net: forward guest DNS to the host's DNS server - #108
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Guest DNS queries were always forwarded to a hard-coded 8.8.8.8. A VPN such as Mullvad blocks DNS to anything but its own resolver while still allowing direct IP traffic, so guest TCP to public addresses worked but hostnames did not resolve. NAT now forwards guest UDP DNS queries to the host's configured DNS server: the first IPv4 nameserver in /etc/resolv.conf on macOS and Linux, or the IPv4 DNS server of the connected adapter with the lowest metric on Windows (GetAdaptersAddresses). The address is re-read every 5 s, so connecting or disconnecting a VPN needs no restart. If none is found, 8.8.8.8 is used as before. GatewayConfig::dns_upstream becomes an Option: None (the default) follows the host, Some pins a fixed server. DHCP still advertises 8.8.8.8 rather than the host's server, which is often a loopback address (a VPN's local resolver, systemd-resolved) that the guest would take as its own. The NAT forwards port-53 UDP whatever the destination address, so guest lookups still reach the host's server.
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Guest DNS queries were always forwarded to a hard-coded 8.8.8.8. A VPN such as Mullvad blocks DNS to anything but its own resolver while still allowing direct IP traffic, so guest TCP to public addresses worked but hostnames did not resolve.
NAT now forwards guest UDP DNS queries to the host's configured DNS server: the first IPv4 nameserver in /etc/resolv.conf on macOS and Linux, or the IPv4 DNS server of the connected adapter with the lowest metric on Windows (GetAdaptersAddresses). The address is re-read every 5 s, so connecting or disconnecting a VPN needs no restart. If none is found, 8.8.8.8 is used as before.
GatewayConfig::dns_upstream becomes an Option: None (the default) follows the host, Some pins a fixed server.
DHCP still advertises 8.8.8.8 rather than the host's server, which is often a loopback address (a VPN's local resolver, systemd-resolved) that the guest would take as its own. The NAT forwards port-53 UDP whatever the destination address, so guest lookups still reach the host's server.