add dns doctoring example

Signed-off-by: Mohammad Shehar Yaar Tausif <[email protected]>

Makefile

@@ -47,6 +47,8 @@ examples_install:
 	${INSTALL} -m 0644 examples/filter/*.lua ${SCRIPTS_INSTALL_PATH}/examples/filter
 	${MKDIR} ${SCRIPTS_INSTALL_PATH}/examples/dnsblock
 	${INSTALL} -m 0644 examples/dnsblock/*.lua ${SCRIPTS_INSTALL_PATH}/examples/dnsblock
+	${MKDIR} ${SCRIPTS_INSTALL_PATH}/examples/dnsdoctor
+	${INSTALL} -m 0644 examples/dnsdoctor/*.lua ${SCRIPTS_INSTALL_PATH}/examples/dnsdoctor
 
 examples_uninstall:
 	${RM} -r ${SCRIPTS_INSTALL_PATH}/examples

README.md

@@ -1148,10 +1148,10 @@ This function receives the following arguments:
   * `match` : function to be called for matching packets. It receives the following arguments:
 	* `skb` (readonly): a `data` object representing the socket buffer.
 	* `par`: a table containing `hotdrop`, `thoff` (transport header offset) and `fragoff` (fragment offset) fields.
-    * `userdata` : a lua string passed from the userspace xtable module.
+    * `userargs` : a lua string passed from the userspace xtable module.
     * The function must return `true` if the packet matches the extension; otherwise, it must return `false`.
-  * `checkentry`: function to be called for checking the entry. This function receives `userdata` as it's argument.
-  * `destroy`: function to be called for destroying the xtable extension. This function receives `userdata` as it's argument.
+  * `checkentry`: function to be called for checking the entry. This function receives `userargs` as its argument.
+  * `destroy`: function to be called for destroying the xtable extension. This function receives `userargs` as its argument.
 
 #### `xtable.target(opts)`
 
@@ -1166,10 +1166,10 @@ This function receives the following arguments:
   * `target` : function to be called for targeting packets. It receives the following arguments:
     * `skb`: a `data` object representing the socket buffer.
     * `par` (readonly): a table containing `hotdrop`, `thoff` (transport header offset) and `fragoff` (fragment offset) fields.
-    * `userdata` : a lua string passed from the userspace xtable module.
+    * `userargs` : a lua string passed from the userspace xtable module.
     * The function must return one of the values defined by the [xtable.action](https://github.com/luainkernel/lunatik#xtableaction) table.
-  * `checkentry`: function to be called for checking the entry. This function receives `userdata` as it's argument.
-  * `destroy`: function to be called for destroying the xtable extension. This function receives `userdata` as it's argument.
+  * `checkentry`: function to be called for checking the entry. This function receives `userargs` as its argument.
+  * `destroy`: function to be called for destroying the xtable extension. This function receives `userargs` as its argument.
 
 #### `xtable.family`
 
@@ -1211,9 +1211,9 @@ netfilter hooks to Lua.
 
 ### luaxt
 
-The `luaxt` [userspace library](usr/lib/xtable) provides support for generating userspace code for [xtable extensions](https://inai.de/projects/xtables-addons/). The user can modify the generated lua code to implement the userspace handlers for the corresponding xtable extension.
+The `luaxt` [userspace library](usr/lib/xtable) provides support for generating userspace code for [xtable extensions](https://inai.de/projects/xtables-addons/).
 
-To generate the library, the following steps are required:
+To build the library, the following steps are required:
 
 1. Go to `usr/lib/xtable` and create a `libxt_<ext_name>.lua` file.
 2. Register your callbacks for the xtable extension by importing the library (`luaxt`) in the created file.
@@ -1229,10 +1229,10 @@ This function receives the following arguments:
   * `revision`: integer representing the xtable extension revision (**must** be same as used in corresponding kernel extension).
   * `family`: address family, one of [luaxt.family](https://github.com/luainkernel/lunatik#luaxtfamily)
   * `help`: function to be called for displaying help message for the extension.
-  * `init`: function to be called for initializing the extension. This function receives an `par` table that can be used to set `userdata`. (`par.userdata = "mydata"`)
-  * `print`: function to be called for printing the arguments. This function recevies `userdata` set by the `init` or `parse` function.
-  * `save`: function to be called for saving the arguments. This function recevies `userdata` set by the `init` or `parse` function.
-  * `parse`: function to be called for parsing the command line arguments. This function receives an `par` table that can be used to set `userdata` and `flags`. (`par.userdata = "mydata"`)
+  * `init`: function to be called for initializing the extension. This function receives an `par` table that can be used to set `userargs`. (`par.userargs = "mydata"`)
+  * `print`: function to be called for printing the arguments. This function recevies `userargs` set by the `init` or `parse` function.
+  * `save`: function to be called for saving the arguments. This function recevies `userargs` set by the `init` or `parse` function.
+  * `parse`: function to be called for parsing the command line arguments. This function receives an `par` table that can be used to set `userargs` and `flags`. (`par.userargs = "mydata"`)
   * `final_check`: function to be called for final checking of the arguments. This function receives `flags` set by the `parse` function.
 
