Last month's feature, "Personal Post," (available online at http://www.linux-mag.com/2003-07/email_01.html) explained how email is delivered, and also described a variety of power tools ideal for untangling a morass of messages.
One of the tools mentioned last month was fetchmail, a great Linux utility that can centralize your email and automate tasks that would otherwise be spread across the network on multiple mail systems on multiple hosts. This month, let’s look at fetchmail in more detail, and apply it to some difficult email problems.
Setting up fetchmail to work well takes some time and requires some familiarity with email transport. Let’s quickly review how email works before we go any further. Here are the basics:
* Users compose, send, and read email with a Mail User Agent (or MUA). mutt, Outlook Express, and Mozilla are examples of an MUA.
* Email is sent from user to user by a Mail Transfer Agent (MTA), a program like sendmail or qmail.
* The header of an email message is composed of a series of fields with names like Received, Message-ID, and Content-Type.
* It’s important to understand that the addresses in header fields like From and To are not necessarily where the message originated from or where the message was delivered to! Instead, the message envelope contains the actual addresses of the sender and recipients — in the same way a paper envelope holds “snail mail” for delivery.
* Years ago, the body of a message was limited to plain ASCII text. But, more recently, the MIME standard provides for reliable delivery of messages with non-ASCII text and data.
* The procmail utility is a delivery agent that processes incoming messages. procmail can file, forward, and possibly edit messages as they arrive. procmail rules are called recipes, and recipes can be fairly elaborate — it’s not unusual for a recipe to invoke other Linux utilities such as formail, sed, and your local MTA.
* Mail messages are typically stored in a folder or other message store. If it’s on your local machine, your MUA can read that folder directly. If it’s on a remote machine, a network protocol like POP can copy the messages to your local host, or IMAP can access that folder directly.
* The POPfile program retrieves messages from a POP server, and analyzes them dynamically, marking the header of each message to help you categorize your incoming email.
* The fetchmail program retrieves messages from remote servers and gives them to your local MTA to deliver.
If you’re familiar with all of these concepts, you’re already well-ahead on the power curve. Now, let’s dig in to what you can do with fetchmail!
(Some of) What Fetchmail Does
fetchmail can retrieve email messages that are waiting for you or multiple users, on multiple remote servers — and deliver them to one or more other mailboxes. It can do this on-demand (getting new mail when you run fetchmail from an hourly cron job or from the command line) or as a daemon (a continuously-running “background” process that you start just once — for instance, each time your system reboots).
What good are all of those features? In the simplest case, fetchmail lets you manage multiple mailboxes from one central host. fetchmail can talk to many remote servers, get your email, and deliver it reliably to your local MTA. All of the details are handled for you: the messages simply appear in your local mailbox. fetchmail can also “mark” message headers so you’ll know which message came from which mailbox, letting you separate incoming messages automatically with a tool like procmail.
fetchmail understands multiple protocols, including obscure ones that most MUAs don’t. It supports SSH and other methods to encrypt and verify mail transfer from remote hosts. It can split huge message deliveries into smaller parts, retry if the connection fails, and much more. This means that your MUA doesn’t need to be super-sophisticated and reliable at retrieving remote mail. In fact, this lets you use any MUA — even one that can’t get email from remote mailboxes.
The fetchmail home page, http://www.catb.org/~esr/fetchmail, has all the links you need — including lots of documentation on installing and using the utility. It’s worth looking through the long list of fetchmail features so you’ll be ready to take advantage of all its capabilities. That’s even true if you use fetchmailconf, a graphical configurator (written in Python). fetchmailconf’s short descriptions make setup easy, but it helps to have some background so you’ll understand exactly what the settings are doing. There’s a screen shot of one panel from fetchmailconf — the Expert-mode Mail Server Options — in Figure One.
Figure One: fetchmailconf’s “Expert Mail Server Options” panel.
In the user interface for fetchmailconf, the “Probe for supported protocols” button is handy if you don’t know a mail server well. For example, the probe might suggest that a POP server also supports APOP — which, if you have popauth set up, will let you avoid sending your password in the clear.
