Running Secure DNS Servers Jay Beale Lead Developer

Running Secure DNS Servers Jay Beale Lead Developer, Bastille Linux Security Team Director, Mandrake. Soft

Running Secure DNS Servers " " " Installing and configuring Unix BIND DNS server Simulating attacks and information probes for security assurance Implementing security "best practices" for DNS configuration Advanced Methods: Chroot the Daemon, Run as Non-superuser Split Horizon?

How Does DNS Work? DNS provides mapping between machine names and IP addresses. Name servers answer queries they have information for and often hunt for the rest in a recursive query. Root Servers are the starting point in a recursive query.

Installing Unix BIND DNS server Grab source for BIND at: ftp: //ftp. isc. org/isc/bind/src/cur/bind-8/bind-src. tar. gz

Building BIND " On Solaris: # tar -xzvf bind-src. tar. gz # cd src # vi port/solaris/Makefile. set Onto configuring build process. . .

Configuring BIND Makefile. set (Solaris) Append DESTINC and DESTLIB lines: 'DESTINC=/usr/local/include' 'DESTLIB=/usr/local/lib' to get the following:

Solaris Makefile. set 'CC=gcc' 'CDEBUG=-g -O 2' 'DESTBIN=/usr/local/bin' 'DESTSBIN=/usr/local/sbin' 'DESTEXEC=/usr/local/sbin' 'DESTMAN=/usr/local/share/man' 'DESTHELP=/usr/local/lib' 'DESTETC=/usr/local/etc' 'DESTRUN=/usr/local/etc' 'LDS=: ' 'AR=/usr/ccs/bin/ar cru' 'LEX=/usr/ccs/bin/lex' 'YACC=/usr/ccs/bin/yacc -d' 'SYSLIBS=-ll -lnsl -lsocket' 'INSTALL=/usr/ucb/install' 'MANDIR=man' 'MANROFF=man' 'CATEXT=$$N' 'PS=ps -p' 'RANLIB=/usr/ccs/bin/ranlib' 'DESTINC=/usr/local/include' 'DESTLIB=/usr/local/lib'

Compiling BIND (Solaris) # make depend # make install

Installing BIND on Linux " Find the location for your Linux distribution's most recent BIND package. # rpm -ivh http: //path_to_package/bind-8. 2. 2_P 7 -x. rpm

Configuring BIND " BIND 8 and 9, unlike BIND 4, use named. conf. Syntax has changed, but you can use namedbootconf to convert your old named. boot to the new format. " Zone (data) files have not changed syntax. "

Sample named. conf File 1/3 acl internal { 192. 168. 1. 0/24 ; }; options { directory "/var/run/named"; listen-on {192. 168. 1. 2 ; }; allow-recursion { internal ; }; allow-query { internal ; }; };

Sample named. conf File 2/3 zone ". " { type hint; file "db. cache"; }; // bastille-linux. org maps to the 192. 168. 1. 0/24 network zone "bastille-linux. org" { type master; file "db. bastille-linux. org"; allow-query { any ; }; };

Sample named. conf File 3/3 zone "1. 168. 192. in-addr. arpa" { type master; file "db. 192. 168. 1"; allow-query { any; }; }; zone "0. 0. 127. in-addr. arpa" { type master; file "db. 127. 0. 0"; allow-query { 127. 0. 0. 1 ; }; };

Best Practice: Logging " BIND 8 adds the capability for enhanced logging. Channel: a method (syslog priority, file, stderr or bit bucket) for logging data - Category: a type of data to log, based on hard-coded names in the BIND 8 program

BIND 8 Logging Channels 1/2 ( For syslog, format is: syslog facility; severity; ) " channel info_syslog { syslog daemon; severity info; }; " channel some_file { file "your_file"; severity error; };

$BIND 8 Logging Channels 2/2 " channel stderr_fd { file "<stderr>"; severity critical; };$

BIND 8 Logging Channels 2/2 " channel stderr_fd { file "<stderr>"; severity critical; }; " channel null_gone { null; };

BIND 8 Logging Categories 1/3 Quoted from DNS and BIND book: " default - wildcard, maps to ALL " cname - CNAME errors " config - configuration file errors " db - database operations " eventlib - system events " insist - internal consistency check failures " lame-servers - Detection of bad delegation

BIND 8 Logging Categories 2/3 Quoted from DNS and BIND book: " load - zone loading messages " maintenance - Maintenance events (e. g. system queries) " ncache - Negative caching events " notify - Asynchronous change notifications " os - problems with the operating system " packet - decodes of packets received and sent " panic - problems that cause the shutdown of the server

