The Linux Operating System

Lecture 7:

Email

Tonga Institute of Higher Education

Email● Email has many aspects that work together to

give people almost instant communication from any computer on the internet to any other computer

● There are three main parts that we will look at: sending email on a server, receiving email on a server and receiving email as a client

● The simplest is to receive email as a client.● You merely put in the name of the server that

holds your email and download it.● On Linux, you can view your email through many

different programs. There are graphical programs like Mozilla, Ximian Evolution and Kmail

● You can also use programs from the command line like 'pine' and 'mutt'

Email on the Server● Getting and reading email is simple on a

client, but setting up a server to get and send email requires a little more understanding of how email works

● There are three parts to have an email server

– The MTA – Mail Transport Agent (most people use the program called 'sendmail')

– The LDA – Local Delivery Agent (most people use the program procmail)

– IMAP and POP3 servers – these are two ways to get email from a server. When a client connects to a server to get email, they will use one of these servers

Setting up Sendmail● Sendmail normally runs with all RedHat

distributions, so you won't need to start any service.

● Sendmail is a Mail Transport Agent – this means that it is a program which moves mail from one computer to another computer. It is estimated that 80% of all email is handled by Sendmail today.

● With Sendmail, you can do many things, like block spam, relay mail, have forwarders and configure ways to route mail automatically across networks.

● Like most Linux programs, Sendmail is changed through configuration files

Setting up Sendmail● Setting up Sendmail can be an enormous task if you

want to do complicated things, but if we just want it to accept email, we can keep it simple.

● First change into /etc/mail where the sendmail files are saved

● Sendmail has a different configuration, where you edit files and then compile them into a form that Sendmail likes to see

● First open the file “access” - This is the file that contains all the domains that will be allowed to use your Sendmail to send emails. You’ll also want to make a file called “relay-domains” and put your domain in there.

● This is to stop people from outside using your server to send spam

● You need to add in your domain here and any domains that might be own your network

Setting up Sendmail● You'll also want to open up the file “local-

host-names” - This will contain other names for your computer, so that Sendmail will still accept mail from these domains.

● This file should contain any other names you have for your computer

● Now your sendmail will know who it is good to accept mail from and who it is not good.

● If you haven’t set these up the right way and you are not receiving mail, you can look inside “/var/log/maillog” for any error messages

Setting up Sendmail● The actual sendmail configuration file is

in “sendmail.cf” but it has become so complicated they made another configuration file, which you then compile to make into the “sendmail.cf”

● In RedHat, they use this program called “m4” to generate the sendmail.cf file

● So after we edit the “sendmail.mc” file, we use m4 to change it, like

[root@comp root]# m4 sendmail.mc > sendmail.cf

Setting up Sendmail● Now that you have the configuration set up

for Sendmail, you can restart the service if you want to take the new changes into affect

● So we use the service command to restart sendmail

● [root@comp root]# service sendmail restart

● And you can check to see if it's running by using 'ps -aux' and you'll see an entry that says, “sendmail: accepting connections” This means that it is up and running and people can send email to your server

● But how does anyone else on the Internet know how to send email to your server?

MX Records● MX Records – Mail Exchange records are part of the

DNS system for the entire Internet. In order for other computers to know where to send you email, you need to have the correct MX records set up on some Name Server on the internet.

● They use a numerical priority determines the order in which servers should be used.

● The server with the lowest priority is the primary. In DNS, the MX records will look like the following.

● In this example, mail.example.com handles mail, but mail.connect.com will also accept mail if the primary is down:

example.com. IN MX 10 mail.example.com

example.com. IN MX 100 mail.connect.com

Local Delivery Agents● In most RedHat distributions, Sendmail will get

the mail from some server on the Internet and then pass it off to another program for local delivery

● This means there is another layer of handling before an email will reach your inbox.

● Procmail is usually the program that is chosen to do the local delivery.

● The reason that there is another layer is that it is easy to do things to mail after it has come in with procmail. For example, you could sort mail into different folders, delete it if it is spam or make copies of everybody's email

Procmail● When a new message comes in, Procmail will

start automatically and deliver the mail to the correct folder for the person to read it

● You can change how mail is delivered though through a procmail configuration file.

