Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION User Manual
Red Hat Enterprise Linux 5
Virtual Server
Administration
Linux Virtual Server (LVS) for Red Hat Enterprise Linux
Virtual Server Administration
Red Hat Enterprise Linux 5 Virtual Server Administration
Linux Virtual Server (LVS) for Red Hat Enterprise Linux
Edition 3
Copyright © 2009 Red Hat, Inc. This material may only be distributed subject to the terms and
conditions set forth in the Open Publication License, V1.0 or later (the latest version of the OPL is
presently available at http://www.opencontent.org/openpub/).
Red Hat and the Red Hat "Shadow Man" logo are registered trademarks of Red Hat, Inc. in the United
States and other countries.
All other trademarks referenced herein are the property of their respective owners.
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Phone: 888 733 4281
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Building a Linux Virtual Server (LVS) system offers highly-available and scalable solution for
production services using specialized routing and load-balancing techniques configured through the
PIRANHA. This book discusses the configuration of high-performance systems and services with Red
Hat Enterprise Linux and LVS for Red Hat Enterprise Linux 5.
iii
Introduction v
1. Document Conventions ................................................................................................... vi
1.1. Typographic Conventions ..................................................................................... vi
1.2. Pull-quote Conventions ........................................................................................ vii
1.3. Notes and Warnings ........................................................................................... viii
2. Feedback ....................................................................................................................... ix
1. Linux Virtual Server Overview 1
1.1. A Basic LVS Configuration ............................................................................................ 1
1.1.1. Data Replication and Data Sharing Between Real Servers ................................... 3
1.2. A Three-Tier LVS Configuration ..................................................................................... 3
1.3. LVS Scheduling Overview ............................................................................................. 4
1.3.1. Scheduling Algorithms ....................................................................................... 5
1.3.2. Server Weight and Scheduling ........................................................................... 6
1.4. Routing Methods .......................................................................................................... 7
1.4.1. NAT Routing ..................................................................................................... 7
1.4.2. Direct Routing ................................................................................................... 8
1.5. Persistence and Firewall Marks ..................................................................................... 9
1.5.1. Persistence ....................................................................................................... 9
1.5.2. Firewall Marks ................................................................................................. 10
1.6. LVS — A Block Diagram ............................................................................................ 10
1.6.1. LVS Components ............................................................................................. 11
2. Initial LVS Configuration 13
2.1. Configuring Services on the LVS Routers .................................................................... 13
2.2. Setting a Password for the Piranha Configuration Tool .............................................. 14
2.3. Starting the Piranha Configuration Tool Service ......................................................... 14
2.3.1. Configuring the Piranha Configuration Tool Web Server Port ........................... 15
2.4. Limiting Access To the Piranha Configuration Tool .................................................... 15
2.5. Turning on Packet Forwarding .................................................................................... 16
2.6. Configuring Services on the Real Servers .................................................................... 16
3. Setting Up LVS 19
3.1. The NAT LVS Network ................................................................................................ 19
3.1.1. Configuring Network Interfaces for LVS with NAT ............................................... 19
3.1.2. Routing on the Real Servers ............................................................................ 20
3.1.3. Enabling NAT Routing on the LVS Routers ........................................................ 21
3.2. LVS via Direct Routing ............................................................................................... 21
3.2.1. Direct Routing and arptables_jf .................................................................. 22
3.2.2. Direct Routing and iptables .......................................................................... 23
3.3. Putting the Configuration Together ............................................................................... 24
3.3.1. General LVS Networking Tips ........................................................................... 25
3.4. Multi-port Services and LVS ........................................................................................ 25
3.4.1. Assigning Firewall Marks .................................................................................. 26
3.5. Configuring FTP ......................................................................................................... 27
3.5.1. How FTP Works .............................................................................................. 27
3.5.2. How This Affects LVS Routing .......................................................................... 27
3.5.3. Creating Network Packet Filter Rules ................................................................ 28
3.6. Saving Network Packet Filter Settings ......................................................................... 29
4. Configuring the LVS Routers with Piranha Configuration Tool 31
4.1. Necessary Software ................................................................................................... 31
4.2. Logging Into the Piranha Configuration Tool .............................................................. 31
Virtual Server Administration
iv
4.3. CONTROL/MONITORING ........................................................................................... 32
4.4. GLOBAL SETTINGS .................................................................................................. 34
4.5. REDUNDANCY .......................................................................................................... 35
4.6. VIRTUAL SERVERS .................................................................................................. 37
4.6.1. The VIRTUAL SERVER Subsection ................................................................. 38
4.6.2. REAL SERVER Subsection ............................................................................. 41
4.6.3. EDIT MONITORING SCRIPTS Subsection ........................................................ 44
4.7. Synchronizing Configuration Files ................................................................................ 46
4.7.1. Synchronizing lvs.cf ..................................................................................... 46
4.7.2. Synchronizing sysctl ..................................................................................... 47
4.7.3. Synchronizing Network Packet Filtering Rules ................................................... 47
4.8. Starting LVS ............................................................................................................... 47
A. Using LVS with Red Hat Cluster 49
B. Revision History 51
Index 53
v
Introduction
This document provides information about installing, configuring, and managing Red Hat Virtual Linux
Server (LVS) components. LVS provides load balancing through specialized routing techniques that
dispatch traffic to a pool of servers. This document does not include information about installing,
configuring, and managing Red Hat Cluster software. Information about that is in a separate
document.
