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Suppose one has configured a bog standard Ethernet connection - there is an IPv4 address, a subnet specification, a default route, even nameservers in resolv.conf, static routes everywhere - is it possible for client software to find other hosts on the same network that route to, say, the Internet?

I guess one could just send packets to, for example, 8.8.8.8 using an Ethernet frame directed at a maybe-router and see whether something else comes back. I don't see why it wouldn't work, but am trying to find out why it's not done in practice. Does modern Windows do this? Wouldn't Skype try something like this?

In security terms, I'm asking whether this possible covert channel can be autodiscovered by "slightly-misbehaved" software like Skype.

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Detection of routers is possible, if typical. In many large networks routers detect each other and configure their own routes, using something called a routing protocol.

On an LAN, it is not entirely uncommonly to see RIP or OSPF running allowing you to detect your routers. On the public Internet, the most common routing protocol is BGP

But most consumer 'router/switch/nat/gateway' boxes do not do RIP, or do not have it enabled by default. So a consumer product like Skype really wouldn't gain anything about this. Also it would be a layering violation for Skype which is a TCP/IP application to start mucking around with the routing tables, and routing protocols. The routing tables belong to the entire system. If a single application was messing around with routing tables, lots of things could get broken.

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Thanks for your input, I shall edit the question to limit its scope to static routing. –  mushiflo Apr 17 '12 at 18:37
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While it's possible, I'm pretty sure applications unless written to do otherwise are going to follow what the operating system's networking stack provides.

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I was thinking along the lines of surreptitious software trying to escape firewalling. So this is actually a matter of security and possible covert channels. Editing. –  mushiflo Apr 17 '12 at 18:37
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