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feat: add DNS-over-TCP support alongside UDP listener #56

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a1fe803
feat: add DNS-over-TCP support alongside UDP listener
Jul 29, 2026
8b75cf1
fix: return SERVFAIL DNS response when upstream DoH query fails
Jul 29, 2026
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254 changes: 107 additions & 147 deletions src/handler.rs
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Original file line number Diff line number Diff line change
@@ -1,185 +1,145 @@
use crate::context::Context;
use crate::{Cache, DohError, DohResult};
use crate::{DohError, DohResult};
use bytes::Bytes;
use dns_message_parser::question::Question;
use dns_message_parser::Dns;
use futures::lock::Mutex;
use std::future::Future;
use dns_message_parser::{Dns, RCode};
use log::{debug, error};
use std::net::SocketAddr;
use std::sync::Arc;
use std::time::Duration;
use tokio::net::UdpSocket;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::time::timeout as create_timeout;

async fn send_response(
dns_response: &mut Dns,
id: u16,
addr: SocketAddr,
sender: Arc<UdpSocket>,
) -> DohResult<()> {
dns_response.id = id;
let bytes = dns_response.encode()?;
sender.send_to(bytes.as_ref(), addr).await?;
Ok(())
}
pub async fn resolve_request(msg: Bytes, context: &Context) -> DohResult<Bytes> {
let mut dns_request = Dns::decode(msg)?;
if dns_request.is_response() {
return Err(DohError::DnsNotRequest(dns_request));
}

enum CacheReturn<'a> {
Found(DohResult<()>),
NotFound(Option<(&'a Mutex<Cache<Question, Dns>>, Question)>),
}
let id = dns_request.id;

#[allow(clippy::needless_lifetimes)]
async fn get_response_from_cache<'a>(
context: &'a Context,
dns_request: &Dns,
addr: &SocketAddr,
) -> CacheReturn<'a> {
// 1. Check cache
if let Some(cache) = &context.cache {
let questions = &dns_request.questions;
if questions.len() == 1 {
let question = &questions[0];
if dns_request.questions.len() == 1 {
let question = &dns_request.questions[0];
let mut guard_cache = cache.lock().await;
let entry = if context.cache_fallback {
guard_cache.get_expired(question)
} else {
guard_cache.get(question)
};

if let Some(dns_response) = entry {
let id = dns_request.id;
let sender = context.sender.clone();
let addr = *addr;
let mut resp = dns_response.clone();
resp.id = id;
debug!("Question is found in cache");
let result = send_response(dns_response, id, addr, sender).await;
CacheReturn::Found(result)
} else {
debug!("Question is not found in cache");
CacheReturn::NotFound(Some((cache, question.clone())))
return Ok(resp.encode()?.into());
}
} else {
debug!("The amount of questions is not equal 1");
CacheReturn::NotFound(None)
}
} else {
debug!("Cache is disable");
CacheReturn::NotFound(None)
}
}

async fn get_response(
context: &Context,
cache_question: &Option<(&Mutex<Cache<Question, Dns>>, Question)>,
response: (
impl Future<Output = DohResult<(Dns, Option<Duration>)>>,
u32,
),
id: u16,
addr: &SocketAddr,
) -> Option<DohResult<()>> {
let (response_future, connection_id) = response;
let timeout = context.timeout;
match create_timeout(timeout, response_future).await {
Ok(Ok((mut dns_response, duration))) => {
let addr = *addr;
let sender = context.sender.clone();
let result = send_response(&mut dns_response, id, addr, sender).await;
if let Some(duration) = duration {
if let Some((cache, question)) = cache_question {
let mut guard_cache = cache.lock().await;
debug!(
"Add records in cache: {}, {}, {:?}",
question, dns_response, duration
);
guard_cache.put(question.clone(), dns_response, duration);
// 2. Fetch from remote DoH
let mut guard_remote = context.remote_session.lock().await;
let remote_res = guard_remote.start_request(&mut dns_request).await;
drop(guard_remote);

if let Ok((response_future, connection_id)) = remote_res {
match create_timeout(context.timeout, response_future).await {
Ok(Ok((mut dns_response, duration))) => {
if let Some(duration) = duration {
if let Some(cache) = &context.cache {
if dns_request.questions.len() == 1 {
let question = &dns_request.questions[0];
let mut guard_cache = cache.lock().await;
debug!(
"Add records in cache: {}, {}, {:?}",
question, dns_response, duration
);
guard_cache.put(question.clone(), dns_response.clone(), duration);
}
}
}
dns_response.id = id;
return Ok(dns_response.encode()?.into());
}
Ok(Err(e)) => {
error!("Could not retrieve DNS response from server: {}", e);
}
Err(e) => {
error!("Timeout: {}", e);
}
return Some(result);
}
Ok(Err(e)) => {
error!("Could not retrieve DNS response from server: {}", e);
}
Err(e) => {
error!("Timeout: {}", e);
}
}
let mut guard_remote_session = context.remote_session.lock().await;
guard_remote_session.disconnect(connection_id);
None
}

async fn get_response_from_remote(
context: &Context,
cache_question: &Option<(&Mutex<Cache<Question, Dns>>, Question)>,
dns_request: &mut Dns,
addr: &SocketAddr,
) -> Option<DohResult<()>> {
let mut guard_remote_session = context.remote_session.lock().await;
let result = guard_remote_session.start_request(dns_request).await;
drop(guard_remote_session);
match result {
Ok(response) => {
let id = dns_request.id;
get_response(context, cache_question, response, id, addr).await
}
Err(e) => {
info!("Could not contact DNS server: {}", e);
None
}
let mut guard_remote = context.remote_session.lock().await;
guard_remote.disconnect(connection_id);
}
}

