feat: Add support for custom DNS servers

Adds an option `dns_servers` which takes IPs for custom servers.
Does not allow for custom DNS ports as this is largely not supported in
tokio and hickory, and would otherwise be a pain to implement properly.

Fixes: #1866
See-Also: https://matrix.to/#/!ksTlboXVgcyWjv5GrlEeKyQuJ8ZCprnwQx2b6-BQ44Q/%24qrBGOzosK4BdRbF5jhf5ehrxQUTaIr-U-UpqTmPhkPY?via=wolfgirl.pet&via=matrix.org

src/core/config/mod.rs

@@ -383,6 +383,15 @@ pub struct Config {
 	#[serde(default = "default_stateinfo_cache_capacity")]
 	pub stateinfo_cache_capacity: u32,
 
+	/// DNS servers to use instead of system defaults. Does NOT accept IP:Port
+	/// pairs, only plain IPs. IPv6 addresses should be without brackets.
+	///
+	/// Uses system configuration (/etc/resolv.conf) if empty.
+	///
+	/// default: []
+	#[serde(default)]
+	pub dns_servers: Vec<String>,
+
 	/// Maximum entries stored in DNS memory-cache. The size of an entry may
 	/// vary so please take care if raising this value excessively. Only
 	/// decrease this when using an external DNS cache. Please note that

src/service/client/mod.rs

@@ -4,7 +4,10 @@
 use conduwuit::{Config, Result, Server, err, trace, utils::response::LimitReadExt, warn};
 use either::Either;
 use hickory_resolver::{
-	TokioResolver, config::ConnectionConfig, net::runtime::TokioRuntimeProvider,
+	TokioResolver,
+	config::{ConnectionConfig, NameServerConfig},
+	net::runtime::TokioRuntimeProvider,
+	proto::rr::IntoName,
 };
 use ipaddress::IPAddress;
 use reqwest::redirect;
@@ -320,7 +323,20 @@ fn get_dns_resolver(server: &Arc<Server>) -> Result<Arc<TokioResolver>> {
 		conf.add_search(sys_conf.clone());
 	}
 
-	for sys_conf in sys_conf.name_servers() {
+	let config_nameservers: Vec<NameServerConfig> = server
+		.config
+		.dns_servers
+		.iter()
+		.filter_map(IntoName::to_ip)
+		.map(NameServerConfig::udp_and_tcp)
+		.collect();
+	let nameservers = if !config_nameservers.is_empty() {
+		config_nameservers
+	} else {
+		sys_conf.name_servers().to_vec()
+	};
+
+	for sys_conf in nameservers {
 		let mut ns = sys_conf.clone();
 
 		if config.query_over_tcp_only {