LINUX NEWS
http://www.Cramsession.com
October 3, 2002 - Issue #101TABLE OF CONTENTS
1) Sean’s Notes
2) Linux News
Red Hat 8.0 Released
Red Hat 8.0 - SNAFU #1
Behind the Name
Mandrake Releases 9.03) Linux Resources
Multiheaded X-Windows
Where Do You Set Kernel Parameters?
Solaris Security Primer
How do Hashes Work?
MD5SUM?4) App o’ the Week
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1) Sean’s Notes
As you’ve probably noticed, Red Hat 8.0 came out this week. I was planning to give you my first impressions of it this week, but busy mirrors, broken parts in my test environment, and general craziness prevented that. But I’ve got a great series lined up on one of the oldest applications of the Internet, email, which will hopefully tide you over until I get my act together.
Linux makes a great email server, whether you’re just doing personal email with your custom domain, or if you’re hosting for thousands of users. Email software in Linux scales beautifully, you could handle a light volume on a Pentium-90, or move to a large cluster of machines handling thousands of mailboxes, with only a few changes in configuration.
This week, we’ll look at the flow of email on the Internet. In upcoming articles, I’ll cover setting up the Postfix mail transfer agent, and how to provide POP and IMAP access to the mailboxes.
How email gets delivered is very elegant. The first part to look at is the communication between Mail Transfer Agents (MTAs), who are responsible for receiving and sending email. When you put an SMTP server in your mail client (called a Mail User Agent (MUA)), you’re specifying the MTA that will forward your email toward its destination. SMTP, the Simple Mail Transfer Protocol, dictates how the MTAs will speak (and how you’ll speak to the MTA). RFC 821 lays out the protocol:
http://www.isi.edu/in-notes/rfc821.txt
A quick summary of the protocol:
Sending MTA contacts receiving MTA on port 25. Receiving MTA answers with a banner identifying itself. Each response from the receiver has a number at the beginning of each line, called the status code. The MTAs aren’t too concerned about anything other than the status code, it’s there for humans to read if something goes wrong.
220 poochie ESMTP Sendmail 8.11.6/8.11.2; Tue, 1 Oct 2002 20:53:09 -0500
That’s the banner from one of my mail servers. The sender then identifies himself with HELO:
HELO me 250 poochie Hello, me, pleased to meet you
The sender’s address is then entered:
MAIL FROM: swalberg@cramsession.com
and the response:
250 2.1.0 swalberg@cramsession.com… Sender ok
Then, the list of recipients (if there are multiple, it’s one per line, with the RCPT TO: command repeated)
RCPT TO: swalberg@cramsession.com 250 2.1.5 swalberg@cramsession.com… Recipient ok
Then, finally, the actual message itself:
DATA 354 Enter mail, end with ”.” on a line by itself From: “Sean Walberg” <swalberg@cramsession.com> To: “Sean Walberg” <swalberg@cramsession.com> Subject: testing
Hi, Sean! .
When I send the last line, the ”.” by itself, I get back:
250 2.0.0 g921vb929510 Message accepted for delivery
So, I finish the conversation: QUIT 221 2.0.0 poochie closing connection
Why am I showing you the protocol itself? As an email administrator, your best troubleshooting tool is to make yourself look like another MTA and to telnet in and send a test message. Only then can you see where the conversation is going. Plus, by seeing how simple SMTP really is (as opposed to SNMP, the Simple Network Management Protocol, which isn’t simple at all), I’m hoping it dispels some of the fears you might have about understanding email.
Why did you specify the to/from addresses both in the RCPT/MAIL commands, and after DATA? Good question! Think of email like its physical counterpart. You’ve got an envelope with a letter inside. The envelope says who the message is to, and who it is from. Likewise, the RCPT and MAIL commands form the envelope of the message. Unless your MTA injects that information in the email’s headers, you’ll never see it again. The message itself (content) comes after the DATA keyword. Within that, we’ve got the headers, and the text itself. Headers are for your MUA to chew on, such as a more friendly version of the From and To addresses, along with a subject and other fun. The first blank line marks the end of the headers, and the beginning of the text that the user reads.
Is this vulnerable to forgery? You bet. I could have entered anything I wanted in the headers (or nothing at all), and the only reliable way to trace would be to go to the MTA logs. But SMTP dates back over 20 years, to a time where the Internet was more friendly.
How does the sending MTA know where to find the receiving MTA? Excellent question! SMTP doesn’t cover this, so we make use of the domain name system, DNS.
Recall that DNS is composed of various records, each specifying information for a name within a zone. The A record for “www” within the “cramsession.com” zone will tell me the IP address of Cramsession’s web servers. The MX record (Mail Exchange) tells me where to deliver the mail. By stripping off everything before the @ in the email address, your MTA knows the domain.
$ host -t mx cramsession.com cramsession.com mail is handled by 10 mail.cramsession.com.
Using the host command, with the -t (type) mx option tells me where to send the mail. The number before the name dictates a priority (called the preference), with lower numbers winning. The MTA’s job is to pick the lowest one, and connect to it. If there is a tie, pick one at random. If you can’t connect, pick the next highest MX. As we’ll see later, a higher priority MX will accept the email, but continue to try to deliver it to the lower priority MX on its own. Nothing’s set in stone about your choice of priorities, but popular convention is to set your preferred MX to 10, and to increase by 10 from there. If you need, for some reason, to set up a higher priority MX somewhere, you’ve got lots of room to play.
