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Suppose I have a networking problem and try to debug it by setting the MSS manually to ex.: 1392. How can I achieve this permanently? I have a native IPv6 address. OS is Debian/CentOS

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I think the proper answer is going to be different for Debian and CentOS. I didn't find a good solution myself, but I found a workaround that at least gets me some of the way. In /etc/rc.local I have this line: ip -6 route replace default advmss 1220 via 2001:db8:2a01:4f8::1. You obviously have to change the gateway address to match your configuration. If the gateway address happens to be a link-local address two lines will be needed, for example ip -6 route del default and ip -6 route add default advmss 1220 via fe80::1 dev eth0. – kasperd Jan 1 at 14:08

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