 #### `luaxt.target(opts)`
@@ -1243,10 +1243,10 @@ This function receives the following arguments:
   * `revision`: integer representing the xtable extension revision (**must** be same as used in corresponding kernel extension).
   * `family`: address family, one of [luaxt.family](https://github.com/luainkernel/lunatik#luaxtfamily)
   * `help`: function to be called for displaying help message for the extension.
-  * `init`: function to be called for initializing the extension. This function receives an `par` table that can be used to set `userdata`. (`par.userdata = "mydata"`)
-  * `print`: function to be called for printing the arguments. This function recevies `userdata` set by the `init` or `parse` function.
-  * `save`: function to be called for saving the arguments. This function recevies `userdata` set by the `init` or `parse` function.
-  * `parse`: function to be called for parsing the command line arguments. This function receives an `par` table that can be used to set `userdata` and `flags`. (`par.userdata = "mydata"`)
+  * `init`: function to be called for initializing the extension. This function receives an `par` table that can be used to set `userargs`. (`par.userargs = "mydata"`)
+  * `print`: function to be called for printing the arguments. This function recevies `userargs` set by the `init` or `parse` function.
+  * `save`: function to be called for saving the arguments. This function recevies `userargs` set by the `init` or `parse` function.
+  * `parse`: function to be called for parsing the command line arguments. This function receives an `par` table that can be used to set `userargs` and `flags`. (`par.userargs = "mydata"`)
   * `final_check`: function to be called for final checking of the arguments. This function receives `flags` set by the `parse` function.
 
