host_firewall: GreenSnow set greensnow_v4 drops inbound new connections of the listed hosts

host_firewall/defaults/main.yml

@@ -83,7 +83,8 @@ firewall:
     # Lets the kernel log the nft log statements of container network
     # namespaces (LXC guests) into the host kernel log.
     all_netns: false
-    # Shared by the Spamhaus (fw-spamhaus-in: , fw-spamhaus-out: ), FireHOL (fw-firehol-in: , fw-firehol-out: ) and scanner (fw-scanners-in: ) chains, one limit each.
+    # Shared by the Spamhaus (fw-spamhaus-in: , fw-spamhaus-out: ), FireHOL (fw-firehol-in: , fw-firehol-out: ), scanner (fw-scanners-in: ) and
+    # GreenSnow (fw-greensnow-in: ) chains, one limit each.
     spamhaus_rate: "6/minute burst 10 packets"
   # Spamhaus DROP, DROPv6 and ASN-DROP: traffic from the listed networks drops
   # (input, forward), traffic to them is rejected (output, forward). The sets
@@ -104,6 +105,13 @@ firewall:
   # of any host, and ones holding an address of the host are left out
   # (/etc/nft-host-scanners.allowlist). Refreshed daily by nft-host-scanners.
   scanners: true
+  # GreenSnow (IPv4 single addresses, compromised and dynamic hosts from the
+  # FireHOL blocklist-ipsets): inbound new connections from the listed hosts
+  # drop, outbound traffic is not filtered. Log prefix fw-greensnow-in: with its
+  # own limit, which CrowdSec whitelists. Entries in private or reserved space
+  # and ones holding an address of the host are left out. Refreshed daily by
+  # nft-host-greensnow.
+  greensnow: true
   # Reject outbound TCP 25; the mail group turns it off.
   block_smtp_out: true
   # EU-27 plus CH, NO, GB, IS, LI (ipdeny country codes). Empty leaves the geo

