Firewall: note on reqid matching.

To be done when we upgrade to Bullseye for more fine-grained control.

1 files changed, 4 insertions, 3 deletions

diff --git a/roles/common/templates/etc/nftables.conf.j2 b/roles/common/templates/etc/nftables.conf.j2
index 2261994..098a66d 100755
--- a/roles/common/templates/etc/nftables.conf.j2
+++ b/roles/common/templates/etc/nftables.conf.j2
@@ -44,43 +44,44 @@ define out-tcp-ports = {
 ###############################################################################
 
 flush ruleset
 
 table inet filter {
     # blackholes
     set fail2ban  { type ipv4_addr; timeout 10m; }
     set fail2ban6 { type ipv6_addr; timeout 10m; }
 
     chain input {
         type filter hook input priority 0
         policy drop
 
         iif lo accept
 
         # XXX Bullseye: this is a rather crude match as nftables 0.9.0 lacks support for ipsec expressions
         # to match match inbound resp. outbound policies and source resp. destination tunnel addresses.
         # https://serverfault.com/questions/971735/how-to-match-reqid-in-nftables
         # https://blog.fraggod.net/2016/09/25/nftables-re-injected-ipsec-matching-without-xt_policy.html
         # (We can't use marks to match post-ESP decapsulation here because that doesn't work well with UDP
-        # encapsulation.)
+        # encapsulation.)  We'll also pin the reqid to the lowest address byte in ipsec.conf(5); that way
+        # peers can't impersonate each other.
         meta l4proto esp accept
-        # ipsec in ip saddr {{ ipsec_subnet }} ip daddr {{ ipsec[inventory_hostname_short] }} accept
+        # ip saddr {{ ipsec_subnet }} ip daddr {{ ipsec[inventory_hostname_short] }} ipsec in reqid $i accept
         ip saddr {{ ipsec_subnet }} ip daddr {{ ipsec[inventory_hostname_short] }} meta secpath exists accept
 
         # rate-limiting is done directly by the kernel (net.ipv4.icmp_{ratelimit,ratemask} runtime options)
         icmp   type { echo-reply, echo-request, destination-unreachable } counter accept
         icmpv6 type { echo-reply, echo-request, destination-unreachable,
                       packet-too-big, time-exceeded, parameter-problem } counter accept
 
         # accept neighbour discovery for autoconfiguration, RFC 4890 sec. 4.4.1
         icmpv6 type { 133,134,135,136,141,142 } ip6 hoplimit 255 counter accept
 
         jump martian
         jump invalid
 
         udp sport  123 udp dport  123 ct state     related,established accept
 {% if groups.all | length > 1 %}
         udp sport  500 udp dport  500 ct state new,related,established accept
 {% if groups.NATed | length > 0 %}
         udp sport 4500 udp dport 4500 ct state new,related,established accept
 {% endif %}
 {% endif %}
@@ -93,41 +94,41 @@ table inet filter {
 {% endif %}
 
         meta l4proto tcp ip  saddr @fail2ban  counter drop
         meta l4proto tcp ip6 saddr @fail2ban6 counter drop
 
         tcp dport $in-tcp-ports  ct state related,established accept
         tcp dport $in-tcp-ports  ct state new counter accept
         tcp sport $out-tcp-ports ct state related,established accept
     }
 
     chain output {
         type filter hook output priority 0
         policy drop
 
         oif lo accept
 
         # XXX Bullseye: unlike for input we can't use marks or test for
         # secpath existence here, because by the time we see a packet to
         # 172.16.0.0/24 we don't know if it'll be encapsulated
         meta l4proto esp accept
-        # ipsec out ip saddr {{ ipsec[inventory_hostname_short] }} ip daddr {{ ipsec_subnet }} accept
+        # ip saddr {{ ipsec[inventory_hostname_short] }} ip daddr {{ ipsec_subnet }} ipsec out reqid $i accept
         ip saddr {{ ipsec[inventory_hostname_short] }} ip daddr {{ ipsec_subnet }} accept
 
         meta l4proto { icmp, icmpv6 } accept
 
         jump martian
         jump invalid
 
         udp sport  123 udp dport  123 ct state new,related,established accept
 {% if groups.all | length > 1 %}
         udp sport  500 udp dport  500 ct state new,related,established accept
 {% if groups.NATed | length > 0 %}
         udp sport 4500 udp dport 4500 ct state new,related,established accept
 {% endif %}
 {% endif %}
 
         udp dport 53 ct state new,related,established accept
         tcp dport 53 ct state new,related,established accept
 {% if 'dhclient' in group_names %}
         ip  version 4 udp sport  68 udp dport  67 ct state new,related,established accept
         ip6 version 6 udp sport 546 udp dport 547 ct state new,related,established accept