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Authenticated SMTP tutorial

IndiMail supports three AUTH methods. LOGIN, PLAIN and CRAM-MD5. Most email clients like thunderbird, outlook, outlook express, evolution support these methods. These methods are provided using checkpassword compatible modules vchkpass(8) and pam-checkpwd(8)

To understand how these methods work is to use telnet and the base64 encoding/decoding utility /usr/bin/base64

For illustration purpose, let's say we have a user '' with the password 'pass'

% echo -n | /usr/bin/base64 -i

% echo -n pass | /usr/bin/base64 -i

% telent 0 smtp
220 Laptop (NO UCE) ESMTP IndiMail 1.28 21 Jun 2003 22:35:24 +0530
auth login
334 VXNlcm5hbWU6
334 UGFzc3dvcmQ6
235 ok, go ahead (#2.0.0)


% printf "\\0pass" | /usr/bin/base64

% telnet 0 smtp
Connected to 0.
Escape character is '^]'.
220 Laptop (NO UCE) ESMTP IndiMail 1.28 21 Jun 2003 23:08:33 +0530
auth plain AHBvc3RtYXN0ZXJAZXhhbXBsZS5jb20AcGFzcw==
235 ok, go ahead (#2.0.0)


The CRAM-MD5 is a challenge-response method where the password is not sent over the network. It is expected that the password is stored in the clear in IndiMail's backend database MySQL.

% sudo /usr/bin/vpasswd -e pass

Next step is to write a script named cram-md5

% cat > cram-md5 <<>"
str=cram_md5_response(sys.argv[1], sys.argv[2], sys.argv[3]);
print "%s" %str

% sudo chmod +x ./cram-md5

Now when you do (see below) auth cram-md5, the server will issue a challenge
e.g. in the below example, the challenge is


if you decode this, i.e.

% echo PDIwMTM3LjEyNjc1ODUxMDBAaW5kaW1haWwub3JnPg== | base64 -d
<20137 .1267585100="""">

The response for the challenge can be generated using the cram-md5 shell script which we created above. i.e.

% ./cram-md5 PDIwMTM3LjEyNjc1ODUxMDBAaW5kaW1haWwub3JnPg==

% telnet 0 smtp
Connected to 0.
Escape character is '^]'.
220 (NO UCE) ESMTP IndiMail 1.137 3 Mar 2010 08:28:17 +0530
auth cram-md5
334 PDIwMTM3LjEyNjc1ODUxMDBAaW5kaW1haWwub3JnPg==
235 ok, go ahead (#2.0.0)

Please do take a look at Erwin Hoffman's excellent tutorial on the same subject at

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