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Using Fetchmail
Fetchmail is an unusual mail program. It is
neither a mail server nor a mail reader, although it contains elements of both.
Fetchmail's job is to fetch mail from a pull mail server and then to pass it on
to some other programmost commonly, Fetchmail uses a push mail protocol to
forward the mail it retrieves from a pull mail server. This last mail push
often occurs entirely on the same computerthat is, Fetchmail acquires mail
from another computer, then mails it to a local user of the computer on which
it runs. There are many variant ways to configure Fetchmail, though. The
program may be set up by individual users by editing its configuration file, .fetchmailconf .
There's also a GUI Fetchmail configuration tool available, known as fetchmailconf ,
that can be used to configure the program for many common tasks.
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Fetchmail's
Place in Mail Delivery Systems
Pull mail protocols were designed to allow
mail reader programs to retrieve e-mail from mail server computers. The idea is
that a computer capable of running a mail server is likely to be available at
all times, and so can run a push mail server to accept mail at the sender's
convenience. Only the end user's connection is likely to be transient or
variable in some other way, such as in having a dynamic IP address. Although
this generalization is true of many traditional networks, it's not true of all
of them. There are situations in which it may be desirable to transfer mail
with a pull mail protocol and then continue sending it through additional mail
server programs:
Dial-up Linux systems Linux computers generally run mail servers locally, even when the
computers are connected to the Internet only through dial-up Point-to-Point
Protocol (PPP) connections. The local mail servers on such systems can exchange
mail between multiple users or can be used to notify a single user of messages
generated by the computer. One way to integrate these messages with those that
arrive at the user's ISP mail account is to pull the mail using POP or IMAP and
then inject the pulled mail into the local mail queue. The user can then read
all mail (local and remote) using a single mail program configured to read the
mail locally rather than using POP or IMAP.
Dial-up with a LAN Some small businesses and even homes use local area networks
(LANs) on which e-mail passes between users. Such LANs may connect to the
Internet via a dial-up account with one or more associated e-mail accounts. You
can retrieve the dial-up e-mail accounts' messages and route them to
appropriate individuals on the LAN's e-mail system.
Multiple pull mail
accounts If you have multiple mail accounts,
managing them all can be a nuisance. As with the dial-up Linux system just
described, it's often possible to use a pull mail protocol to collect mail from
these multiple sources and send them on to a single account. (This account
could be local to the computer on which Fetchmail runs, but it might also be an
account on a remote server.) Alternative ways to handle this situation include
using the forwarding features of many mail accounts or using a mail reader that
can handle multiple accounts.
One mailbox, multiple
users Sometimes multiple users may share a
single e-mail account. If you have some way to filter the mail intended for
different users, you can retrieve it, filter it with features built into
Fetchmail or available in separate programs, and send the mail to separate mail
queues that can be used by appropriate individuals.
Converting POP to IMAP Your main external mail
account might use POP, but if you want to use IMAP and if you control a Linux
computer with appropriate network connections, you can retrieve the mail from
the POP account and send it to a local mail queue. If you then run an IMAP
server on this computer, you can read your mail using IMAP.
Put another way, pull mail accounts are
usually intended for single mail users using fairly simple local
configurations. This is especially true of POP, which works best when the user
reads mail from a single computer using just one mail program. When the mail
setup associated with an account becomes more complex, it may be helpful to add
another step to mail delivery in order to merge mail from multiple sources or
to separate mail into separate streams. Fetchmail is a tool that allows you to
do this.
It's important to realize that Fetchmail
functions as a client to both a pull mail server and a push mail server. (Alternatively,
it can deliver mail in ways that are unrelated to a push mail server, but such
configurations are rarer.) Because Fetchmail fills a dual-client role, it's in
charge of the scheduling of e-mail acquisition and delivery, assuming both
servers to which it connects are available. Fetchmail can run in both a batch
mode, in which it runs, checks one or more pull mail accounts, forwards any
mail it retrieves, and exits; or in a daemon mode, in which it runs constantly
and performs a check for new mail at some interval you specify. The latter mode
sounds convenient, but Fetchmail occasionally crashes or runs into problems
when run in daemon mode for long periods, which can result in a failure to
retrieve mail. I therefore favor running Fetchmail in batch mode except for
configurations in which daemon mode is used for limited periods of time. If you
want to schedule regular checks of the pull mail account, you can use cron to
schedule Fetchmail runs on a regular basis. You can also run Fetchmail
manually, checking for mail only when you decide to do so.
