nftables can do this

[squarooticus]

In trying to debug my Google Cast issues last night, I discovered your daemon. It works great, presumably because it doesn't ingest and then create new (and presumably somehow mangled) mDNS payloads like avahi-daemon does. But then I realized nftables can do the same thing. To wit:

table ip mangle {
    chain prerouting {
        type filter hook prerouting priority mangle; policy accept;
        ip daddr 224.0.0.251 iif enp2s0.30 ip saddr set 192.168.1.1 dup to 224.0.0.251 device enp2s0.10 notrack
        ip daddr 224.0.0.251 iif enp2s0.10 ip saddr set 192.168.3.1 dup to 224.0.0.251 device enp2s0.30 notrack
    }
}

table ip6 mangle {
    chain prerouting {
        type filter hook prerouting priority mangle; policy accept;
        ip6 daddr ff02::fb iif enp2s0.30 ip6 saddr set fd20:1111:1111:1::1 dup to ff02::fb device enp2s0.10 notrack
        ip6 daddr ff02::fb iif enp2s0.10 ip6 saddr set fd20:1111:1111::3::1 dup to ff02::fb device enp2s0.30 notrack
    }
}

The above repeats mDNS packets from enp2s0.10 to enp2s0.30 and vice versa. Works great. Kinda nuts how simple this is.