a secure bind 9 – best practices

BIND 9Security best practices

10 ways to improve your BIND 9 security

BIND 9 Security

a collection of security best practice tips from the Men & Mice services team

• this is no exhaustive list, but a good start

• if you have an important tip we've missed in this webinar, let us know on Twitter @menandmice(and we will retweet)

Keep BIND 9 "up-to-date"

keeping BIND 9 up-to-date

from time to time there are security vulnerabilities in BIND 9

• update BIND 9 via the package manager of your Linux/Unix

• also monitor security of BIND 9 library dependencies (openssl, libxml2)

•ISC announcement mailing list is a good source for BIND 9 security related information

Keep your BIND 9 software up-to-date

•if BIND 9 is not updated by the OS vendor (e.g. MacOS X), use a cross-platform package manager

•pkggsrc — https://www.pkgsrc.org

•Nix — https://nixos.org/nix/

•subscribe to BIND 9 announce mailing list (low volume, new versions and security announcements only) https://lists.isc.org/mailman/listinfo/bind-announce

consider using a new(er) version of BIND 9

•new security features (like DNSSEC negative trust anchors)

•newer/better crypto algorithms for DNSSEC/TSIG

•HMAC-SHA256 for TSIG

•new elliptic curves for DNSSEC signatures

•new privacy features (QNAME minimisation, DNS over TLS … )

Separating authoritative and recursive

Separating authoritative and recursive DNS

authoritative DNS server and DNS resolver are separate functions in the DNS infrastructure

•they have different security requirements

•while BIND 9 can operate in "hybrid" mode (default), it is strongly recommended to separate the two functions

•can run on the same hardware with operating system containers or virtualization

Functions of a DNS Server

cached data

Smart resolver

authoritative data

2 Functions of a DNS Server

!!!!!!!!!

authoritative DNS Server

authoritative dataCaching DNS Server

!!!!!!!!

cached data

Smart resolver

Separating authoritative and recursive DNS

benefits of separate authoritative and recursive DNS

•required for DNSSEC validation of own zones

•security configuration optimised for the function (for example query ACLs)

•helps troubleshooting (logging)

•easier maintenance (Updates)

process isolation

chroot is the classic process isolation function in Unix operating systems

•chroot confines a process into a subtree of the filesystem

•in a proper chroot setup, the process cannot access files outside the chroot subtree

process isolation

//dev/etc/bin/sbin/usr/var/chroot/etc /dev /var/named

full Unix filetree view

process isolation

//dev/etc/bin/sbin/usr/var/chroot/dev /etc /var/named

files and directories seen by the chrooted

process

process isolation

chrooting BIND 9 is easy (compared with other daemon processes or BIND 4/8: named -t /var/named

•all files BIND 9 needs during operation must be located inside the chroot directory

•all file references in the BIND 9 configuration file are relative to the chroot

•this is a source of confusion for some admins

chroot directory

process isolation

shortcomings of the chroot function

• chroot was never designed as a security feature

• a real "root" user (UID 0) can find ways to break out of the chroot

• isolation is only on the filesystem level, not on process, environment and network level

process isolation

modern alternatives to chroot:

•GRSecurity Linux-Kernel enhanced chroot https://grsecurity.net/features.php

•Container virtualization:

•Linux container (LXC, LXD, docker, firejail, systemd-nspawn)

•FreeBSD "jails"

•Solaris/Illumnos "zones"

process isolation

benefits of modern alternatives

•process, environment and network isolation

•manageability

•separation of application (BIND 9 DNS server and utilities) and data (zonefiles, configuration)

•enables quick clean rebuild after intrusion

DNSSEC

classic DNS is vulnerable to a large number of attacks on the content of DNS answers

•DNSSEC (digital signatures on DNS data) guards against many of these attacks

•the DNS root-zone, all gTLDs and nTLDs and many ccTLDs are DNSSEC signed

•many second level domains are also DNSSEC secured

DNSSEC

BIND 9 comes with a trust-anchor for the Internet Root-Zone build-in

•DNSSEC validation can be enabled with just one line of configuration:options { dnssec-validation auto; };

DNSSEC

enable DNSSEC validation on a DNS resolver

•test DNSSEC validation is enabled:

# rndc validation check DNSSEC validation is enabled (view _default)# dig soa . @127.0.0.1 +adflag; <<>> DiG 9.11.0 <<>> soa . @127.0.0.1 +adflag;; global options: printcmd;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46337 ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 13, ADDITIONAL: 0

AD = authentic data

DNSSEC

consider DNSSEC signing your zones

• modern BIND 9 versions make DNSSEC signing and management easy

•the Men & Mice DNSSEC workshop training gives you a place to learn, test DNSSEC and build routine

• management can be automated (dnssec-keymgr, CDS/CDNSKEY)

•never store private key material on a DNS server exposed to the Internet!

minimal responses

RFC 1034 defines the additional section in a DNS answer as "Carries RRs which may be helpful in using the RRs in the other sections."

•in the default configuration, BIND 9 tries to be very helpful, sending additional information ...