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Using fetchmailconf
Fetchmail configuration involves editing a
text file that's traditionally stored in the home directory of the user who
runs the program. Thus, several users can run Fetchmail on a single computer,
if desired. If you prefer GUI configuration tools, though, Fetchmail supports
one, known as fetchmailconf . I describe this tool in this section; the next section, "
Most distributions include fetchmailconf in a separate package from Fetchmail proper, so you may need to install two
packages to get both programs. As an X program that's based on Tcl/Tk, fetchmailconf also requires that you have additional libraries installed. Once you've
installed it, you can configure Fetchmail to retrieve e-mail from a pull mail
account as follows:
As an ordinary user, type fetchmailconf in an xterm window. This action opens the Fetchmail Launcher
window, which allows you to click one of four buttons to configure Fetchmail,
test Fetchmail, run Fetchmail, or quit.
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Click the Configure Fetchmail
button in the Fetchmail Launcher window. This action produces a Fetchmail
Configurator dialog box that allows you to choose from novice or expert
configuration modes. The former produces dialog boxes that contain a subset of
the options in the latter. Although I don't emphasize the expert-only options
in this procedure, I do describe this path so that you can see what some of
these options are.
3.
Click the Expert Configuration
button in the Fetchmail Configurator dialog box. The result is the Fetchmail
Expert Configurator dialog box shown in
you want to run Fetchmail in daemon mode, enter the number of seconds between
mail checks in the Poll Interval field (for instance, 1200 for a
check every 20 minutes). Leave this field at 0 if you intend to run Fetchmail
in batch mode. The Postmaster field is the name of the local mail account
that's to receive reports of problems with Fetchmail's activities. It defaults
to the user who runs the program. You can leave most of the other options in
the middle section of the window at their defaults, or click the upper Help button
to learn more about them.
name=ch11fig02> Figure
11.2. The Fetchmail Expert Configurator dialog box lets you enter global
options and name specific mail servers from which Fetchmail will retrieve mail.
11.2. The Fetchmail Expert Configurator dialog box lets you enter global
options and name specific mail servers from which Fetchmail will retrieve mail.
> 4.
The most important item in the
Fetchmail Expert Configurator dialog box is the bottom panel, in which you
enter the name of the mail server from which you want to retrieve mail. Type in
the hostname and press the Enter key, and a new dialog box will appear called
Fetchmail Host Hostname, as shown in
hostname should also appear in the scrollable list below the New Server data
entry field in the Fetchmail Expert Configurator dialog box. If you want to
retrieve mail from multiple servers, you may enter multiple server hostnames,
although you may want to do so after configuring the first server.
name=ch11fig03> Figure
11.3. You enter most information about specific pull mail servers in the
Fetchmail Host Hostname dialog box.
11.3. You enter most information about specific pull mail servers in the
Fetchmail Host Hostname dialog box.
> 5.
The most important sections of the
Fetchmail Host Hostname dialog box are the
Protocol, User Entries for Hostname, and
Security sections. The Run Controls section sets options related to the timing
of retrieval and the server name if it's different from what you specified
earlier. The Multidrop Options area allows you to enter rules with which to
duplicate or redirect mail based on mail headers that you specify. You can use
this to let multiple users share a single pull mail account, although this use
can result in problems with certain types of mail (such as mailing lists), and
so is discouraged.
6.
In the Protocol section of
Fetchmail Host Hostname, you specify the pull
mail protocol you want to use. Auto is the default, and works with some
servers, but you may want to specify the protocol if you know what's supported
on your server. You can click Probe for Supported Protocols to check the server
for the protocols that Fetchmail supports, but this option doesn't always work
correctly. If necessary, you can ask your ISP or try using telnet manually, as illustrated in
7.
The Security section is
particularly helpful if you use a dial-up network interface that's not always
active. Enter an interface name, such as ppp0 , in the Interface to
Monitor field to have Fetchmail poll a server only when that interface has been
used by another program since the last poll. (This behavior prevents Fetchmail
from keeping a link up unnecessarily if it would otherwise be brought down
automatically.) The IP Range to Check Before Poll field can be used to check
for the presence of an IP address on a given interface. Enter an interface
name, IP address, and netmask, separated by slashes ( / ), and
Fetchmail will run only if an IP address within the specified range and on the
specified device exists. For instance, ppp0/172.20.0.0/255.255.0.0 requires that the computer have an address in the 172.20.0.0/16
network on ppp0 before Fetchmail will poll the host.
8.
