Databases, Systems & Networks

Reverse SSH is a technique that can be used to access systems (that are behind a firewall) from the outside world.

As you already know SSH is a network protocol that supports cryptographic communication between network nodes. Using this protocol, you can do a secure remote login, secure copy from/to a remote machine etc.

You’ll typically do the following to connect to a remote server securely using ssh command.

$ ssh [your-account-login]@[server-ip]

What is Reverse SSH?

SSH is very good tool to access remote machine or server securely. But, the problem arises when you try to connect to a remote server which is behind a firewall and this firewall denies any incoming connection or data transfer request that has no prior outgoing request. This means that only those connections would be allowed which are initiated by the remote server machine. This is a real problem for those who want to access this server machine remotely. Lire la suite…

Source: Comment ça marche

Les commandes suivantes nécessitent d’avoir un compte sur la machine sur laquelle on veut se connecter et qu’un serveur SSH y soit installé.

Sous Linux, la syntaxe est simple (le client étant intégré dans la plupart des distributions) :

Dans un terminal (n’importe lequel), tapez :

ssh login:motDePasse@MachineSurLaquelleJeveuxmeconnecter
ou
login@(machine sur laquelle je veux me connecter )

Dans ce cas-là, si la machine accepte la connexion, le mot de passe vous sera demandé juste après.
Pour se logguer en mode graphique (utiliser un serveur X), il faut utiliser l’option -X.

Utilisation de SSH à travers un proxy

Installez tout d’abord le paquet proxy-connect :

sudo aptitude install proxy-connect

Modifiez le fichier /etc/ssh/ssh_config pour permettre l’utilisation de SSH en passant par le proxy :

sudo echo 'ProxyCommand /usr/bin/connect-proxy -4 -S monproxy.domaine.com:port %h %p' >> /etc/ssh/ssh_config

Veillez à bien remplacer “monproxy.domaine.com” et “port” par le nom de votre proxy et son numéro de port.

Remote Linux Commands

You would like to execute a command on a remote Linux/FreeBSD/Solaris/UNIX host and have the result displayed locally. Once result obtained it can be used by local script or program. A few examples:
=> File system and disk information

=> Get user information

=> Find out all running process

=> Find out if particular service is running or not etc

You can use rsh or ssh for this purpose. However, for security reason you should always use the ssh and NOT rsh. Please note that remote system must run the OpenSSH server.

Syntax for running command on a remote host:
ssh [USER-NAME]@[REMOTE-HOST] [command or script]

Where,

• ssh: ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine.
• USER-NAME: Remote host user name.
• REMOTE-HOST: Remote host ip-address or host name, such as my-site.com.
• command or script: Command or shell script is executed on the remote host instead of a login shell.

Examples

(A) Get disk information from a server called my-site.com:
$ ssh user@my-site.com df -h

(B) List what ports are open on remote host
$ ssh user@my-site.com netstat -vatn

(C) Reboot remote host:
$ ssh root@my-site.com reboot

(D) Restart mysql server (please note enclosed multiple command line arguments using a single or double quotes)
$ ssh root@my-site.com '/etc/init.d/mysql restart'

(E) Get memory information and store result/output to local file /tmp/memory.status:
$ ssh user@my-site.com 'free -m' > /tmp/memory.status

(G) You can also run multiple command or use the pipes, following command displays memory in format of “available memory = used free memory” :
$ ssh user@debian.test.com free -m | grep "Mem:" | awk '{ print "Total memory (used free): " $3 " " $4 " = " $2 }'

See how to configure ssh for password less login using public key based authentication.

=> Related: shell script to get uptime, disk usage, cpu usage, RAM usage, system load, etc. from multiple Linux servers and output the information on a single server in a html format.

Source: NixCraft

Monitor your Linux system

As a Linux sysadmin it feels great power when monitoring system resources like cpu, memory on the commandline. To peek inside the system is a good habit here atleast, because that’s one way of driving your Linux system safe. Plenty of tools like Htop, Nmon, Collectl, top and iotop etc help you accomplish the task. Today lets try another tool called Glances.

Glances

Glances is a tool similar to Nmon that has a very compact display to provide a complete overview of different system resources on just a single screen area. It does not support any complex functionality but just gives a brief overview CPU, Load, Memory, Network rate, Disk IO, file system, process number and details.

As a bonus, glances is actually cross platform, which means you can use it on obsolete OSes like windows :P.

Here’s a quick glimpse of it.



The output is color highlighted. Green indicates optimum levels of usage whereas red indicates that the particular resource is under heavy use.

$ glances -v
Glances version 1.6 with PsUtil 0.6.1

Project homepage https://github.com/nicolargo/glances http://nicolargo.github.io/glances/

Lire la suite…

Source: nixCraft

How can I mount remote directory with ssh on a Linux bases system? How do I use SSHFS to mount remote file systems over SSH on a Ubuntu or Debian/RHEL/CentOS/Arch Linux system?

SSH is a secure protocol and you can use it to mount a directory on a remote server or local laptop with the help of the SSHF service. With SSHFS you can mount remote server file system to your local

More on SSHFS

sshfs is a filesystem based on the SSH file transfer protocol. It is used on a client system i.e. you need to install sshfs package on your local computer/laptop powered by CentOS/RHEL/Ubuntu/Debian/Arch Linux. No need to install anything on server (server1.cyberciti.biz). You only need an openssh server installed on server side. Our sample setup:

Lire la suite…