•... creating larger than needed DNS answer packets

•this is sometimes exploited by attackers in distributed denial of service attacks

minimal responses

DNS is UDP "stateless" communication

source IP addresses can be spoofed

some DNS Server on the Internet

Source IP Address

"spoofed"

Answer is delivered to the

owner of the "spoofed" IP

Address

minimal responses

There are many many DNS servers to be found in the

Internet

minimal responses

% dig @ns2.xb.nl. mx ncsc.nl !; <<>> DiG 9.9.2-vjs287.12 <<>> @ns2.xb.nl. mx ncsc.nl ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60070 ;; flags: qr aa rd; QUERY: 1, ANSWER: 6, AUTHORITY: 2, ADDITIONAL: 10 ;; WARNING: recursion requested but not available !;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ;; QUESTION SECTION: ;ncsc.nl. IN MX !;; ANSWER SECTION: ncsc.nl. 60 IN MX 20 min3.govcert.nl. ncsc.nl. 60 IN MX 20 min4.govcert.nl. ncsc.nl. 60 IN MX 30 min5.govcert.nl. ncsc.nl. 60 IN MX 40 smtp.espritxb.nl. ncsc.nl. 60 IN MX 10 min1.govcert.nl. ncsc.nl. 60 IN MX 10 min2.govcert.nl. !;; AUTHORITY SECTION: ncsc.nl. 60 IN NS ns1.xb.nl. ncsc.nl. 60 IN NS ns2.xb.nl. !;; ADDITIONAL SECTION: min1.govcert.nl. 60 IN A 193.172.9.50 min2.govcert.nl. 60 IN A 193.172.9.51 min3.govcert.nl. 60 IN A 31.161.17.13 min4.govcert.nl. 60 IN A 31.161.17.14 min5.govcert.nl. 60 IN A 217.169.231.54 smtp.espritxb.nl. 60 IN A 80.248.34.142 smtp.espritxb.nl. 60 IN A 80.248.34.141 ns1.xb.nl. 300 IN A 80.248.34.15 ns2.xb.nl. 300 IN A 212.67.179.100 !;; Query time: 39 msec ;; SERVER: 212.67.179.100#53(212.67.179.100) ;; WHEN: Fri Jan 18 13:02:08 2013 ;; MSG SIZE rcvd: 362

large additional section

minimal responses

configure "minimal-responses" in BIND 9options { minimal-responses yes; };

BIND 9 will only return the datarequired for the DNS protocol to work

this reduces the "ammo" available to attackers

minimal responses enabled during an attack

minimal "ANY"

Minimal ANY

•a BIND 9 server getting a query with type ANY (QTYPE 255) will answer with all records matching the requested domain name and class

•this can create large UDP DNS answer packets ;; QUESTION SECTION:;menandmice.com. IN ANY ;; ANSWER SECTION:menandmice.com. 86400 IN SOA dns1.menandmice.com. hostmaster.menandmice.com. 2016052701 900 300 604800 900menandmice.com. 3600 IN TXT "HhnTdT3K" menandmice.com. 3600 IN TXT "MS=ms81797768" menandmice.com. 3600 IN TXT "v=spf1 include:spf.protection.outlook.com a:smtp.menandmice.is a:support.menandmice.com a:otrs.menandmice.com a:imap2.skyrr.is a:mx.hysing.is ~all" ns2.c.is. 84985 IN A 213.176.143.102 dns1.menandmice.com. 171385 IN A 217.151.171.7 dns2.menandmice.com. 171385 IN A 217.151.171.21 dns3.menandmice.com. 171385 IN A 45.79.153.125 […];; Query time: 97 msec;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Mon Aug 15 10:49:15 CEST 2016 ;; MSG SIZE rcvd: 719

DNS answer

Minimal ANY

•starting with BIND 9.11, BIND 9 can be configured to only return the first entry of a matching ANY query

•this mitigates the problem without causing (too much) breakage of older software (qmail etc)

options { minimal-any yes; };

Minimal ANY•same query as before with minimal-any enabled:

# dig menandmice.com any

; <<>> DiG 9.11.0b3 <<>> menandmice.com any;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32396;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096; COOKIE: f0a6921ce7023ebc646d789357b1837a0962c60d534b251e (good);; QUESTION SECTION: ;menandmice.com. IN ANY

;; ANSWER SECTION: menandmice.com. 86033 IN SOA dns1.menandmice.com. hostmaster.menandmice.com. 2016052701 900 300 604800 900

;; Query time: 0 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Mon Aug 15 10:55:22 CEST 2016;; MSG SIZE rcvd: 123

DNS answer size 123 < 719 Byte

Platform for BIND 9

OS platform to run BIND 9

•Choose an operating system you as an administrator are most familiar with

•Linux, Solaris, OpenBSD, FreeBSD, NetBSD, DragonflyBSD, Windows, AIX, HP-UX, Minix 3, MacOS X

• all work well if configured correctly

•choose a version that still receives security patches

Minimal Installs

•choose a minimal footprint install

•no GUI

•no unnecessary software (databases, webserver, mailserver, development tools, scripting languages)

