Encrypting and forwarding local email to an external email address


Last week I set out to find a reliable and permanent solution to my problems with unread system mail and undelivered cron error messages, and I can now see that sending, delivering, and receiving email is far more complicated than I had thought. I have had to learn more about SMTP, SSL/TLS, and email delivery than I ever wanted to know, and I still only know the bare basics.

When I started I had two requirements:

  1. All local email sent to the root user (and preferably everyone else) must be forwarded to an external SMTP server and email account
  2. All email that leave my network must be encrypted

After experimenting with number of different applications (GNU Anubis, Nullmailer, ESMTP, MSMTP, etc) I finally found something that worked; Postfix with GPG-Mailgate. If all you need is the ability to send email to an external account then there are other applications you could use instead (I had some success with MSMTP and Nullmailer). The advantage of using Postfix is its flexibility and maturity. All other applications I tried had some small thing they could not do reliably or would fail in some edge cases, such as only forwarding some of the emails sent to root, but not quite all of them.

This guide will show each step needed to set this up, and a few mistakes to look out for. The first part will cover Postfix and how to configure it to forward all emails sent to a local user to an external email account. The second part will show how to set up GPG-Mailgate so that all emails that are sent to certain accounts are encrypted with GPG before they leave the server. If all you need is mail forwarding then you can stop after the first part, and if you already have Postfix configured you can jump straight to the second part.

A few assumptions will be made in this guide.

  1. You are using the root account. If you are not then you will have to prepend ‘sudo’ to some commands.
  2. You are using Debian. Everything should still work if you are using a different Linux distribution, but you may have to make some minor changes.
Names and servers

I will be using the following user names, email accounts, and servers in this guide.

User names

root : The local root user

gpgmap : The user account used for GPG-Mailgate

Email accounts

[email protected] : The fully qualified email address of the root user

[email protected] : An external email account that we want to send email through

[email protected] : An external email account that we want to forward all local emails to

Domains

yourdomain.com : Your local domain

emailprovider.com : The domain of your email provider

anotherdomain.com : The domain of the email provider for the account you want to forward all local email to

Servers

smtp.emailprovider.com : The SMTP server of your email provider

server.yourdomain.com : The computer you want to forward local emails from

Part 1 – Forwarding local email to an external SMTP server

The first step is to install and configure Postfix to forward all emails sent to the root user (or any user you want) to an external SMTP server and email account. This is not difficult, but it does require a few steps.

First, if you do not already have Postfix install it and some dependencies.

apt-get install postfix libsasl2-modules

Fill in any domain names the installer asks for and choose the “satellite” option.

If you are not already using Postfix then you are probably running Sendmail. Before starting Postfix you will have to turn it off. To avoid future error messages and warnings I also recommend you completely uninstall it.

service sendmail stop

apt-get remove sendmail-base sendmail-cf sendmail-doc

Next, start postfix.

service postfix start

Make sure that it started correctly.

service postfix status

Next comes the hard part (not really); configuring Postfix. The exact details depends on your email provider. Everything I describe here will be completely compatible with Gmail.

Most of the settings you need should have been configured for you by the installer.

Open ‘/etc/postfix/main.cf’ and make sure that the relay host is set to your email provider’s SMTP server.

relayhost = smtp.emailprovider.com:587

And add these lines:

smtp_use_tls=yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_sasl_tls_security_options = noanonymous
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Next, create a password file ‘/etc/postfix/sasl_passwd’ and add the following to it:

smtp.emailprovider.com:587 [email protected]:yourpassword

The SMTP hostname and port must match relayhost in the main.cf file.

Update Postfix’s password database and settings.

postmap /etc/postfix/sasl_passwd
service postfix reload

Everything you need to send emails through Postfix using your email provider’s SMTP server should now be in place. Before proceeding you should test that it is working.

echo "Testing Postfix email delivery" | mail -s "Test email" [email protected]

If the email doesn’t arrive check in ‘/var/log/sysIog’ for errors and make sure that you can successfully send emails using the same settings from an email client (such as Thunderbird).