If there are no MX records, an A record will be tried. Since your web site and mail server may not be on the same server, it’s good practice to have an MX record.
$ host cramsession.com cramsession.com has address 63.146.189.41
Recapping, you send your message from your MUA (email client) to your chosen MTA (email server) using the SMTP protocol, where it is queued for delivery. The MTA then looks at the email address, and figures out the domain name(s) of the recipient(s). MX records are pulled for the domain(s), and the MTA contacts to the remote MTA, speaks SMTP to deliver the message, where it is queued again.
There are a few possible cases at the remote MTA. It could be configured to statically send all email to another server, which it will then do (think of the case of a mail gateway, or virus scanner). The remote MTA might not be the lowest preference MTA for the domain, in which case it will start trying to contact the better MTA. Finally, the MTA could be the one responsible for delivering the email to the destination.
This last step is fairly boring compared to what’s happened so far. When an MTA receives an email, it says “What do I do with this?” After consulting various tables, or even DNS, it might say “Hey, that’s for one of my users!”, in which case it dumps it to a local database. This phase is called “local delivery”.
Depending on your mail server, local delivery can take on various forms. Traditional UNIX delivery agents dump the message to /var/spool/mail/USERNAME (or sometimes /var/mail/USERNAME). The format (called mbox format) is pretty simple, it’s just the messages smashed together with a brief header. Some mail servers like Cyrus (or MS Exchange) store the message in a database for faster access. Some, like qmail, store it in the user’s home directory. They’re all good.
The last phase is for the recipient to get the email. This can happen with POP, IMAP, read from the mbox directly, or specialized applications. It’s actually the easiest and least complex part of the whole process.
Well, that’s the flow of email, from sender to receiver. A successful email administrator understands it completely, so that any breaks in the chain can be found and fixed. In upcoming articles, I’ll cover setting up the MTA, and how to get your POP and IMAP set up. If there is any demand, I’ll walk through the setup of a web-based mail reader too.
Long live the Penguin,
Sean mailto:swalberg@cramsession.com
2) Linux News
Red Hat 8.0 Released
It’s that time again. Red Hat has a new point-oh release ready for purchase or download. Big features this time around are the “Bluecurve” theme, which makes GNOME and KDE look similar out of the box, and the inclusion of Open Office.
http://redhat.com/about/presscenter/2002/press_eightoh.html
Red Hat 8.0 - SNAFU #1
Over concerns about patents and royalties, XMMS in Red Hat 8 was stripped of MP3 capabilities. When you tried to play an MP3, you should have received a popup saying this, and where you could get an RPM to fix it. Guess who forgot to include that? Here’s a link to the MP3 decoder.
http://soraas.student.nlh.no/~havardk/xmms/xmms-1.2.7-rh8-rpm/
Behind the Name
I’ve noticed some relationships between various names that Red Hat has used for distributions, such as Valhalla->Limbo->Null, and the choice of painters for a few releases. Someone has come up with all the names ever used, and put together some plausible (and not-so-plausible) linkages. Fun reading!
http://www.smoogespace.com/documents/behind_the_names.html
Mandrake Releases 9.0
The more desktop oriented distribution, Mandrake, has also reached a major milestone with the release of Mandrake Linux 9.0. The ProSuite edition is Linux Standards Base certified, which I call a sign of a good trend.
http://www.mandrakesoft.com/company/press/pr?n=/pr/products/2393
3) Linux Resources
Multiheaded X-Windows
“Multiheaded” is a term applied to a machine with more than one monitor. Most often, the monitors are bonded together so that you have one virtual desktop that spans both (or more than two). Here’s an article on how to do it… I’ve been wanting to for some time, but one limitation is that both displays must be at the same colour depth.
http://www.linuxgazette.com/issue82/ward.html
Where Do You Set Kernel Parameters?
It used to be that setting kernel parameters like ip_forward (for firewalls and routers) was done within /etc/sysconfig/network on Red Hat systems. It’s still just as easy, but more comprehensive. Check this link out for how you go about setting just about any kernel parameter you need. This’ll also work on any distribution, and it goes for any kernel parameter.
http://www.redhat.com/docs/manuals/linux/RHL-7.3-Manual/ref-guide/s 1-proc-sysctl.html
Solaris Security Primer
Here’s a good primer on securing Solaris. Some examples include the equivalent Linux command or file, where appropriate. Good techniques and ideas here…
http://www.sunperf.com/Security.html
How do Hashes Work?
Maybe it’s some of my computer science courses making a comeback, but I found this article on Perl hashes quite interesting. It explains how hashes are represented in Perl, and how to make your use of them more efficient.
http://www.perl.com/pub/a/2002/10/01/hashes.html
MD5SUM?
When you download files from an FTP site, often a file called MD5SUM is there. This file contains the MD5 hashes of the files in the directory. By comparing their hashes to the ones you downloaded, you can ensure the download was successful.
http://boards.cramsession.com/boards/vbm.asp?md4554
4) App o’ the Week
Got a bunch of MP3s that you want to put on a CD, but they’re all at different volume levels? You want to normalize it, with the aptly named “normalize” tool.
http://www.cs.columbia.edu/~cvaill/normalize/
(C) 2002 BrainBuzz.com, Inc. All Rights Reserved.
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