Added dnstc service that serves any UDP DNS-like request with TC flag.

TC flag means `truncated`, so it actually means "ask me via TCP" for
compliant resolver.

Makefile

@@ -1,4 +1,4 @@
-OBJS := parser.o main.o redsocks.o log.o http-connect.o socks4.o socks5.o http-relay.o base.o base64.o md5.o http-auth.o utils.o redudp.o
+OBJS := parser.o main.o redsocks.o log.o http-connect.o socks4.o socks5.o http-relay.o base.o base64.o md5.o http-auth.o utils.o redudp.o dnstc.o
 SRCS := $(OBJS:.o=.c)
 CONF := config.h
 DEPS := .depend

README

@@ -114,8 +114,6 @@ TODO
 Test OpenBSD (pf) and FreeBSD (ipfw) and write setup examples for those
 firewall types.
 
-Allow redirecting of DNS packets.
-
 
 Author
 ======

dnstc.c

@@ -0,0 +1,259 @@
+/* redsocks - transparent TCP-to-proxy redirector
+ * Copyright (C) 2007-2011 Leonid Evdokimov <[email protected]>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License.  You may obtain a copy
+ * of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+#include <assert.h>
+
+#include "list.h"
+#include "log.h"
+#include "parser.h"
+#include "main.h"
+#include "redsocks.h"
+#include "dnstc.h"
+#include "utils.h"
+
+#define dnstc_log_error(prio, msg...) \
+	redsocks_log_write_plain(__FILE__, __LINE__, __func__, 0, &clientaddr, &self->config.bindaddr, prio, ## msg)
+#define dnstc_log_errno(prio, msg...) \
+	redsocks_log_write_plain(__FILE__, __LINE__, __func__, 1, &clientaddr, &self->config.bindaddr, prio, ## msg)
+
+static void dnstc_fini_instance(dnstc_instance *instance);
+static int dnstc_fini();
+
+typedef struct dns_header_t {
+	uint16_t id;
+	uint8_t qr_opcode_aa_tc_rd;
+	uint8_t ra_z_rcode;
+	uint16_t qdcount;
+	uint16_t ancount;
+	uint16_t nscount;
+	uint16_t arcount;
+} PACKED dns_header;
+
+#define DNS_QR 0x80
+#define DNS_TC 0x02
+#define DNS_Z  0x70
+
+/***********************************************************************
+ * Logic
+ */
+static void dnstc_pkt_from_client(int fd, short what, void *_arg)
+{
+	dnstc_instance *self = _arg;
+	struct sockaddr_in clientaddr;
+	union {
+		char raw[0xFFFF]; // UDP packet can't be larger then that
+		dns_header h;
+	} buf;
+	ssize_t pktlen, outgoing;
+
+	assert(fd == EVENT_FD(&self->listener));
+	pktlen = red_recv_udp_pkt(fd, buf.raw, sizeof(buf), &clientaddr);
+	if (pktlen == -1)
+		return;
+
+	if (pktlen <= sizeof(dns_header)) {
+		dnstc_log_error(LOG_INFO, "incomplete DNS request");
+		return;
+	}
+
+	if (1
+		&& (buf.h.qr_opcode_aa_tc_rd & DNS_QR) == 0 /* query */
+		&& (buf.h.ra_z_rcode & DNS_Z) == 0 /* Z is Zero */
+		&& buf.h.qdcount /* some questions */
+		&& !buf.h.ancount && !buf.h.nscount && !buf.h.arcount /* no answers */
+	) {
+		buf.h.qr_opcode_aa_tc_rd |= DNS_QR;
+		buf.h.qr_opcode_aa_tc_rd |= DNS_TC;
+		outgoing = sendto(fd, buf.raw, pktlen, 0,
+		                  (struct sockaddr*)&clientaddr, sizeof(clientaddr));
+		if (outgoing != pktlen)
+			dnstc_log_errno(LOG_WARNING, "sendto: I was sending %zd bytes, but only %zd were sent.",
+			                pktlen, outgoing);
+		else
+			dnstc_log_error(LOG_INFO, "sent truncated DNS reply");
+	}
+	else {
+		dnstc_log_error(LOG_INFO, "malformed DNS request");
+	}
+}
+
+/***********************************************************************
+ * Init / shutdown
+ */
+static parser_entry dnstc_entries[] =
+{
+	{ .key = "local_ip",   .type = pt_in_addr },
+	{ .key = "local_port", .type = pt_uint16 },
+	{ }
+};
+
+static list_head instances = LIST_HEAD_INIT(instances);
+
