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--- en:dpi:faq:cgnat:start [2024/08/05 14:00] – elena.krasnobryzh
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-====== CG-NAT and NAT (Service 11)======
-{{indexmenu_n>4}}
-<accordion autoclose>
-<accordion-item title="1. Why is it Recommended to Create a Pool of at Least 2 or 4 Addresses?">
-The non-blocking dispatch algorithm in DPI, which distributes sessions across worker threads, imposes a limitation on which public IP address can be assigned to a subscriber from the pool:
-  - To ensure that a subscriber receives their designated public address, the pool must contain at least as many addresses as there are worker threads (typically 2 for SSG-6 and 4 for SSG-10 and higher). You can determine the number of worker threads with the command <code bash>expr $(ps -p `pidof fastdpi` H -o comm | grep wrk | wc -l) / $(ps -p `pidof fastdpi` H -o comm | grep rx | wc -l)</code>
-  - If the pool contains only one address, it may not be assigned to all subscribers but only to those who fit the load balancing algorithm.
-</accordion-item>
-<accordion-item title="2. How to Determine Which Public Address from the Pool a Subscriber Will Get?">
-You can see which public address has been assigned to a private one with the command <code bash>fdpi_ctrl list status --service 11 --ip 192.168.4.20</code>. In NAT 1:1, the public address is allocated immediately upon service assignment; in CG-NAT, it is allocated at the start of the session. The assigned public address is also reported in Radius Accounting for billing logging purposes.
-Predicting the exact address that will be assigned to a subscriber from the pool in advance is impossible; it depends on various factors, particularly the current pool load.
-</accordion-item>
-<accordion-item title="3. Inactive SSH Sessions are Closed After Enabling NAT">
-<note>Parameter descriptions [[dpi:opt_cgnat:сgnat_settings:start#параметры_и_возможные_значения|at this link]]</note>
-Indeed, the session lifetime in NAT is limited because the number of sessions per subscriber is a limited resource, and a large number of inactive sessions in the pool reduces NAT and overall performance.
-NAT cannot distinguish whether a session became inactive due to an accident or simply because there is no activity, and it closes such long-hanging sessions by inactivity timeout. This behavior is standard and supported by most CG-NAT manufacturers.
-In SSG, session lifetimes can be adjusted with the following parameters:
-  * ''lifetime_flow_long=600'' — lifetime in seconds of inactive TCP sessions
-  * ''lifetime_flow=60'' — lifetime in seconds of other sessions
-<note warning>These parameters should not be set to excessively large values, as the session table may grow too large, impacting CG-NAT performance, and the subscriber may run out of session limits (set in the nat pool parameters).</note>
-To maintain long inactive connections, it is recommended to use the ''tcp keep-alive'' mechanism, where an empty packet is periodically sent in the session, signaling that the session is still active.
-You can configure ''tcp keep-alive'' both individually for the application on the server or client side and at the operating system level for all applications.
-**Example** configuration on the ssh server: add the line to the ''/etc/ssh/ssh_config'' file:
-<code bash>
-ServerAliveInterval 60
-</code>
-**Example** configuration on the ssh client: add the lines to the ''~/.ssh/config'' file:
-<code bash>
-Host *
-  ServerAliveInterval 60
-</code>
-or in the command line:
-<code bash>
-ssh -o TCPKeepAlive=yes -o ServerAliveInterval=60 user@example.com
-</code>
-**Example** configuration for all applications in CentOS: add the lines to the ''/etc/sysctl.conf'' file:
-<code bash>
-net.ipv4.tcp_keepalive_time = 600
-net.ipv4.tcp_keepalive_intvl = 60
-net.ipv4.tcp_keepalive_probes = 20
-</code>
-</accordion-item>
-<accordion-item title="4. How Many Private IP Addresses Can Be Hidden Behind One Public IP in CG-NAT?">
-<note important>It is recommended to maintain a ratio from 1:10 (better) to 1:100 (worse).</note>
-Details:
-By default, 64512 ports (65535-1023, as the first 1024 ports are not used because they are system ports) are available for CG-NAT on one public IP. Each port represents one TCP session and one UDP. The number of sessions created by subscribers varies: individuals create fewer sessions, while corporate users create more (thus, corporate users should use a separate pool with different session limits). A subscriber with torrents can create up to 1000 sessions at peak.
-On average, an individual creates 50-60 simultaneous sessions, meaning 64512/60=1075 individuals can be hidden behind one private IP. However, such significant oversubscription is not recommended in practice, as many popular services (mail, video, search) use IP address-based botnet attack protection. If too many requests come from one address, they may be considered an attack and block some requests or enable captcha, causing inconvenience to subscribers.
