Enable BBR correctly (second run)

In this section, we verify and enable TCP BBR (Bottleneck Bandwidth and Round-trip propagation time) on Debian 12.

Goal: confirm the kernel supports BBR and it is actually in use to optimize proxy network speeds.

7.1 Background: what went wrong before

We previously tried a risky approach meant for older Debian versions:

• adding an old backports source (buster-backports) to /etc/apt/sources.list .
• installing a different kernel from that source.
• rebooting.

Result:

• system behavior became unstable; SSH broke, returning Connection refused, and the custom SSH port was unexpectedly wiped out and reset to 22 .
• conclusion: do NOT mix old Debian backports on Debian 12 just to get BBR.

So we switched to the correct approach:

• use the Debian 12 kernel as-is. Debian 12's stock kernel already supports BBR natively.

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7.2 Verify BBR is available

We checked the current congestion control:

sudo sysctl net.ipv4.tcp_congestion_control

We also checked what algorithms are available:

sudo sysctl net.ipv4.tcp_available_congestion_control

Expected output pattern:

• available includes bbr (e.g., reno cubic bbr).
• current is bbr (or you can switch to it).

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7.3 Common problem: sysctl: command not found

When running the checks without sudo, we saw :

• -bash: sysctl: command not found

Cause:

• sysctl is located in /usr/sbin/, which is usually not in a normal user's $PATH .

Fix:

• run the command with sudo, which includes /usr/sbin in its path.
• if the binary is truly missing, install the procps package :

  sudo apt update
  sudo apt install -y procps

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7.4 Enable BBR via sysctl config (The Modern Way)

Instead of editing the main /etc/sysctl.conf directly, it is best practice on modern Debian to drop a clean file into /etc/sysctl.d/ .

• create the configuration file and add the BBR parameters :

  echo "net.core.default_qdisc=fq" | sudo tee /etc/sysctl.d/99-bbr.conf
  echo "net.ipv4.tcp_congestion_control=bbr" | sudo tee -a /etc/sysctl.d/99-bbr.conf

• apply the settings without needing to reboot :

  sudo sysctl --system

• verify BBR is loaded :

  lsmod | grep bbr

  (Note: It usually loads automatically when first used, so don't worry if it doesn't show up immediately ).

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7.5 Quick checklist for this stage

After finishing this section, you should have:

• Verified that the stock Debian 12 kernel supports bbr.
• Confirmed tcp_available_congestion_control includes bbr.
• Confirmed tcp_congestion_control is actively set to bbr.
• A dedicated /etc/sysctl.d/99-bbr.conf file ensuring BBR persists across reboots.
• Avoided any risky backports or kernel mixing.