The expert configurator can probably get you a working setup in a flash (if you browsed the documentation first), but you may be able to optimize it. Let’s take a look at one sample setup, then look at a few (perhaps unusual) ways to integrate fetchmail into a personal setup on your workstation.
A Sample .fetchmailrc File
Let’s say that you get mail from four mailboxes on other hosts. You always want to monitor your office mailbox at pop.xyz.com. You also get mail from your office “support” mailbox from the same host, but only while you’re on support duty from noon to 1 PM weekdays. Third, you’re testing POPfile on your workstation, running its POP server on port 9999, constantly getting copies of “marketing” mail to see how well POPfile can classify them; you’re sending those messages to the local address alexa. Fourth, you sometimes want to get copies of your family’s email from mail.marslink.net — but also leave the messages on that server for your family to read from home.
Listing One shows part of the .fetchmailrc file you’d put in your home directory (the set option-name lines have been omitted). The fetchmail manual page and FAQ have details, but let’s see the highlights:
Listing One: Part of a .fetchmailrc file
defaults proto pop3 tracepolls user ‘zoe’ here
poll office via ‘pop.xyz.com’
user ‘zsmith’ there with password ‘X’
skip support via ‘pop.xyz.com’
user ‘support’ there with password ‘X’
poll 127.0.0.1 port 9999
user ‘pop.xyz.com:marketing’ there
with password ‘X’ is ‘alexa’ here
skip home via ‘mail.marslink.net’
user ‘sm2736′ there with password ‘X’ keep
* The defaults line defines options used in all of the other entries. The first line specifies that all servers use the POP3 protocol; the tracepolls option marks message headers with the remote mailbox name; and fetchmail delivers most remote messages (except the marketing mail) to the local user zoe.
* The other entries describe four remote mailboxes. The two starting with poll are checked every time fetchmail runs — unless you specify particular mailboxes on the command line. Conversely, the two entries starting with skip aren’t polled unless you specify them on the command line.
* The password entries are needed for fetchmail’s daemon mode. You can also use them if you aren’t concerned about someone reading the passwords from your .fetchmailrc file. Without them, fetchmail will prompt you for each POP3 server password. However, there are other ways to authenticate; see the man page for fetchmail.
* via is used in three of the entries to identify three remote mailboxes by names that we choose: office, support, and home. Otherwise, fetchmail identifies remote mailboxes by the server’s hostname or IP address, as we have with the local POPfile server. Here, via is necessary because we’ve got two different mailboxes with different configurations on the same server pop.xyz.com, and we don’t want to poll both of them every time.
* A local POPfile daemon is running at port 9999 on your workstation, overriding the default POP port 110. fetchmail accesses it through the local IP address 127.0.0.1, getting marketing mail from the pop.xyz.com mail server.
* The keep option on the home entry ensures that messages won’t be deleted from the remote mailbox.
There are many other options. Also, you can override the .fetchmailrc configuration when you call fetchmail from the command line. For instance, the command fetchmail -K home would get mail from the home mailbox and delete messages after downloading them.
How often should fetchmail poll remote mailboxes? To get mail from all entries that start with poll, daemon mode can do the job: it puts fetchmail in the background and polls at an interval (in seconds) that you set. For example, fetchmail -d 360 causes fetchmail to poll every six minutes.
(Tip: if you configure fetchmail to poll an external mail server often, your local machine may not even need an incoming SMTP server accessible to the outside world! This “pull-only” operation can be a security win.)
Other than its interval option for individual servers, though, fetchmail’s daemon mode doesn’t give much control over the times that particular servers are polled. For more control over your own fetchmail process, you can run it from the command line, from a GUI button on your desktop, from a cron job, and so on.
For instance, you can add a desktop button that runs fetchmail to poll all servers, or runs fetchmail mail. marslink.xyz to poll only that one server. You can also write a shell alias that checks for email on your home ISP’s server when you type hm at a shell prompt. Or, you can set up cron jobs to do the right thing, polling your office and marketing mailboxes 24 hours a day, polling the support mailbox every five minutes from 1200 to 1300 hours, and polling your home account only after 6 PM. (The February 2003 “Power Tools” column, “Running Jobs Unattended,” available online at http://www.linux-mag.com/2003-02/power_01.html, covers cron.)
