Internet Filters

Internet Filters

Post by Jay Barn » Tue, 25 Jul 1995 04:00:00

Can anyone help me with _generic_ filters/commands/configurations that
should be applied to a cisco 2500 running 10.3 software operating as an
Internet router (between our ISP and our firewall)?  I'm not worried about
site-specific ACLs, protection policies, etc.  Those I will sort out myself
in conjunction with a seperate firewall.  What I'm looking for is, for
example, in the same vein as "no source routing", "no telnet", etc.

Perhaps there's a FAQ I should be reading ...

            Web surfing is for people  <|  who don't know how to SAIL
                                 _____/ | \________  
                                /    /  |  \       \
                               /    /   |   \       \    /\
                          /\  /    /____|    \       \  /  \
                         /  \/   _______|_____\__     \/    \
                        /        \  Jay Barnes  /            \


1. Need starting Internet filter rules for IOS/700 (Cisco 776M)

I saw a note about this from somebody else in comp.dcom.isdn, and I am
pretty much in the same situation.  Since I haven't seen an answer yet...

I have a Cisco 776M for connecting to the Internet.  On the local side of
the router it's a SOHO LAN, TCP/IP over Ethernet, with four to five
Win95-based machines and, later, one NT box and possibly a Linux box.  
There are a couple of printers on micro-servers, so I'm not doing any
printer sharing, however one selected directory on each machine *is* set
up for read-only file sharing.

Does anyone have a group of filter rules for the 700 series that is
reasonably safe for connecting a LAN supporting file-sharing Win95 to the

I have some rules from Cisco that will prevent loopback attacks and so
on, and I know how to block NetBios queries and local LAN traffic from
making it out to the Internet.  What I *don't* have is a clear enough
understanding of things to be sure I'm correctly setting up filters to
prevent people from getting into the LAN.  

I will be offering no services, and there will be no ISDN dial-in
capabilities.  Basically, anything that comes to me unrequested (with the
SYN flag set, I assume?) from the WAN side, I want to reject with extreme
prejudice... but I still want to be able to FTP, etc.  I have a dynamic
IP address, and I am running NAT inside the box so *supposedly*, I am
told, it wouldn't be possible to get anything useful back onto the LAN
even via IP spoofing.  But before I get complacent, I wanted to ask the

So, how about it?  Has anyone who would be willing to share, set up a
batch of IOS-700 filters that makes a reasonable firewall (well, as best
as is possible with packet-level filtering within a router)?

(BTW, I have an old 486 or a slow Pentium I could press into service with
a couple of NICs to insert a "real" firewall (proxy server, etc.)  If
anyone has thoughts or recommendations I would love to hear them... but
I'd still like to do the best possible job with the router filtering,


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