Mart Frauenlob wrote:
netfilter-owner@xxxxxxxxxxxxxxx wrote:
Mart Frauenlob wrote:
netfilter-owner@xxxxxxxxxxxxxxx wrote:
William Fitzgerald wrote:
Dear Experts,
My query is how to interpret TCP flag semantics.
Consider that you have a web server and you want to permit access
to it. And lets assume that there are no other communications or
rules for other servers. From a security point of view, a web
server should not be initiating a connection (syn flag) and
clients should be.
From what I was reading on the web I could write the following rules:
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -m tcp --dport 80 --tcp-flags SYN -j
ACCEPT
iptables -A FORWARD -o eth1 -m tcp --sport 80 --tcp-flags ACK -j
ACCEPT
My question is what happens after the 3-way-handshake?
Would these rules enable continued traffic communication?
I realise that if I wrote the rules in the following format, there
would be no issue, as the filter does not care about the flags.
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -m tcp --dport 80 -j ACCEPT
iptables -A FORWARD -o eth1 -m tcp --sport 80 -j ACCEPT
Similarly if I chose the *stateful* method I could right the rules
as:
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -m tcp --dport 80 -m state --state
NEW,ESTABLISHED -j ACCEPT
iptables -A FORWARD -o eth1 -m tcp --sport 80 -m state --state
ESTABLISHED -j ACCEPT
In those stateful rules, TCP flags are handled implicitly and
automatically making life easier ;-)
However, lets suppose I want to write *stateless* rules that
include TCP flags like above. As I read books like that of
Cheswick, I see references to packet filters in the early years
and given that Netfilter, while is stateful, can perform in a
stateless manner, I would like to know more about what it means
for packet filtering using additional options such as TCP flags
and how it impacts on the semantics of a configuration.
Perhaps the rules with the SYN and ACK flags set as shown at the
top of this email can handle connections after the initial TCP
handshake.
Does the rule:
iptables -A FORWARD -i eth0 -m tcp --dport 80 --tcp-flags SYN -j
ACCEPT
state incoming traffic that has at least one flag set to SYN
(regardless of any other inapropriate flags being simultaneously
set) must be allowed?
Actually the more I think about that last statement, the more I
realise this can't be how it works. In that once a connection has
been established, incoming packets will no longer have the SYN flag
set and so packets will be dropped. Since bidirectional
communication fails, access to the web server fails. So I guess the
final question below still stands: "If the above *stateless* TCP
flag rules do not handle traffic after the TCP handshake, then what
rules need to come before or after the rules defined at the top of
this email?"
If that is the case, then I presume that adding flags in this way
handles both TCP initial handshake connection and ongoing
established connections. Of course if this is true, then I would
need to put a number of rules before this rule to catch malformed
TCP flag packets (nmap scans), for example iptables -A TCP_FLAGS
-p tcp --tcp-flags SYN,RST SYN,RST -j DROP.
However, if the above *stateless* TCP flag rules do not handle
traffic after the TCP handshake, then what rules need to come
before or after the rules defined at the top of this email?
kind regards,
Will.
Hello,
IMHO you should forget thinking about creating a stateless firewall
to protect your webserver.
Thanks once again Mart.
I agree. However, I am curious to know how this can be achieved using
only *stateless* rules.
From various resources (books, web etc) packet filters are
susceptible for example to forged TCP header attributes.
An attacker can forge a TCP header that will bypass stateless rules.
Consider an attacker sending a forged packet, to scan a network, that
mimics the expected return packets for outbound http traffic
requests. However, the same attack against a *stateful* firewall will
fail because it will consult both its rules and the current state
table. For example "is there an entry in the state table for an
existing outbound http request, sourced on some internal LAN IP with
source port >1024, bound for destination port 80 over TCP on host
hacker IP? No, so drop packet!"
worried about scans?
take a look at this:
http://jengelh.medozas.de/documents/Chaostables.pdf
from the xtables-addons.
The link further enhances my understanding of why certain flag
combinations are wrong and why such rules are required.
So, I agree entirely that stateful firewalling is essential.
However, this has made me think a little more about stateless packet
filters and how they operated in the past. Again from various
resources, (rules often represented in table format to be vendor
neutral), you see situations where SYN and ACK flags are used. For
example, to stop an attacker making connections to internal systems a
rule would state that inbound packets should only be responses and so
only the ACK flag should be set. Makes perfect sense. Yes, it
prevents a full connection but will not prevent a TCP-ACK scan as
mentioned above, but never the less the firewall is doing what it can!
The trouble I have, is understanding what happens after I introduce
these (protection) rules? How are established TCP connections handled
after the 3-way-handshake. How did the stateless packet filters
control who could send a syn flag for example while at the same time
allow ongoing traffic? There will no longer be a Syn flag for
example. Presumably there are ongoing ACK packets that simultaneously
include PSH flags when delivering data. Typically books discussing
TCP only ever detail the TCP-handshake and don't mention what flags
are set after the connection.
