From 2332d9b7c1e2575048c2fe158753a1c7e51260e0 Mon Sep 17 00:00:00 2001
From: Ming Aldrich-Gan <mingaldrichgan@gmail.com>
Date: Sun, 24 Mar 2019 16:01:32 -0400
Subject: [PATCH] Add local_addr configuration for doh-server (#39)

* Add local_addr configuration for doh-server

This commit adds a `local_addr` string value to `doh-server.conf`, specifying the IP address and port from which outgoing calls to upstream DNS resolvers should originate. This value is set as the `udpClient`'s and `tcpClient`'s `Dialer.LocalAddr` when initializing a `NewServer`. If the value is left empty in `doh-server.conf`, it defaults to the first `listen` address (which in turn defaults to `"127.0.0.1:8053"`).

One use case for this would be if `doh-server` is proxying requests to a local DNS resolver (e.g. `unbound` or Pi-hole). Up to version 2.0.0, all DNS queries from `doh-server` are sent from `127.0.0.1` (even if the `listen` address is set to a different loopback IP address), making it hard to distinguish them from all other local DNS queries from the same machine in the query logs.

* Revert defaulting of local_addr to listen address

This commit reverts to the existing behavior when `conf.LocalAddr == ""`, i.e. letting `dns.Client` instantiate its own `Dialer` with the default local address.

* Fixup comment in configuration file

* Log errors from Dialer instantiation (e.g. if LocalAddr port is missing)

* Fixup other comment in configuration file

* Return error and log fatal
---
 doh-server/config.go       |  1 +
 doh-server/doh-server.conf |  4 ++++
 doh-server/main.go         |  5 ++++-
 doh-server/server.go       | 29 ++++++++++++++++++++++++-----
 4 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/doh-server/config.go b/doh-server/config.go
index e51c07c..f937c14 100644
--- a/doh-server/config.go
+++ b/doh-server/config.go
@@ -31,6 +31,7 @@ import (
 
 type config struct {
 	Listen           []string `toml:"listen"`
+	LocalAddr        string   `toml:"local_addr"`
 	Cert             string   `toml:"cert"`
 	Key              string   `toml:"key"`
 	Path             string   `toml:"path"`
diff --git a/doh-server/doh-server.conf b/doh-server/doh-server.conf
index 335fe86..eaa7f53 100644
--- a/doh-server/doh-server.conf
+++ b/doh-server/doh-server.conf
@@ -4,6 +4,10 @@ listen = [
     "[::1]:8053",
 ]
 
+# Local address and port for upstream DNS
+# If left empty, a local address is automatically chosen.
+local_addr = ""
+
 # TLS certification file
 # If left empty, plain-text HTTP will be used.
 # You are recommended to leave empty and to use a server load balancer (e.g.
diff --git a/doh-server/main.go b/doh-server/main.go
index a6c24e5..813bee5 100644
--- a/doh-server/main.go
+++ b/doh-server/main.go
@@ -110,6 +110,9 @@ func main() {
 		conf.Verbose = true
 	}
 
-	server := NewServer(conf)
+	server, err := NewServer(conf)
+	if err != nil {
+		log.Fatalln(err)
+	}
 	_ = server.Start()
 }
diff --git a/doh-server/server.go b/doh-server/server.go
index 22ffce3..e758767 100644
--- a/doh-server/server.go
+++ b/doh-server/server.go
@@ -56,22 +56,41 @@ type DNSRequest struct {
 	errtext         string
 }
 
-func NewServer(conf *config) (s *Server) {
-	s = &Server{
+func NewServer(conf *config) (*Server, error) {
+	timeout := time.Duration(conf.Timeout) * time.Second
+	s := &Server{
 		conf: conf,
 		udpClient: &dns.Client{
 			Net:     "udp",
 			UDPSize: dns.DefaultMsgSize,
-			Timeout: time.Duration(conf.Timeout) * time.Second,
+			Timeout: timeout,
 		},
 		tcpClient: &dns.Client{
 			Net:     "tcp",
-			Timeout: time.Duration(conf.Timeout) * time.Second,
+			Timeout: timeout,
 		},
 		servemux: http.NewServeMux(),
 	}
+	if conf.LocalAddr != "" {
+		udpLocalAddr, err := net.ResolveUDPAddr("udp", conf.LocalAddr)
+		if err != nil {
+			return nil, err
+		}
+		tcpLocalAddr, err := net.ResolveTCPAddr("tcp", conf.LocalAddr)
+		if err != nil {
+			return nil, err
+		}
+		s.udpClient.Dialer = &net.Dialer{
+			Timeout:   timeout,
+			LocalAddr: udpLocalAddr,
+		}
+		s.tcpClient.Dialer = &net.Dialer{
+			Timeout:   timeout,
+			LocalAddr: tcpLocalAddr,
+		}
+	}
 	s.servemux.HandleFunc(conf.Path, s.handlerFunc)
-	return
+	return s, nil
 }
 
 func (s *Server) Start() error {