stunnel: Authentication

Either the TLS client, the TLS server, or both need to be authenticated:

  • Server authentication prevents Man-In-The-Middle (MITM) attacks on the encryption protocol.
  • Client authentication allows for restricting access for individual clients (access control).


The easiest way to configure authentication is with PSK (Pre-Shared Key). It provides both client and server authentication. PSK is also the fastest TLS authentication.


PSK authentication requires stunnel version 5.09 or higher.

Server Configuration

A trivial configuration example:

[PSK server]
accept = <server_port>
connect = <dst_port>
ciphers = PSK
PSKsecrets = psk.txt

The psk.txt file contains one line for each client:


Client Configuration

A trivial configuration example:

[PSK client 1]
client = yes
accept =<src_port>
connect = <server_host>:<server_port>
PSKsecrets = psk1.txt

The psk1.txt file only needs a single line:


Each client needs a separate secret. Otherwise, all the clients sharing the same key will have to be reconfigured if the key is compromised.


For simplicity, this tutorial only covers server authentication. The advantage of this configuration is that it does not require individual secrets for each of the clients.

Server Configuration

Unless PSK authentication is configured, each stunnel server needs a certificate with the corresponding private key. The Windows installer of stunnel automatically builds a certificate. On Unix platforms, a certificate can be built with "make cert". A certificate can also be purchased from one of the available commercial certificate authorities.

A trivial configuration example:

[certificate-based server]
accept = <server_port>
connect = <dst_port>
cert = cert.pem
key = key.pem

The "key" option may be omitted if cert.pem also contains the private key.

Client Configuration

stunnel can use an existing PKI (Public Key Infrastructure). The following configuration requires stunnel 5.15 or later:

[PKI client]
client = yes
accept =<src_port>
connect = <server_host>:<server_port>
verifyChain = yes
CAfile = ca-certs.pem
checkHost = <server_host>

The ca-certs.pem file contains the certificates of trusted certificate authorities.

Alternatively, a technique known as certificate pinning can be used. The following configuration requires stunnel version 4.46 or higher:

[pinning client]
client = yes
accept =<src_port>
connect = <server_host>:<server_port>
verifyPeer = yes
CAfile = peer-certificate.pem

The peer-certificate.pem file needs to contain the server certificate.

0 (0)
Article Rating (No Votes)
Rate this article
There are no attachments for this article.
There are no comments for this article. Be the first to post a comment.
Full Name
Email Address
Security Code Security Code
Related Articles RSS Feed
Linux - How to monitor memory usage
Viewed 430 times since Fri, Jun 8, 2018
CentOS / RHEL 7 : Configuring an NFS server and NFS client Linux NFS
Viewed 452 times since Fri, Feb 21, 2020
watchdog How to restart a process out of crontab on a Linux/Unix
Viewed 1460 times since Tue, Jul 31, 2018
LVM: Move allocated PE between Physical Volumes
Viewed 747 times since Sat, Jun 2, 2018
RHCS6: Install a two-node basic cluster
Viewed 695 times since Sun, Jun 3, 2018
CentOS / RHEL : Configure yum automatic updates with yum-cron service
Viewed 2086 times since Fri, Oct 26, 2018
Set Up SSH Tunneling on a Linux / Unix / BSD Server To Bypass NAT
Viewed 87 times since Fri, May 15, 2020
debian Install a newer kernel in Debian 9 (stretch) stable
Viewed 466 times since Sun, Sep 23, 2018
Check Detailed CPU Information In Linux With CoreFreq [Advanced]
Viewed 571 times since Thu, Apr 18, 2019
SYS: Configure a local repository. local repo
Viewed 614 times since Mon, Oct 29, 2018