host_firewall/files/nft-host-greensnow

@@ -0,0 +1,129 @@
+#!/usr/bin/python3
+# Managed by Ansible (role host_firewall), do not edit.
+"""Rebuilds the GreenSnow set greensnow_v4 of table inet host.
+
+Source: the GreenSnow list of the FireHOL blocklist-ipsets (single IPv4
+addresses, comment lines start with #). A failed download or sanity check
+exits non-zero before anything changes, so the previous set stays in place.
+"""
+import ipaddress
+import json
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import time
+import urllib.request
+
+PERSISTED = "/etc/nftables.d/host-greensnow.nft"
+URL = "https://raw.githubusercontent.com/firehol/blocklist-ipsets/master/greensnow.ipset"
+USER_AGENT = "ruekov-host-firewall (nft-host-greensnow)"
+
+# Lower bound that catches an empty or truncated answer (the list holds about
+# 4700 entries).
+MIN_ENTRIES = 500
+# A prefix this wide is a parse error, not a bad address.
+MIN_PREFIXLEN = 8
+# Private, CGNAT (NetBird), loopback, link-local and multicast/reserved space:
+# an entry overlapping one of these never lands in the set, so the own
+# infrastructure cannot block itself.
+EXCLUDED = [ipaddress.ip_network(n) for n in (
+    "0.0.0.0/8", "10.0.0.0/8", "100.64.0.0/10", "127.0.0.0/8",
+    "169.254.0.0/16", "172.16.0.0/12", "192.168.0.0/16", "224.0.0.0/3")]
+
+
+def fail(message):
+    print(f"nft-host-greensnow: {message}", file=sys.stderr)
+    sys.exit(1)
+
+
+def fetch():
+    error = None
+    for attempt in range(3):
+        try:
+            request = urllib.request.Request(URL, headers={"User-Agent": USER_AGENT})
+            with urllib.request.urlopen(request, timeout=120) as response:
+                return response.read().decode("utf-8")
+        except OSError as e:
+            error = e
+            time.sleep(5 * (attempt + 1))
+    fail(f"download of {URL} failed: {error}")
+
+
+def own_addresses():
+    result = subprocess.run(["ip", "-j", "-4", "address", "show"], capture_output=True, text=True)
+    if result.returncode != 0:
+        fail(f"ip address show failed: {result.stderr.strip()}")
+    return [ipaddress.ip_address(a["local"]) for link in json.loads(result.stdout)
+            for a in link.get("addr_info", []) if a.get("family") == "inet"]
+
+
+def main():
+    networks = []
+    for line in fetch().splitlines():
+        line = line.strip()
+        if not line or line.startswith("#"):
+            continue
+        try:
+            networks.append(ipaddress.IPv4Network(line))
+        except ValueError:
+            fail(f"unparsable line: {line!r}")
+    if len(networks) < MIN_ENTRIES:
+        fail(f"only {len(networks)} entries")
+
+    own = own_addresses()
+    kept, excluded, own_skipped = [], 0, 0
+    for network in networks:
+        if network.prefixlen < MIN_PREFIXLEN:
+            fail(f"suspiciously wide prefix {network}")
+        if any(network.overlaps(e) for e in EXCLUDED):
+            excluded += 1
+        elif any(address in network for address in own):
+            own_skipped += 1
+        else:
+            kept.append(network)
+    merged = list(ipaddress.collapse_addresses(kept))
+    print(f"greensnow_v4: {len(networks)} entries, {excluded} in private or reserved space and "
+          f"{own_skipped} holding an own address skipped -> {len(merged)} merged")
+
+    elements = ",\n\t\t".join(str(n) for n in merged) + ","
+    persisted = ("# Generated by nft-host-greensnow, do not edit.\n"
+                 "table inet host {\n"
+                 "\tset greensnow_v4 { type ipv4_addr; flags interval; auto-merge; elements = {\n"
+                 f"\t\t{elements}\n\t}} }}\n}}\n")
+
+    # Checked under another table name: a dry run against the live table
+    # name changes the live sets for its duration and unlisted addresses match.
+    check_rules = persisted.replace("table inet host {", "table inet host_greensnow_check {")
+    checked = subprocess.run(["nft", "-c", "-f", "-"], input=check_rules, capture_output=True, text=True)
+    if checked.returncode != 0:
+        fail(f"nft rejects the set: {checked.stderr.strip()}")
+
+    with tempfile.TemporaryDirectory() as tmp:
+        previous = None
+        if os.path.exists(PERSISTED):
+            previous = os.path.join(tmp, "previous.nft")
+            shutil.copyfile(PERSISTED, previous)
+        with open(PERSISTED + ".new", "w", encoding="utf-8") as f:
+            f.write(persisted)
+        os.chmod(PERSISTED + ".new", 0o644)
+        os.replace(PERSISTED + ".new", PERSISTED)
+
+        # A reload replaces the whole table, sets included, in one transaction.
+        active = subprocess.run(["systemctl", "-q", "is-active", "nft-host.service"]).returncode == 0
+        if active:
+            reloaded = subprocess.run(["systemctl", "reload", "nft-host.service"], capture_output=True, text=True)
+            if reloaded.returncode != 0:
+                if previous:
+                    shutil.copyfile(previous, PERSISTED)
+                fail(f"reloading the rule set failed, kept the previous set: {reloaded.stderr.strip()}")
+        print("reloaded the rule set" if active else "persisted only")
+
+
+try:
+    main()
+except SystemExit:
+    raise
+except Exception as e:  # keep the previous set on any surprise
+    fail(f"{type(e).__name__}: {e}")

host_firewall/files/nft-host-greensnow.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Refresh the GreenSnow set of the host firewall
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/sbin/nft-host-greensnow

host_firewall/files/nft-host-greensnow.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Daily refresh of the host firewall GreenSnow set
+
+[Timer]
+OnCalendar=daily
+RandomizedDelaySec=6h
+Persistent=true
+
+[Install]
+WantedBy=timers.target

host_firewall/tasks/main.yml

@@ -59,7 +59,7 @@
     src: "{{ item }}"
     dest: "/etc/systemd/system/{{ item }}"
     mode: "0644"
-  loop: [nft-host.service, nft-host-geo.service, nft-host-geo.timer, nft-host-spamhaus.service, nft-host-spamhaus.timer, nft-host-firehol.service, nft-host-firehol.timer, nft-host-scanners.service, nft-host-scanners.timer]
+  loop: [nft-host.service, nft-host-geo.service, nft-host-geo.timer, nft-host-spamhaus.service, nft-host-spamhaus.timer, nft-host-firehol.service, nft-host-firehol.timer, nft-host-scanners.service, nft-host-scanners.timer, nft-host-greensnow.service, nft-host-greensnow.timer]
   register: firewall_units
 
 - name: Install the rollback script
@@ -287,6 +287,59 @@
         path: /etc/nftables.d/host-scanners.nft
         state: absent
 
