[tor-relays] Dropping packets with TCP MSS=1400 to foil GFW active probing

[David Fifield]

On Sun, Aug 19, 2018 at 07:41:26PM -0400, Nathaniel Suchy wrote:
> Is China successfully probing OBFS4 bridges? Or does this apply more to non
> obfs bridges?

China doesn't dynamically probe obfs4 bridges. (More precisely: they may
try to probe, but the probes don't result in blocks for obfs4.) They
only block obfs4 bridges whose addresses they learn in other ways. The
MSS=1400 trick works, the authors report, even for non-obfuscated
bridges.