[Feature] Set final DNS to avoid issues when using `templates.extend` with `templates.servers`

[john-jane-doe]

Commit: 474ab7e

According to the source code¹, using templates.extend to extend a template, fields of type list in the derived template will be prepended to the field in the base template (which is a good feature, especially when overriding DNS/route rules). However, it raises an issue when extending servers (DNS servers) in a template.

For example, for the following configuration:

{
  "templates": [
    {
      "name": "base",
      "dns": "tls://8.8.8.8",
      "dns_local": "https://223.5.5.5/dns-query"
    },
    {
      "name": "derived",
      "servers": [
        {
          "tag": "internal",
          "address": "100.100.100.100"
        }
      ]
    }
  ]
}

the derived template produces the following DNS config:

{
  "dns": {
    "servers": [
      {
        "tag": "internal",
        "address": "100.100.100.100"
      },
      {
        "tag": "default",
        "address": "tls://8.8.8.8"
      },
      {
        "tag": "local",
        "address": "https://223.5.5.5/dns-query",
        "strategy": "prefer_ipv4",
        "detour": "direct"
      }
    ],
    "strategy": "ipv4_only"
  }
}

The newly added DNS server internal is the first in the list, therefore becomes the final DNS server², which is not desired. We desire the original default DNS server to be the final one.

Therefore, we propose serenity to set dns.final to default (or user-specified value) to avoid this issue. It seems to require only one line of change:

--- a/template/render_dns.go
+++ b/template/render_dns.go
@@ -63,6 +63,7 @@ func (t *Template) renderDNS(metadata M.Metadata, options *option.Options) error
 		defaultDNSOptions.AddressResolver = DNSLocalTag
 	}
 	options.DNS.Servers = append(options.DNS.Servers, defaultDNSOptions)
+	options.DNS.Final = DNSDefaultTag  // or user-specified value
 	var (
 		localDNSOptions  option.DNSServerOptions
 		localDNSIsDomain bool

Footnotes

1. https://github.com/SagerNet/sing/blob/9f69e7f9f7e2c4fc26828d98ec9f81ab72fdec98/common/json/badjson/merge.go#L111-L120 ↩

2. https://github.com/SagerNet/sing-box/blob/77e4d86fa02d2f476e7a6767e8ea1de830a0b8b0/route/router.go#L268 ↩