Linux: How to Load Balance with DNS Round Robin

If you are running multiple application server instances, you will need to use something to balance the HTTP traffic from the network. For example, you can use a DNS server or a hardware load balancer (e.g. F5) to load balance HTTP traffic.

In this article, we will cover how to configure a DNS server to round-robin network traffic across all application servers.

Load Balancing with DNS Round Robin

Domain Name Service (DNS) is an Internet service that map human-memorable domain names and hostnames into the corresponding numeric Internet Protocol (IP) addresses which are used to identify and locate computer systems and resources on the Internet. In this way, DNS alleviates the need to remember IP addresses. Computers that run DNS are called name servers. Both Red Hat and Ubuntu ship with BIND (Berkley Internet Naming Daemon),^[12] the most widely deployed DNS server.

Using a DNS server, even with one application server, makes it very easy if you need to change to different machines — you just need to update the IP address on the DNS server instead of on every other machine.

Load balancing allows organizations to distribute inbound traffic across multiple back-end nodes or servers (represented as the IP address of a physical server). Round robin is a local balancing mechanism used by DNS servers to share and distribute network resource loads. Compared to hardware load balancer, DNS round robin is a poor man’s load balancing solution.^[1]

Configuring Name Servers

Managing BIND's named daemon is easy to do, but the procedure differs between Linux distributions. Here are some things to keep in mind:

• Firstly, different Linux distributions use different daemon management systems. Each system has its own set of commands to do similar operations.
• Secondly, the daemon name needs to be known. In this case the name of the daemon is named.

Armed with this information you can know how to:

• Start your daemons automatically on booting
• Stop, start and restart them later on during troubleshooting or when a configuration file change needs to be applied.

For more details, read [11] and [12]. The Linux release used in this article is:

$ cat /etc/*-release
Enterprise Linux Enterprise Linux Server release 5.4 (Carthage)
Red Hat Enterprise Linux Server release 5.4 (Tikanga)

Assume that your named is up and running—To enable DNS services on the name server, the /etc/host.conf file should look like this:

# Lookup names via /etc/hosts first, then by DNS query
order hosts, bind

Next you need to configure DNS tables for DNS services. If you use the BIND 8.x package with the Red Hat distribution, the configuration of DNS tables can be done in:

• /etc/named.conf

For example, the /etc/named.config could look like this:

options {
// DNS tables are located in the /var/named directory
directory "/var/named";
dump-file "/var/named/data/cache_dump.db";
statistics-file "/var/named/data/named_stats.txt";
listen-on { any; };
allow-query { any; };
rrset-order {
order cyclic;
};
};

controls {
inet 127.0.0.1 allow {
localhost;
};
};

zone "." {
type forward;
forward first;
forwarders {
123.12.40.17;
};
};

// All our DNS information is stored in /var/named/foobar
zone "foobar.com" {
type master;
notify no;
file "foobar.com";
};

The rrset-order substatement accepts fixed, random and cyclic as arguments. By specifying cyclic as the argument, you can configure the name server to return matching records in cyclic (round robin) order.^[7]

The DNS table named /var/named/foobar.com translates host names (such as jassut) to IP addresses. could have the contents like this:^[8]

; Zone file for foobar.com
;
$TTL 3D
@ IN SOA nameserver.foobar.com. hostmaster.foobar.com. (
2005061504 ; serial # (today's date + increment)
1S ; refresh
1S ; retry
1S ; expire
1S) ; minumum ttl
;
NS nameserver.foobar.com.
;
; sorted alphnumerically by hostname (first column) then by IP address
;
nameserver A 192.168.11.190

jassut A 192.168.10.14
jasapp1dom1inst1nic1 A 192.168.10.14
jassut A 192.168.12.14
jasapp1dom1inst1nic2 A 192.168.12.14
jassut A 192.168.14.14
jasapp1dom1inst1nic3 A 192.168.14.14
jassut A 192.168.16.14
jasapp1dom1inst1nic4 A 192.168.16.14

Configuring DNS Clients

There are three main client configuration files associated with DNS:^[9]

• /etc/hosts
• /etc/nsswitch.conf
• /etc/resolv.conf

When your computer looks for another computer on a network such as the Internet, it typically looks in two places: /etc/hosts and any DNS servers that you've set up for your network.

/etc/hosts

The /etc/hosts file keeps a local name database. This file helps in local name resolution if your local DNS server is not functioning. Network adinistrators should manually populate entries in this file. A sample /etc/hosts file is copied below.

