Sep 25, 2016

nftables re-injected IPSec matching without xt_policy

As of linux-4.8, something like xt_policy is still - unfortunately - on the nftables TODO list, so to match traffic pre-authenticated via IPSec, some workaround is needed.

Obvious one is to keep using iptables/ip6tables to mark IPSec packets with old xt_policy module, as these rules interoperate with nftables just fine, with only important bit being ordering of iptables hooks vs nft chain priorities, which are rather easy to find in "netfilter_ipv{4,6}.h" files, e.g.:

enum nf_ip_hook_priorities {
  NF_IP_PRI_FIRST = INT_MIN,
  NF_IP_PRI_CONNTRACK_DEFRAG = -400,
  NF_IP_PRI_RAW = -300,
  NF_IP_PRI_SELINUX_FIRST = -225,
  NF_IP_PRI_CONNTRACK = -200,
  NF_IP_PRI_MANGLE = -150,
  NF_IP_PRI_NAT_DST = -100,
  NF_IP_PRI_FILTER = 0,
  NF_IP_PRI_SECURITY = 50,
  NF_IP_PRI_NAT_SRC = 100,
  NF_IP_PRI_SELINUX_LAST = 225,
  NF_IP_PRI_CONNTRACK_HELPER = 300,
  NF_IP_PRI_CONNTRACK_CONFIRM = INT_MAX,
  NF_IP_PRI_LAST = INT_MAX,
};

(see also Netfilter-packet-flow.svg by Jan Engelhardt for general overview of the iptables hook positions, nftables allows to define any number of chains before/after these)

So marks from iptables/ip6tables rules like these:

*raw
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A PREROUTING -m policy --dir in --pol ipsec --mode transport -j MARK --or-mark 0x101
-A OUTPUT -m policy --dir out --pol ipsec --mode transport -j MARK --or-mark 0x101
COMMIT

Will be easy to match in priority=0 input/ouput hooks (as NF_IP_PRI_RAW=-300) of nft ip/ip6/inet tables (e.g. mark and 0x101 == 0x101 accept)

But that'd split firewall configuration between iptables/nftables, adding more hassle to keep whole "iptables" thing initialized just for one or two rules.

xfrm transformation (like ipsec esp decryption in this case) seem to preserve all information about the packet intact, including packet marks (but not conntrack states, which track esp connection), which - as suggested by Florian Westphal in #netfilter - can be utilized to match post-xfrm packets in nftables by this preserved mark field.

E.g. having this (strictly before ct state {established, related} accept for stateful firewalls, as each packet has to be marked):

define cm.ipsec = 0x101
add rule inet filter input ip protocol esp mark set mark or $cm.ipsec
add rule inet filter input ip6 nexthdr esp mark set mark or $cm.ipsec
add rule inet filter input mark and $cm.ipsec == $cm.ipsec accept

Will mark and accept both still-encrypted esp packets (IPv4/IPv6) and their decrypted payload.

Note that this assumes that all IPSec connections are properly authenticated and trusted, so be sure not to use anything like that if e.g. opportunistic encryption is enabled.

Much simplier nft-only solution, though still not a full substitute for what xt_policy does, of couse.

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