T5169: Add PoC for generating CGNAT rules rfc6888

Add PoC for generating CGNAT rules
https://datatracker.ietf.org/doc/html/rfc6888
Not all requirements are implemented, but some of them.
Implemented:

REQ-2
```
A CGN MUST have a default "IP address pooling" behavior of "Paired"
CGN must use the same external IP
      address mapping for all sessions associated with the same internal
      IP address, be they TCP, UDP, ICMP, something else, or a mix of
      different protocols.
```

REQ-3
```
The CGN function SHOULD NOT have any limitations on the size
      or the contiguity of the external address pool
```

REQ-4
```
A CGN MUST support limiting the number of external ports (or,
      equivalently, "identifiers" for ICMP) that are assigned per
      subscriber
```

CLI:
```
set nat cgnat pool external ext1 external-port-range '1024-65535'
set nat cgnat pool external ext1 per-user-limit port '1000'
set nat cgnat pool external ext1 range 192.0.2.222/32
set nat cgnat pool internal int1 range '100.64.0.0/28'
set nat cgnat rule 10 source pool 'int1'
set nat cgnat rule 10 translation pool 'ext1'
```

1 files changed, 288 insertions, 0 deletions

diff --git a/src/conf_mode/nat_cgnat.py b/src/conf_mode/nat_cgnat.py
new file mode 100755
index 000000000..f41d66c66
--- /dev/null
+++ b/src/conf_mode/nat_cgnat.py
@@ -0,0 +1,288 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2024 VyOS maintainers and contributors
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import ipaddress
+import jmespath
+import os
+
+from sys import exit
+
+from vyos.config import Config
+from vyos.template import render
+from vyos.utils.process import cmd
+from vyos.utils.process import run
+from vyos import ConfigError
+from vyos import airbag
+
+airbag.enable()
+
+
+nftables_cgnat_config = '/run/nftables-cgnat.nft'
+
+
+class IPOperations:
+    def __init__(self, ip_prefix: str):
+        self.ip_prefix = ip_prefix
+        self.ip_network = ipaddress.ip_network(ip_prefix) if '/' in ip_prefix else None
+
+    def get_ips_count(self) -> int:
+        """Returns the number of IPs in a prefix or range.
+
+        Example:
+        % ip = IPOperations('192.0.2.0/30')
+        % ip.get_ips_count()
+        4
+        % ip = IPOperations('192.0.2.0-192.0.2.2')
+        % ip.get_ips_count()
+        3
+        """
+        if '-' in self.ip_prefix:
+            start_ip, end_ip = self.ip_prefix.split('-')
+            start_ip = ipaddress.ip_address(start_ip)
+            end_ip = ipaddress.ip_address(end_ip)
+            return int(end_ip) - int(start_ip) + 1
+        elif '/31' in self.ip_prefix:
+            return 2
+        elif '/32' in self.ip_prefix:
+            return 1
+        else:
+            return sum(
+                1
+                for _ in [self.ip_network.network_address]
+                + list(self.ip_network.hosts())
+                + [self.ip_network.broadcast_address]
+            )
+
+    def convert_prefix_to_list_ips(self) -> list:
+        """Converts a prefix or IP range to a list of IPs including the network and broadcast addresses.
+
+        Example:
+        % ip = IPOperations('192.0.2.0/30')
+        % ip.convert_prefix_to_list_ips()
+        ['192.0.2.0', '192.0.2.1', '192.0.2.2', '192.0.2.3']
+        %
+        % ip = IPOperations('192.0.0.1-192.0.2.5')
+        % ip.convert_prefix_to_list_ips()
+        ['192.0.2.1', '192.0.2.2', '192.0.2.3', '192.0.2.4', '192.0.2.5']
+        """
+        if '-' in self.ip_prefix:
+            start_ip, end_ip = self.ip_prefix.split('-')
+            start_ip = ipaddress.ip_address(start_ip)
+            end_ip = ipaddress.ip_address(end_ip)
+            return [
+                str(ipaddress.ip_address(ip))
+                for ip in range(int(start_ip), int(end_ip) + 1)
+            ]
+        elif '/31' in self.ip_prefix:
+            return [
+                str(ip)
+                for ip in [
+                    self.ip_network.network_address,
+                    self.ip_network.broadcast_address,
+                ]
+            ]
+        elif '/32' in self.ip_prefix:
+            return [str(self.ip_network.network_address)]
+        else:
+            return [
+                str(ip)
+                for ip in [self.ip_network.network_address]
+                + list(self.ip_network.hosts())
+                + [self.ip_network.broadcast_address]
+            ]
+
+
+def generate_port_rules(
+    external_hosts: list,
+    internal_hosts: list,
+    port_count: int,
+    global_port_range: str = '1024-65535',
+) -> list:
+    """Generates list of nftables rules for the batch file."""
+    rules = []
+    proto_map_elements = []
+    other_map_elements = []
+    start_port, end_port = map(int, global_port_range.split('-'))
+    total_possible_ports = (end_port - start_port) + 1
+
+    # Calculate the required number of ports per host
+    required_ports_per_host = port_count
+
+    # Check if there are enough external addresses for all internal hosts
+    if required_ports_per_host * len(internal_hosts) > total_possible_ports * len(
+        external_hosts
+    ):
+        raise ConfigError("Not enough ports available for the specified parameters!")
+
+    current_port = start_port
+    current_external_index = 0
+
+    for internal_host in internal_hosts:
+        external_host = external_hosts[current_external_index]
+        next_end_port = current_port + required_ports_per_host - 1
+