•minimize the open sockets to the Internet

•Port 53 - DNS

•Port 22 - SSH for remote administration (no RDP, VNC or TeamViewer)

• Port 123 - Network Time Protocol (NTP)

•Precision Time Protocol (PTP) might be an alternative (https://en.wikipedia.org/wiki/Precision_Time_Protocol)

plan for recovery

•after detecting a security breach, don't put the compromised system(s) back in business

•have a plan to rebuild your services

•configuration orchestration tools (Ansible, SaltStack, Puppet, cfengine …) or Container management tools (Docker) can be of great help

•only restore data from backup, no executable content (no programs, scripts)

•peer-review scripts and configuration with a colleague/service partner

Hidden Primary

•consider a hidden primary DNS setup for zone management

Internet

Internal Network

zone transfer

server visible to the

Internet

hidden management

master

OS platform to run BIND 9

•monitor your DNS server for uptime and content

•AA-Flag responses for zones on authoritative servers

•AD-Flag responses for important query answers from a DNS resolver

•more on DNS/DNSSEC monitoring (including tools) in an upcoming Webinar in 2016

process image hardening

Linux (and other modern Unix systems) program binaries can be hardened at compile time

•unfortunately, not all distributions enable this security feature

process image hardeningSome available features:

• RELRO

• PIE - Position Independent Executables

• ASLR - Address Space Layout Randomization

• Fortify Source - additional security checks added by the compiler

• Stack protector/StackCanary - detect and mitigate stack overflow attacks

• NX - Software/Hardware "Non-Executable" Memory tagging

•See Compiler options for creating secure DNS Server binaries (on Linux) https://dnsworkshop.org/roller/dnsworkshop/entry/compiler_options_for_creating_secure

CheckSec script: http://www.trapkit.de/tools/checksec.html # wget http://www.trapkit.de/tools/checksec.sh# bash ./checksec.sh --file /usr/sbin/named

RELRO STACK CANARY NX PIE RPATH RUNPATH FILEFull RELRO Canary found NX enabled PIE enabled No RPATH No RUNPATH /usr/sbin/named

BIND 9 access control

for a DNS resolver without zones, restrict the networks that can use the resolver options { allow-query { networkblock-acl; }; };

for a BIND 9 server running as a resolver with authoritative zones, restrict the networks that can use recursive queries to the resolver options { allow-recursion { networkblock-acl; }; };

on an all authoritative server, disable recursion options { recursion no; };

on an authoritative server, secure zone transfer and updates with TSIG (use tsig-keygen utility) key "tsig-server01-server02" { algorithm hmac-sha256; secret "JlI8ARXJmNGWj4A4YEezpBqX1hPTYqF2sBJmAmp+uRY=";};

server 2001:db8:100::/64 { keys { tsig-server01-server02"; }; };

TSIG signs SOA-queries, notifies and

zone-transfer

local private toplevel domain

private top level domain

using a Local Private Top Level Domain for the internal DNS namespace is a BAD IDEAtm

Examples:

•.company

•.local (collides with mDNS)

•.lan

•.work

private top level domain

Problems with private top level domains:

•not delegated from the official root — creates delegation and lookup issues

•authenticated denial of existence from the ROOT — DNSSEC proves that the domain is not delegated

•data leaks

•possible collisions with new official top level domains

Empty Zones

The root server system sees a high number of queries for non-delegated TLD queries

best practice for local namespace

if you need a private DNS namespace for internal use

•register a second level domain on the Internet

•there is now an abundance of choice thanks to the new TLDs — even short names

•not expensive ( 9 - 50 US$/year)

•delegate the zone to a dummy (empty) zone on the Internet (many registrars provide this "out-of-the box")

•use the domain for your internal namespace

next @ Men & Mice

Upcoming training classes

KEA-DHCP

October 13 – 14, 2016 - West Coast, USA

October 17 – 18, 2016 - East Coast, USA

November 21 – 22, 2016 - Amsterdam, The Netherlands

Two days - Hands-On class

US$ 1795

https://www.menandmice.com/support-training/training/kea-dhcp-training/

more training

•October 3 – 5, 2016 Introduction to DNS & BIND Hands-On class Arlington (VA), USA

•October 3 – 7, 2016 Introduction & Advanced DNS and BIND Topics Hands-on class Arlington (VA), USA

•October 5 – 7, 2016 DNSSEC Technical Workshop – Implementation and Deployment Arlington (VA), USA

•October 10-14, 2014 "DNS und BIND / DNS Sicherheit" (German) @ Linuxhotel, Essen, Germany

https://www.menandmice.com/support-training/training/

our next webinars

We are working on a couple new webinars for Q3/Q4 2016, dates will be announced soon

in the pipeline:

• "DNS reconnaissance" - fighting spam with DNS monitoring data

• DNS and DNSSEC monitoring

• DNS high availability tools https://www.menandmice.com/resources/educational-resources/webinars/

Thank you! !

Questions? Comments?

a secure bind 9 – best practices

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