BIND 8 Logging Categories 3/3 Quoted from DNS and BIND book: " parser - Parsing of the configuration file. " queries - Analogous to BIND 4's query logging " response-checks - Malformed responses, unrelated addt'l information, . . . " security - approved / unapproved requests " statistics - periodic reports of activities " update - dynamic update events " xfer-in / xfer-out - Zone transfers from remote/local nameserver to local/remote nameserver

Additional Logging BIND 8 has strong default logging, but we can add a security log for certain important categories. Append to named. conf: logging { channel security { file "/var/adm/bind-security" versions 4 size 10 m; print-time yes; }; category insist {security; }; category os { security; }; category panic {security; }; category security {security; }; category xfer-out { security; }; category update {security; }; };

Interesting Part of the Talk " Take the role of an attacker! DNS gives out a whole lot of useful information " " Most syadmins don't think from the attacker's point of view! Bad habits from when the Internet was a more exclusive neighborhood.

Interesting Part of the Talk " Take the role of an attacker! DNS servers have traditionally made good cracking targets! " " BIND has had a number of security vulnerabilities BIND runs as root by default

"Profiling the Target" " Find out the target's primary nameservers: [jay@max jay]$ nslookup Default Server: ns. my. isp Address: 10. 0. 0. 1 > set q=ns > dumb. target. jay Server: ns. my. isp Address: 10. 0. 0. 1 Non-authoritative answer: dumb. target. jay nameserver = ns 2. dumb. target. jay Authoritative answers can be found from: dumb. target. jay internet address = 192. 168. 1. 85

Zone Transfer of their Zone 1/2 [jay@max zone]# dig @192. 168. 1. 85 dumb. target. jay axfr ; <<>> Di. G 8. 2 <<>> @192. 168. 1. 85 dumb. target. jay axfr ; (1 server found) $ORIGIN dumb. target. jay. @ 20 H IN SOA ns 1 hostmaster. dumb. target. jay ( 2000111001 ; serial 5 H ; refresh 1 H ; retry 4 d 4 h ; expire 1 D ) ; minimum 1 H IN NS dumb. target. jay. 20 H IN NS ns. dumbs. isp. 20 H IN A 192. 168. 1. 85 1 D IN HINFO "Pentium 133" "Red Hat 6. 1" 1 D IN MX 10 mail

Zone Transfer of their Zone 2/2 mail 1 D IN A 192. 168. 1. 2 really 1 D IN A 192. 168. 1. 20 1 D IN TXT "Admin's Trusted Workstation" 1 D IN HINFO "Athlon 850" "Red Hat 6. 1" rather 1 D IN A 192. 168. 1. 15 1 D IN HINFO "Pentium 266" "Mandrake 7. 1" serious 1 D IN CNAME extra 1 D IN A 192. 168. 1. 80 ns 1 D IN A 192. 168. 1. 30 r_g 1 D IN A 192. 168. 1. 68 roblimo 1 D IN MX 10 r_g 1 D IN A 192. 168. 1. 44 tahara 1 D IN A 192. 168. 1. 27

Reactions? Two commands and we have a full dump of their zone data! Hey, check out that TXT record! Which host appears to be the Sys. Admin's workstation? Will other machines potentially trust this one more? Maybe they all trust it for rsh-based backups even? !!!!

Reactions? 2/2 Check out the HINFO records! I note the Sys. Admin's trusted box, running Red Hat 6. 1. I have a small kit of Red Hat 6. 1 exploits -- which machine should I try them on?

Defense to Zone Transfers Should I really let everyone have this much information? Directives: allow-transfer { }; allow-recursion { }; allow-query { };

Defense to Zone Transfers 2/2 Let's look at the named. conf file from before, this time with a few additions!

$named. conf. restricted_axfr 1/3 acl internal { 192. 168. 1. 0/24 ; }; acl$

named. conf. restricted_axfr 1/3 acl internal { 192. 168. 1. 0/24 ; }; acl secondaries { 192. 168. 1. 3 ; 192. 168. 2. 3 ; }; options { directory "/var/named/"; listen-on {192. 168. 1. 2 ; }; allow-recursion { internal ; }; allow-query { internal ; }; allow-transfer { none; }; };

$named. conf. restricted_axfr 2/3 zone ". " { type hint; file "db. cache"; };$

named. conf. restricted_axfr 2/3 zone ". " { type hint; file "db. cache"; }; // bastille-linux. org maps to the 192. 168. 1. 0/24 network zone "bastille-linux. org" { type master; file "db. bastille-linux. org"; allow-query { any ; }; allow-transfer { secondaries; }; };

$named. conf. restricted_axfr 3/3 zone "1. 168. 192. in-addr. arpa" { type master; file$

named. conf. restricted_axfr 3/3 zone "1. 168. 192. in-addr. arpa" { type master; file "db. 192. 168. 1"; allow-query { any; }; allow-transfer { secondaries; }; }; zone "0. 0. 127. in-addr. arpa" { type master; file "db. 127. 0. 0"; allow-query { 127. 0. 0. 1 ; }; // Note: no one needs to get zone transfers of this one! };

Additional Data Mining Defense Notice those allow-query lines? Those stop outside attackers from mining us for data! They also fight cache poisoning. We set default deny on queries, by specifying this in our options block.

