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FS#3830 - OpenVPN Client Using TCP Connection Has MTU or TCPMSS Issue #8828

openwrt-bot opened this issue May 26, 2021 · 0 comments
flyspray release/21.02


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openwrt-bot commented May 26, 2021


OpenVPN Client connects to a TCP based OpenVPN server connects fine. However, the connections to remote network servers connect but can't transfer data. The mangle rule with clamp-mss-to-pmtu won't receive any data. Setting tcpmss to something around 1000 will only receive the first 100 - 200 bytes and hang. It's not working until reduced tcpmss to 59.

iptables -t nat -A postrouting_rule -o tun0 -j MASQUERADE
iptables -t mangle -A POSTROUTING -o tun0 -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 59
#iptables -t mangle -A POSTROUTING -o tun0 -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

@aparcar aparcar added the release/21.02 label Feb 22, 2022
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