In the User Entries for Hostname section, enter your username on the mail
server in the New User field and press the Enter key. This produces the
Fetchmail User Username Querying Hostname dialog box shown in
when you enter a pull mail server name, you may finish with this dialog box and
enter additional accounts to query on the same server.
name=ch11fig04> Figure
11.4. Many Fetchmail options apply to specific pull mail server accounts.
11.4. Many Fetchmail options apply to specific pull mail server accounts.
> 9.
The most important item to specify
in the Fetchmail User Username Querying Hostname dialog box is the Password field in the
Authentication section. This information is required for retrieving mail from
most servers. You should also be sure that the Local Names field lists all the
local users who are to receive the mail retrieved from this account. The
default is to use the same username locally as is used on the pull mail server,
but you may need to change this. The Forwarding Options field lets you change
the host that's to receive the mailthe default is to use the local mail
system, but you can use Fetchmail to retrieve mail from one system and forward
it to another. The Forwarding Options, Processing Options, and Resource Limits
sections all let you set assorted options, most of which are self-explanatory,
the rest of which are required only in some circumstances. When you first set
up Fetchmail, you may want to select the Suppress Deletion of Messages After
Reading option in Processing Options, so you can test Fetchmail without fear of
losing mail. You'll want to remove this option once you're convinced Fetchmail
is working, though. Remote Folders lets you set the names of IMAP folders that
Fetchmail is to check in addition to the default INBOX .
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Click OK in the Fetchmail User Username Querying Hostname
dialog box, then in Fetchmail Host Hostname. You
can then click Save in the Fetchmail Expert Configurator dialog box to save
your changes to .fetchmailrc .
11.
To test your setup, click Test Fetchmail in the
Fetchmail Launcher window. This runs Fetchmail with debugging options, so you
can see the commands that Fetchmail sends to the pull and push mail servers,
and the replies that Fetchmail receives in return. This information can be
invaluable in tracking down problems. When your configuration works, exit from
Fetchmail by clicking Quit.
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For many people, Fetchmail configuration via fetchmailconf is quite adequate.
Particularly in Expert mode, fetchmailconf
can set the options you're most likely to need for retrieving mail from one or
more mail accounts, and even for delivering mail from multiple pull mail
accounts to multiple local users. Some people, though, prefer editing text
files to using GUI configuration tools; or you might want to edit a text file
to make a quick change or to create a particularly complex configuration. For
whatever reason you want to do it, manual editing of .fetchmailrc is certainly possible.
Configuring
.fetchmailrc
When you run fetchmailconf ,
as just described, the program converts your selections into entries in the .fetchmailrc file in your home directory.
The structure of and options in this file therefore mirror those in fetchmailconf .
Listing
11.3 A sample .fetchmailrc file
# Fetchmail file for retrieving mail from mail.abigisp.net # and imap.asmallisp.com set postmaster rodsmith set bouncemail set daemon 1800 set syslog poll mail.abigisp.net with proto POP3 user rodericksmith there with password abc123 is rodsmith here fetchall forcecr smtphost speaker.rodsbooks.com poll imap.asmallisp.com with proto IMAP user rodsmith there with password A1B2C3 is rodsmith here
As with many configuration files, a pound sign ( # ) denotes a comment, so Fetchmail ignores
the first two lines. The remainder of
a series of set statements set
global options, most of which relate to features you can set in the Fetchmail
Expert Configurator dialog box's Fetchmail Run Controls section (
overridden using command-line arguments, as well. Second,
remote e-mail account to be accessed, including how the mail retrieved from
that account should be delivered. (This information appears in the Fetchmail
Host Hostname and Fetchmail User Username Querying Hostname
dialog boxes in fetchmailconf ,
as shown in
statements may exceed one line in length, without
using special characters to indicate a line continuation, as some other file
formats require. Line breaks in
Fetchmail supports a huge number of options, so I can't describe
all of them here. For more information, consult the Fetchmail man page or other
Fetchmail documentation. Some options take a string, such as a username, as an
argument. If this string contains spaces, you should enclose it in quotes ( " ). Some of the more important global
options include the following:
set postmaster
username This
option sets the username used for mail delivery if other options fail to
specify one. This user also receives certain types of error messages. It's
normally an ordinary user, but you can use postmaster
or root for username , if you like. (All SMTP mail
servers are supposed to support a user called postmaster
to handle queries concerning the computer's mail system. The Fetchmail postmaster may or may not be the same as
the SMTP server's postmaster .)
