localsubnetsetd, a daemon to track the local subnet for nftables

5 files changed, 398 insertions, 0 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+# How to Contribute
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diff --git a/README.md b/README.md
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+# localsubnetsetd
+
+localsubnetsetd maintains [nftables](https://nftables.org/) sets for all subnets
+directly attached to the current machine. The original use case for
+localsubnetsetd was to give special access to traffic on an IPv6 subnet with an
+unpredictable network number (as is common with prefix delegations from consumer
+ISPs). More generally, localsubnetsetd allows you to treat local network traffic
+specially without involving a border firewall.
+
+## Dependencies
+
+localsubnetsetd is written in Python 3 and requires the nftables and pyroute2
+libraries. On Debian, you want the `python3`, `python3-nftables`, and
+`python3-pyroute2` packages.
+
+## Setup
+
+localsubnetsetd requires that `local_subnets4` and `local_subnets6` sets exist
+in the `inet filter` table. Create them with
+
+    nft add set inet filter local_subnets4 { type ipv4_addr; flags interval; }
+    nft add set inet filter local_subnets6 { type ipv6_addr; flags interval; }
+
+---
+
+This is not an official Google product.
diff --git a/localsubnetsetd b/localsubnetsetd
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+#!/usr/bin/python3
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+from socket import AF_INET, AF_INET6
+
+import nftables
+import pyroute2
+
+
+def send_command(nft, s):
+    '''Sends a command to nftables.
+
+    Sends the specified command string to the specified nftables instance. Waits
+    for acknowledgement. Raises RuntimeError if the command fails.
+    '''
+    print(s)
+    failed, stdout, stderr = nft.cmd(s)
+    if failed:
+        raise RuntimeError('nftables command failed: %s' % stderr)
+
+
+def set_name(family):
+    '''Generates the relevant nftables set name for the specified protocol.'''
+    if family == AF_INET:
+        return 'local_subnets4'
+    elif family == AF_INET6:
+        return 'local_subnets6'
+    raise ValueError("don't have a set for family %d" % family)
+
+
+def flush(family):
+    '''Creates an nftables command to flush the local subnet set.'''
+    return 'flush set inet filter %s' % set_name(family)
+
+
+def add(msg):
+    '''Creates an nftables command to add a subnet.
+
+    Processes a netlink message to extract its subnet, and creates an nftables
+    command to add that subnet to the appropriate set.
+    '''
+    for k, v in msg['attrs']:
+        if k == 'IFA_ADDRESS':
+            subnet = v + '/' + str(msg['prefixlen'])
+            break
+    else:
+        raise ValueError("message lacks 'IFA_ADDRESS' attribute")
+
+    return 'add element inet filter %s { %s }' % (set_name(
+        msg['family']), subnet)
+
+
+def add_current(netlink, family=None):
+    '''Creates an nftables command to add all current subnets.'''
+    r = []
+    for msg in netlink.get_addr():
+        if family and msg['family'] != family:
+            continue
+        r.append(add(msg))
+    return '; '.join(r)
+
+
+def maintain_sets(nft, netlink):
+    # Start listening for netlink messages now so we don't miss any while
+    # we're querying the current IP address set.
+    netlink.bind()
+
+    # Populate the sets with the initial set of IP addresses.
+    send_command(
+        nft, '; '.join([flush(AF_INET),
+                        flush(AF_INET6),
+                        add_current(netlink)]))
+
+    # Watch for changes.
+    while True:
+        for msg in netlink.get():
+            if msg['event'] == 'RTM_NEWADDR':
+                send_command(nft, add(msg))
+            elif msg['event'] == 'RTM_DELADDR':
+                # We might still be holding another address on the same subnet,
+                # so we can't just delete this subnet. Rebuild the subnet set
+                # from scratch.
+                fam = msg['family']
+                send_command(nft,
+                             flush(fam) + '; ' + add_current(netlink, fam))
+
+
+if __name__ == '__main__':
+    nft = nftables.Nftables()
+    with pyroute2.IPRoute() as netlink:
+        maintain_sets(nft, netlink)
diff --git a/localsubnetsetd.service b/localsubnetsetd.service
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+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+[Unit]
+Description=local subnet set maintenance daemon
+Requires=nftables.service
+After=nftables.service
+
+[Service]
+ExecStart=localsubnetsetd
+Restart=always
+NoNewPrivileges=true
+ProtectSystem=strict
+ProtectHome=true
+PrivateDevices=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectControlGroups=true
+RestrictAddressFamilies=AF_NETLINK
+RestrictNamespaces=true
+LockPersonality=true
+MemoryDenyWriteExecute=true
+RestrictRealtime=true
+SystemCallFilter=@system-service
+SystemCallArchitectures=native
+
+[Install]
+WantedBy=sysinit.target