[GH-ISSUE #3878] Abuse complaint from ISP: NetBird client sending unsolicited traffic to CGNAT subnet #8018

New Issue

saavagebueno · 2026-08-05T01:15:39-04:00

saavagebueno commented

2026-08-05 01:15:39 -04:00

Originally created by @mrchupatek on GitHub (May 26, 2025).
Original GitHub issue: https://github.com/netbirdio/netbird/issues/3878

Describe the problem

We received an abuse complaint from our ISP due to NetBird client sending unsolicited UDP and TCP traffic to IPs in the 100.79.0.0/16 CGNAT subnet. The ISP flagged this as a network scan attempt.

This subnet is used internally by NetBird on interface wt0, but external probing of CGNAT addresses appears suspicious from the ISP's point of view and may lead to service suspension.

To Reproduce

Install NetBird (default setup, no configuration changes)
Join a self-hosted network where 100.79.0.0/16 is assigned to wt0
Observe outbound connections to 100.79.x.x addresses
Receive abuse message from provider reporting "Netscan" activity

Expected behavior

NetBird should avoid initiating connections to RFC6598 (CGNAT) IP ranges unless explicitly allowed. At a minimum, this behavior should be configurable.

Are you using NetBird Cloud?

No, using self-hosted NetBird.

NetBird version

Daemon version: 0.44.0
CLI version: 0.44.0

Is any other VPN software installed?

No.

Firewall (UFW) settings

# ufw status verbose
Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
Anywhere                   ALLOW IN    100.64.0.0/10             
Anywhere                   ALLOW IN    192.168.0.0/24            
Anywhere                   ALLOW IN    46.101.102.23             

10.0.0.0/8                 DENY OUT    Anywhere on eno1          
172.16.0.0/12              DENY OUT    Anywhere on eno1          
192.168.0.0/16             DENY OUT    Anywhere on eno1

Abuse

TIME (UTC)           SRC             SRC-PORT -> DST             DST-PORT SIZE PROT
-------------------------------------------------------------------------------
2025-05-26 07:35:35  37.27.133.173  8301     -> 100.79.0.94      8301     215 UDP
2025-05-26 07:35:35  37.27.133.173  8301     -> 100.79.2.178     8301     215 UDP
2025-05-26 07:35:25  37.27.133.173 50222     -> 100.79.3.2       8300      78 TCP
2025-05-26 07:35:26  37.27.133.173 50222     -> 100.79.3.2       8300      78 TCP
2025-05-26 07:35:24  37.27.133.173 50222     -> 100.79.3.2       8300      78 TCP
2025-05-26 07:35:14  37.27.133.173  8301     -> 100.79.3.2       8301     141 UDP
2025-05-26 07:35:30  37.27.133.173  8301     -> 100.79.3.2       8301     243 UDP
2025-05-26 07:35:31  37.27.133.173 54762     -> 100.79.3.2       8301      78 TCP
2025-05-26 07:34:46  37.27.133.173  8301     -> 100.79.3.62      8301     150 UDP
2025-05-26 07:35:06  37.27.133.173  8301     -> 100.79.3.62      8301     144 UDP
2025-05-26 07:35:15  37.27.133.173  8301     -> 100.79.4.71      8301     266 UDP

Additional context

We use the default NetBird configuration out-of-the-box, no modifications.
The internal network is assigned the 100.79.0.0/16 subnet on interface wt0.
UFW explicitly blocks outbound access to other private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), but not CGNAT, as this is needed for NetBird.
The packets appear to be part of peer discovery or connection attempts, but to the provider this behavior mimics a port scan.

Originally created by @mrchupatek on GitHub (May 26, 2025). Original GitHub issue: https://github.com/netbirdio/netbird/issues/3878 **Describe the problem** We received an abuse complaint from our ISP due to NetBird client sending unsolicited UDP and TCP traffic to IPs in the 100.79.0.0/16 CGNAT subnet. The ISP flagged this as a network scan attempt. This subnet is used internally by NetBird on interface `wt0`, but external probing of CGNAT addresses appears suspicious from the ISP's point of view and may lead to service suspension. --- **To Reproduce** 1. Install NetBird (default setup, no configuration changes) 2. Join a self-hosted network where 100.79.0.0/16 is assigned to `wt0` 3. Observe outbound connections to 100.79.x.x addresses 4. Receive abuse message from provider reporting "Netscan" activity --- **Expected behavior** NetBird should avoid initiating connections to RFC6598 (CGNAT) IP ranges unless explicitly allowed. At a minimum, this behavior should be configurable. --- **Are you using NetBird Cloud?** No, using self-hosted NetBird. --- **NetBird version** Daemon version: 0.44.0 CLI version: 0.44.0 --- **Is any other VPN software installed?** No. --- **Firewall (UFW) settings** ``` # ufw status verbose Status: active Logging: on (low) Default: deny (incoming), allow (outgoing), disabled (routed) New profiles: skip To Action From -- ------ ---- Anywhere ALLOW IN 100.64.0.0/10 Anywhere ALLOW IN 192.168.0.0/24 Anywhere ALLOW IN 46.101.102.23 10.0.0.0/8 DENY OUT Anywhere on eno1 172.16.0.0/12 DENY OUT Anywhere on eno1 192.168.0.0/16 DENY OUT Anywhere on eno1 ``` --- **Abuse** ``` TIME (UTC) SRC SRC-PORT -> DST DST-PORT SIZE PROT ------------------------------------------------------------------------------- 2025-05-26 07:35:35 37.27.133.173 8301 -> 100.79.0.94 8301 215 UDP 2025-05-26 07:35:35 37.27.133.173 8301 -> 100.79.2.178 8301 215 UDP 2025-05-26 07:35:25 37.27.133.173 50222 -> 100.79.3.2 8300 78 TCP 2025-05-26 07:35:26 37.27.133.173 50222 -> 100.79.3.2 8300 78 TCP 2025-05-26 07:35:24 37.27.133.173 50222 -> 100.79.3.2 8300 78 TCP 2025-05-26 07:35:14 37.27.133.173 8301 -> 100.79.3.2 8301 141 UDP 2025-05-26 07:35:30 37.27.133.173 8301 -> 100.79.3.2 8301 243 UDP 2025-05-26 07:35:31 37.27.133.173 54762 -> 100.79.3.2 8301 78 TCP 2025-05-26 07:34:46 37.27.133.173 8301 -> 100.79.3.62 8301 150 UDP 2025-05-26 07:35:06 37.27.133.173 8301 -> 100.79.3.62 8301 144 UDP 2025-05-26 07:35:15 37.27.133.173 8301 -> 100.79.4.71 8301 266 UDP ``` --- **Additional context** 1. We use the default NetBird configuration out-of-the-box, no modifications. 2. The internal network is assigned the 100.79.0.0/16 subnet on interface wt0. 3. UFW explicitly blocks outbound access to other private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), but not CGNAT, as this is needed for NetBird. 4. The packets appear to be part of peer discovery or connection attempts, but to the provider this behavior mimics a port scan.

saavagebueno added the triage-needed label 2026-08-05 01:15:39 -04:00

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Reference: DYNR/netbird#8018