.

Running Commands on Remote Hosts

Run a single command remotely:

ssh server1 "df -h"

Or execute a script:

ssh server1 'bash -s' < local_script.sh

For multiple servers:

for host in server1 server2 device1; do ssh $host "uptime"; done

2. Transferring Files Between Hosts

SCP: Simple and Direct File Transfers

Copy files to a remote host:

scp file.txt server1:/home/myuser/

Copy files from a remote host:

scp server1:/home/myuser/file.txt ./

Copy directories recursively:

scp -r myfolder server1:/home/myuser/

Using Rsync for Large or Incremental Transfers

Rsync is more efficient than SCP for large or recurring transfers:

rsync -avz myfolder/ server1:/home/myuser/myfolder/

For transferring files between two remote hosts via an intermediary:

rsync -avz -e "ssh -J jumphost" server1:/data/ server2:/backup/

3. Working with Air-Gapped or Restricted Hosts

SSH Tunneling for Indirect Access

If a target machine is not directly reachable, use an SSH tunnel:

ssh -L 8080:target:80 jumphost

Now, accessing localhost:8080 will route traffic to target:80 via jumphost.

For persistent tunnels:

autossh -M 0 -f -N -L 8080:target:80 jumphost

Transferring Files When SCP or Rsync Won’t Work

For devices without SCP/SFTP:

ssh user@jumphost "cat /remote/file" > local_copy

Using tar for multi-file transfers:

ssh server1 "tar czf - /important/data" | ssh server2 "tar xzf - -C /backup/"

4. Automating Repetitive Operations

Using Ansible for Remote Automation

Ansible allows running commands on multiple hosts with minimal setup:

ansible all -i inventory -m shell -a "uptime"

A simple inventory file (inventory):

[servers]
server1
server2
[devices]
device1

To install software across all hosts:

ansible all -i inventory -m apt -a "name=htop state=present" --become

SSH Multiplexing for Faster Connections

If you're running multiple SSH commands in quick succession, enable connection reuse:

Host *
    ControlMaster auto
    ControlPath ~/.ssh/sockets/%r@%h-%p
    ControlPersist 10m

This significantly speeds up repeated SSH operations.

5. Debugging and Troubleshooting

Checking SSH Connectivity

If you have trouble connecting, use:

ssh -v server1

For even more details:

ssh -vvv server1

Finding Network Bottlenecks

Check latency:

ping server1

Measure network speed:

iperf3 -c server1

Recovering Lost Connections

If an SSH session disconnects, use:

screen -r

Or with tmux:

tmux attach-session -t mysession

By implementing these techniques, you can simplify remote system operations, automate tedious tasks, and efficiently manage Linux hosts across complex network environments. Whether handling a fleet of servers or remotely managing devices, these strategies will save time and effort while improving security and reliability.

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