Frequently Asked Questions

When something doesn’t work, look up here… and if it still doesn’t work, report how what was suggested here went.

It’s also worth reading how to ask questions before posting on the mailing list or opening an issue in Github.

Getting more info

In general, if something doesn’t work, you’ll want to run sslh with lots of logging, and the logging directly in the terminal (Otherwise, logs are sent to syslog, and usually end up in /var/log/auth.log). You will achieve this by running sslh in foreground with verbose:

sslh -v 1 -f -F myconfig.cfg

Higher values of verbose produce more information. 1 is usually sufficient. 2 will also print incoming packets used for probing.

forward to [PROBE] failed:connect: Connection refused

Usually this means sslh is configured to forward a protocol somewhere, but no service is listening on the target address. Check your sslh configuration, check the corresponding server really is listening and running. Finally, check the server is listening where you expect it to:

netstat -lpt

I get a segmentation fault!

Well, it’s not yours (fault): a segfault is always a bug in the programme. Usually standard use cases are well tested, so it may be related to something unusual in your configuration, or even something wrong, but it should still never result in a segfault.

Thankfully, when they are deterministic, segfaults are usually fairly easy to fix if you’re willing to run a few diagnostics to help the developer.

First, make sure you have debug symbols:

$ file sslh-select
sslh-select: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/, for GNU/Linux 3.2.0, BuildID[sha1]=a758ac75ff11f1ace577705b4d6627e301940b59, with debug_info, not stripped

Note with debug_info, not stripped at the end. If you don’t have that, your distribution stripped the binary: you will need to get the source code and compile it yourself (that way, you will also get the latest version).

Install valgrind and run sslh under it:

valgrind --leak-check=full ./sslh-fork -v 2 -f -F yourconfig.cfg

Report the full output to the mailing list or github. Valgrind is very powerful and gives precise hints of what is wrong and why. For example on sslh issue (#273)[]:

sudo valgrind --leak-check=full ./sslh-fork -v 2 -f -F /etc/sslh.cfg
==20037== Memcheck, a memory error detector
==20037== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==20037== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==20037== Command: ./sslh-fork -v 2 -f -F /etc/sslh.cfg
sslh-fork v1.21b-1-g2c93a01-dirty started
--20037-- WARNING: unhandled arm-linux syscall: 403
--20037-- You may be able to write your own handler.
--20037-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--20037-- Nevertheless we consider this a bug.  Please report
--20037-- it at
==20040== Conditional jump or move depends on uninitialised value(s)
==20040==    at 0x112A3C: parse_tls_header (tls.c:162)
==20040==    by 0x111CEF: is_tls_protocol (probe.c:214)
==20040==    by 0x11239F: probe_client_protocol (probe.c:366)
==20040==    by 0x10A8F7: start_shoveler (sslh-fork.c:98)
==20040==    by 0x10AE9B: main_loop (sslh-fork.c:200)
==20040==    by 0x1114FB: main (sslh-main.c:322)

Here we see that something wrong is happening at tls.c line 162, and it’s linked to an uninitialised value.

Using sslh for virtual hosting

Virtual hosting refers to having several domain names behind a single IP address. All Web servers handle this, but sometimes it can be useful to do it with sslh.

TLS virtual hosting with SNI

For TLS, this is done very simply using Server Name Indication, SNI for short, which is a TLS extension whereby the client indicates the name of the server it wishes to connect to. This can be a very powerful way to separate several TLS-based services hosted behind the same port: simply name each service with its own hostname. For example, we could define,,, all of which point to the same IP address. sslh uses the sni_hostnames setting of the TLS probe to do this, e.g.:

protocols: (
    {   name: "tls";
        host: "localhost";
        port: "993";
        sni_hostnames: [ "" ];
    {   name: "tls";
        host: "localhost";
        port: "xmpp-client";
        sni_hostnames: [ "" ];
    {   name: "tls";
        host: "localhost";
        port: "4443";
        sni_hostnames: [ "" ];

HTTP virtual hosting with regex

If you wish to serve several Web domains over HTTP through sslh, you can do this simply by using regular expressions on the Host specification part of the HTTP query.

The following example forwards connections to host_A.acme to, and connections to host_B.acme to

protocols: (
    {   name: "regex";
        host: "";
        port: "80";
                ["^(GET|POST|PUT|OPTIONS|DELETE|HEADER) [^ ]* HTTP/[0-9.]*[\r\n]*Host: host_A.acme"] },
    {   name: "regex";
        host: "";
        port: "80";
                ["^(GET|POST|PUT|OPTIONS|DELETE|HEADER) [^ ]* HTTP/[0-9.]*[\r\n]*Host: host_B.acme"] }