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.github | 1 month ago | |
letmein | 1 month ago | |
letmein-conf | 1 month ago | |
letmein-proto | 1 month ago | |
letmein-systemd | 1 month ago | |
letmeind | 1 month ago | |
maintenance | 2 months ago | |
pic | 2 months ago | |
.gitignore | 3 months ago | |
Cargo.lock | 1 month ago | |
Cargo.toml | 1 month ago | |
LICENSE-APACHE | 3 months ago | |
LICENSE-MIT | 3 months ago | |
README.md | 1 month ago | |
SECURITY.md | 1 month ago | |
build.cmd | 2 months ago | |
build.sh | 2 months ago | |
install-client.sh | 1 month ago | |
install-server.sh | 1 month ago | |
install.sh | 3 months ago | |
nftables.conf | 3 months ago |
Letmein is a simple port knocker with a simple and secure authentication mechanism. It can be used to harden against pre-authentication attacks on services like SSH, VPN, IMAP and many more.
Letmein hides services on a server behind a knock authentication barrier to reduce the attack surface of a service. The service will not be accessible unless a knock authentication is successful. In case of a successful knock, the letmeind server will only open the knocked port for the client IP address that performed the knocking. Machines with different IP addresses still won't have access to the protected service.
Machines that can't successfully authenticate the knock sequence won't be able to access the protected service.
They will receive a TCP/ICMP reject
on the protected service port with the provided example nftables.conf
.
(You can also decide to drop
the packets in your nftables.conf
instead).
Letmein requires an nftables
based firewall.
It will not work with iptables
.
If you use an iptables
based firewall, please convert to nftables
before installing letmein.
There are descriptions about how to do that on the Internet.
It's not as hard and as much work as it sounds. :)
On the server install the letmein server software (see sections below).
On the client install the letmein client software (see sections below).
Please read the nftables.conf example configuration file provided with this project.
Adding a letmein specific input chain to your existing nftables
configuration is required.
Modify your nftables.conf
accordingly.
Generate shared secret key and a user identifier to be installed on the server and client with the following client command:
letmein gen-key -u 00000000
The gen-key command will print the generated key string to the console.
By default this will generate a secure random key for the user identifier 00000000
.
You can manually edit the user identifier, if you want, or you can just leave it as-is.
Add the generated string (user identifier and the shared secret) to the server configuration in /opt/letmein/etc/letmeind.conf
.
Put the generated key string together with the user identifier into the [KEYS]
section of the configuration file.
Add the same generated string (user identifier and shared secret) to the client configuration in /opt/letmein/etc/letmein.conf
.
Put the generated key string together with the user identifier into the [KEYS]
section of the configuration file.
Create a resource
in the server that describes the sshd
port that can be opened.
In the [RESOURCES]
section of the server configuration file /opt/letmein/etc/letmeind.conf
all ports that may be opened must be specified. A resource consists of a resource identifier followed by a port identifier like that:
[RESOURCES]
00000022 = port: 22
The resource identifier is an 8 digit hexdecimal number. In this example it is 22(hex), but it can be any number. It just has to be the same number on the server and the client. After port:
the port number (in decimal) that can be knocked-open is specified.
Add the same resource with the same resource identifier and the same port number to the client configuration in /opt/letmein/etc/letmein.conf
.
Restart the letmein server:
systemctl restart letmeind.service
Now remove your static sshd
port (22) accept
from your nftables.conf
firewall configuration.
Letmein will install such a rule dynamically into the letmein input chain after successful knock authentication.
Then restart nftables:
systemctl restart nftables.service
Done! You should now be able to knock-open the sshd
port on your server:
# This must fail! No successful knock authentication, yet.
# If this does not fail, check if you have removed the sshd accept rule from nftables.conf.
ssh your-server.com
# Knock-open port 22 (sshd) on the server using user-id/key 00000000:
# (You do not have to specify -u 00000000 if that is your default user (see config).)
letmein knock -u 00000000 your-server.com 22
# Now you should be able to ssh into your server successfully:
ssh your-server.com
To automatically knock the port before connecting with ssh, you can add a Match exec
rule to your ~/.ssh/config
file:
Match host your-server.com exec "letmein knock -u 00000000 your-server.com 22"
The Rust compiler must be installed to build letmein. It is recommended to use the latest version of the stable Rust compiler:
The Rust installer will install the compiler and the build tool cargo
.
The build requires the additional cargo-audit
and cargo-auditable
tools to be installed.
Run this command to install both tools:
cargo install cargo-audit cargo-auditable
At runtime the nftables nft
binary is required to be installed reachable in the $PATH
.
On Debian please install the nftables
package:
apt install nftables
Run the build.sh
script to build letmein.
After installing all build prerequisites, run the build script:
./build.sh
After building, run the install-server.sh
to install the letmeind server to /opt/letmein/
:
./install-server.sh
Installing the server will also install the service and socket into systemd and start the letmeind server.
The server is used to receive knock packets from the client.
Upon successful knock authentication, the server will open the knocked port in its nftables
firewall.
Then run the install-client.sh
to install the letmein client to /opt/letmein/
:
./install-client.sh
The client is used to send a knock packet to the server.
Please be aware that the user identifiers and resource identifiers from the configuration files are transmitted over the network without encryption in clear text.
Make sure the user identifiers and resource identifiers do not include any private information.
These identifiers are merely meant to be an abstract identification for managing different letmein
keys, installations and setups.
The server application letmeind
is Linux-only, because it only supports nftables
as firewall backend.
The client application letmein
is portable and should run on all major platforms.
Tested platforms are:
The main design goals of letmein are:
-4
and -6
client command line options.The program has carefully been designed to be secure, to the best of my knowledge.
However, nobody is infallible.
I am interested to hear your opinion.
If you found a security vulnerability, see the vulnerability reporting process for how to proceed.
There are a couple of known weaknesses that exist in letmein. In this paragraph we discuss why these weaknesses exist.
These weaknesses are not addressed by the design of letmein to make the design simpler. It is a tradeoff between a simple design and a weakness that doesn't practically impact security.
It is believed that these weaknesses do not make letmein insecure in practical use. The simple design is supposed to reduce the attack surface and as such improve security.
weakness: The user identifiers and resource identifiers from the configurations are transmitted in plain text over the network.
weakness: The first Knock
packet is not protected against a replay attack.
Knock
packet can successfully be replayed by an attacker.
But that doesn't mean much.
The attacker will still not be able to successfully solve the Challenge
.
The authentication of the Knock
is only in place, because it's easy to implement in the given design and it stops port knocks that don't have a key at all early.weakness: After a successful knock sequence from legitimate user an MiM attacker can use the knocked-open port, if she is able to use the same sender-IP-address as the legitimate user.
weakness: If you knock a port open from behind a NAT, then the port will be opened for the whole NATed network, because from the outside the NATed network has only one IP address. Everybody from within the NATed network will be able to access the knocked-open port.
weakness: The authentication key is a shared secret that is stored in plain text on the server and on the client.
weakness: All users that can successfully authenticate to letmein can start to attack the protected service.
weakness: The wire protocol does not have mechanisms for future changes and updates.
Copyright (c) 2024 Michael Büsch m@bues.ch
Licensed under the Apache License version 2.0 or the MIT license, at your option.