 #### `luaxt.family`
@@ -1410,6 +1410,46 @@ sudo lunatik run examples/dnsblock/dnsblock false	# runs the Lua kernel script
 sudo iptables -A OUTPUT -m dnsblock -j DROP     	# this initiates the netfilter framework to load our extension
 ```
 
+### dnsdoctor
+
+[dnsdoctor](examples/dnsdoctor) is a kernel script that uses the lunatik xtable library to change the DNS response 
+from Public IP to a Private IP if the destination IP matches the one provided by the user. For example, if the user
+wants to change the DNS response from `192.168.10.1` to `10.1.2.3` for the domain `lunatik.com` if the query is being sent to `10.1.1.2` (a private client), this script can be used.
+
+#### Usage
+
+```
+sudo make examples_install              # installs examples
+cd examples/dnsdoctor
+setup.sh                                # sets up the environment
+
+# test the setup, a response with IP 192.168.10.1 should be returned
+dig lunatik.com
+
+# run the Lua kernel script
+sudo lunatik run examples/dnsdoctor/dnsdoctor false
+
+# copy the userspace extension to luaxt directory
+cp libxt_dnsdoctor.lua ../../usr/lib/xtable/
+cd ../../usr/lib/xtable
+
+# build and install the userspace extension for netfilter
+LUAXTABLE_MODULE=dnsdoctor make
+sudo LUAXTABLE_MODULE=dnsdoctor make install
+
+# add rule to the mangle table
+sudo iptables -t mangle -A PREROUTING -p udp --sport 53 -j dnsdoctor
+
+# test the setup, a response with IP 10.1.2.3 should be returned
+dig lunatik.com
+
+# cleanup
+sudo iptables -t mangle -D PREROUTING -p udp --sport 53 -j dnsdoctor # remove the rule
+sudo lunatik unload
+cd ../../../examples/dnsdoctor
+cleanup.sh
+```
+
 ## References
 
 * [Scripting the Linux Routing Table with Lua](https://netdevconf.info/0x17/sessions/talk/scripting-the-linux-routing-table-with-lua.html)

examples/dnsdoctor/Makefile

@@ -0,0 +1,14 @@
+# SPDX-FileCopyrightText: (c) 2024 Mohammad Shehar Yaar Tausif <[email protected]>
+# SPDX-License-Identifier: MIT OR GPL-2.0-only
+
+all:
+	LUAXTABLE_MODULE=dnsdoctor $(MAKE) -C ../../usr/lib/xtable
+
+install:
+	sudo LUAXTABLE_MODULE=dnsdoctor $(MAKE) -C ../../usr/lib/xtable install
+
+uninstall:
+	sudo rm -f ${XTABLES_SO_DIR}/libxt_${LUAXTABLE_MODULE}.so
+
+clean:
+	LUAXTABLE_MODULE=dnsdoctor $(MAKE) -C ../../usr/lib/xtable clean

examples/dnsdoctor/cleanup.sh

@@ -0,0 +1,36 @@
+# SPDX-FileCopyrightText: (c) 2024 Mohammad Shehar Yaar Tausif <[email protected]>
+# SPDX-License-Identifier: MIT OR GPL-2.0-only
+
+#!/bin/bash
+
+set -eux
+
+rm dnstest -rf
+
+# backup resolv config
+if [[ -f /etc/resolv.conf.lunatik ]] then
+	echo "Restoring dns config from resolv.conf.lunatik"
+	sudo rm /etc/resolv.conf
+	sudo cp /etc/resolv.conf.lunatik /etc/resolv.conf
+	sudo rm /etc/resolv.conf.lunatik
+fi
+
+# down the interfaces
+sudo ip -n ns1 link set veth1 down
+sudo ip -n ns2 link set veth3 down
+sudo ip link set veth2 down
+sudo ip link set veth4 down
+
+sudo ip addr delete 10.1.1.2/24 dev veth2
+sudo ip -n ns1 addr delete 10.1.1.3/24 dev veth1
+sudo ip addr delete 10.1.2.2/24 dev veth4
+sudo ip -n ns2 addr delete 10.1.2.3/24 dev veth3
+
+# delete link between host and the namespaces
+sudo ip -n ns1 link delete veth1
+sudo ip -n ns2 link delete veth3
+
+# delete namespaces ns1 for dns server ns2 for server
+sudo ip netns delete ns1
+sudo ip netns delete ns2
+

examples/dnsdoctor/dnsdoctor.lua

@@ -0,0 +1,68 @@
+--
+-- SPDX-FileCopyrightText: (c) 2024 Mohammad Shehar Yaar Tausif <[email protected]>
+-- SPDX-License-Identifier: MIT OR GPL-2.0-only
+--
+
+-- DNS Doctoring : Rewrite DNS type A record to a private address for local clients
+
+local xt = require("xtable")
+local linux = require("linux")
+local string = require("string")
+local action = xt.action
+local family = xt.family
+
+local udp = 0x11
+local dns = 0x35
+
+local function nop() end
+
+local function get_domain(skb, off)
+	local i = tostring(skb):sub(off+1):find('\0')
+	return skb:getstring(off, i - 1), i
+end
+
+local function dnsdoctor_tg(skb, par, userargs)
+	local target_dns, dst_ip, target_ip = string.unpack(">s4 I4 I4", userargs)
