4 files changed, 324 insertions, 0 deletions
diff --git a/modules/renumber/.packaging/test.config b/modules/renumber/.packaging/test.config
new file mode 100644
index 0000000..37f136a
--- /dev/null
+++ b/modules/renumber/.packaging/test.config
@@ -0,0 +1,4 @@
+-- SPDX-License-Identifier: GPL-3.0-or-later
+modules.load('renumber')
+assert(renumber)
+quit()
diff --git a/modules/renumber/README.rst b/modules/renumber/README.rst
new file mode 100644
index 0000000..2e68991
--- /dev/null
+++ b/modules/renumber/README.rst
@@ -0,0 +1,36 @@
+.. SPDX-License-Identifier: GPL-3.0-or-later
+
+.. _mod-renumber:
+
+IP address renumbering
+======================
+
+The module renumbers addresses in answers to different address space.
+e.g. you can redirect malicious addresses to a blackhole, or use private address ranges
+in local zones, that will be remapped to real addresses by the resolver.
+
+
+.. warning:: While requests are still validated using DNSSEC, the signatures
+   are stripped from final answer. The reason is that the address synthesis
+   breaks signatures. You can see whether an answer was valid or not based on
+   the AD flag.
+
+Example configuration
+---------------------
+
+.. code-block:: lua
+
+	modules = {
+		renumber = {
+			-- Source subnet, destination subnet
+			{'10.10.10.0/24', '192.168.1.0'},
+			-- Remap /16 block to localhost address range
+			{'166.66.0.0/16', '127.0.0.0'},
+			-- Remap /26 subnet (64 ip addresses)
+			{'166.55.77.128/26', '127.0.0.192'},
+			-- Remap a /32 block to a single address
+			{'2001:db8::/32', '::1!'},
+		}
+	}
+
+.. TODO: renumber.name() hangs in vacuum, kind of.  No occurrences in code or docs, and probably bad UX.
diff --git a/modules/renumber/renumber.lua b/modules/renumber/renumber.lua
new file mode 100644
index 0000000..60803d5
--- /dev/null
+++ b/modules/renumber/renumber.lua
@@ -0,0 +1,181 @@
+-- SPDX-License-Identifier: GPL-3.0-or-later
+-- Module interface
+local ffi = require('ffi')
+local prefixes_global = {}
+
+-- get address from config: either subnet prefix or fixed endpoint
+local function extract_address(target)
+	local idx = string.find(target, "!", 1, true)
+	if idx == nil then
+		return target, false
+	end
+	if idx ~= #target then
+		error("[renumber] \"!\" symbol in target is only accepted at the end of address")
+	end
+	return string.sub(target, 1, idx - 1), true
+end
+
+-- Create bitmask from integer mask for single octet: 2 -> 11000000
+local function getOctetBitmask(intMask)
+	return bit.lshift(bit.rshift(255, 8 - intMask), 8 - intMask)
+end
+
+-- Merge ipNet with ipHost, using intMask
+local function mergeIps(ipNet, ipHost, intMask)
+	local octetMask
+	local result = ""
+
+	if #ipNet ~= #ipHost then
+		return nil
+	end
+
+	if intMask == nil then
+		return ipNet
+	end
+
+	for currentOctetNo = 1, #ipNet do
+		if intMask >= 8 then
+			result = result .. ipNet:sub(currentOctetNo,currentOctetNo)
+		elseif (intMask <= 0) then
+			result = result .. ipHost:sub(currentOctetNo,currentOctetNo)
+		else
+			octetMask = getOctetBitmask(intMask)
+			result = result .. string.char(bit.bor(
+					bit.band(string.byte(ipNet:sub(currentOctetNo,currentOctetNo)), octetMask),
+					bit.band(string.byte(ipHost:sub(currentOctetNo,currentOctetNo)), bit.bnot(octetMask))
+			))
+		end
+		intMask = intMask - 8
+	end
+
+	return result
+end
+
+-- Create subnet prefix rule
+local function matchprefix(subnet, addr)
+	local is_exact
+	addr, is_exact = extract_address(addr)
+	local target = kres.str2ip(addr)
+	if target == nil then error('[renumber] invalid address: '..addr) end
