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Full Discussion: Dynamic Tunnel
Top Forums UNIX for Advanced & Expert Users Dynamic Tunnel Post 302382636 by Smiling Dragon on Thursday 24th of December 2009 05:39:23 AM
Old 12-24-2009
Ah, now I think I understand what you are after - I've implemented something similar in the past as a low budget proof of concept. If you want something more solid - stick with NAT rules on the routers but as a test case ssh tunnels should do the job for you.

You are still pretty light on the details so I'll suggest an example as it might help show the process:
Suppose you have 4 servers, 2 of which are customer facing webservers and 2 are back-end application servers. These servers operate in an active-standby mode where any pair of 2 servers (one webserver, one app server) are up and running while the other two are on standby. If the app itself is too stupid to handle this scenario, we can use ssh tunnels instead:
App server A opens a connection via ssh to webserver A and webserver B, it listens to the localhost loopback interface on some high port (use a high port so you don't need to trust an ssh as root on your webserver). Any traffic to these high ports is directed down the tunnel and to the back-end app server. Configure both webservers to connect to "back ends" on localhost:<high port>.
Traffic from either webserver will arrive at the app server automagically.

If you want to switch to the other app server (even if that's in a completly different network). You kill the ssh tunnels and reopen them from App Server B instead on the same high port. The webserver sees a brief connection loss but everything comes back right away and it carries on oblivious to the move.

You'll not be able to switch without outage at all, but the outage can be reduced to a few seconds at least this way and avoids config changes on the webservers.

Is that what you had in mind?
 

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gateways(4)						     Kernel Interfaces Manual						       gateways(4)

NAME
gateways - Specifies Internet routing information to the routed daemon SYNOPSIS
/etc/gateways DESCRIPTION
The /etc/gateways file identifies gateways for the routed daemon. Ordinarily, the routed daemon queries the network and builds routing tables. The routed daemon builds the tables from routing information transmitted by other hosts directly connected to the network. How- ever, there may be gateways that this command cannot identify through its queries. These unidentified gateways are known as distant gate- ways. Such gateways should be identified in the /etc/gateways file, which the routed daemon reads when it starts. The general format of an file entry in the /etc/gateways file is: Destination Name1 gateway Name2 metric Value Type The following is a brief description of each element in an /etc/gateways file entry: A keyword that indicates whether the route is to a network or to a specific host. The two possible keywords are net and host. The name associated with Destination. Name1 can be either a symbolic name (as used in the /etc/hosts or /etc/networks file) or an Internet address specified in dotted-decimal format. An indicator that the following string identifies the gateway host. The name or address of the gateway host to which messages should be forwarded. An indicator that the next string represents the hop count to the destination host or network. The hop count, or number of gateways, from the local network to the destination network. A keyword that indicates whether the gateway should be treated as active, passive, or external. The three possible keywords are as follows: An active gateway is treated like a network interface. That is, it is expected to exchange RIP (Routing Information Protocol) routing information. Information about it is maintained in the internal routing tables as long as it is active and is included in any routing information that is transmitted through RIP. If it does not respond for a period of time, the route associated with it is deleted from the internal routing tables. A passive gateway is not expected to exchange RIP routing information. Information about it is maintained in the routing tables indefinitely and is included in any routing information that is transmitted through RIP. An external gateway is identified to inform the routed daemon that another routing process will install such a route and that alternative routes to that destination should not be installed. Information about external gateways is not maintained in the internal rout- ing tables and is not transmitted through RIP. Note that these routes must be to networks. EXAMPLES
To specify a route to a network through a gateway host with an entry in the gateways file, enter: net net2 gateway host4 metric 4 passive This example specifies a route to a network, net2, through the gateway host4. The hop count metric to net2 is 4, and the gateway is treated as passive. To specify a route to a host through a gateway host with an entry in the gateways file, enter: host host2 gate- way host4 metric 4 passive This example specifies a route to a host, host2, through the gateway host4. The hop count metric to host2 is 4, and the gateway is treated as passive. To specify a route to a host through an active Internet gateway with an entry in the gateways file, enter: host host10 gateway 192.100.11.5 metric 9 active This example specifies a route to a specific host, host10, through the gateway 192.100.11.5. The hop count metric to host10 is 9 and the gateway is treated as active. To specify a route to a host through a passive Internet gateway with an entry in the gateways file, enter: host host10 gateway 192.100.11.5 metric 9 passive This example specifies a route to a specific host, host10, through the gateway 192.100.11.5. The hop metric count to host10 is 9 and the gateway is treated as passive. To specify a route to a network through an external gateway, enter a line in the following format: net net5 gateway host7 metric 11 external This example specifies a route to a network, net5, through the gateway host7. The hop count metric to net5 is 11 and the gateway is treated as external (that is, it is not advertised through RIP, but is advertised through an unspecified routing protocol). RELATED INFORMATION
Daemons: gated(8), routed(8) delim off gateways(4)
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