Re: Query: TCP Flag Semantics post 3-way-handshake?

[William Fitzgerald <wfitzgerald@tssg.org>]

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.
>
>
>
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