Letters to the Editor
I read with interest Chris Kostick's article in Linux Journal July 1996 (Issue 27). I was a bit disappointed that he concentrated on 1.2.x kernels and didn't mention the advantages of using a 1.3.85+ or pre2.x kernel, and that he didn't explain the use of ipfwadm. For instance, although many ICMP packet programs do not work, with recent kernels ICMP support has been added for some programs such as traceroute (ping still does not, and probably never will work).
I was also bothered by his insistence that you couldn't pass remote X clients to a masqueraded server. I was sure you could, because I remembered doing it! Using ssh (a secure rlogin variant available at http://www.cs.hut.fi/ssh/) to connect to a remote host from the masqueraded server, your X display environment variable is automatically set, and “X11 connection forwarding provides secure X11 sessions !”
So you can pass X clients through a masquerading machine if you initiate a connection from the masqueraded machine using ssh.
—Daniel Morrison email@example.com
At the time I wrote the article, 1.3.57 was the latest “stable” release. The 60's were available but most of them were to be avoided. I was using 1.3.56 so that's what I based the article on, pointing out that 1.2.x kernels were compatible.
Actually, ping could work. ICMP echo request/echo reply messages can be masqueraded for. The key is the identifier field in the ICMP message. The problem is matching replies that come back from the external networks to the originator. The masquerading would function as shown in Figure 1.
The masquerading machine will have to maintain a cache that will map the originating IP_Addr+ID --> ID numbers of the requests originating from the Masq host. The ID numbers start at a value that should not conflict with 'real' ICMP echos from the Masq host. I chose 60000 to keep in concert with the masquerading port numbers. The ICMP code within the kernel would also have to be modified so as not to conflict with internal ID numbers never exceeding 59999.
Matching the replies is a matter of checking the cache for the return mapping, and sending the ICMP echo reply to the original host.
There are other protocol details not discussed here, and I haven't looked at all of the nuances of the protocol so I leave the theory of whether it would really work up for discussion.
I didn't consider passing X (or any other protocol) encapsulated the same as sending the real thing. Therefore because the X client has to identify the address of the server in the DISPLAY variable or -display argument, and the server's address is hidden, it won't work.
—Chris Kostick firstname.lastname@example.org
Thanks again for another great issue of the Linux Journal. On the day it arrives all activity in the house (on my part anyway) stops until I have read it from front to back.
I would like to make a comment on Michael Johnson's article Serving Two Masters. The “Recovery Recipe” that Michael presents was something that I had to discover the hard way myself a month or two ago. The situation had nothing to do with Win95, although it did involve a dual boot PC. This PC runs both Linux and DOS/Win3.1 (thank you LILO), and in one of the DOS sessions I managed to acquire a virus that infected the MBR. No problem says I—I'll just use McAfee Virus Clean, and all will be back to normal. Virus Clean worked fine, but all of a sudden LILO was gone. It appears that (at least with McAfee) the process of cleaning a boot sector virus just restores the DOS flavour MBR. I was not brave enough to re-infect my PC with the same virus to see if other virus cleaning programs act the same way, but I would suspect that they do (as most do not take periodic copies of the MBR to restore from).
I found the second edition of Æleen Frisch's book Essential System Administration (published by O'Reilly) absolutely invaluable in this effort. If you are a Linux user—run, don't walk and get this book!! Make sure that it is the second edition though as the first edition does not really deal with Linux.
—Kris Boyle email@example.com
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
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.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
Free to Linux Journal readers.Register Now!
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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