$ cat /etc/hosts

# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 localhost.localdomain localhost
192.168.11.49 clientserver1.foobar.com clientserver1

/etc/nsswitch.conf

The order of name resolution process is determined by a single line in /etc/nsswitch.conf:

hosts: files dns

In this case, the name resolution process will begin with checking /etc/hosts file, and if the name cannot be resolved, the name resolution will happen with the DNS server.

/etc/resolv.conf

The /etc/resolv.conf file contains directives with the IP addresses of nameservers available to a host. A sample /etc/resolv.conf file is copied below.

$ cat /etc/resolv.conf
options attempts: 5
options timeout: 15
search foobar.com

nameserver 192.168.11.190

How to Test?

nslookup is a network administration command-line tool available for many computer operating systems for querying the DNS server(s) to obtain domain name or IP address mapping or for any other specific DNS record.

For example, our sample setup uses four server instances in one WebLogic domain, which listens on four Network Interface Cards.

On the client computer, you can query host name jassut by typing:
$ nslookup jassut

Server: 192.167.11.190
Address: 192.167.11.190#53

Name: jassut.foobar.com
Address: 192.168.16.14

Name: jassut.foobar.com
Address: 192.168.10.14

Name: jassut.foobar.com
Address: 192.168.12.14

Name: jassut.foobar.com
Address: 192.168.14.14

Other commands for debugging DNS besides nslookup are host and dig. Note that ping is not useful for debugging DNS, as it will use whatever is configured in /etc/nsswitch.conf to do the name-lookup. Also, noted that our host (i.e., jassut.foobar.com) has multiple network interface controllers (NIC).^[16]

DNS Caching

The Internet's Domain Name System (DNS) involves caching on both DNS servers and on the client computers that contact DNS servers. To minimize the load on the actual DNS server, we want to use a local DNS cache. There are two levels of local name caching on DNS clients:

• OS
• To enable DNS caching on the client computers, you can install nscd and run it.
• A daemon that provides a cache for the most common name service requests.
• To launch the daemon, do:
• /etc/init.d/nscd start
• JVM
• By default, the JVM will cache DNS information.

However, if client-side DNS caching is enabled, it can lessen the effectiveness of the round-robin DNS load balancing. In other words, due to the client-side DNS query caching, your round-robin DNS load balancing is not likely to distribute the load on your remote servers on a per-request basis. It will be more like a per-session basis.

To effectively support DNS round robin, you want to:

• Disable JVM local caching by setting
• -Dsun.net.inetaddr.ttl=0 -Dnetworkaddress.cache.ttl=0
• Decrease the local DNS cache (i.e., nscd) default time-to-live to the minimum of 1 second (see instructions below).

On Windows, under HKEY_LOCAL_MACHINE\System\CurrentControlSet set Services\Dnscache\Parameters\MaxCacheEntryTtlLimit to 1

For UNIX machines with a name service cache daemon (nscd),^[5] ensure that /etc/nscd.conf contains

enable-cache hosts yes
positive-time-to-live hosts 1
negative-time-to-live hosts 0

References

1. Configuring DNS round robin on Windows
2. Load Balancing With Round Robin DNS
3. port 53
• Default port listened by DNS
4. Domain Name Server (DNS) Configuration and Administration
• BIND 8.x
• /etc/named.conf file should be configured to point to your DNS tables
• BIND9 (Berkley Internet Naming Daemon)
• BIND9 Configuration files are stored in /etc/bind
5. nscd (Name Service Cache Daemon)The default configuration file, /etc/nscd.conf, determines the behavior of the cache daemon
• See also: nsswitch.conf - System Databases and Name Service Switch configuration file
6. The Domain Name System (good)
7. Round Robin Load Distribution
8. Zone files (Red Hat Linux 5)
9. Linux Domain Name System (DNS) client configuration files
10. Linux / Unix: Dig Command
• The chief difference between nslookup and dig is that dig has no interactive mode: you specify everything at the command line.
11. Quick HOWTO : Ch18 : Configuring DNS (good)
12. Berkeley Internet Name Domain (BIND) (Red Hat Linux 5)
13. Why 192.168.*.* for local addresses?
14. Load Balancing T3 InitialContext Retrieval for WebLogic using Oracle Traffic Director
• For bootstrapping the InitialContext, you can choose two approaches:
• DNS round robin
• External load blancer such as OTD
15. DNS and BIND, 5th edition, by Cricket Liu and Paul Albitz
16. Multihoming