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SSH / SSL multiplexer
===== sslh -- A ssl/ssh multiplexer. ===== 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. ==== Compile and install ==== sslh uses libconfig (http://www.hyperrealm.com/libconfig/) and libwrap. For Debian, these are contained in packages libwrap0-dev and libconfig8-dev. For OpenSUSE, these are contained in packages libconfig9 and libconfig-dev in repository http://download.opensuse.org/repositories/multimedia:/libs/openSUSE_12.1/ For Fedora, this package should work: https://admin.fedoraproject.org/pkgdb/acls/name/libconfig (feedback from Fedorans appreciated). If you can't find libconfig, or just don't want a configuration file, set 'USELIBCONFIG=' in the Makefile. After this, the Makefile should work: make install There are a couple of configuration options at the beginning of the Makefile: USELIBWRAP compiles support for host access control (see hosts_access(3)), you will need libwrap headers and library to compile (libwrap0-dev in Debian). USELIBCONFIG compiles support for the configuration file. You will need libconfig headers to compile (libconfig8-dev in Debian). The Makefile produces two different executables: sslh-fork and sslh-select. sslh-fork forks a new process for each incoming connection. It is well-tested and very reliable, but incurs the overhead of many processes. sslh-select uses only one thread, which monitors all connections at once. It is more recent and less tested, but only incurs a 16 byte overhead per connection. Also, if it stops, you'll lose all connections, which means you can't upgrade it remotely. If you are going to use sslh for a "small" setup (less than a dozen ssh connections and a low-traffic https server) then sslh-fork is probably more suited for you. If you are going to use sslh on a "medium" setup (a few thousand ssh connections, and another few thousand ssl connections), sslh-select will be better. If you have a very large site (tens of thousands of connections), you'll need a vapourware version that would use libevent or something like that. To install: make cp sslh-fork /usr/local/sbin/sslh cp scripts/etc.default.sslh /etc/default/sslh For Debian: cp scripts/etc.init.d.sslh /etc/init.d/sslh For CentOS: cp scripts/etc.rc.d.init.d.sslh /etc/rc.d/init.d/sslh and probably create links in /etc/rc<x>.d so that the server start automatically at boot-up, e.g. under Debian: update-rc.d sslh defaults ==== Configuration ==== You can edit settings in /etc/default/sslh: LISTEN=ifname:443 SSH=localhost:22 SSL=localhost:443 A good scheme is to use the external name of the machine in $LISTEN, and bind httpd to localhost:443 (instead of all binding to all interfaces): that way, https connections coming from inside your network don't need to go through sslh, and sslh is only there as a frontal for connections coming from the internet. Note that 'external name' in this context refers to the actual IP address of the machine as seen from your network, i.e. that that is not 127.0.0.1 in the output of ifconfig(8). ==== Libwrap support ==== Sslh can optionnaly perform libwrap checks for the sshd service: because the connection to sshd will be coming locally from sslh, sshd cannot determine the IP of the client. ==== OpenVPN support ==== OpenVPN clients connecting to OpenVPN running with -port-share reportedly take more than one second between the time the TCP connexion is established and the time they send the first data packet. This results in sslh with default settings timing out and assuming an SSH connexion. To support OpenVPN connexions reliably, it is necessary to increase sslh's timeout to 5 seconds. Instead of using OpenVPN's port sharing, it is more reliable to use sslh's -o option to get sslh to do the port sharing. ==== Using proxytunnel with sslh ==== If you are connecting through a proxy that checks that the outgoing connection really is SSL and rejects SSH, you can encapsulate all your traffic in SSL using proxytunnel (this should work with corkscrew as well). On the server side you receive the traffic with stunnel to decapsulate SSL, then pipe through sslh to switch HTTP on one side and SSL on the other. In that case, you end up with something like this: ssh -> proxytunnel -e --------ssh/ssl------> stunnel ---ssh---> sslh --> sshd Web browser --------http/ssl------> stunnel ---http---> sslh --> http:80 Configuration goes like this: On the server side, using stunnel3: stunnel -f -p mycert.pem -d thelonious:443 -l /usr/local/sbin/sslh -- sslh -i --http localhost:80 --ssh localhost:22 stunnel options: -f for foreground/debugging, -p specifies the key + certificate, -d specifies which interface and port we're listening to for incoming connexions, -l summons sslh in inetd mode. sslh options: -i for inetd mode, --http to forward http connexions to port 80, and SSH connexions to port 22. ==== IP_TPROXY support ==== There is a netfilter patch that adds an option to the Linux TCP/IP stack to allow a program to set the source address of an IP packet that it sends. This could let sslh set the address of packets to that of the actual client, so that sshd would see and log the IP address of the client, making sslh transparent. This is not, and won't be, implemented in sslh for the following reasons (in increasing order of importance): * It's not vital: the real connecting IP address can be found in logs. Little gain. * It's Linux only: it means increased complexity for no gain to some users. * It's a patch: it means it'd only be useful to Linux users who compile their own kernel. * Only root can use the feature: that's a definite no-no. Sslh should not, must not, will never run as root. This isn't to mean that it won't eventually get implemented, when/if the feature finds its way into the main kernel and it becomes usuable by non-root processes. ==== Comments? Questions? ==== You can subscribe to the sslh mailing list here: http://rutschle.net/cgi-bin/mailman/listinfo/sslh This mailing list should be used for discussion, feature requests, and will be the prefered channel for announcements.
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