+	local addrtype = string.find(addr, ':', 1, true) and kres.type.AAAA or kres.type.A
+	local subnet_cd = ffi.new('char[16]')
+	local bitlen = ffi.C.kr_straddr_subnet(subnet_cd, subnet)
+	if bitlen < 0 then error('[renumber] invalid subnet: '..subnet) end
+	return {subnet_cd, bitlen, target, addrtype, is_exact}
+end
+
+-- Create name match rule
+local function matchname(name, addr)
+	local is_exact
+	addr, is_exact = extract_address(addr) -- though matchname() always leads to replacing whole address
+	local target = kres.str2ip(addr)
+	if target == nil then error('[renumber] invalid address: '..addr) end
+	local owner = todname(name)
+	if not name then error('[renumber] invalid name: '..name) end
+	local addrtype = string.find(addr, ':', 1, true) and kres.type.AAAA or kres.type.A
+	return {owner, nil, target, addrtype, is_exact}
+end
+
+-- Add subnet prefix rewrite rule
+local function add_prefix(subnet, addr)
+	local prefix = matchprefix(subnet, addr)
+	table.insert(prefixes_global, prefix)
+end
+
+-- Match IP against given subnet or record owner
+local function match_subnet(subnet, bitlen, addrtype, rr)
+	local addr = rr.rdata
+	return addrtype == rr.type and
+	       ((bitlen and (#addr >= bitlen / 8) and (ffi.C.kr_bitcmp(subnet, addr, bitlen) == 0)) or subnet == rr.owner)
+end
+
+-- Renumber address record
+local function renumber_record(tbl, rr)
+	for i = 1, #tbl do
+		local prefix = tbl[i]
+		local subnet = prefix[1]
+		local bitlen = prefix[2]
+		local target = prefix[3]
+		local addrtype = prefix[4]
+		local is_exact = prefix[5]
+
+		-- Match record type to address family and record address to given subnet
+		-- If provided, compare record owner to prefix name
+		if match_subnet(subnet, bitlen, addrtype, rr) then
+			if is_exact then
+				rr.rdata = target
+			else
+				local mergedHost = mergeIps(target, rr.rdata, bitlen)
+				if mergedHost ~= nil then rr.rdata = mergedHost end
+			end
+
+			return rr
+		end
+	end
+	return nil
+end
+
+-- Renumber addresses based on config
+local function rule(prefixes)
+	return function (state, req)
+		if state == kres.FAIL then return state end
+		local pkt = req.answer
+		-- Only successful answers
+		local records = pkt:section(kres.section.ANSWER)
+		local ancount = #records
+		if ancount == 0 then return state end
+		-- Find renumber candidates
+		local changed = false
+		for i = 1, ancount do
+			local rr = records[i]
+			if rr.type == kres.type.A or rr.type == kres.type.AAAA then
+				local new_rr = renumber_record(prefixes, rr)
+				if new_rr ~= nil then
+					records[i] = new_rr
+					changed = true
+				end
+			end
+		end
+		-- If not rewritten, chain action
+		if not changed then return state end
+		-- Replace section if renumbering
+		local qname = pkt:qname()
+		local qclass = pkt:qclass()
+		local qtype = pkt:qtype()
+		pkt:recycle()
+		pkt:question(qname, qclass, qtype)
+		for i = 1, ancount do
+			local rr = records[i]
+			-- Strip signatures as rewritten data cannot be validated
+			if rr.type ~= kres.type.RRSIG then
+				pkt:put(rr.owner, rr.ttl, rr.class, rr.type, rr.rdata)
+			end
+		end
+		req:set_extended_error(kres.extended_error.FORGED, "DUQR")
+		return state
+	end
+end
+
+-- Export module interface
+local M = {
+	prefix = matchprefix,
+	name = matchname,
+	rule = rule,
+	match_subnet = match_subnet,
+}
+
+-- Config
+function M.config (conf)
+	if conf == nil then return end
+	if type(conf) ~= 'table' or type(conf[1]) ~= 'table' then
+		error('[renumber] expected { {prefix, target}, ... }')
+	end
+	for i = 1, #conf do add_prefix(conf[i][1], conf[i][2]) end
+end
+
+-- Layers
+M.layer = {
+	finish = rule(prefixes_global),
