sslh

Warning:

These instructions were only tested on Debian. It will probably work for other Linux distributions, but you might need to adapt the provided instructions.

sslh is a program that allows you to run several programs on port 443. Mainly it allows your SSH server and web server to share the same port.

This guide will show how to install sslh in transparent mode with nftables. It will also use the lighter sslh-select instead of the simpler sslh-fork. For other modes of operation, refer to the documentation.

Warning: Using SSH can be a violation of your corporate internet use policy. Please act responsibly. In particular, never ever create a reverse tunnel from your company network. Also this tool is not disguising SSH traffic as web but simply changing the port and can be easily detected by your network administrator.

Prerequisite

Before running this guide, you will need:

• nftables
• SSH
• Nginx

Install

$ sudo apt install sslh

When it ask you how sslh should be run, choose standalone.

Configure

sslh has several modes of operation. In this tutorial, we will use transparent mode with sslh-select.

OpenSSH

We will start by configuring OpenSSH to listen on a second port. We do that by modifying /etc/ssh/ssd_config

# What ports, IPs and protocols we listen for
Port 2200
# Use these options to restrict which interfaces/protocols sshd will bind to
ListenAddress 203.0.113.23:2200 # direct access
ListenAddress 203.0.113.23:2201 # access through sslh

You can now restart your SSH server.

Nginx

Now we will need to free port 443 so that it can be used by sslh. Edit file /etc/nginx/snippets/listen-https.conf and change the port for IPV4

listen [2001:db8:3:47d0::2e:7]:443 ssl spdy;
listen 203.0.113.23:4433 ssl spdy;

Do not restart Nginx yet.

Routing

Create file /etc/nftables/sslh.conf

#!/usr/sbin/nft -f

# Use ip as we want to configure sslh only for IPV4
table ip sslh {
        chain output {
                type route  hook output  priority -150;
                oif eth0  tcp sport { 2201, 4433 }  counter  mark set 0x4155;
        }
}

and register it in /etc/nftables.conf

include "/etc/nftables/sslh.conf"

and manually activate it with

$ sudo nft -f  /etc/nftables/sslh.conf

sslh

Create file /etc/systemd/system/sslh.service.d/override.conf (you will have to create the folder)

[Service]

# Replace the start command and make it use sslh-select 
ExecStart=
ExecStart=/usr/sbin/sslh-select --foreground $DAEMON_OPTS

# Run sslh as an user and use capabilities to bind ports
User=sslh
# Systemd 229
AmbientCapabilities=CAP_NET_BIND_SERVICE CAP_NET_ADMIN
# Systemd 228 and bellow
#SecureBits=keep-caps
#Capabilities=cap_net_bind_service,cap_net_admin+pie

# Limit access
PrivateTmp=true
PrivateDevices=true
ProtectSystem=full
ProtectHome=true

# Set routing rules automaticaly on script start
PermissionsStartOnly=true

# Check for mark 0x4155 (set by nftables) and forward packet to table 0x4155
ExecStartPre=/sbin/ip rule add fwmark 0x4155 lookup 0x4155
ExecStopPost=/sbin/ip rule del fwmark 0x4155

# Table 0x4155 to route all packets back to loopback interface
ExecStartPre=/sbin/ip route add local 0.0.0.0/0 dev lo table 0x4155
ExecStopPost=/sbin/ip route del local 0.0.0.0/0 dev lo table 0x4155

If using the Capabilities= rule for systemd 228-, you need to give the binary some capabilities as well

$ sudo setcap cap_net_bind_service,cap_net_admin+ei /usr/sbin/sslh-select

Next edit file /etc/default/sslh

# Default options for sslh initscript
# sourced by /etc/init.d/sslh

# Disabled by default, to force yourself
# to read the configuration:
# - /usr/share/doc/sslh/README.Debian (quick start)
# - /usr/share/doc/sslh/README, at "Configuration" section
# - sslh(8) via "man sslh" for more configuration details.
# Once configuration ready, you *must* set RUN to yes here
# and try to start sslh (standalone mode only)

RUN=no

# binary to use: forked (sslh) or single-thread (sslh-select) version
# systemd users: don't forget to modify /lib/systemd/system/sslh.service
DAEMON=/usr/sbin/sslh

DAEMON_OPTS="-n --transparent --listen 203.0.113.23:443 --tls 203.0.113.23:4433 --ssh 203.0.113.23:2201"

Changes done from the default

• -n Don't resolve domain name of connecting ip in logs. This allow to not loose time doing a DNS lookup for each new client
• --transparent SSH and webserver will see connection as if it where coming directly from the outside. In particular, you will get the correct connecting IP address in the logs.
• --listen 203.0.113.23:443 IP and port sslh listen to
• --tls 203.0.113.23:4433 IP and port of Nginx
• --ssh 203.0.113.23:2201 IP and port of OpenSSH
• Remove --user option. The modification in the systemd script make it run with the correct user from the start.
• Remove --pidfile option. It is useless with systemd.

Start

You can now restart Nginx and start sslh

$ sudo systemctl reload nginx && systemctl start sslh