Why is there access problems with some websites?

Recent Linux kernels enable ECN by default, which may cause access problems with some websites on bad routers. To check ECN status:

# cat /proc/sys/net/ipv4/tcp_ecn
or
# sysctl net.ipv4.tcp_ecn

To turn it off, use:

# echo "0" > /proc/sys/net/ipv4/tcp_ecn
or
# sysctl -w net.ipv4.tcp_ecn=0

To disable TCP ECN on every boot, edit /etc/sysctl.conf and add:

net.ipv4.tcp_ecn = 0

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