Additional Data Mining Defense What about the allow-recursion lines? These prevent outsiders from using our nameservers to query for zones that we don't run. Again, a defense for cache poisioning.

Cache Poisoning? By taking over an authoritative nameserver, an attacker obtains the ability to put data into our nameserver's cache. He accomplish this, if he can get us to ask his nameserver for information! Remember that DNS is a distributed database. The steps we're taking here attempt to minimize this possibility.

Safe From Data Mining Yet? [jay@max jay]$ dig @localhost dumb. target. jay hinfo ; <<>> Di. G 8. 2 <<>> @localhost dumb. target. jay hinfo ; (1 server found) ; ; res options: init recurs defnam dnsrch ; ; got answer: ; ; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6 ; ; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ; ; QUERY SECTION: ; ; dumb. target. jay, type = HINFO, class = IN (continued)

Nope. ; ; ANSWER SECTION: dumb. target. jay. 1 D IN HINFO "Pentium 166" "Red Hat 6. 1" ; ; Total query time: 1 msec ; ; FROM: max. fictional. attacker to SERVER: localhost 127. 0. 0. 1 ; ; WHEN: Sun Nov 12 00: 17: 08 2000 ; ; MSG SIZE sent: 33 rcvd 69

The Simple Solution? Simple. Don't give away so much information! But how do we reconcile this with the need to keep easy online data about hosts?

Two Choices 1) Remove all HINFO and TXT records. Consider choosing less informative machine names than sysadmin, printspool, and intranet. 2) Implement Split Horizon DNS. Basically, you maintain two separate DNS servers, with two separate sets of data.

Split Horizon DNS 1/3 This is an overview: Two DNS servers, generally separated via a firewall/packet filter. Possibly both DNS servers are on the same machine, which restricts external hosts to the external BIND process. Possibly both DNS servers sit on your firewall.

Split Horizon DNS 2/3 Internal DNS Server: Only accepts requests from internal hosts. Possesses a richer zone database. Forwards all requests for external data to the external DNS server. No zone transfers, unless we have other internal DNS servers.

Split Horizon DNS 3/3 External DNS Server: does recursion for internal hosts, usually Only only for internal DNS server. Possesses a sparser zone database. " Potentially much sparse, if used w/ NAT. transfers allowed from this host, but only Zone to the slave servers.

Playing the Attacker Some More Cracking the DNS Server. . . 8. 2. 0 and 8. 2. 1 was vulnerable to a BIND Remote Root exploit! numbers of exploitable systems, when this Huge exploit was released. Red Hat 6. 0, among many others, shipped with one of these versions.

Cracking the DNS Server The Cracker HOW-TO is available here: http: //www. enteract. com/~lspitz/NXT-Howto. txt. But how does an attacker find vulnerable machines?

Finding Vulnerable DNS Servers Try this command against one of your DNS servers: dig @your_ns txt chaos version. bind Here's the output against my test system:

Grabbing the BIND Version Number ; <<>> Di. G 8. 2 <<>> @dumb. target. jay txt chaos version. bind ; (1 server found) ; ; res options: init recurs defnam dnsrch ; ; got answer: ; ; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6 ; ; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ; ; QUERY SECTION: ; ; version. bind, type = TXT, class = CHAOS ; ; ANSWER SECTION: VERSION. BIND. 0 S CHAOS TXT "8. 2. 2" (truncated)

Security through Obscurity? Security through Obscurity can be helpful, though it should never be your only defense. Against Script Kiddies, it can be extremely effective, because of their Modus Operandi. ( see http: //www. securityportal. com/topnews/tighten 20000720. html) We can obscure the version number through the version directive in the options section of named. conf.

named. conf Revisited We'll just add one directive to the options block: options { directory "/var/named"; listen-on {192. 168. 1. 2 ; }; allow-recursion { internal ; }; allow-query { internal ; }; allow-transfer { none; }; version "Go away!"; };

Go Away? This is a judgment call. The script kiddies generally use scanners that pattern-match. Your goal is to not get matched. At the same time, you may want to "blend in". Suggestions: version "9. 0. 0"; version "4. 9. 7"; version "8. 3. 0";

Other Defenses Against Cracking Well, suppose a script kiddie gets lucky and tries an attack, even though a scanner didn't pick you out? Or, worse, suppose a more capable cracker comes after you? What defenses do you have?

