Configurable MTU for IPv6 and other non-1500 configurations

[mk-fg]

Hi,

Current shared/src/wg.rs code has ip link set mtu 1420 ... hardcoded, which is presumably derived from 1420 B payload + 54 B wg header + 20 B IPv4 header, and sums up to just under 1500 B, which is a typical MTU you'd see on eth0 interface.

Such calculation of course breaks down with IPv6, where header is 40B, causing fragmentation on 1500B ifaces.
This might be somewhat related to #15.

But more generally, if innernet is used over e.g. batman-adv mesh (which makes sense to use with such overlay network), MTU can be something like 1450 there anyway, so switching 1420 -> 1400 in the code won't cover this use-case like configurable MTU value might.

Given that "inn up" operation creates interface, sets mtu and immediately tries to contact innernet-server over it, I think such mtu adjustment might need to be done by innernet tool itself, as doing that after it runs or from some netlink event-monitor might be too late or at least racy, and tool will fail if mtu prevents it talking to server for long enough.

Thanks!

[kbknapp]

Adding to this, a long term goal could be to do something like PMTU discovery dynamically.

[mcginty]

Sorry for the late reply! Just pushed a change to make MTU configurable via the CLI, ex. inn --mtu 1350 up foo. Will also add some logic in to set the default MTU to 1400 if the network's CIDR range is IPv6.