Here's the scoop: DSL router with dynamic WAN IP servicing up NATed DHCP connected to the WAN port on a Sonicwall firewall which is then serving up its own NATed network. Is it possible, knowing the public IP address (e.g. via dyndns) to access resources on the internal network (esp. remote desktop)?
Some clarifications on the request: I have no control over the DSL router and I'd like to avoid 3rd party infrastructure (e.g. logmein, etc).
If your external router supports port forwarding, you can have it forward the ports to the services you need.
You could also look at VPN solutions, either as a function of the router or having your internal client and external client access a common server on the internet.
If you don't mind heavy latency, you can set up a client on the inside to be a TOR client with a hidden SSH service. Then you can ssh into that client from any other TOR client (so long as you know the service name), and use port SSH's forwarding or SOCKS proxy capability to access various internal services.
There are many options here, and they all depend on what services you require access to, how much control over the setup you have, and how much effort you're willing to put into the setup.
port redir on both nats [if you can] or vpn [ eg openvpn ] established from the host behind nats to some publicly reachable machine [ eg vps or friend's server provided your friend has public ip or port redirected from one ] are your best options.
alternativly you can take a look at services like logmein / gotomypc.
