Bypassing Firewalls

[09Sparky () gmail com]

I am a new Pentester and have some general questions as to how to bypass
Firewalls.  I have attempted the standard FTP bounce and fragmentation attempts in nmap, but have been unsuccessful.  
My main question I guess would be:  How do you get past a firewall, so you could for instance perform your nessus 
scanning, etc?  I have a good understanding of all the steps involved, I just am having problems actually getting them 
to work.  Do I need to find exploits in the Firewall, or are there other more effective ways?  
For instance, I want to do a standard Pen test on a company (class B network, no NAT).  I have their IP range, but they 
have a firewall blocking everything but HTTP and DNS.  How do I get past the firewall to perform the test on there IP 
range?

Any help/suggestions would be great,
Thanks,
Sparky

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