Scroll to navigation

SSLH(8) SSLH(8)

NAME

 sslh - protocol demultiplexer

SYNOPSIS

sslh [-Fconfig file] [-t num] [--transparent] [-p listening address [-p listening address ...] [--tls target address for TLS] [--ssh target address for SSH] [--openvpn target address for OpenVPN] [--http target address for HTTP] [--xmpp target address for XMPP] [--tinc target address for TINC] [--anyprot default target address] [--on-timeout protocol name] [-u username] [-C chroot] [-P pidfile] [-v] [-i] [-V] [-f] [-n]

DESCRIPTION

sslh accepts connections on specified ports, and forwards them further based on tests performed on the first data packet sent by the remote client.

Probes for HTTP, SSL, SSH, OpenVPN, tinc, XMPP are implemented, and any other protocol that can be tested using a regular expression, can be recognised. A typical use case is to allow serving several services on port 443 (e.g. to connect to ssh from inside a corporate firewall, which almost never block port 443) while still serving HTTPS on that port.

Hence sslh acts as a protocol demultiplexer, or a switchboard. Its name comes from its original function to serve SSH and HTTPS on the same port.

Libwrap support

One drawback of sslh is that the servers do not see the original IP address of the client anymore, as the connection is forwarded through sslh.

For this reason, sslh can be compiled with libwrap to check accesses defined in /etc/hosts.allow and /etc/hosts.deny. Libwrap services can be defined using the configuration file.

Configuration file

A configuration file can be supplied to sslh. Command line arguments override file settings. sslh uses libconfig to parse the configuration file, so the general file format is indicated in <http://www.hyperrealm.com/libconfig/libconfig_manual.html>. Please refer to the example configuration file provided with sslh for the specific format (Options have the same names as on the command line, except for the list of listen ports and the list of protocols).

The configuration file makes it possible to specify protocols using regular expressions: a list of regular expressions is given as the regex_patterns parameter, and if the first packet received from the client matches any of these expressions, sslh connects to that protocol.

Probing protocols

When receiving an incoming connection, sslh will read the first bytes sent by the connecting client. It will then probe for the protocol in the order specified on the command line (or the configuration file). Therefore --anyprot should alway be used last, as it always succeeds and further protocols will never be tried.

If no data is sent by the client, sslh will eventually time out and connect to the protocol specified with --on-timeout, or ssh if none is specified.

Logging

As a security/authorization program, sslh logs to the LOG_AUTH facility, with priority LOG_INFO for normal connections and LOG_ERR for failures.

OPTIONS

Uses filename as configuration file. If other command-line options are specified, they will override the configuration file's settings.
Timeout before forwarding the connection to the timeout protocol (which should usually be SSH). Default is 2s.
Name of the protocol to connect to after the timeout period is over. Default is to forward to the first specified protocol. It usually makes sense to specify 'ssh' as the timeout protocol, as the SSH specification does not tell who is supposed to speak first and a large number of SSH clients wait for the server to send its banner.
Makes sslh behave as a transparent proxy, i.e. the receiving service sees the original client's IP address. This works on Linux only and involves iptables settings. Refer to the README for more information.
Interface and port on which to listen, e.g. foobar:443, where foobar is the name of an interface (typically the IP address on which the Internet connection ends up).

This can be specified several times to bind sslh to several addresses.

Interface and port on which to forward SSL connection, typically localhost:443.

Note that you can set sslh to listen on ext_ip:443 and httpd to listen on localhost:443: this allows clients inside your network to just connect directly to httpd.

Also, sslh probes for SSLv3 (or TLSv1) handshake and will reject connections from clients requesting SSLv2. This is compliant with RFC6176 which prohibits the usage of SSLv2. If you wish to accept SSLv2, use --anyprot instead.

Interface and port on which to forward SSH connections, typically localhost:22.
Interface and port on which to forward OpenVPN connections, typically localhost:1194.
Interface and port on which to forward XMPP connections, typically localhost:5222.
Interface and port on which to forward HTTP connections, typically localhost:80.
Interface and port on which to forward tinc connections, typically localhost:655.

This is experimental. If you use this feature, please report the results (even if it works!)

Interface and port on which to forward if no other protocol has been found. Because sslh tries protocols in the order specified on the command line, this should be specified last. If no default is specified, sslh will forward unknown protocols to the first protocol specified.
Increase verboseness.
Do not attempt to resolve hostnames: logs will contain IP addresses. This is mostly useful if the system's DNS is slow and running the sslh-select variant, as DNS requests will hang all connections.
Prints sslh version.
Requires to run under the specified username.
Requires to run under the specified chroot.
Specifies a file in which to write the PID of the main server.
Runs as an inetd server. Options -P (PID file), -p (listen address), -u (user) are ignored.
Runs in foreground. The server will not fork and will remain connected to the terminal. Messages normally sent to syslog will also be sent to stderr.
Runs in background. This overrides foreground if set in the configuration file (or on the command line, but there is no point setting both on the command line unless you have a personality disorder).

FILES

/etc/init.d/sslh
Start-up script. The standard actions start, stop and restart are supported.
/etc/default/sslh
Server configuration. These are environment variables loaded by the start-up script and passed to sslh as command-line arguments. Refer to the OPTIONS section for a detailed explanation of the variables used by sslh.

SEE ALSO

The latest version is available from <http://www.rutschle.net/tech/sslh>, and can be tracked from <http://freecode.com/projects/sslh>.

AUTHOR

Written by Yves Rutschle.

2024-09-22 2.1.1-2