In the book by the highly esteemed William Cheswick: "Firewalls and
Internet Security," he states that when an ACK is seen in a rule it
is interpreted as "return ACK packets are OK nothing else is OK".
So perhaps I was correct in my original email when I said, incoming
traffic that has at least one particular flag (regardless of any
other inappropriate flags being simultaneously set) must be allowed?
Therefore would the correct stateless rules be:
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -m tcp --dport 80 --tcp-flags SYN -j
ACCEPT # allow outside to initiate web server connection.
iptables -A FORWARD -i eth0 -m tcp --dport 80 --tcp-flags ACK -j
ACCEPT # allow ongoing communication after 3-way-handshake.
iptables -A FORWARD -o eth1 -m tcp --sport 80 --tcp-flags ACK -j
ACCEPT # allow ACK in 3-way-handshake and ongoing communication.
iptables -A FORWARD -o eth1 -m tcp --sport 80 --tcp-flags SYN -j DROP
# redundant to default policy. disallow server to initiate
communication.
During ongoing communication there might be other flags like PSH etc
but so long as we have the ACK flag set we are happy to believe that
packets have correct simultaneous flags set. Of course we need a set
of preceding rules to filter malformed TCP flag packets as part and
parcel of security in depth.
Comments to my understanding of how it works?
I think syntax for --tcp-flags is wrong - compare value missing.
Your right.
What about FIN and RST? Don't you want connection allow to end?
What about sequence numbers, you can't track them, but the conntrack can.
What about fragments? With conntrack/NAT you don't have to care about
them either.
Another downside: so many rules to process for every packet.
Really, it has it's reasons that every modern firewall system is
stateful.
As you talked about magic before, the magic of conntrack is, that it
does monitor the traffic and therefore already does more than you
can ever do with tcp flag rules.
I'll try to give you a short example how it could be done (of course
there might be other ways):
webserver=10.1.1.10
ext_if=eth0
dmz_if=eth1
iptables -N web_server
iptables -A web_server -d $webserver -p tcp --dport 80 -m state
--state NEW,ESTABLISHED -j ACCEPT
iptables -A web_server -s $webserver -p tcp --sport 80 -m state
--state ESTABLISHED -j ACCEPT # a global rule might do that - your
choice
iptables -A web_server -j DROP # policy might do that - your choice
iptables -A FORWARD -i $ext_if -d $webserver -j web_server
iptables -A FORWARD -i $dmz_if -s $webserver -j web_server
Don't forget to allow only valid tcp connections (referring to our
previous mails) before.
In relation to the extra "checks and balances", thanks for
consolidating that for me.
If you keep insisting on stateless firewalling - No more talk on this
from my side :-p
No problem. I'm like a dog with a bone trying to understand how it can
be achieved.
The various book seem to "wave" the idea around that its just a matter
of controlling who can send a SYN packet and who can send a ACK packet.
That's fine in the TCP handshake, but where I get lost is what happens
afterwords. None of the books nor online material I have sourced to
date, provide such information. And you are correct, I need to also
worry about connection tear down, adding further complications for
stateless packet filters!
The reason I ask, is that I want to write a piece about how how
stateless rules compare to stateful rules. Hence the reason for being so
persistent :-)
For example Stateless Port Attack Surface Reduction versus Stateful Port
Attack Surface Reduction, where the former can reduce ports to varying
degrees of subsets of ports. for example source port range of >1024 and
a destination port of 80. From the servers point of view there is a port
reduction of 65535 ports to 1 and on the client side 64511 ports that
need to be statically opne. The latter reduces the ports to a singleton
for source and destination ports. That is, only open the correct ports
involved for source and destination, so its a reduction of 65535 ports
to 1 for both directions.
However, it wasn't until I said to myself, how do I compare Stateless
TCP Flags against Stateful Connection Tracking that I realised that my
common understanding of how packet filters work fell apart :-(
I used to think of them as, simple rules, for example, to prevent nmap
scans or IP spoofing of RFC3330 and RFC1918 that are placed before the
stateful rules that do all the hard work of intelligently maintaining
traffic flow. This knowledge was implicit from various books etc I read
as I mentioned above, but there is more to this. And given your reaction
(mine included when I first thought about it) it seems crazy to even
consider stateless rules to manage traffic flow. However, one thing is
for sure, somewhere somehow the packet filter administrators in the
early days could do it!
Thats why in my example, I chose a simple example of configuring a Web
server on its own. However, its not so simple after all ;-)
Listen, thanks for your input. I do appreciate your time on this.
regards,
Will.
Regards
Mart
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