+}
+
+return M
diff --git a/modules/renumber/renumber.test.lua b/modules/renumber/renumber.test.lua
new file mode 100644
index 0000000..97d6a6f
--- /dev/null
+++ b/modules/renumber/renumber.test.lua
@@ -0,0 +1,103 @@
+local function gen_rrset(owner, rrtype, rdataset)
+	assert(type(rdataset) == 'table' or type(rdataset) == 'string')
+	if type(rdataset) ~= 'table' then
+		rdataset = { rdataset }
+	end
+	local rrset = kres.rrset(todname(owner), rrtype, kres.class.IN, 3600)
+	for _, rdata in pairs(rdataset) do
+		assert(rrset:add_rdata(rdata, #rdata))
+	end
+	return rrset
+end
+
+local function prepare_cache()
+	cache.open(100*MB)
+	cache.clear()
+
+	local ffi = require('ffi')
+	local c = kres.context().cache
+
+	assert(c:insert(
+		gen_rrset('a10-0.test.',
+			kres.type.A, kres.str2ip('10.0.0.1')),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+	assert(c:insert(
+		gen_rrset('a10-2.test.',
+			kres.type.A, kres.str2ip('10.2.0.1')),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+	assert(c:insert(
+		gen_rrset('a10-0plus2.test.',
+			kres.type.A, {
+				kres.str2ip('10.0.0.1'),
+				kres.str2ip('10.2.0.1')
+			}),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+	assert(c:insert(
+		gen_rrset('a10-3plus4.test.',
+			kres.type.A, {
+				kres.str2ip('10.3.0.1'),
+				kres.str2ip('10.4.0.1')
+			}),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+	assert(c:insert(
+		gen_rrset('a166-66.test.',
+			kres.type.A, kres.str2ip('166.66.42.123')),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+	assert(c:insert(
+		gen_rrset('a167-81.test.',
+			kres.type.A, kres.str2ip('167.81.254.221')),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+	assert(c:insert(
+		gen_rrset('aaaa-db8-1.test.',
+			kres.type.AAAA, {
+				kres.str2ip('2001:db8:1::1'),
+				kres.str2ip('2001:db8:1::2'),
+			}),
+		nil, ffi.C.KR_RANK_SECURE + ffi.C.KR_RANK_AUTH))
+
+	c:commit()
+end
+
+local check_answer = require('test_utils').check_answer
+
+local function test_renumber()
+	check_answer('unknown IPv4 range passes through unaffected',
+		'a10-0.test.', kres.type.A, kres.rcode.NOERROR, '10.0.0.1')
+	check_answer('known IPv4 range is remapped when matching first-defined rule',
+		'a10-2.test.', kres.type.A, kres.rcode.NOERROR, '192.168.2.1')
+	check_answer('mix of known and unknown IPv4 ranges is remapped correctly',
+		'a10-0plus2.test.', kres.type.A, kres.rcode.NOERROR, {'192.168.2.1', '10.0.0.1'})
+	check_answer('mix of known and unknown IPv4 ranges is remapped correctly to exact address',
+		'a10-3plus4.test.', kres.type.A, kres.rcode.NOERROR, {'10.3.0.1', '192.168.3.10'})
+	check_answer('known IPv4 range is remapped when matching second-defined rule',
+		'a166-66.test.', kres.type.A, kres.rcode.NOERROR, '127.0.42.123')
+	check_answer('known IPv4 range is remapped when matching a rule with netmask not on a byte boundary',
+		'a167-81.test.', kres.type.A, kres.rcode.NOERROR, {'127.0.30.221'})
+
+	check_answer('two AAAA records',
+		'aaaa-db8-1.test.', kres.type.AAAA, kres.rcode.NOERROR,
+		{'2001:db8:2::2', '2001:db8:2::1'})
+end
+
+net.ipv4 = false
+net.ipv6 = false
+
+trust_anchors.remove('.')
+policy.add(policy.all(policy.DEBUG_ALWAYS))
+policy.add(policy.suffix(policy.PASS, {todname('test.')}))
+prepare_cache()
+
+log_level('debug')
+modules.load('renumber < cache')
+renumber.config({
+	-- Source subnet, destination subnet
+	{'10.2.0.0/24', '192.168.2.0'},
+	{'10.4.0.0/24', '192.168.3.10!'},
+	{'166.66.0.0/16', '127.0.0.0'},
+	{'167.81.255.0/19', '127.0.0.0'},
+	{'2001:db8:1::/48', '2001:db8:2::'},
+})
+
+return {
+	test_renumber,
+}