Beyond Linux® From Scratch (systemd Edition) - Version r12.0-816

Chapter 4. Security

stunnel-5.71

Introduction to stunnel

The stunnel package contains a program that allows you to encrypt arbitrary TCP connections inside SSL (Secure Sockets Layer) so you can easily communicate with clients over secure channels. stunnel can also be used to tunnel PPP over network sockets without changes to the server package source code.

[Note]

Note

Development versions of BLFS may not build or run some packages properly if LFS or dependencies have been updated since the most recent stable versions of the books.

Package Information

stunnel Dependencies

Optional

libnsl-2.0.1, netcat (required for tests), tcpwrappers, and TOR

Installation of stunnel

The stunnel daemon will be run in a chroot jail by an unprivileged user. Create the new user and group using the following commands as the root user: 

groupadd -g 51 stunnel &&
useradd -c "stunnel Daemon" -d /var/lib/stunnel \
        -g stunnel -s /bin/false -u 51 stunnel
[Note]

Note

A signed SSL Certificate and a Private Key is necessary to run the stunnel daemon. After the package is installed, there are instructions to generate them. However, if you own or have already created a signed SSL Certificate you wish to use, copy it to /etc/stunnel/stunnel.pem before starting the build (ensure only root has read and write access). The .pem file must be formatted as shown below: 

-----BEGIN PRIVATE KEY-----
<many encrypted lines of private key>
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
<many encrypted lines of certificate>
-----END CERTIFICATE-----
-----BEGIN DH PARAMETERS-----
<encrypted lines of dh parms>
-----END DH PARAMETERS-----

Install stunnel by running the following commands: 

./configure --prefix=/usr        \
            --sysconfdir=/etc    \
            --localstatedir=/var &&
make

If you have installed the optional netcat application, the regression tests can be run with make check.

Now, as the root user: 

make docdir=/usr/share/doc/stunnel-5.71 install

Install the included systemd unit by running the following command as the root user: 

install -v -m644 tools/stunnel.service /usr/lib/systemd/system

If you do not already have a signed SSL Certificate and Private Key, create the stunnel.pem file in the /etc/stunnel directory using the command below. You will be prompted to enter the necessary information. Ensure you reply to the 

Common Name (FQDN of your server) [localhost]:

prompt with the name or IP address you will be using to access the service(s).

To generate a certificate, as the root user, issue: 

make cert

Command Explanations

make docdir=... install: This command installs the package and changes the documentation installation directory to standard naming conventions.

Configuring stunnel

Config Files

/etc/stunnel/stunnel.conf

Configuration Information

As the root user, create the directory used for the .pid file created when the stunnel daemon starts: 

install -v -m750 -o stunnel -g stunnel -d /var/lib/stunnel/run &&
chown stunnel:stunnel /var/lib/stunnel

Next, create a basic /etc/stunnel/stunnel.conf configuration file using the following commands as the root user: 

cat > /etc/stunnel/stunnel.conf << "EOF"
; File: /etc/stunnel/stunnel.conf

; Note: The pid and output locations are relative to the chroot location.

pid    = /run/stunnel.pid
chroot = /var/lib/stunnel
client = no
setuid = stunnel
setgid = stunnel
cert   = /etc/stunnel/stunnel.pem

;debug = 7
;output = stunnel.log

;[https]
;accept  = 443
;connect = 80
;; "TIMEOUTclose = 0" is a workaround for a design flaw in Microsoft SSL
;; Microsoft implementations do not use SSL close-notify alert and thus
;; they are vulnerable to truncation attacks
;TIMEOUTclose = 0

EOF

Finally, add the service(s) you wish to encrypt to the configuration file. The format is as follows: 

[<service>]
accept  = <hostname:portnumber>
connect = <hostname:portnumber>

For a full explanation of the commands and syntax used in the configuration file, issue man stunnel.

Systemd Unit

To start the stunnel daemon at boot, enable the previously installed systemd unit by running the following command as the root user: 

systemctl enable stunnel

Contents

Installed Programs: stunnel and stunnel3
Installed Library: libstunnel.so
Installed Directories: /{etc,lib,var/lib}/stunnel and /usr/share/doc/stunnel-5.71

Short Descriptions

stunnel

is a program designed to work as an SSL encryption wrapper between remote clients and local or remote servers

stunnel3

is a Perl wrapper script to use stunnel 3.x syntax with stunnel 4.05 or later

libstunnel.so

contains the API functions required by stunnel