Stunnel Setup

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Stunnel is a SSL proxy designed to add TLS encryption to existing clients and servers without changes to the daemon's themselves. One (or more) endpoint is run in server mode, the other endpoint is run in client mode. The daemon software connects to a localhost port, the connection is proxied over the SSL tunnel, then handed to the server localhost port as defined.

Wiki Conventions

In this wiki, a traditional MariaDB replication configuration will be used to exemplify use as compatible version 5.5.x is available on both distributions. One server is located in one area of the USA, the second server in another USA region, and standard public IPv4 networking to connect the two servers. Two different Linux distributions will be used to verify the technology is agnostic.

  • s1 = CentOS 7 as server, IP
  • s2 = Ubuntu 14 as client, IP

Actual public IPs would be used in implementation as appropriate. This wiki will not cover setting up MariaDB replication as it's a standard, by the book process however two notes:

  1. On the MariaDB master GRANT REPLICATION SLAVE ON ... stanza for the user, use <user>@localhost not the actual IP of the remote slave like you would normally.
  2. For both master and slave MariaDB instances, implement bind-address = to lock the daemons to localhost.
  3. Remember to use MASTER_HOST='localhost' in your CHANGE MASTER TO ... stanza on the slave to match the user created in note 1

MariaDB traffic will travel over the stunnel proxy, so they should not listen on the public IPs for security best practices.


The stunnel package may be a part of the base distribution or it may be required to use a third party repository such as EPEL or a PPA to obtain. In these examples with CentOS 7 and Ubuntu 14 the package is readily available for both in the base repositories.

RHEL / CentOS 7 as Server

First install the base package:

yum install stunnel

Next, create a user and directories to run the software - on this platform the RPM package does not create the user or directories:

# the user and directory for immediate use:
useradd -r -m -d /var/run/stunnel -s /bin/false stunnel

# the tmpfiles.d configuration to recreate the directory on reboot:
echo "d /var/run/stunnel 0770 stunnel stunnel -" > /etc/tmpfiles.d/stunnel.conf

Lastly, create the systemd unit file to run stunnel as a service:

cat << XYZZY > /etc/systemd/system/stunnel.service
Description=SSL tunnel for network daemons



Ubuntu 14 as Client

First install the base package:

apt-get update && apt-get install stunnel

Next, update /etc/default/stunnel4 to enable it at boot:

sed -i -e 's/^ENABLED=0/ENABLED=1/' /etc/default/stunnel4

Ubuntu creates the stunnel4 user and group, and /var/run/stunnel4 directory as part of the package.

Server Configuration

First, create a basic self-signed certificate to use on the server; if a real SSL cert is available from a certificate authority it can be used, however a self-signed cert works for the basic point-to-point setup.

openssl req -new -newkey rsa:2048 -days 3650 \
  -nodes -x509 -sha256 \
  -subj '/CN=' \
  -keyout /etc/stunnel/stunnel.pem \
  -out /etc/stunnel/stunnel.pem

Next, create the stunnel server oriented config file; in our example we're using MariaDB so we'll choose the ports accordingly to have stunnel accept the connection on the public IP port 3307, then pass the connection to the localhost port 3306:

chroot = /var/run/stunnel
setuid = stunnel
setgid = stunnel
pid    = /
fips   = no

client     = no
accept     =
connect    =
cert       = /etc/stunnel/stunnel.pem
key        = /etc/stunnel/stunnel.pem
# stunnel 4.53 (Ubuntu 14) only supports TSLv1 not TLSv1.2
# stunnel 4.56 (CentOS 7) supports both TLSv1 and TSLv1.2
sslVersion = TLSv1

Last, as appropriate open up an iptables/firewalld/ufw rule that allows the client to connect on port 3307; a very basic iptables rule with no port restrictions would look like:

-A INPUT -p tcp -m tcp -s --dport 3307 -j ACCEPT

Tailor the ACL on your firewall(s) as needed to meet your desired security posture. Assuming MariaDB is up and running, start and enable the stunnel service:

systemctl start stunnel

Client Configuration

The client does not require a SSL certificate; create the client oriented config file that accepts a connection on local port 3307 and talks to the remote stunnel on 3307:

chroot = /var/run/stunnel4
setuid = stunnel4
setgid = stunnel4
pid    = /

client     = yes
accept     =
connect    =
# stunnel 4.53 (Ubuntu 14) only supports TSLv1 not TLSv1.2
# stunnel 4.56 (CentOS 7) supports both TLSv1 and TSLv1.2
sslVersion = TLSv1

Once again, assuming the basic MariaDB is up and running, start the stunnel service:

service stunnel start


From the Ubuntu client, use the standard mysql command to connect to remote instance and observe the hostname (or some other test of your design) to ensure you're connecting to the remote MariaDB, not local:

root@s2:~# mysql -p --host= --port=3307 -e "show variables like '%hostname%';"
Enter password: 
| Variable_name | Value    |
| hostname      | s1.local |

Given that it connected, you are clear to proceed with configuring normal replication between the two instances.