+static int dnstc_onenter(parser_section *section)
+{
+	dnstc_instance *instance = calloc(1, sizeof(*instance));
+	if (!instance) {
+		parser_error(section->context, "Not enough memory");
+		return -1;
+	}
+
+	INIT_LIST_HEAD(&instance->list);
+	instance->config.bindaddr.sin_family = AF_INET;
+	instance->config.bindaddr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+
+	for (parser_entry *entry = &section->entries[0]; entry->key; entry++)
+		entry->addr =
+			(strcmp(entry->key, "local_ip") == 0)   ? (void*)&instance->config.bindaddr.sin_addr :
+			(strcmp(entry->key, "local_port") == 0) ? (void*)&instance->config.bindaddr.sin_port :
+			NULL;
+	section->data = instance;
+	return 0;
+}
+
+static int dnstc_onexit(parser_section *section)
+{
+	dnstc_instance *instance = section->data;
+
+	section->data = NULL;
+	for (parser_entry *entry = &section->entries[0]; entry->key; entry++)
+		entry->addr = NULL;
+
+	instance->config.bindaddr.sin_port = htons(instance->config.bindaddr.sin_port);
+
+	list_add(&instance->list, &instances);
+
+	return 0;
+}
+
+static int dnstc_init_instance(dnstc_instance *instance)
+{
+	/* FIXME: dnstc_fini_instance is called in case of failure, this
+	 *        function will remove instance from instances list - result
+	 *        looks ugly.
+	 */
+	int error;
+	int fd = -1;
+
+	fd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (fd == -1) {
+		log_errno(LOG_ERR, "socket");
+		goto fail;
+	}
+
+	error = bind(fd, (struct sockaddr*)&instance->config.bindaddr, sizeof(instance->config.bindaddr));
+	if (error) {
+		log_errno(LOG_ERR, "bind");
+		goto fail;
+	}
+
+	error = fcntl_nonblock(fd);
+	if (error) {
+		log_errno(LOG_ERR, "fcntl");
+		goto fail;
+	}
+
+	event_set(&instance->listener, fd, EV_READ | EV_PERSIST, dnstc_pkt_from_client, instance);
+	error = event_add(&instance->listener, NULL);
+	if (error) {
+		log_errno(LOG_ERR, "event_add");
+		goto fail;
+	}
+
+	return 0;
+
+fail:
+	dnstc_fini_instance(instance);
+
+	if (fd != -1) {
+		if (close(fd) != 0)
+			log_errno(LOG_WARNING, "close");
+	}
+
+	return -1;
+}
+
+/* Drops instance completely, freeing its memory and removing from
+ * instances list.
+ */
+static void dnstc_fini_instance(dnstc_instance *instance)
+{
+	if (event_initialized(&instance->listener)) {
+		if (event_del(&instance->listener) != 0)
+			log_errno(LOG_WARNING, "event_del");
+		if (close(EVENT_FD(&instance->listener)) != 0)
+			log_errno(LOG_WARNING, "close");
+		memset(&instance->listener, 0, sizeof(instance->listener));
+	}
+
+	list_del(&instance->list);
+
+	memset(instance, 0, sizeof(*instance));
+	free(instance);
+}
+
+static int dnstc_init()
+{
+	dnstc_instance *tmp, *instance = NULL;
+
+	// TODO: init debug_dumper
+
+	list_for_each_entry_safe(instance, tmp, &instances, list) {
+		if (dnstc_init_instance(instance) != 0)
+			goto fail;
+	}
+
+	return 0;
+
+fail:
+	dnstc_fini();
+	return -1;
+}
+
+static int dnstc_fini()
+{
+	dnstc_instance *tmp, *instance = NULL;
+
+	list_for_each_entry_safe(instance, tmp, &instances, list)
+		dnstc_fini_instance(instance);
+
+	return 0;
+}
+
+static parser_section dnstc_conf_section =
+{
+	.name    = "dnstc",
+	.entries = dnstc_entries,
+	.onenter = dnstc_onenter,
+	.onexit  = dnstc_onexit
+};
+
+app_subsys dnstc_subsys =
+{
+	.init = dnstc_init,
+	.fini = dnstc_fini,
+	.conf_section = &dnstc_conf_section,
+};
+
+/* vim:set tabstop=4 softtabstop=4 shiftwidth=4: */
+/* vim:set foldmethod=marker foldlevel=32 foldmarker={,}: */