If you want multiple configurations, you can name a specific configuration file on the command line, and you can even feed a configuration setup to fetchmail’s standard input. You, or a cron job, can also simply update the .fetchmailrc control file when you want its configuration to change; the fetchmail daemon will notice the change and automatically restart itself.
So, for example, here’s a cron job that would use your daytime setup between 8 AM and 5 PM weekdays, and your nighttime setup the rest of the time. (We’re using relative pathnames to the files; a personal cron job typically runs from your home directory.)
If you run cron jobs like the ones mentioned, what happens if you aren’t at work? Will cron keep pulling in the “support” mail while you aren’t there? A simple personal crontab entry to fix that would look like this (which you should enter on a single crontab line):
Every minute between 12 Noon and 12:59 PM, that command runs who to get a list of logged-on users. The grep command looks for your username (you may need $LOGNAME instead) at the start of a line, with a space after it (to be sure it’s unique). grep output is redirected to the Linux “bit bucket” /dev/null, but grep also returns an exit status. If the status is 0 (true), because grep found your account logged on, then the shell’s && operator executes fetchmail support. If you aren’t logged on, cron’s shell won’t run fetchmail.
When fetchmail polls, how can you know if it retrieved new mail? If you’re running it from the command line and don’t have the syslog option set, you’ll see progress messages that tell you what’s happening. Otherwise, you might set your MUA, or an X utility like xbiff, to ring a bell or do something else when new mail arrives in your local mailbox.
And here’s another idea: if you’re running fetchmail from a command line (a button, a cron job, and so on), you can test its exit status. Fetchmail returns a zero exit status if it’s delivered new mail, or a non-zero status otherwise. (See “Exit Codes” in the man page.)
If you’re using the X Window System and you want to be notified right away when mail comes in, try the getmail script shown in Listing Two. It runs fetchmail, passing it any command line arguments that you specify through the shell’s special parameter $@. For instance, to check support and home mail, you’d run getmail support home. The shell’s if statement checks the fetchmail exit status; if it’s zero, a yellow xmessage window opens near your mouse cursor.
Listing Two: getmail script, version one
if fetchmail “$@”
then xmessage -near -bg yellow “NEW MAIL”
While getmail is useful, you can’t run it from a cron job. Why? A cron job usually doesn’t run under the control of your window system, so it doesn’t have access to your X display.
And there could be another problem with getmail. If your setup does some mail filtering after fetchmail runs — for example, POPfile looking for spam or procmail tossing it — it’s possible that the “NEW MAIL” window might pop up when there’s no new email that you’d want to see.
The Daemon’s in the Details
The second version of getmail, in Listing Three, runs fetchmail on the remote support mailbox, then tests the local support mail folder. If the folder contains mail, the xmessage window opens. (This still isn’t optimal because, if there’s any old mail in the support folder, xmessage tells you about it. It’s just a simple example of what you can do. A smarter program that watches the mailbox state, like xbiff, would avoid false notifications.)
Listing Three: getmail script, version two
fetchmail support &&
test -s /home/zoe/mail/support &&
xmessage -near -bg yellow “NEW MAIL”
A program like getmail doesn’t work with fetchmail’s daemon mode, of course, because it waits for fetchmail to terminate. If you want to run a program after each time the fetchmail daemon polls a server, add the program name and command-line arguments to a postconnect option in your .fetchmailrc file, like this:
skip support via ‘pop.xyz.com’
user ‘support’ there with password ‘X’
But it’s hard to use an X utility like xmessage from fetchmail’s daemon mode because, by default, a fetchmail daemon detaches from your tty. The fetchmail -N or –nodetach command-line option keeps the daemon running in the foreground. So, if you start fetchmail from your shell’s command line, it will have access to your X display. You could put fetchmail -N into the background, from your shell — and then it can access your terminal to send messages and open X windows.
Jerry Peek (email@example.com) is a freelance writer and instructor who has used Unix and Linux for over 20 years.
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