Re: Firewall best practices

One other point I always like to make--for outgoing traffic, if you use default:deny you do your part to stop a lot of attacks that use forged IP.  Your network, at least, will not be a source.  E.g.

Outgoing Packets
Default: deny all SIP, allow only your assigned IP space, only ports X,Y,Z,P,D,Q.

Of course it is even better when you mate an SIP and a port (e.g. smtp to/from your smtp gateway/server).

--p


On 14 April 2010 14:10, Jason Lewis <[email protected]> wrote:
> While I believe the only allow what you need is a good rule, it's 
> impossible to enforce in a lot of scenarios.  How many small 
> businesses have no firewall admins and do the configuration 
> themselves?  Do you think they are going to spend the time examining 
> what ports should be open based on what their users are using?  No, 
> they will open ports until it works.  Last time I checked every 
> linksys router comes with allow all outbound by default.  How many 
> people change that?
>
> The point of my question was if you're forced into a position to open 
> everything, what ports *should* you always block and why.  The 
> response below doesn't help that IT guy with no experience or time to 
> research everything.
>
> For example,  blocking SMB and NT RPC inbound and outbound should be a 
> high priority.  Ports 135,137-139, 445.  A lot of worms are propagated 
> via these ports and when you attempt to do DNS lookups, windows will 
> also try to connect outbound via SMB.  I had hoped someone had already 
> put this info on the web somewhere, but I have yet to find it.
>
> Marcus's thoughts on default permit are here:
> http://www.ranum.com/security/computer_security/editorials/dumb/index.
> html
>  Again, I agree with the thoughts, but for a hardware vendor selling 
> to a home user or a SMB, it's never going to happen.  The user wants 
> to buy a device, plug it in and have it work.  They don't want to 
> spend time configuring things.  That's reality, default deny is a 
> dream.
>
> jas
>