Now that that is working we only need to tell Postfix to forward all emails sent to the root user to an external email account. Open ‘/etc/aliases’ and add the following line:

root: [email protected]

If you want to forward emails for other users than root you simply add them to the list as well. When you are done run:

newaliases

That’s it. If everything is configured properly all emails sent to the root user will now be forwarded to [email protected]

echo "Testing Postfix forwarding" | mail -s "Test email" root

If that was all you needed then you can stop here, but you are now sending unencrypted emails through the internet containing potentially sensitive information. If you think that sounds dangerous then continue to the second part were we will set up some automated GPG encryption.

A warning about restrictive SMTP servers and the From header

It is possible that these settings won’t work with your particular email provider. I’m using a Gmail account to forward email. It isn’t my primary email provider, but there are practical reasons for using it in this case. Unlike some other email providers Gmail does not care about what you specify in the ‘From’ header. It will overwrite whatever is in it with the Gmail user you used. This simplifies things greatly when forwarding local emails since you would otherwise have to change the contents of the header before sending it out. Postfix has the ability to replace addresses in outbound email (just google smtp_generic_maps or sender_canonical_maps and you should find instructions for how to configure Postfix), however, depending on how an application fills out the headers in emails they send out this may not work reliably in all situations. I tried to set this up with a more restrictive Zoho.com email account, but I could not get it to forward cron error messages.

If you need this for your email provider, and if you manage to set it up successfully, please leave a comment explaining how you did it. For everyone else; just use a Gmail account. It’s easier.

Part 2 – Encrypting outgoing email with GPG

It is possible that some of the applications sending email to your local accounts might send data that you don’t want any stranger on the internet to see. In my paranoid mind that just isn’t acceptable, and so we will be adding a layer of encryption to Postfix using GPG-Mailgate. GPG-Mailgate is a content filter script for Postfix that will encrypt a received email if there is a public GPG key available for its recipient, and if the email is not already encrypted. It is relatively easy to install and configure, but be warned that if you do not configure it correctly it will probably fail silently and send out empty emails.

First, either download the source code from Github or clone it.

git clone https://github.com/ajgon/gpg-mailgate.git

Next, manually put everything where it needs to be. Replace python2.6 with your python version.

cd gpg-mailgate
cp gpg-mailgate.py /usr/local/bin/gpg-mailgate.py
cp -r GnuPG /usr/lib/python2.6/

Make sure that all permissions are correct.

chown root:root /usr/local/bin/gpg-mailgate.py
chmod 755 /usr/local/bin/gpg-mailgate.py
chown -R root:root /usr/lib/python2.6/GnuPG
chmod 755 /usr/lib/python2.6/GnuPG
chmod 644 /usr/lib/python2.6/GnuPG/__init__.py

Create a user to run the GPG-Mailgate script as and import the public key you want to encrypt forwarded email with.

useradd -s /bin/false -d /var/gpg -M gpgmap
mkdir -p /var/gpg/.gnupg
chown -R gpgmap:gpgmap /var/gpg/.gnupg
chmod 700 /var/gpg/.gnupg
sudo -u gpgmap /usr/bin/gpg --import yourpublic.key

yourpublic.key is the public part of your GPG key pair. If you don’t have one already then your will need to create one. How to do that is beyond the scope of this guide, but it isn’t difficult.

Check that everything worked and that the key is in place.

sudo -u gpgmap /usr/bin/gpg --list-keys  --keyid-format long

This should give you something like this.

/var/gpg/.gnupg/pubring.gpg
 ---------------------------
pub 4096R/0123456789ABCDEF 2014-09-17
uid                        Your Name <[email protected]>
sub 4096R/FEDCBA9876543210 2014-09-17

Save whatever you have instead of ‘0123456789ABCDEF’. This is the identifier of your public key and you will need it later.