+        # If the port range exceeds the end_port, move to the next external host
+        while next_end_port > end_port:
+            current_external_index = (current_external_index + 1) % len(external_hosts)
+            external_host = external_hosts[current_external_index]
+            current_port = start_port
+            next_end_port = current_port + required_ports_per_host - 1
+
+        # Ensure the same port is not assigned to the same external host
+        if any(
+            rule.endswith(f'{external_host}:{current_port}-{next_end_port}')
+            for rule in rules
+        ):
+            raise ConfigError("Not enough ports available for the specified parameters")
+
+        proto_map_elements.append(
+            f'{internal_host} : {external_host} . {current_port}-{next_end_port}'
+        )
+        other_map_elements.append(f'{internal_host} : {external_host}')
+
+        current_port = next_end_port + 1
+        if current_port > end_port:
+            current_port = start_port
+            current_external_index += 1  # Move to the next external host
+
+    return [proto_map_elements, other_map_elements]
+
+
+def get_config(config=None):
+    if config:
+        conf = config
+    else:
+        conf = Config()
+
+    base = ['nat', 'cgnat']
+    config = conf.get_config_dict(
+        base,
+        get_first_key=True,
+        key_mangling=('-', '_'),
+        no_tag_node_value_mangle=True,
+        with_recursive_defaults=True,
+    )
+
+    return config
+
+
+def verify(config):
+    # bail out early - looks like removal from running config
+    if not config:
+        return None
+
+    if 'pool' not in config:
+        raise ConfigError(f'Pool must be defined!')
+    if 'rule' not in config:
+        raise ConfigError(f'Rule must be defined!')
+
+    # As PoC allow only one rule for CGNAT translations
+    # one internal pool and one external pool
+    if len(config['rule']) > 1:
+        raise ConfigError(f'Only one rule is allowed for translations!')
+
+    for pool in ('external', 'internal'):
+        if pool not in config['pool']:
+            raise ConfigError(f'{pool} pool must be defined!')
+        for pool_name, pool_config in config['pool'][pool].items():
+            if 'range' not in pool_config:
+                raise ConfigError(
+                    f'Range for "{pool} pool {pool_name}" must be defined!'
+                )
+
+    for rule, rule_config in config['rule'].items():
+        if 'source' not in rule_config:
+            raise ConfigError(f'Rule "{rule}" source pool must be defined!')
+        if 'pool' not in rule_config['source']:
+            raise ConfigError(f'Rule "{rule}" source pool must be defined!')
+
+        if 'translation' not in rule_config:
+            raise ConfigError(f'Rule "{rule}" translation pool must be defined!')
+
+
+def generate(config):
+    if not config:
+        return None
+    # first external pool as we allow only one as PoC
+    ext_pool_name = jmespath.search("rule.*.translation | [0]", config).get('pool')
+    int_pool_name = jmespath.search("rule.*.source | [0]", config).get('pool')
+    ext_query = f"pool.external.{ext_pool_name}.range | keys(@)"
+    int_query = f"pool.internal.{int_pool_name}.range"
+    external_ranges = jmespath.search(ext_query, config)
+    internal_ranges = [jmespath.search(int_query, config)]
+
+    external_list_count = []
+    external_list_hosts = []
+    internal_list_count = []
+    internal_list_hosts = []
+    for ext_range in external_ranges:
+        # External hosts count
+        e_count = IPOperations(ext_range).get_ips_count()
+        external_list_count.append(e_count)
+        # External hosts list
+        e_hosts = IPOperations(ext_range).convert_prefix_to_list_ips()
+        external_list_hosts.extend(e_hosts)
+    for int_range in internal_ranges:
+        # Internal hosts count
+        i_count = IPOperations(int_range).get_ips_count()
+        internal_list_count.append(i_count)
+        # Internal hosts list
+        i_hosts = IPOperations(int_range).convert_prefix_to_list_ips()
+        internal_list_hosts.extend(i_hosts)
+
+    external_host_count = sum(external_list_count)
+    internal_host_count = sum(internal_list_count)
+    ports_per_user = int(
+        jmespath.search(f'pool.external.{ext_pool_name}.per_user_limit.port', config)
+    )
+    external_port_range: str = jmespath.search(
+        f'pool.external.{ext_pool_name}.external_port_range', config
+    )
+
+    proto_maps, other_maps = generate_port_rules(
+        external_list_hosts, internal_list_hosts, ports_per_user, external_port_range
+    )
+
+    config['proto_map_elements'] = ', '.join(proto_maps)
+    config['other_map_elements'] = ', '.join(other_maps)
+
+    render(nftables_cgnat_config, 'firewall/nftables-cgnat.j2', config)
+
+    # dry-run newly generated configuration
+    tmp = run(f'nft --check --file {nftables_cgnat_config}')
+    if tmp > 0:
+        raise ConfigError('Configuration file errors encountered!')
+
+
+def apply(config):
+    if not config:
+        # Cleanup cgnat
+        cmd('nft delete table ip cgnat')
+        if os.path.isfile(nftables_cgnat_config):
+            os.unlink(nftables_cgnat_config)
+        return None
+    cmd(f'nft --file {nftables_cgnat_config}')
+
+
+if __name__ == '__main__':
+    try:
+        c = get_config()
+        verify(c)
+        generate(c)
+        apply(c)
+    except ConfigError as e:
+        print(e)
+        exit(1)