1 files changed, 181 insertions, 0 deletions
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