Split DNS. Same domain names with different values in internal and external world

Question

I am searching for possible solutions for splitted DNS for internal/external worlds. Thing is, those two "worlds" (in reality different sets of nameservers) are partialy overlaping.

Here is example for domain example.com.

External world zone data:

@       IN SOA          ns01.example.com. hostmaster.example.com. (
                                2022082908      ; Serial
                                10800           ; Refresh
                                3600            ; Retry
                                864000          ; Expire
                                10800 )        ; Minimum
                                
@       IN NS    ns01.example.com.
@       IN NS    ns02.example.com.
@       IN MX   50      smtp01.example.com.
@       IN MX   20      smtp02.example.com.
www     IN A    90.80.70.60
portal  IN A    90.80.70.60

Internal world zone data:

@       IN SOA          ins01.example.com. hostmaster.example.com. (
                                2022082908      ; Serial
                                10800           ; Refresh
                                3600            ; Retry
                                864000          ; Expire
                                10800 )        ; Minimum
                                
@       IN NS    ins01.example.com.
@       IN NS    ins02.example.com.
@       IN MX   50      exchange.example.com.
portal  IN A    192.168.10.100

What is a purpose is to have some "resolver" (IP address) which can resolve both worlds for internal clients. So when internal client is accessing portal.example.com he needs to go to internal website. But he also needs to have access to www.example.com. Are there any possible solution for this situation?

Current solution is some very old software on internal side, which is doing (from cron) AXFR from external nameserver and then do some modifications for a zone based on data in internal world. Which produces it's own set of problems. (for example ignoring TTL in internal world)

The best solution that I can imagine could be some resolver/dns-proxy which is able to query internal nameserver first (which would contain only internal DNS records). If internal NS returns NXDOMAIN or just anything besides answer with value, it should try to resolve query like resolver (or just forward query to some real resolver). Does something like that exists?

Another kinda obvious solution is to update/rewrite that internal DNS management software for modern OS's. And get rid of it's known problems (but some of them just cannot be solved I believe).

Or is there any other way that just does not come to my mind?

Answer 1

If the number of hosts that need to be different for internal clients is small, one option would be to serve those clients using dnsmasq. You could point dnsmasq at your upstream DNS server, and then override specific entries.

A configuration like this would cause dnsmasq to reply with 192.168.10.100 for portal.example.com, but all other queries would be passed on to an upstream server (which is either extracted from /etc/resolv.conf or specified explicitly with server= directives in your dnsmasq configuration):

address=/portal.example.com/192.168.10.100

Answer 2

Thanks for the hints, especially to @Patrick Mevzek. It showed me right direction.

The solution that I found in the end is a combination of dnsdist and coredns.

Example config files if anyone is interested.

For coredns it is /etc/coredns/Corefile:

.:53 {
    forward . 127.0.0.1:5353
    alternate SERVFAIL,NXDOMAIN . 8.8.8.8:53
}

For dnsdist it is /etc/dnsdist/dnsdist.conf:

-- Set control console and secret key to control
-- to generate key use makeKey() command on dnsdist console (launched from shell - dnsdist -l)
controlSocket('127.0.0.1:5299')
setConsoleACL('127.0.0.1/32')
setKey("KEYHASH")

-- HOW TO CONNECT TO CONSOLE
-- Run command: dnsdist -k "ENCODED KEY" -c 127.0.0.1:5299

-- Set addressess and ports where to accept DNS queries
addLocal('127.0.0.1:5353')

-- Set web server with statistics and run it in specified port
-- to generate password hash use hashPassword() command on dnsdist console
webserver('0.0.0.0:8084')
setWebserverConfig({password="PASSWORDHASH", acl="10.0.0.0/8, 127.0.0.1, ::1"})

-- Add internal nameservers pool and set it's caching policy
newServer({address="10.0.0.253:53", pool="internal"})
newServer({address="10.0.0.254:53", pool="internal"})
internalpc = newPacketCache(10000, {maxTTL=86400, minTTL=0, temporaryFailureTTL=60, staleTTL=60, dontAge=false})
getPool("internal"):setCache(internalpc)

-- Set ACL to allow anyone to query
setACL({'::/0','0.0.0.0/0'})

-- Send all queries to internal DNS
addAction(AllRule(), PoolAction("internal"))

-- LUA function that simply returns SERVFAIL RCODE
function NoAnswerAction(dq)
    return DNSResponseAction.ServFail
end


-- If RESPONSE does not contain data in answer section, run LUA function
addResponseAction(NotRule(RecordsCountRule(DNSSection.Answer, 1, 999)), LuaResponseAction(NoAnswerAction))

It's not nice, but it works. You can also add another dnsdist instance in front of coredns to not send all queries to internal servers...