-Also, consider the feature of port release mechanism in the NAT Pool:
-  - When Service 11 is enabled for a subscriber, [[dpi:faq:cgnat:start#how_to_determine_which_public_address_from_the_pool_a_subscriber_will_get|a Public IP is assigned based on the distribution algorithm]]
-  - When a subscriber starts establishing sessions, ports are taken from the common SSG DPI queue and [[dpi:faq:cgnat:start#inactive_ssh_sessions_are_closed_after_enabling_nat|assigned with certain timeouts]]
-  - If a specific Public IP has many subscribers competing for free ports, subscribers may experience access issues.
-Recommendations for creating and operating NAT Pools:
-  - Subscribers under blocking (Service 5 + policing) should be placed in a separate NAT Pool to avoid impacting active subscribers. For instance, an iPhone may establish many sessions searching for an active service.
-  - Create sparse pools and separate clients into different NAT Pools by type: individuals and corporate users.
-  - Monitor clients generating high load and work with them. For receiving, processing, and storing NetFlow from DPI, we suggest using the [[dpi:dpi_components:qoestor:start|QoE Store software product for statistics collection]] and the [[dpi:dpi_components:dpiui:start|DPIUI2 graphical interface]]. You can analyze subscriber traffic and conclude that their PC is infected.
-</accordion-item>
-<accordion-item title="5. How to Change the Parameters of an Existing and Used Pool?">
-<note>Parameter descriptions [[dpi:opt_cgnat:сgnat_settings:start#параметры_и_возможные_значения|at this link]]</note>
-  - Changing the session limit:<code bash>fdpi_ctrl load profile --service 11 --profile.name test_nat_2000 --profile.json '{ "nat_ip_pool" : "111.111.111.0/24", "nat_tcp_max_sessions" : 2000, "nat_udp_max_sessions" : 2000, "nat_type" : 0 }'</code>Use the pool creation command identical to the previous one but with different ''nat_tcp_max_sessions'' and ''nat_udp_max_sessions'' settings.
-  - Adding additional addresses to the pool:<code bash>fdpi_ctrl load profile --service 11 --profile.name test_nat_2000 --profile.json '{ "nat_ip_pool" : "111.111.111.0/24,222.222.222.0/25", "nat_tcp_max_sessions" : 2000, "nat_udp_max_sessions" : 2000, "nat_type" : 0 }'</code>Use the pool creation command identical to the previous one but with an additional pool specified by a comma.
-  - Reducing the pool
-<note important>The current version does not support dynamic pool size reduction and address exclusion.
-In this case, you will need to free the pool, delete it, and create it with new parameters.</note>
-For convenience, install jq (a utility for working with JSON data):
-<code bash>
-yum install epel-release yum-utils
-yum-config-manager --disable epel
-yum --enablerepo epel install jq
-</code>
-Then save the information about the subscribers of the current pool, delete and recreate the pool, and reconnect the subscribers:
-<code bash>
-fdpi_ctrl list all --service 11 --profile.name test_nat_4000 --outformat json | jq '.lservices[] | .login | select(. != null)' > save_users.txt
-fdpi_ctrl list all --service 11 --profile.name test_nat_4000 --outformat json | jq -r '.lservices[] | .ipv4 | select(. != null)' >> save_users.txt
-fdpi_ctrl del all --service 11 --profile.name test_nat_4000
-fdpi_ctrl del profile --service 11 --profile.name test_nat_4000
-fdpi_ctrl load profile --service 11 --profile.name test_nat_4000 --profile.json '{ "nat_ip_pool" : "111.111.111.0/30", "nat_tcp_max_sessions" : 4000, "nat_udp_max_sessions" : 4000, "nat_type" : 0 }'
-fdpi_ctrl load --service 11 --profile.name test_nat_4000 --file save_users.txt
-</code>
-Do not forget to change the pool name and its new parameters in the commands to the ones you need.
-</
-accordion-item>
-<accordion-item title="6. Is There a Radius Accounting Message When Assigning/Revoking a Public Address?">
-<note>When working with Radius Accounting, you can verify which IP was assigned to which user using the command <code bash>fdpi_ctrl list status --service 11 --ip 192.168.4.20</code></note>
-Yes, the IP is reflected in the accounting messages, such as Start and Interim. For example, using Cisco AVPair attributes in FreeRadius.
-**Example:**
-<code bash>
-echo "User-Name = test, Cisco-AVPair = "ipv6:addr=2001:db8::1234", Cisco-AVPair = "ipv4:addr=192.168.4.20"" | radclient -f - localhost acct testing123
-</code>
-</accordion-item>
-</accordion>

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