✅ Layer 1 (RECOMMENDED): Disable IPv6 via sysctl (permanent)

1️⃣ Create a dedicated sysctl file

sudo nano /etc/sysctl.d/99-disable-ipv6.conf

Paste exactly this:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Save and exit.

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2️⃣ Apply immediately (no reboot needed)

sudo sysctl --system

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3️⃣ Verify IPv6 is disabled

ip a | grep inet6

✅ Expected result: no output
(or only ::1 disappears as well)

Also check:

cat /proc/sys/net/ipv6/conf/all/disable_ipv6

Should return:

1