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diff --git a/README.tun b/README.tun new file mode 100644 index 0000000..5e1cb07 --- /dev/null +++ b/README.tun @@ -0,0 +1,132 @@ +How to use OpenSSH-based virtual private networks +------------------------------------------------- + +OpenSSH contains support for VPN tunneling using the tun(4) network +tunnel pseudo-device which is available on most platforms, either for +layer 2 or 3 traffic. + +The following brief instructions on how to use this feature use +a network configuration specific to the OpenBSD operating system. + +(1) Server: Enable support for SSH tunneling + +To enable the ssh server to accept tunnel requests from the client, you +have to add the following option to the ssh server configuration file +(/etc/ssh/sshd_config): + + PermitTunnel yes + +Restart the server or send the hangup signal (SIGHUP) to let the server +reread it's configuration. + +(2) Server: Restrict client access and assign the tunnel + +The OpenSSH server simply uses the file /root/.ssh/authorized_keys to +restrict the client to connect to a specified tunnel and to +automatically start the related interface configuration command. These +settings are optional but recommended: + + tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... reyk@openbsd.org + +(3) Client: Configure the local network tunnel interface + +Use the hostname.if(5) interface-specific configuration file to set up +the network tunnel configuration with OpenBSD. For example, use the +following configuration in /etc/hostname.tun0 to set up the layer 3 +tunnel on the client: + + inet 192.168.5.1 255.255.255.252 192.168.5.2 + +OpenBSD also supports layer 2 tunneling over the tun device by adding +the link0 flag: + + inet 192.168.1.78 255.255.255.0 192.168.1.255 link0 + +Layer 2 tunnels can be used in combination with an Ethernet bridge(4) +interface, like the following example for /etc/bridgename.bridge0: + + add tun0 + add sis0 + up + +(4) Client: Configure the OpenSSH client + +To establish tunnel forwarding for connections to a specified +remote host by default, use the following ssh client configuration for +the privileged user (in /root/.ssh/config): + + Host sshgateway + Tunnel yes + TunnelDevice 0:any + PermitLocalCommand yes + LocalCommand sh /etc/netstart tun0 + +A more complicated configuration is possible to establish a tunnel to +a remote host which is not directly accessible by the client. +The following example describes a client configuration to connect to +the remote host over two ssh hops in between. It uses the OpenSSH +ProxyCommand in combination with the nc(1) program to forward the final +ssh tunnel destination over multiple ssh sessions. + + Host access.somewhere.net + User puffy + Host dmzgw + User puffy + ProxyCommand ssh access.somewhere.net nc dmzgw 22 + Host sshgateway + Tunnel Ethernet + TunnelDevice 0:any + PermitLocalCommand yes + LocalCommand sh /etc/netstart tun0 + ProxyCommand ssh dmzgw nc sshgateway 22 + +The following network plan illustrates the previous configuration in +combination with layer 2 tunneling and Ethernet bridging. + ++--------+ ( ) +----------------------+ +| Client |------( Internet )-----| access.somewhere.net | ++--------+ ( ) +----------------------+ + : 192.168.1.78 | + :............................. +-------+ + Forwarded ssh connection : | dmzgw | + Layer 2 tunnel : +-------+ + : | + : | + : +------------+ + :......| sshgateway | + | +------------+ +--- real connection Bridge -> | +----------+ +... "virtual connection" [ X ]--------| somehost | +[X] switch +----------+ + 192.168.1.25 + +(5) Client: Connect to the server and establish the tunnel + +Finally connect to the OpenSSH server to establish the tunnel by using +the following command: + + ssh sshgateway + +It is also possible to tell the client to fork into the background after +the connection has been successfully established: + + ssh -f sshgateway true + +Without the ssh configuration done in step (4), it is also possible +to use the following command lines: + + ssh -fw 0:1 sshgateway true + ifconfig tun0 192.168.5.1 192.168.5.2 netmask 255.255.255.252 + +Using OpenSSH tunnel forwarding is a simple way to establish secure +and ad hoc virtual private networks. Possible fields of application +could be wireless networks or administrative VPN tunnels. + +Nevertheless, ssh tunneling requires some packet header overhead and +runs on top of TCP. It is still suggested to use the IP Security +Protocol (IPSec) for robust and permanent VPN connections and to +interconnect corporate networks. + + Reyk Floeter + +$OpenBSD: README.tun,v 1.4 2006/03/28 00:12:31 deraadt Exp $ |