The --postmaster username command-line option overrides
this configuration option.
set bouncemail
This option tells Fetchmail to send error messages to the mail's sender. You
can alter this option by using set no
bouncemail instead; in this case, the Fetchmail postmaster receives the error messages.
set daemon interval This option tells
Fetchmail to run in daemon mode, and to query mail servers every interval seconds. If you want to run
Fetchmail in batch mode, omit this option. You can override this value with the
--daemon interval command-line argument. Using
the --daemon 0 command-line
argument causes Fetchmail to do one fetch operation, even if it's configured to
use daemon mode in .fetchmailrc .
set logfile
filename You can
tell Fetchmail to log its activities to a particular file with this option.
set syslog
If you want Fetchmail to log its activities through the system logger, you can
do so with this option.
The poll
statements can be fairly simple or quite complex. The format of a poll statement is as follows:
poll server.name server-options user-descriptions
The keyword server
is synonymous with poll . You can
replace either of these with skip
to tell Fetchmail to skip this entry. You can use this feature to easily
disable an entry without deleting it from the .fetchmailrc
file. The server options define how Fetchmail is to interact with the server,
and the user descriptions define accounts on the server and locally. You can
intermix options within each of these two categories, but not between them. (A
common source of problems in manually configuring Fetchmail is to intermix
these two option classes.) The words and ,
with , has , wants ,
and options are all ignored, as
are the punctuation symbols colon ( : ),
semicolon ( ; ), and comma ( , ). You can place these anywhere within
the server options or user descriptions to make the poll entry easier to understand.
Some of the more common or important server options include
the following:
proto name or
protocol name These synonymous options
specify the pull mail protocol to be used. The name will most commonly be POP3 or IMAP ,
but Fetchmail supports several others, including some fairly obscure pull mail
protocols. You can override this value with the -p command-line argument.
interface interface /IP.address/net.mask
You can specify a network interface that must be up before Fetchmail will poll
a given server with this option. You specify the interface with a device name ( interface ), such as eth1 or ppp0 ;
an IP address, such as 192.168.1.0 ;
and a netmask that determines the range of acceptable IP addresses. For
instance, interface
eth1/192.168.1.0/255.255.255.0 specifies that the computer must
have an address between 192.168.1.1 and 192.168.1.254 on eth1 before Fetchmail will try to contact
the server. You can also provide this information with the -I command-line option.
monitor interface This option causes
Fetchmail in daemon mode to watch a network interface for activity. If
Fetchmail detects no activity between the previous poll and a scheduled one,
Fetchmail skips the poll. This behavior can keep Fetchmail from keeping an
auto-dial device up with its own polling. You can use the -M command-line argument to provide this
information, as well.
Some of the more common user options include
the following:
user name or username name
These options usually mark the start of the user description portion of the poll option,
and they specify a username. This is normally the remote username, but if the
sequence is followed by here , the username is local. The keyword there forces
interpretation of the user as a remote account. The -u command-line argument overrides this
option.
pass pw or password pw
These options specify the password ( pw ) to be used on the
pull mail account. You provide the password in cleartext (unencrypted), and
it's stored that way in the configuration file.
is name or to name
These options link a pull account to a local user. One of these options
normally appears after the description of the pull mail account (using a
sequence such as user name with pass pw ). When the remote account description precedes the local account
description, here follows these options to identify the account as being local. The
keyword there identifies a
remote account.
smtphost host.name Normally, Fetchmail attempts to use the address localhost ,
which corresponds to the computer on which it runs, to send mail. You can have
the program use another computer's mail server for sending mail on its way,
though, by using this option. You can also use this option and specify your
computer's regular hostname, in which case the addresses in your mail headers
corresponding to Fetchmail transfers will include that hostname rather than localhost . The
-S command-line argument
overrides this setting.
keep By default, Fetchmail deletes messages from the server once it's
retrieved them. This option causes Fetchmail to leave messages. You might use
this when testing a new configuration. The -k command-line argument
is another way to specify this option.
fetchall Normally, Fetchmail doesn't retrieve messages that it has already
retrieved. You can force the program to retrieve all messages with this option,
which has a command-line equivalent of -a .
forcecr Technically, mail messages should have lines that end with a
carriage return/line feed pair of characters. Most mail programs tolerate
messages that lack the carriage return, and a few systems send such messages. The
qmail push mail server can't tolerate such messages, though, so the forcecr option
corrects matters for users of qmail.
When you specify more than one local name,
Fetchmail examines the mail headers to try to determine who should receive the
mail. For instance, if you specify local accounts of jack and jill , and if
incoming mail is addressed to jill , Fetchmail delivers the mail to jill . This is multidrop mode, in which mail from a single account
may be processed in different ways according to the headers in the mail.
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