Patch the Server! As boring as this is, it's wholly necessary! The BIND DNS server has had a number of security problems. We believe it will have more. You must continually patch/upgrade the server. Follow the security announcements to know when it is necessary.

Remote Root Vulnerability Remember how we talked about a "remote root" vulnerability? You can't remove the "remote" part, but you can remove the "root" part in a very simple way: Don't run named as root! The attacker will get access as an ordinary user instead of superuser access!

Run as a Non-Root User 1/3 named -u dns_user -g dns_group Create a separate user for the DNS server, with shell equal to /bin/false. # vipw /etc/passwd dns: x: 53: dns: /var/named: /bin/false # vipw /etc/shadow dns: NP: 6445: : :

Run as a Non-Root User 2/3 named -u dns_user -g dns_group Create a separate group for the DNS server, with shell equal to /bin/false. # echo "dns: : 53: dns" >> /etc/group

Run as a Non-Root User 3/3 Modify the named init script to use: named -u dns -g dns Major changes: This forces the DNS server to drop root privileges after binding to port 53. DNS server won't be able to rebind to port 53, when you send it a SIGHUP. When you make config file changes, restart named.

What Did This Defense Do? When the attacker uses that sort of exploit, he still gets shell access on the machine. In this case, though, he isn't root! But wait, he has a great deal of access to walk the filesystem, as much as an ordinary user. He might be able to escalate privilege. Reference: http: //www. securityportal. com/coverstory 20000626. html

Additional Defense He'd escalate privilege by finding a Set-UID program, or other program running as root, and try to exploit it. If there was a vulnerable on the system that an ordinary user can run, he could get root. We can stop this by doing a Set-UID audit, but also by cutting off his access to most of the filesystem. Chroot the named process, and thus the attacker!

Chroot? Short for "Change Root", where we change the root of the process's filesystem. As an ordinary user, it shouldn't be able to read files outside this root. This can be difficult, unless you have someone telling you how to do it ahead of time.

Chrooting BIND under Solaris We have to build a functional subset of the filesystem, with every program, library and data file that named needs. We'll choose very restrictive permissions, so that if an attacker compromises named and gets user dns access, he won't be able to change much. (Note that the DESTLIB and DESTINC lines that we appended to the Makefile. set file during our build make this slightly less hairy. )

Creating the Chroot Prison 1/6 Set up directories and permissions: mkdir -p /chroot/named chmod 111 /chroot chmod 511 /chroot/named cd /chroot/named mkdir -p dev var/run/named var/tmp maps/master usr/lib /usr/local/etc usr/local/sbin usr/share/lib/zoneinfo/US chmod -R 111 dev var maps usr chmod 1711 var/tmp /usr/ucb/chown -R root /chroot

Creating the Chroot Prison 2/6 Copy over binaries: cp /usr/local/sbin/named{, -xfer} /chroot/named/usr/local/sbin chmod 111 /usr/local/sbin/named{, -xfer} /usr/ucb/chown root /usr/local/sbin/named{, -xfer} Copy over Timezone files: cp /usr/share/lib/zoneinfo/US/Eastern /chroot/named/usr/share/lib/zoneinfo/US /usr/ucb/chown root /usr/share/lib/zoneinfo/US/Eastern chmod 444 /usr/share/lib/zoneinfo/US/Eastern

Creating the Chroot Prison 3/6 Create devices: cd /chroot/named/dev mknod tcp c 11 42 mknod udp c 11 41 mknod conslog c 21 0 mknod log c 21 5 mknod null c 13 2 mknod zero c 13 12 /usr/ucb/chown root * chgrp sys conslog null zero chmod 666 tcp udp conslog null zero

Creating the Chroot Prison 4/6 Determine necessary libraries: # ldd /usr/local/sbin/named{, -xfer} /usr/local/sbin/named: libl. so. 1 => /usr/libl. so. 1 libnsl. so. 1 => /usr/libnsl. so. 1 libsocket. so. 1 => /usr/libsocket. so. 1 libc. so. 1 => /usr/libc. so. 1 libdl. so. 1 => /usr/libdl. so. 1 libmp. so. 2 => /usr/libmp. so. 2 /usr/platform/SUNW, Ultra-5_10/libc_psr. so. 1 /usr/local/sbin/named-xfer: libl. so. 1 => /usr/libl. so. 1 libnsl. so. 1 => /usr/libnsl. so. 1 libsocket. so. 1 => /usr/libsocket. so. 1 libc. so. 1 => /usr/libc. so. 1 libdl. so. 1 => /usr/libdl. so. 1 libmp. so. 2 => /usr/libmp. so. 2 /usr/platform/SUNW, Ultra-5_10/libc_psr. so. 1 (example on Sol 2. 7)

