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    SubjectRe: Firewalling Rules (Was: Linux Kernels)
    On Fri, 31 Oct 1997, Rogier Wolff wrote:

    > Richard B. Johnson wrote:
    > >
    > > On Fri, 31 Oct 1997, Matthew Kirkwood wrote:
    > > [SNIPPED]
    > > >
    > > > Having heard various (and unconvincing) horror stories about the effect
    > > > that having many firewalling rules can have on network latency, I thought
    > > > up this foolish plan: firewall modules
    > >
    > > I have not looked at the firewall code. However, I think that firewall
    > > rules just create and/or modify entries within a hash-table or two. If so,
    > > you are just adding/modifying entries to an existing table when using
    > > the cumbersome utility. The code should already be somewhat optimized so
    > > your net-gain from a lot of work might be near zero.

    > I don't think many people are hitting performance problems on firewall
    > rules, but there was a discussion a while back where I showed that it
    > could be done to translate a firewall config file into a C program.
    > Aren't your firewall rules wrong if you cannot lump many of them
    > together?
    > Roger.

    Probably. What I have done is when I find another Micro$garbage program
    sneeking its crap into my PPP Link, I identify it with tcpdump and then
    add another entry to the firewall rules. Basically, because Netbouis
    is NETBIOS within BROACDAST packets, I have to filter them "per machine".
    Then there are the "SNMP" programs that come with 'NT' that make a
    pretty screen showing all the nodes on a network. This runs by arp-ing
    every possible machine and recording those that respond. It also requests
    a reverse-lookup on my DNS once a responding IP Address is found. This
    junk has to be filtered or I get 254 arp requests every second on my
    PPP Link. You don't notice this stuff on Ethernet, but once you try
    to extend your LAN to a subnet connected by dialup PPP, you get to
    hate all the junk.

    Dick Johnson

    Richard B. Johnson
    Project Engineer
    Analogic Corporation
    Penguin : Linux version 2.1.60 on an i586 machine (66.15 BogoMips).
    Warning : It's hard to remain at the trailing edge of technology.

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