Add the following to the end of ‘/etc/postfix/master.cf’:

#GPG-Mailgate
gpg-mailgate unix - n n - - pipe
flags= user=gpgmap argv=/usr/local/bin/gpg-mailgate.py ${recipient}

127.0.0.1:10028 inet n - n - 10 smtpd
  -o content_filter=
  -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
  -o smtpd_helo_restrictions=
  -o smtpd_client_restrictions=
  -o smtpd_sender_restrictions=
  -o smtpd_recipient_restrictions=permit_mynetworks,reject
  -o mynetworks=127.0.0.0/8
  -o smtpd_authorized_xforward_hosts=127.0.0.0/8

Make certain that the user name and script location matches what you used above.

Add the following to ‘/etc/postfix/main.cf’:

content_filter = gpg-mailgate

And reload Postfix’s settings.

service postfix reload

Create a configuration file for GPG-Mailgate at ‘/etc/gpg-mailgate.conf’ and add the following to it:

[default]
# whether gpg-mailgate should add a header after it has processed an email
# this may be useful for debugging purposes
add_header = yes

# whether we should only sign emails if they are explicitly defined in
# the key mappings below ([keymap] section)
# this means gpg-mailgate won't automatically detect PGP recipients
keymap_only = yes

[gpg]
# the directory where gpg-mailgate public keys are stored
# (see INSTALL for details)
keyhome = /var/gpg/.gnupg

[logging]
# For logging to syslog. 'file = syslog', otherwise use path to the file.
file = syslog
verbose = no

[relay]
# the relay settings to use for Postfix
# gpg-mailgate will submit email to this relay after it is done processing
# unless you alter the default Postfix configuration, you won't have to modify this
host = 127.0.0.1
port = 10028

[keymap]
# You can find these by running the following command:
# gpg --list-keys --keyid-format long [email protected]
# Which will return output similar to:
# pub 1024D/AAAAAAAAAAAAAAAA 2007-10-22
# uid Joe User <[email protected]>
# sub 2048g/BBBBBBBBBBBBBBBB 2007-10-22
# You want the AAAAAAAAAAAAAAAA not BBBBBBBBBBBBBBBB.
#[email protected] = <gpg key id>
[email protected] = 0123456789ABCDEF
[email protected] = 0123456789ABCDEF

Replace the addresses and keys on the last two lines with the email address you want to forward emails to and the identifier of the public key you imported earlier.

The final line is a bit of a hack. GPG-Mailgate uses the ‘To’ header to find the GPG key to use when encrypting email. If it can’t find a matching entry in the configuration file it will not encrypt it. This creates a minor complication since Postfix doesn’t rewrite the ‘To’ header when forwarding local email. Any email sent to the root user will therefore have a ‘To’ address of ‘[email protected]’; which GPG-Mailgate won’t recognise. To fix this we add the local email address to the configuration file as well.

This should be all. Now test that everything is working.

echo "Testing GPG encryption" | mail -s "Test GPG" [email protected]
echo "Testing GPG encryption to root user" | mail -s "Test GPG root" root

These commands should both send an encrypted email to [email protected] If it doesn’t work check ‘/var/log/syslog’ for errors.

If GPG-Mailgate did not encrypt your emails then it is likely that it did not find a matching public key. Make sure that the keys and addresses in ‘/etc/gpg-mailgate.conf’ are correct.

If you get empty messages then it is likely that GPG returned an error to GPG-Mailgate. This will cause GPG-Mailgate to fail silently. Make sure that the gpgmap user has the permissions needed to use the GPG keys in ‘/var/gpg/.gnupg’ and that gpgmap is used by Postfix when running GPG-Mailgate.

If you get an error about a missing GnuPG module then you either set the wrong permissions for the GnuPG folder and its contents, or you placed it in the wrong python folder.

If everything is working than this should be it.

  • You can now send email through an external SMTP server from the command line
  • All emails sent to the root user are forwarded to an external email address
  • All emails sent to [email protected] are automatically encrypted before they leave the server

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