conntrack nftables rules from wg-quick

[Demi M. Obenour]

What is the purpose of the premangle and postmangle chains created
by wg-quick(8)?  Is it to ensure that rogue packets cannot bypass
stateful firewall rules?

More precisely, I am using wg-quick in a Qubes VM.  I want all
traffic on vif interfaces to go through WireGuard, which will
ultimately send the packets on eth0.  All incoming traffic on eth0
that is not protected by WireGuard should be blocked.  I currently
use hand-written nftables rules to ensure this, but I am wondering
if the default QubesOS firewall is sufficient.  The default QubesOS
IPv4 firewall rules follow:

# Generated by iptables-save v1.4.5 on Mon Sep  6 08:57:46 2010
*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:PR-QBS - [0:0]
:PR-QBS-SERVICES - [0:0]
-A PREROUTING -j PR-QBS
-A PREROUTING -j PR-QBS-SERVICES
-A POSTROUTING -o vif+ -j ACCEPT
-A POSTROUTING -o lo -j ACCEPT
-A POSTROUTING -j MASQUERADE
COMMIT
# Completed on Mon Sep  6 08:57:46 2010
# Generated by iptables-save v1.4.5 on Mon Sep  6 08:57:46 2010
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:QBS-FORWARD - [0:0]
-A INPUT -m state --state INVALID -j DROP
-A INPUT -i vif+ -p udp -m udp --dport 68 -j DROP
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i vif+ -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -i vif+ -j REJECT --reject-with icmp-host-prohibited
-A INPUT -j DROP
-A FORWARD -m state --state INVALID -j DROP
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -j QBS-FORWARD
-A FORWARD -i vif+ -o vif+ -j DROP
-A FORWARD -i vif+ -j ACCEPT
-A FORWARD -j DROP
COMMIT
# Completed on Mon Sep  6 08:57:46 2010

And for IPv6:

*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:PR-QBS - [0:0]
:PR-QBS-SERVICES - [0:0]
-A PREROUTING -j PR-QBS
-A PREROUTING -j PR-QBS-SERVICES
-A POSTROUTING -o vif+ -j ACCEPT
-A POSTROUTING -o lo -j ACCEPT
-A POSTROUTING -j MASQUERADE
COMMIT
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:QBS-FORWARD - [0:0]
-A INPUT -m state --state INVALID -j DROP
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i vif+ -p icmpv6 --icmpv6-type router-advertisement -j DROP
-A INPUT -i vif+ -p icmpv6 --icmpv6-type redirect -j DROP
-A INPUT -i vif+ -p icmpv6 -j ACCEPT
-A INPUT -i vif+ -j REJECT --reject-with icmp6-adm-prohibited
-A INPUT -p icmpv6 -j ACCEPT
-A INPUT -j DROP
-A FORWARD -m state --state INVALID -j DROP
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -j QBS-FORWARD
-A FORWARD -i vif+ -o vif+ -j DROP
-A FORWARD -i vif+ -j ACCEPT
-A FORWARD -j DROP
COMMIT

Sincerely,

Demi

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