I configured my first ASA this week and what a mess it was! Oh well it was a good intro to getting hands on with some of the security side of things. When one of our remote sites was initially built, there wasn't any internet availability in the area due to it being in a more rural location. To remedy this, all internet traffic was delivered over our MPLS and then out of our corporate site to access any external website. Recently the area finally offered simple business DSL connectivity (that was more than enough for this location) and we asked for this service pretty quickly.
Well of course we didn't want to just connect a non-secure DSL connection to our network so we needed a simple firewall aka ASA 5505 to provide a buffer between our remote sites network and the outside world. Once all was said and done/configured it was actually pretty simple but I ran into many bumps along the way. The first thing was that the new ASA had NO configuration whatsoever on the device, not even the factory-default settings. The device was asking for a username and password which wasn't configured so after about 3 hours of mucking around I ended up finally getting in by using the ROMMON mode and changing the configuration register to 0x41. This tells the ASA to ignore the saved start-up configuration, from there I entered the command configure factory-default which put the right factory default settings on the device it should of had to begin with.
The only thing I could think of is that maybe they forgot to throw this command on the device before shipping it out lol. After that fiasco now I had to figure out to configure it finally! It wasn't to bad but I ran into some weird things mainly due to the silly DSL modem itself. The DSL provider configured the modem to work in "pass through" mode so it didn't do anything besides provide a bridge between the ISP and the ASA, it took me a while to realize that. So I had to battle NAT configuration, IP Addresses, and ASA port configuration.
What confused me the most was how I had to tell the ASA what it should and shouldn't know. The ASA had no problems reaching the internet or the internal network. However internal devices could only reach the ASA and not the external world, not even the ASA's outside interface. At first I thought this was due to access-lists I had configured. Turns out it was because NATing wasn't configured all the way and I hadn't specified the internal network IP range the ASA should know about.
I used ASDM and even a little CLI to finally get the thing working. Commands I'll never forget is the route inside and route outside which is similar to the ip route command used on routers and L3 switches.