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The first rule of dn42: Always disable rp_filter.
The second rule of dn42: Always disable rp_filter.
The third rule of dn42: Allow ip forwarding!
No seriously, in case some packets are dropped, first check if your settings are correct.
rp_filter also known as reverse path filtering is a security measure. When the route to return a packet uses a different interface than it arrived from, the packet is dropped. Some attackers will set a wrong return address on their packets. This security measure was created to address when this happens. Core internet routing can however be asymmetric. This means that packets can take different routes on the return path. That is why rp_filter needs to be disabled.
Note using sysctl is not persistent. Depending on your linux distribution put it into /etc/sysctl.conf or /etc/sysctl.d
Check that its really disabled:
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Note The max value from conf/{all,interface}/rp_filter is used when doing source validation on the {interface}. In case you don't want to disable rp_filter for the entire system, the correct setup is to set both net.ipv4.conf.all.rp_filter=0 and net.ipv4.conf.{interface}.rp_filter=0, where {interface} is your vpn interface.
Also the following options must be set.
Check that ALL your vpn interfaces allow ip forwarding for ipv6/ipv4.
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Note on firewalls, conntrack and asymmetric routing
Do not configure iptables/nftables to drop packets with invalid conntrack state in forward chain.
In some cases your router will not see traffic from both sides e.g. requests are sent via different path not including your networks but responses are fowarded via your network. This will prevent conntrack from assigning any meaningful state information to these packets and your firewall will drop it if it is configured to drop packets with invalid state.
Happy Routing!