Eleven SSH Tricks
Ports are the numbers representing different services on a server; such as port 80 for HTTP and port 110 for POP3. You can find the list of standard port numbers and their services in /etc/services. SSH can translate transparently all traffic from an arbitrary port on your computer to a remote server running SSH. The traffic then can be forwarded by SSH to an arbitrary port on another server. Why would you want to do this? Two reasons: encryption and tunneled connections.
Many applications use protocols where passwords and data are sent as clear text. These protocols include POP3, IMAP, SMTP and NNTP. SSH can encrypt these connections transparently. Say your e-mail program normally connects to the POP3 port (110) on mail.example.net. Also, say you can't SSH directly to mail.example.net, but you have a shell login at shell.example.net. You can instruct SSH to encrypt traffic from port 9110 (chosen arbitrarily) on your local computer and send it to port 110 on mail.example.net, using the SSH server at shell.example.net:
ssh -L 9110:mail.example.net:110 shell.example.net
That is, send local port 9110 to mail.example.net port 110, over an SSH connection to shell.example.net.
Then, simply tell your e-mail program to connect to port 9110 on localhost. From there, data is encrypted, transmitted to shell.example.net over the SSH port, then decrypted and passed to mail.example.net over port 110. As a neat side effect, as far as the POP3 dæmon on mail.example.net knows, it is accepting traffic from shell.example.net.
SSH can act as a bridge through a firewall whether the firewall is protecting your computer, a remote server or both. All you need is an SSH server exposed to the other side of the firewall. For example, many DSL and cable-modem companies forbid sending e-mail from your own machine over port 25 (SMTP).
Our next example is sending mail to your company's SMTP server through your cable-modem connection. In this example, we use a shell account on the SMTP server, which is named mail.example.net. The SSH command is:
ssh -L 9025:mail.example.net:25 mail.example.net
Then, tell your mail transport agent to connect to port 9025 on localhost to send mail. This exercise should look quite similar to the last example; we are tunneling from local port 9025 to mail.example.net port 25 over mail.example.net. As far as the firewall sees, it is passing normal SSH data on the normal SSH port, 22, between you and mail.example.net.
A final example is connecting through an ISP firewall to a mail or news server inside a restricted network. What would this look like? In fact, it would be the same as the first example; mail.example.net can be walled away inside the network, inaccessible to the outside world. All you need is an SSH connection to a server that can see it, such as shell.example.net. Is that neat or what?
If a port is reassigned on a computer (the local port in the examples above), every user of that computer sees the reassigned port. If the local system has multiple users, tunnel only from unused, high-numbered ports to avoid confusion. If you want to forward a privileged local port (lower than 1024), you need to do so as root. Forwarding a lower-numbered port might be useful if a program won't let you change its port, such as standard BSD FTP.
By default, a tunneled local port is accessible only to local users and not by remote connection. However, any user can make the tunneled port available remotely by using the -g option. Again, you can do this to privileged ports only if you are root.
Any user who can log in with SSH can expose any port inside a private network to the outside world using port forwarding. As an administrator, if you allow incoming SSH connections, you're really allowing incoming connections of any kind. You can configure the OpenSSH dæmon to refuse port forwarding with AllowTcpForwarding no, but a determined user can forward anyway.
A config file option is available to forward ports; it is called LocalForward. The first port-forwarding example given above could be written as:
Host forwardpop Hostname shell.example.com LocalForward 9110 mail.example.com:110
Practical Task Scheduling Deployment
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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|The Firebird Project's Firebird Relational Database||Jul 29, 2016|
|Stunnel Security for Oracle||Jul 28, 2016|
|SUSE LLC's SUSE Manager||Jul 21, 2016|
|My +1 Sword of Productivity||Jul 20, 2016|
|Non-Linux FOSS: Caffeine!||Jul 19, 2016|
|Murat Yener and Onur Dundar's Expert Android Studio (Wrox)||Jul 18, 2016|
- The Firebird Project's Firebird Relational Database
- Stunnel Security for Oracle
- My +1 Sword of Productivity
- Managing Linux Using Puppet
- Non-Linux FOSS: Caffeine!
- SUSE LLC's SUSE Manager
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Parsing an RSS News Feed with a Bash Script
- Doing for User Space What We Did for Kernel Space
- Google's SwiftShader Released
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide