Hi, On Tue, Mar 09, 2021 at 03:33:56PM +0000, tincanteksup wrote: > what is the final effect of using `--mssfix 0` in a client > config ? > > What I mean is, how would openvpn interpret this and what would > it tell the TCP stack ? > > I don't need to know about MSS, I just want to know what openvpn > would do with a setting of 0.
I would expect things to break in most interesting ways.
From what I can see, the code does not enforce a minimum value, but
isn't directly *using* that value either. It will be modified by
this macro
mss_fixup_ipv6(&ipbuf, MTU_TO_MSS(TUN_MTU_SIZE_DYNAMIC(&c->c2.frame)));
to figure out the final MSS value to put into the TCP SYN/SYN ACK
packets. Since "0" is the expected *outer* maximum, the net result
is likely something negative, which will end up as a large positive
number in the 16 bit MSS field.
Run tcpdump/wireshark and find out :-)
> Also, would openvpn behave differently on *nix vs Windows with
> this setting.
No. The code in question is not OS dependent (forward.c, mss.c, no
#ifdef _WIN32 anywhere close to "mss").
gert
--
"If was one thing all people took for granted, was conviction that if you
feed honest figures into a computer, honest figures come out. Never doubted
it myself till I met a computer with a sense of humor."
Robert A. Heinlein, The Moon is a Harsh Mistress
Gert Doering - Munich, Germany g...@greenie.muc.de
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