● Initially, there is no configuration file for procmail, because it will just give whatever mail comes in to the person who should receive it

● You can make one yourself though by creating a file called “/etc/procmailrc”

● This is the file where you can put rules that will change how mail is delivered

Procmail configuration● Procmail configuration let's you do almost anything

you can imagine to email, but you have to know the right way to configure

● Once you have the procmailrc file open, you can start putting rules.

● For example, if you want to make a copy of everyone's email so you can read it yourself:

:0c/home/mycopy-email

● The ':0' part says that a new rule is starting. The 'c' says copy all email and the following line says where to copy it.

● If you left out the 'c', it would instead send all email to that file

ProcmailExample: If you wanted to delete all email that

came from a certain domain

:0* ^From.*bad-person@bad-domain.com/dev/null

● The first line says a new rule is starting. The next line checks if the 'From' field is from 'bad-person@bad-domain.com'

● The last line says move that message to /dev/null if the email is from that person

● /dev/null is like the trash bin for linux. If you move something there, it delete's it automatically.

IMAP and POP3● The last part of setting email on the server is to

have a way for users to get that email. ● The most popular way is through to services called

IMAP and POP3● IMAP - Internet Message Access Protocol● It permits a "client" email program to access

remote message stores as if they were local. ● For example, email stored on an IMAP server can

be manipulated from a desktop computer at home, a workstation at the office, and a notebook computer while traveling, without the need to transfer messages or files back and forth between these computers.

Turning on IMAP● To get IMAP and POP3 working on your server, it

is not difficult, but can be confusing because there are no startup scripts in the “rc.d” folder

● To get them started, you need to edit a file called “/etc/inetd.conf”

● This file has a list of all the services that are running and what ports they are connected on.

● Look down the list until you see the info for port “139” – pop3 and “143” – IMAP.

● Uncomment those lines and the next time you restart the server, IMAP and pop3 should be started

POP3● The other way to get email is through the POP3

service● POP – Post Office Protocol● It was the first way to get email from a server.

POP3 is the latest version, which has replaced POP2.

● POP is different from IMAP in that everything is downloaded to the client machine. Thus, if you make a change to your mail, it will only be changed on the client machine and not the server

● You can turn it on using the same procedures from IMAP

SMTP● You will also hear about SMTP servers if you use

computers.● SMTP – Simple Mail Transfer Protocol – is a program that

runs on a server and will send messages● When we set up and turned on Sendmail, the SMTP server

also started. ● When you send email from a client computer, you need a

SMTP server to send it through. This is what is responsible for sending those messages to the rest of the Internet

● It operates on port 25. A good way to see if it is running, try– [root@comp root]# telnet localhost 25

● What this command will do is use the telnet program to connect to port 25 on the computer you are using. You will then be able to see the protocol and server messages coming from the SMTP server

IMAP and POP● You can also see how the IMAP and POP

servers work by using the same telnet idea● IMAP runs on port 143● POP3 runs on port 110

– [root@comp root]# telnet localhost 143– [root@comp root]# telnet localhost 110

● What happens then is that the IMAP or POP server thinks it is connected to something that will know how to speak it's language. If you knew the protocol that IMAP or POP used, you could get your email this way too.

Local and Remote Email● You should be aware of when you are getting email from

remotely and when you are accessing it locally on the server

● If you telnet into a server, and use a program like 'pine' or 'mutt' this means that you are accessing the local copy of the email.

● Your email is saved in a file in the “/var/mail” directory under your username. These programs open up that file and break it up so that it looks like there are many messages

● IMAP and POP3 are services that run on a server that allow client computers to use different programs (like Microsoft Outlook) to access email over a network. IMAP and POP will also open up your mail file, but they are not programs you use to read email. They are servers that will send your email to program that asks for it

Updating programs● Sendmail, especially, but also IMAP and POP3

have been vulnerable to many security holes in the past.

● This is because they run as root, so if you can break in through the program, you can have root access to the system

● So one of the first things to do is to update Sendmail (get the newest from sendmail.org) and then IMAP as well.

● Generally, the installations are pretty easy, but we might need to change startup scripts.

Summary● Again, we've condensed a lot of information into a

short sequence of notes. Sendmail, procmail, IMAP and POP are all powerful programs with many different possible configurations

● For more information about Sendmail:

– http://www.sendmail.org/● More information about Procmail

– http://userpages.umbc.edu/~ian/procmail.html● More info about IMAP

– http://www.imap.org/about/● More info about SMTP

– http://www2.rad.com/networks/1998/smtp/smtp.htm