dnstc.h

@@ -0,0 +1,16 @@
+#ifndef DNSTC_H
+#define DNSTC_H
+
+typedef struct dnstc_config_t {
+	struct sockaddr_in bindaddr;
+} dnstc_config;
+
+typedef struct dnstc_instance_t {
+	list_head       list;
+	dnstc_config    config;
+	struct event    listener;
+} dnstc_instance;
+
+/* vim:set tabstop=4 softtabstop=4 shiftwidth=4: */
+/* vim:set foldmethod=marker foldlevel=32 foldmarker={,}: */
+#endif /* REDUDP_H */

main.c

@@ -29,11 +29,13 @@
 extern app_subsys redsocks_subsys;
 extern app_subsys base_subsys;
 extern app_subsys redudp_subsys;
+extern app_subsys dnstc_subsys;
 
 app_subsys *subsystems[] = {
 	&redsocks_subsys,
 	&base_subsys,
 	&redudp_subsys,
+	&dnstc_subsys,
 };
 
 static const char *confname = "redsocks.conf";

redsocks.conf.example

@@ -78,4 +78,11 @@ redudp {
 	udp_timeout_stream = 180;
 }
 
+dnstc {
+	// fake and really dumb DNS server that returns "truncated answer" to
+	// every query via UDP
+	local_ip = 127.0.0.1;
+	local_port = 5300;
+}
+
 // you can add more `redsocks' and `redudp' sections if you need.

redudp.c

@@ -80,33 +80,6 @@ static struct evbuffer* socks5_mkassociate(void *p)
 	return socks5_mkcommand_plain(socks5_cmd_udp_associate, &sa);
 }
 
-static int recv_udp_pkt(int fd, char *buf, size_t buflen, struct sockaddr_in *inaddr)
-{
-	socklen_t addrlen = sizeof(*inaddr);
-	ssize_t pktlen;
-
-	pktlen = recvfrom(fd, buf, buflen, 0, (struct sockaddr*)inaddr, &addrlen);
-	if (pktlen == -1) {
-		log_errno(LOG_WARNING, "recvfrom");
-		return -1;
-	}
-
-	if (addrlen != sizeof(*inaddr)) {
-		log_error(LOG_WARNING, "unexpected address length %u instead of %zu", addrlen, sizeof(*inaddr));
-		return -1;
-	}
-
-	if (pktlen >= buflen) {
-		char buf[INET6_ADDRSTRLEN];
-		const char *addr = inet_ntop(inaddr->sin_family, &inaddr->sin_addr, buf, sizeof(buf));
-		log_error(LOG_WARNING, "wow! Truncated udp packet of size %zd from %s:%u! impossible! dropping it...",
-		          pktlen, addr ? addr : "?", ntohs(inaddr->sin_port));
-		return -1;
-	}
-
-	return pktlen;
-}
-
 /***********************************************************************
  * Logic
  */
@@ -458,11 +431,9 @@ static void redudp_pkt_from_socks(int fd, short what, void *_arg)
 
 	assert(fd == EVENT_FD(&client->udprelay));
 
-	pktlen = recv_udp_pkt(fd, pkt.buf, sizeof(pkt.buf), &udprelayaddr);
-	if (pktlen == -1) {
-		redudp_log_errno(client, LOG_WARNING, "recv_udp_pkt");
+	pktlen = red_recv_udp_pkt(fd, pkt.buf, sizeof(pkt.buf), &udprelayaddr);
+	if (pktlen == -1)
 		return;
-	}
 
 	if (memcmp(&udprelayaddr, &client->udprelayaddr, sizeof(udprelayaddr)) != 0) {
 		char buf[INET6_ADDRSTRLEN];
@@ -518,10 +489,9 @@ static void redudp_pkt_from_client(int fd, short what, void *_arg)
 	redudp_client *tmp, *client = NULL;
 
 	assert(fd == EVENT_FD(&self->listener));
-	pktlen = recv_udp_pkt(fd, buf, sizeof(buf), &clientaddr);
-	if (pktlen == -1) {
+	pktlen = red_recv_udp_pkt(fd, buf, sizeof(buf), &clientaddr);
+	if (pktlen == -1)
 		return;
-	}
 
 	// TODO: this lookup may be SLOOOOOW.
 	list_for_each_entry(tmp, &self->clients, list) {