#[allow(clippy::needless_lifetimes)]
async fn get_response_from_cache_fallback<'a>(
context: &'a Context,
cache_question: Option<(&Mutex<Cache<Question, Dns>>, Question)>,
dns_request: &Dns,
addr: SocketAddr,
) -> Option<DohResult<()>> {
// 3. Fallback cache
if context.cache_fallback {
if let Some((cache, question)) = &cache_question {
let mut guard_cache = cache.lock().await;
if let Some(dns_response) = guard_cache.get_expired_fallback(question) {
let id = dns_request.id;
let sender = context.sender.clone();
debug!("Question is found in cache fallback");
let result = send_response(dns_response, id, addr, sender).await;
Some(result)
} else {
debug!("Question is not found in cache fallback");
None
if let Some(cache) = &context.cache {
if dns_request.questions.len() == 1 {
let question = &dns_request.questions[0];
let mut guard_cache = cache.lock().await;
if let Some(dns_response) = guard_cache.get_expired_fallback(question) {
let mut resp = dns_response.clone();
resp.id = id;
debug!("Question is found in cache fallback");
return Ok(resp.encode()?.into());
}
}
} else {
debug!("Question cannot be cached");
None
}
} else {
debug!("Cache fallback is disable");
None
}

// Upstream failed: return SERVFAIL DNS response to client instead of dropping TCP connection
let mut servfail_resp = dns_request;
servfail_resp.flags.qr = true;
servfail_resp.flags.rcode = RCode::ServFail;
Ok(servfail_resp.encode()?.into())
}

pub async fn request_handler(msg: Bytes, addr: SocketAddr, context: &Context) -> DohResult<()> {
let mut dns_request = Dns::decode(msg)?;
if dns_request.is_response() {
return Err(DohError::DnsNotRequest(dns_request));
}
let resp_bytes = resolve_request(msg, context).await?;
context.sender.send_to(resp_bytes.as_ref(), addr).await?;
Ok(())
}

let cache = get_response_from_cache(context, &dns_request, &addr).await;
let cache_question = match cache {
CacheReturn::Found(result) => return result,
CacheReturn::NotFound(cache_question) => cache_question,
};
pub async fn tcp_connection_handler(
mut stream: tokio::net::TcpStream,
context: Arc<Context>,
) -> DohResult<()> {
let peer_addr = stream.peer_addr()?;
debug!("New TCP DNS connection from {}", peer_addr);

let remote = get_response_from_remote(context, &cache_question, &mut dns_request, &addr).await;
if let Some(result) = remote {
return result;
}
loop {
let mut len_buf = [0u8; 2];
if stream.read_exact(&mut len_buf).await.is_err() {
break;
}
let packet_len = u16::from_be_bytes(len_buf) as usize;
if packet_len == 0 {
break;
}

let fallback =
get_response_from_cache_fallback(context, cache_question, &dns_request, addr).await;
if let Some(result) = fallback {
return result;
}
let mut msg_buf = vec![0u8; packet_len];
if stream.read_exact(&mut msg_buf).await.is_err() {
break;
}

Err(DohError::CouldNotGetResponse(dns_request))
let msg = Bytes::from(msg_buf);
match resolve_request(msg, context.as_ref()).await {
Ok(resp_bytes) => {
let resp_len = (resp_bytes.len() as u16).to_be_bytes();
if stream.write_all(&resp_len).await.is_err() {
break;
}
if stream.write_all(resp_bytes.as_ref()).await.is_err() {
break;
}
if stream.flush().await.is_err() {
break;
}
}
Err(e) => {
debug!("TCP query handling failed for {}: {}", peer_addr, e);
break;
}
}
}
Ok(())
}
32 changes: 30 additions & 2 deletions src/run.rs
View file
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Original file line number Diff line number Diff line change
@@ -1,17 +1,45 @@
use crate::config::Config;
use crate::error::Result as DohResult;
use crate::handler::request_handler;
use crate::handler::{request_handler, tcp_connection_handler};
use bytes::Bytes;
use dns_message_parser::MAXIMUM_DNS_PACKET_SIZE;
use log::{debug, error, info};
use std::sync::Arc;
use tokio::spawn;

/// Run the `doh-client` with a specific configuration.
pub async fn run(config: Config) -> DohResult<()> {
let (recv, context) = config.into().await?;

let local_addr = recv.local_addr()?;
let context = Arc::new(context);

match tokio::net::TcpListener::bind(local_addr).await {
Ok(tcp_listener) => {
info!("Listening for DNS requests on UDP and TCP address {}", local_addr);
let tcp_context = context.clone();
spawn(async move {
loop {
match tcp_listener.accept().await {
Ok((stream, _)) => {
let c = tcp_context.clone();
spawn(async move {
if let Err(e) = tcp_connection_handler(stream, c).await {
debug!("TCP connection closed: {}", e);
}
});
}
Err(e) => {
error!("TCP accept error: {}", e);
}
}
}
});
}
Err(e) => {
error!("Failed to bind TCP listener on {}: {}", local_addr, e);
}
}

let mut buffer: [u8; MAXIMUM_DNS_PACKET_SIZE] = [0; MAXIMUM_DNS_PACKET_SIZE];
loop {
let (n, addr) = recv.recv_from(&mut buffer[..]).await?;
Expand Down