> Putting a stateful firewall in front of that would be dumb; the server
> is completely capable of coping with the superfluous SYN's in a much
> more competent manner than the firewall.The trouble with blanket statements about "all stateful firewalls" and
"all servers" is there are lots of different firewall and server
platforms.
Yes, yes there are, but only an idiot buys a Ford Pinto to haul pallets
of freight. When you get serious about hauling lots of freight, you buy
something appropriate, and there are suddenly a lot fewer combinations.
Stateful firewalls can implement SYN cookies, and at least
a couple do. Firewalls do not need to build a state entry for
partial TCP sessions, there are a few different things that can be
done, such as the firewall answering on behalf of the server (using
SYN cookies) and negotiating connection with the server after the
final ACK.
And how much of that is done in silicon? Because if it's not in silicon,
then it's being done by a CPU, and if it's being done by a CPU, why not
just let the server do it? Commodity server hardware is cheap compared
to specialized silicon offerings...
As a result, spoofed TCP packets don't consume state. Multiple IPs
they can _receive_ traffic to required, next?Spoofed UDP is a much bigger problem, because there is no connection
establishment. And it's probably not sane to put certain
public-facing UDP services such as large public DNS service IPs
(e.g. 8.8.8.8) behind most forms of stateful filter.But that's not the average case, by any means, most servers are not
DNS servers.
Servers consume state just like firewalls do....E.g. A public FTP server that opens a process for each connection,
goes down in a connection flood, when kernel process slots are used
up, long before the firewall.
Again, Ford Pinto... you can always design a system that will fail.
That's like shooting fish in a barrel. If you're worried about kernel
process slots, you *choose* an appropriate service. Like a threaded
ftp server.
Servers running a robust OS completely and correctly configured to
perfectly protect themsleves (resource limits, etc), no Windows
OSes, with unwanted open ports, is a wholly unwarranted assumption
for real-world server environments.
And so is a high-performance non-crashable stateful firewall that's so
talented that it can keep a poorly configured server operational under
all circumstances. Wheee.
In the best cases it does hold up (to a great extent).
In other cases, it's an operational fantasy; it would be nice if that
could be relied upon....
Some of us are still failing to understand why it is that it's better to
buy an extremely expensive stateful firewall device which is likely to
collapse under load because the salesman lied; it seems like it'd be
easier to go and scale capacity with some cheap stateless firewalls to
do packet filtering in silicon, backended by some additional servers
and some good admins who have a clue about what they're doing.
... JG