News in conntracking...

I've just upgraded some of my systems to debian 'stretch' (kernel 4.9.144-3.1
and iptables 1.6.0+snapshot20161117-6) and got on logs:

	May  7 18:32:48 fouc kernel: [   19.097030] nf_conntrack: default automatic helper assignment has been turned off for security reasons and CT-based  firewall rule not found. Use the iptables CT target to attach helpers instead.

and googling around lead to feel that i'm a bit 'left behind' in my
scripts. ;(
I need to clarify some point... please help me.

a) seems that '-m state --state X' is deprecated, better use '-m conntrack --ctstate X'


b) AFAI've understood, expecially reading:

	https://home.regit.org/netfilter-en/secure-use-of-helpers/

 now the conntrack helpers does not load a predefined set of (implicit)
rules (eg, all traffic to/from TCP/UDP port 5060,5061 is SIP), but you
have to explicitly setup some rule. Good.
But the provided example:
	iptables -A FORWARD -m conntrack --ctstate RELATED -m helper \
		--helper sip -d $ISP_RTP_SERVER -p udp -j ACCEPT

it is a bit unclear to me... this example seems to me like: «match
packet related to some existing traffic, of type SIP».
But, if is a 'related' packet, the conntracking just know that is a SIP
packet! Why add a strict check?

Because the new conntrack really need that? Or for a better security
(eg, to prevent match packet that are related but NOT SIP)?


c) How all that work with marking (mangle table)? Now i do:

	iptables -t mangle -A mrk-post-fwd -m helper --helper sip \
		-j MARK --set-mark 0x1/0xf

 i need also here to do insted '-m conntrack --ctstate RELATED -m helper --helper
sip'?


Thanks.

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