+	local thoff = par.thoff
+
+	local packetdst = skb:getuint32(16)
+	if packetdst ~= linux.hton32(dst_ip) then
+		return action.ACCEPT
+	end
+
+	local srcport = linux.ntoh16(skb:getuint16(thoff))
+	if srcport == dns then
+		local dnsoff = thoff + 8
+		local nanswers = linux.ntoh16(skb:getuint16(dnsoff + 6))
+
+		-- check the domain name
+		dnsoff = dnsoff + 12
+		local domainname, nameoff = get_domain(skb, dnsoff)
+
+		if domainname == target_dns then
+			dnsoff = dnsoff + nameoff + 4 -- skip over type, label fields
+			-- iterate over answers
+			for i = 1,nanswers do
+				local atype = linux.hton16(skb:getuint16(dnsoff + 2))
+				if atype == 1 then
+					skb:setuint32(dnsoff + 12, linux.hton32(target_ip))
+				end
+				dnsoff = dnsoff + 16
+			end
+		end
+	end
+
+	return action.ACCEPT
+end
+
+xt.target{
+	name = "dnsdoctor",
+	revision = 0,
+	family = family.UNSPEC,
+	proto = 0,
+	target = dnsdoctor_tg,
+	checkentry = nop,
+	destroy = nop,
+	hooks = 0,
+}
+

examples/dnsdoctor/libxt_dnsdoctor.lua

@@ -0,0 +1,28 @@
+local luaxt = require("luaxt")
+local family = luaxt.family
+
+local function noop() end
+
+local function dnsdoctor_init(par)
+	local target_ip = "10.1.2.3"
+	local target = 0
+	target_ip:gsub("%d+", function(s) target = target * 256 + tonumber(s) end)
+
+	local src_ip = "10.1.1.2"
+	local src = 0
+	src_ip:gsub("%d+", function(s) src = src * 256 + tonumber(s) end)
+
+	par.userargs = string.pack(">s4 I4 I4", "\x07lunatik\x03com", src, target)
+end
+
+luaxt.target{
+    revision = 0,
+    family = family.UNSPEC,
+    help = noop,
+    init = dnsdoctor_init,
+    print = noop,
+    save = noop,
+    parse = noop,
+    final_check = noop
+}
+

examples/dnsdoctor/setup.sh

@@ -0,0 +1,61 @@
+# SPDX-FileCopyrightText: (c) 2024 Mohammad Shehar Yaar Tausif <[email protected]>
+# SPDX-License-Identifier: MIT OR GPL-2.0-only
+
+#!/bin/bash
+
+set -eux
+
+# add namespaces ns1 for dns server ns2 for server
+sudo ip netns add ns1
+sudo ip netns add ns2
+
+# add link between host and the namespaces
+sudo ip link add veth1 netns ns1 type veth peer name veth2
+sudo ip link add veth3 netns ns2 type veth peer name veth4
+
+# add ip address to the links
+# DNS IP : 10.1.1.3
+# Server IP : 10.1.2.3
+sudo ip addr add 10.1.1.2/24 dev veth2
+sudo ip -n ns1 addr add 10.1.1.3/24 dev veth1
+sudo ip addr add 10.1.2.2/24 dev veth4
+sudo ip -n ns2 addr add 10.1.2.3/24 dev veth3
+
+# up the interfaces
+sudo ip -n ns1 link set veth1 up
+sudo ip -n ns2 link set veth3 up
+sudo ip link set veth2 up
+sudo ip link set veth4 up
+
+# make a directory to setup dns server
+mkdir dnstest
+cd dnstest
+python -m venv .venv
+source .venv/bin/activate
+pip install dnserver
+
+# backup resolv config
+echo "Backing up resolver config to /etc/resolver.conf.lunatik"
+sudo cp -f /etc/resolv.conf /etc/resolv.conf.lunatik && \
+sudo sed -i 's/nameserver/#nameserver/g' /etc/resolv.conf && \
+echo "nameserver 10.1.1.3" | sudo tee -a /etc/resolv.conf && \
+
+# add zone info and run dns server in ns1
+echo """
+[[zones]]
+host = 'lunatik.com'
+type = 'A'
+answer = '192.168.10.1'
+
+[[zones]]
+host = 'lunatik.com'
+type = 'NS'
+answer = 'ns1.lunatik.com.'
+
+[[zones]]
+host = 'lunatik.com'
+type = 'NS'
+answer = 'ns2.lunatik.com.'
+""" > zones.toml
+sudo ip netns exec ns1 .venv/bin/dnserver --no-upstream zones.toml
+

lib/luaxtable.c

@@ -56,7 +56,7 @@ static int luaxtable_docall(lua_State *L, luaxtable_t *xtable, luaxtable_info_t
 
 	lua_insert(L, base + 1); /* op */
 	lua_pop(L, 1); /* table */
-	lua_pushstring(L, info->userdata); /* userdata */
+	lua_pushlstring(L, info->userargs, LUAXTABLE_USERDATA_SIZE); /* userargs */
 
 	if (lua_pcall(L, nargs + 1, nret, 0) != LUA_OK) {
 		pr_err("%s error: %s\n", op, lua_tostring(L, -1));

lib/luaxtable.h

@@ -6,10 +6,12 @@
 #ifndef luaxtable_h
 #define luaxtable_h
 
+#define LUAXTABLE_USERDATA_SIZE 256
+
 struct luaxtable_s;
 
 typedef struct luaxtable_info_s {
-	char userdata[256];
+	char userargs[LUAXTABLE_USERDATA_SIZE];
 
 	/* used internally by the luaxtable kernel module */
 	struct luaxtable_s *data __attribute__((aligned(8)));