how to prevent Route53 from exposing our VPC RFC1918 address space to the Internet

Question

I need to know if it’s possible (and if possible, how) to prevent Route53 from exposing our VPC RFC1918 address space to the Internet. As you can see, these addresses are leaked out onto the Internet where they do no good except to expose the endpoints of various AWS services:

From inside Corp: ps@site:tmp$ dig test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com

; <<>> DiG 9.11.5-P1-1ubuntu2.5-Ubuntu <<>> test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1636 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 65494 ;; QUESTION SECTION: ;test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com. IN A

;; ANSWER SECTION: test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com. 4 IN A 172.31.58.126

;; Query time: 380 msec ;; SERVER: 127.0.0.53#53(127.0.0.53) ;; WHEN: Tue Sep 24 07:10:57 CDT 2019 ;; MSG SIZE rcvd: 106

From my home Linux system:

ps@plex:~$ dig test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com

; <<>> DiG 9.11.3-1ubuntu1.9-Ubuntu <<>> test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9577 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ;; QUESTION SECTION: ;test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com. IN A

;; ANSWER SECTION: test-do-not-use.cmqvubhjfrhv.us-east-1.rds.amazonaws.com. 3600 IN A 172.31.58.126

;; Query time: 210 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Tue Sep 24 07:11:04 CDT 2019 ;; MSG SIZE rcvd: 106

Ideally this external query should return NOTHING.

I’ve been unsuccessful in my document digging in the AWS doc repository.

Accepted Answer

This comes up frequently, especially with public sector customers.  To the best of my knowledge, this is something that cannot be disabled (normally due to requirements to resolve DNS for failover, service-to-service integration, etc).  It also affects other services such as internal load-balancers, which can be queried externally and return the VPC IP addresses that have been assigned to them.

The way I typically handle this conversation is to explain to customers that although the internal IP addresses will be returned, since these are not routable from outside of the VPC, there is little that an attacker can do, just from the knowledge of them.  Also, the DNS records include a random string, and do not reference the customer account ID, and so there is little risk of account discovery from brute-forcing DNS resolution across this space.

how to prevent Route53 from exposing our VPC RFC1918 address space to the Internet

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