+- name: GreenSnow set
+  when: firewall.greensnow
+  block:
+    - name: Install the GreenSnow set updater
+      ansible.builtin.copy:
+        src: nft-host-greensnow
+        dest: /usr/local/sbin/nft-host-greensnow
+        mode: "0755"
+
+    # The rule set includes this file, so a missing one would fail the load at boot.
+    - name: Build the GreenSnow set on first run
+      ansible.builtin.command: /usr/local/sbin/nft-host-greensnow
+      args:
+        creates: /etc/nftables.d/host-greensnow.nft
+
+    - name: Read the GreenSnow set file
+      ansible.builtin.stat:
+        path: /etc/nftables.d/host-greensnow.nft
+      register: firewall_greensnow
+
+    - name: Require non-empty GreenSnow set
+      ansible.builtin.assert:
+        that: firewall_greensnow.stat.exists and firewall_greensnow.stat.size > 0
+        fail_msg: /etc/nftables.d/host-greensnow.nft is missing or empty, refusing to apply the firewall
+      # Check mode neither installs the updater nor builds the set.
+      when: not ansible_check_mode
+
+    - name: Enable the GreenSnow set timer
+      ansible.builtin.systemd:
+        name: nft-host-greensnow.timer
+        enabled: true
+        state: started
+        daemon_reload: "{{ firewall_units is changed }}"
+      # Check mode does not install the unit.
+      ignore_errors: "{{ ansible_check_mode }}"
+
+- name: Stop the GreenSnow set refresh
+  when: not firewall.greensnow
+  block:
+    - name: Disable the GreenSnow set timer
+      ansible.builtin.systemd:
+        name: nft-host-greensnow.timer
+        enabled: false
+        state: stopped
+        daemon_reload: "{{ firewall_units is changed }}"
+      # Check mode does not install the unit.
+      ignore_errors: "{{ ansible_check_mode }}"
+
+    - name: Remove the GreenSnow set file
+      ansible.builtin.file:
+        path: /etc/nftables.d/host-greensnow.nft
+        state: absent
+
 - name: Render the rule set
   ansible.builtin.template:
     src: host.nft.j2

host_firewall/templates/host.nft.j2

@@ -82,6 +82,9 @@ include "/etc/nftables.d/host-firehol.nft"
 {% if firewall.scanners %}
 include "/etc/nftables.d/host-scanners.nft"
 {% endif %}
+{% if firewall.greensnow %}
+include "/etc/nftables.d/host-greensnow.nft"
+{% endif %}
 
 table inet host {
 	{{ addr_set('admin_v4', 'ipv4_addr', firewall.admin.v4) }}
@@ -154,6 +157,12 @@ table inet host {
 		ip saddr @scanners_v4 jump scanners_in
 		ip6 saddr @scanners_v6 jump scanners_in
 
+{% endif %}
+{% if firewall.greensnow %}
+
+		# After the established rule: only inbound connections of GreenSnow hosts drop.
+		ip saddr @greensnow_v4 jump greensnow_in
+
 {% endif %}
 		ct state invalid jump drop_log
 		iif lo accept
@@ -317,6 +326,16 @@ table inet host {
 		drop
 	}
 {% endif %}
+{% if firewall.greensnow %}
+
+	# Own limit and prefix, so GreenSnow hits use up neither the fw-in-drop budget
+	# nor the one of the blocklists. CrowdSec whitelists the prefix: the listed
+	# hosts are known and need no decision.
+	chain greensnow_in {
+		limit rate {{ firewall.log.spamhaus_rate }} log prefix "fw-greensnow-in: "
+		drop
+	}
+{% endif %}
 {% if firewall.block_smtp_out or blocklists %}
 
 	# Outbound mail goes through the mail host only. Loopback stays open for a
@@ -394,7 +413,7 @@ table inet host {
 		ip6 saddr @geo_v6 accept
 {% endif %}
 	}
-{% if forward_guard or blocklists or firewall.scanners or rl_forward %}
+{% if forward_guard or blocklists or firewall.scanners or firewall.greensnow or rl_forward %}
 
 	chain forward {
 		type filter hook forward priority filter; policy accept;
@@ -423,6 +442,10 @@ table inet host {
 		ct state new ip saddr @scanners_v4 jump scanners_in
 		ct state new ip6 saddr @scanners_v6 jump scanners_in
 {% endif %}
+{% if firewall.greensnow %}
+
+		ct state new ip saddr @greensnow_v4 jump greensnow_in
+{% endif %}
 {% for guard in forward_guard %}
 
 		# {{ guard.key }}