Creating the Chroot Prison 5/6 Copy over necessary libraries: cp /usr/libl. so. 1 /usr/libnsl. so. 1 /usr/libsocket. so. 1 /usr/libc. so. 1 /usr/libdl. so. 1 /usr/libmp. so. 2 /chroot/named/usr/lib Additionally the following is necessary: cp /usr/lib/ld. so. 1 /usr/lib/nss_files. so. 1 /chroot/named/usr/lib And fix the permissions: /usr/ucb/chown root /chroot/named/usr/lib/* chmod 555 /chroot/named/usr/lib/*

Creating the Chroot Prison 6/6 Copy in the data files: (Configuration File) cp /usr/local/etc/named. conf /chroot/named/usr/local/etc/named. conf /usr/ucb/chown root /chroot/named/usr/local/etc/named. conf chmod 444 /chroot/named/usr/local/etc/named. conf (Zone Files) cp /var/run/named/db* /chroot/named/var/run/named/ /usr/ucb/chown root /chroot/named/var/run/named/db* chmod 444 /chroot/named/var/run/named/db*

Running named chroot-ed Try this: chroot /chroot/named /usr/local/sbin/named -u dns -g dns Alternatively: /chroot/named/usr/local/sbin/named -u dns -g dns -t /chroot/named

Wrapping Up Chroot Now, go modify your init script to work with the new command line! To harden the system slightly more, you might go delete or "chmod 000 foo" the named executable.

Going Further From here, you can look toward Split Horizon DNS. If you combine this with chroot'd named, we'd suggest the following: "Run each instance of named in it's own chroot'd environment. "Run each instance of named as a separate user. "Use NAT to have your internal hosts appear as a single host. This allows serious sparsity in your external DNS tables.

Going Even Further Go read the DNS and BIND O'Reilly book, by Paul Albitz and Cricket Liu. While this talk covers best practices and puts you way ahead of the game, there is nothing that compares with a deep understanding of the Basics.

Off the Beaten Path. . . Not for the timid: djbdns Daniel Bernstein's DNS server, made to compete with BIND. Much safer design than BIND -- break out functions into small programs with less privilege.

Less Privilege? Instead of a central program, we have several smaller ones: dnscache Local site caching-nameserver, but answers no queries for authoritative zones

Less Privilege? 2/2 tinydns Authoritative nameserver to answer queries about our specific machines axfrdns / axfrget Responsible for serving and getting zone transfers

Setting up djbdns Kuro 5 hin. org has an automated setup script in a great article: http: //www. kuro 5 hin. org/? op=displaystory; sid=200 1/4/9/17053/40631

Security? djbdns does much of what we do here very easily: 1) All servers run by default as a non-root user 2) All servers run chrooted, by default, in their own directories 3) djbdns hasn't had a security hole and promises a $500 prize to the finder of the first one

Security 2/3? djbdns does much of what we do here very easily: 4) walldns -- generates generic forward/reverse information to feed to outside servers, like this: 1. 2. 3. 4 --rev--> 4. 3. 2. 1. in-addr. arpa 4. 3. 2. 1. in-addr-arpa --> 1. 2. 3. 4

Security 3/3? djbdns does much of what we do here very easily: 5) Queries for other people's domains are automatically restricted, since tinydns won't answer them -- this is the internal dnscache's job 6) It's darn easy, by design, to go split horizon with djbdns.

Should You Use It? PRO: Much better security history than BIND PRO: Much better architecture for security CON: Can you sell your boss on the switch? PRO: Relatively easy to set up split-horizon DNS CON: Relatively difficult to configure and use, as few people have the experience or training OTOH: http: //cr. yp. to/djbdns/ad/easeofuse. html Do I like it? Heck yeah!

Wrap Up Go learn more about this stuff! DNS Servers are extremely important in maintaining the security of your organization. THINK about this one for a minute. BIND vulnerabilities are coming out too quickly. . .

Speaker Bio Jay Beale is the Lead Developer of Bastille Linux and the Security Team Director for Linux Mandrake. He writes a column for Security. Portal. com and is currently working on a book on Securing Linux and Unix for Addison Wesley. Read more of his articles on: http: //www. bastille-linux. org/jay