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.
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.
Regards
Mart
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