Linux, ECN and old firewalls

[Lee Watterworth]

Hello all,

Bumped into a problem where my firewall was refusing connections from a
linux machine, found the reason and thought I would share:

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CONFIG_INET_ECN:

  Explicit Congestion Notification (ECN) allows routers to notify
  clients about network congestion, resulting in fewer dropped packets
  and increased network performance. This option adds ECN support to
  the Linux kernel, as well as a sysctl (/proc/sys/net/ipv4/tcp_ecn)
  which allows ECN support to be disabled at runtime.

  Note that, on the Internet, there are many broken firewalls which
  refuse connections from ECN-enabled machines, and it may be a while
  before these firewalls are fixed. Until then, to access a site behind
  such a firewall (some of which are major sites, at the time of this
  writing) you will have to disable this option, either by saying N now
  or by using the sysctl.