pub async fn map_domain_ipv4(&self, domain: &Name) -> FakeDnsResult<(Ipv4Addr, Duration)> {
        map_domain_ip!(self, domain, Ipv4Addr, ipv4_addr, ipv4_network)
    }
Get or create an IPv4 mapping for `domain`
crates/shadowsocks-service/src/local/fake_dns/manager.rs