I have tested this on the server and client test rigs ("nothing broke").
Also, explicitly tested this using "--mssfix 500 fixed" and tcpdumped
on the tunnel interface - not caring about outside packet size, this time.
TUN mode:
IPv4 MSS: 460
Family: IP (2)
Total Length: 500
Transmission Control Protocol, Src Port: 22, Dst Port: 36351, Seq: 5005,
Ack: 37, Len: 448
TCP payload (448 bytes)
IPv6 MSS: 440
Family: IPv6 (28)
Payload Length: 460
Transmission Control Protocol, Src Port: 22, Dst Port: 27345, Seq: 8518,
Ack: 3034, Len: 428
[TCP Segment Len: 428]
TCP payload (428 bytes)
(tshark / tcpdump claim "504 bytes on the wire" in both cases, but
I assume this is some FreeBSD/tun overhead artefact... maybe the
"is this ipv4 or or ipv6?" extra header)
TAP mode:
IPv4 MSS: 460
514 bytes on wire (4112 bits), 514 bytes captured (4112 bits) on interface
tap0, id 0
...
TCP payload (448 bytes)
IPv6 MSS: 440
514 bytes on wire (4112 bits), 514 bytes captured (4112 bits) on interface
tap0, id 0
...
TCP payload (428 bytes)
So, in tun mode, this does what it says on the lid ("MSS is lowered so
the resulting IPv4/IPv6 packet is not exceeding this value"). For TAP,
there is overhead, which gets added.
Arguably this is *correct* behaviour - this goes hand in hand with
"--tun-mtu $small --mssfix (default)", and if you set a "--tun-mtu 500"
on a *TAP* interface, this is what you get: IP packets "up to 500",
plus ethernet header. The system-set MSS value will be exactly the
same number that OpenVPN uses in this case (tested!), so -> perfect.
Your patch has been applied to the master branch.
commit 47671d6d6814eadb3dd5e742ebc40c6f21038224
Author: Arne Schwabe
Date: Thu Feb 24 15:42:45 2022 +0100
Implement fixed MSS value for mssfix and use it for non default MTUs
Acked-by: Lev Stipakov <[email protected]>
Message-Id: <[email protected]>
URL:
https://www.mail-archive.com/[email protected]/msg23886.html
Signed-off-by: Gert Doering <[email protected]>
--
kind regards,
Gert Doering
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