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Stunnel

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stunnel (“Secure Tunnel”) is a

cross-platform application used to provide a universal TLS/SSL tunneling service. It is a sort of proxy designed to add TLS encryption functionality to existing clients and servers without any changes in the programs' code. It is designed for security, portability, and scalability (including load-balancing), making it suitable for large deployments. It uses OpenSSL, and distributed under GNU GPL version 2 or later with OpenSSL exception.

Stunnel input can only be TCP packets. Its FAQ has some work around for UDP. WireGuard also has UDP capabilities. Conceptually, stunnel can be thought of as a generalization of https, or a light tunnel. Basically, it just wrap/unwrap the data within TLS/SSL. It can be used with traditional network interfaces and TCP/IP stack.

Authentication can also be used by the server to allow access only to approved clients.

Installation

Install the stunnel package.

Depending on your usage, you might also edit the provided systemd units to better handle dependencies. In order for the stunnel to start up automatically at system boot you must enable it.

Configuration

The main configuration file is read from /etc/stunnel/stunnel.conf. It is an ini-style file. It is composed from a global section, followed by one, or more, service sections.

Tip /etc/stunnel/stunnel.conf-sample has examples for a few configurations.

A client is one to accept non TLS encrypted data. Stunnel will encrypt its data with TLS and connect to the stunnel server. The stunnel server accepts TLS encrypted data and decrypts it. It then connects to where the data should be sent to.

The default debug value is notice, which is very verbose. After verifying correct operation, it is worth explicitly setting lower value in the configuration file. 

/etc/stunnel/stunnel.conf
debug = err

For better security, it is advised to explicitly set an appropriate uid and gid, other then root, for the global section and the per service sections. The configuration tokens setuid and setgid are available for this purpose.

Byte order mark (BOM)

The configuration file should have a UTF-8 byte order mark (BOM), at the beginning of the file. A BOM is the unicode character U+FEFF. Its UTF-8 representation is the (hexadecimal) byte sequence 0xEF, 0xBB, 0xBF. Creating a file with these bytes at its beginning can be done by 

# echo -e '\xef\xbb\xbf; BOM composed of non printable characters. It is here, before the semicolon!' > /etc/stunnel/stunnel.conf

To test if those bytes appear, one can use 

% od --address-radix=n --format=x1c --read-bytes=8 /etc/stunnel/stunnel.conf
  ef  bb  bf  3b  20  42  4f  4d
 357 273 277   ;       B   O   M

Note that when printing the file to the screen, such as with cat, or when editing the file with a text editor, the BOM bytes are usually not displayed. They should be there, though. Which is why you might want to verify that they are still there after editing is completed with the above od, or similar, command.

Tip When using Kate, add a comment containing kate: bom on; in the first or last 10 lines and the UTF-8 BOM will be automatically added to the file.

Pre Shared Key

At least one of the client and the server, and optionally both, should be authenticated. Either a pre shared secret, or a key and certificate pair, can be used for authentication. A pre shared secret has to be transferred to all involved machines a priory by other means, such as SCP and SFTP. When such transfer is acceptable, pre shared key is the fastest method. Its speed might help mitigating attacks. A simple configuration for a single server with a single client that are using a pre shared secret is:

Client: 

/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!
setuid = stunnel
setgid = stunnel

[trivial client]
client     = yes
accept     = 127.0.0.1:src_port
connect    = server_host:server_port
debug      = 3
PSKsecrets = /etc/stunnel/psk.txt
setuid     = stunnel
setgid     = stunnel

Server: 

/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!
setuid = stunnel
setgid = stunnel

[trivial server]
accept     = server_port
connect    = dst_port
debug      = 3
PSKsecrets = /etc/stunnel/psk.txt
setuid     = stunnel
setgid     = stunnel

where /etc/stunnel/psk.txt could be created on one machine by 

# openssl rand -base64 -out /etc/stunnel/psk.txt 180
# sed --in-place --null-data 's/\n//g;1s/^/psk:/' /etc/stunnel/psk.txt
# chmod 640 /etc/stunnel/psk.txt

and copied to the other machine by secure means before starting stunnel. The permissions for each psk.txt file should be set appropriately, neither world-readable nor world-writable. The psk string from the sed command is just a random name for the sake of the example. Do read stunnel(8).

Using Certificates

Per the upstream examples, such as those in /etc/stunnel/stunnel.conf-sample, there is an option to get Stunnel to run with certificates. With proper Stunnel configurations, the certificates used need not be trusted by the system. However, when the certificates chain is not trusted by the system, the clients, and optionally the server, must approved it before using it. For example, by holding a copy of the whole chain, including the root certificates, in advance.

Follows an example to allow only a limited number of clients to actually use the traditional tcp version of the echo service. With this example, even if the external tcp port of the echo server, 12345 in the example, is open to anyone, the server will deny any client whose certificates it does not have. In effect, with such usage of certificates, the clients certificates are actually like pre shared secrets.

Client: 

/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!

[traditional echo client]
CAfile       = /etc/stunnel/server_req.pem
accept       = ::1:54321
cert         = /etc/stunnel/clients_req.pem
client       = yes
connect      = Server_IPv6:12345
debug        = 5
key          = /etc/stunnel/clients_key.pem
setgid       = stunnel
setuid       = stunnel
verifyChain  = yes
verifyPeer   = yes

Server: 

/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!

[traditional echo server]
CAfile       = /etc/stunnel/clients_req.pem
accept       = Server_IPv6:12345
cert         = /etc/stunnel/server_req.pem
connect      = ::1:7
debug        = 5
key          = /etc/stunnel/server_key.pem
setgid       = stunnel
setuid       = stunnel
verifyChain  = yes
verifyPeer   = yes

The required certificates can be generated as self-signed certificates. For example, by 

# openssl req -new -x509 -subj "/C=GB/ST=LONDON/O=TOWER OF LONDON/CN=DOMAINS" -key <(openssl genpkey -algorithm ED448) -noenc -days 365 -out server_req.pem -keyout server_key.pem

And a similar command can be used to generate the certificate for each client. When it is acceptable for all the clients to share a single certificate, this reduces to 

# openssl req -new -x509 -subj "/C=CA/ST=ONTARIO/O=TOWER OF LONDON ASSOCIATION/CN=CA BRANCH" -key <(openssl genpkey -algorithm ED448) -noenc -days 365 -out clients_req.pem -keyout clients_key.pem

Test it: 

$ echo traditional echo protocol | ncat ::1 54321
traditional echo protocol

See also