The audience of this document should have advanced working knowledge of Red Hat Enterprise Linux
and understand the concepts of clusters, storage, and server computing.
This document is organized as follows:
• Chapter 1, Linux Virtual Server Overview
• Chapter 2, Initial LVS Configuration
• Chapter 3, Setting Up LVS
• Chapter 4, Configuring the LVS Routers with Piranha Configuration Tool
• Appendix A, Using LVS with Red Hat Cluster
For more information about Red Hat Enterprise Linux 5, refer to the following resources:
• Red Hat Enterprise Linux Installation Guide — Provides information regarding installation of Red
Hat Enterprise Linux 5.
• Red Hat Enterprise Linux Deployment Guide — Provides information regarding the deployment,
configuration and administration of Red Hat Enterprise Linux 5.
For more information about Red Hat Cluster Suite for Red Hat Enterprise Linux 5, refer to the following
resources:
• Red Hat Cluster Suite Overview — Provides a high level overview of the Red Hat Cluster Suite.
• Configuring and Managing a Red Hat Cluster — Provides information about installing, configuring
and managing Red Hat Cluster components.
• Logical Volume Manager Administration — Provides a description of the Logical Volume Manager
(LVM), including information on running LVM in a clustered environment.
• Global File System: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS (Red Hat Global File System).
• Global File System 2: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS2 (Red Hat Global File System 2).
• Using Device-Mapper Multipath — Provides information about using the Device-Mapper Multipath
feature of Red Hat Enterprise Linux 5.
• Using GNBD with Global File System — Provides an overview on using Global Network Block
Device (GNBD) with Red Hat GFS.
• Red Hat Cluster Suite Release Notes — Provides information about the current release of Red Hat
Cluster Suite.
Introduction
vi
Red Hat Cluster Suite documentation and other Red Hat documents are available in HTML,
PDF, and RPM versions on the Red Hat Enterprise Linux Documentation CD and online at http://
www.redhat.com/docs/.
1. Document Conventions
This manual uses several conventions to highlight certain words and phrases and draw attention to
specific pieces of information.
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts1 set. The
Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not,
alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes
the Liberation Fonts set by default.
1.1. Typographic Conventions
Four typographic conventions are used to call attention to specific words and phrases. These
conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight
key caps and key-combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current
working directory, enter the cat my_next_bestselling_novel command at the
shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a key cap, all presented in Mono-spaced Bold
and all distinguishable thanks to context.
Key-combinations can be distinguished from key caps by the hyphen connecting each part of a keycombination. For example:
Press Enter to execute the command.
Press Ctrl-Alt-F1 to switch to the first virtual terminal. Press Ctrl-Alt-F7 to return
to your X-Windows session.
The first sentence highlights the particular key cap to press. The second highlights two sets of three
key caps, each set pressed simultaneously.
If source code is discussed, class names, methods, functions, variable names and returned values
mentioned within a paragraph will be presented as above, in Mono-spaced Bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for
directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialogue
box text; labelled buttons; check-box and radio button labels; menu titles and sub-menu titles. For
example:
1
https://fedorahosted.org/liberation-fonts/
Pull-quote Conventions
vii
Choose System > Preferences > Mouse from the main menu bar to launch Mouse
Preferences. In the Buttons tab, click the Left-handed mouse check box and click
Close to switch the primary mouse button from the left to the right (making the mouse
suitable for use in the left hand).
To insert a special character into a gedit file, choose Applications > Accessories
> Character Map from the main menu bar. Next, choose Search > Find… from the
Character Map menu bar, type the name of the character in the Search field and click
Next. The character you sought will be highlighted in the Character Table. Double-
click this highlighted character to place it in the Text to copy field and then click the
Copy button. Now switch back to your document and choose Edit > Paste from the
gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific
menu names; and buttons and text found within a GUI interface, all presented in Proportional Bold and
all distinguishable by context.
Note the > shorthand used to indicate traversal through a menu and its sub-menus. This is to avoid
the difficult-to-follow 'Select Mouse from the Preferences sub-menu in the System menu of the main
menu bar' approach.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether Mono-spaced Bold or Proportional Bold, the addition of Italics indicates replaceable or
variable text. Italics denotes text you do not input literally or displayed text that changes depending on
circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at
a shell prompt. If the remote machine is example.com and your username on that
machine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file
system. For example, to remount the /home file system, the command is mount -o
remount /home.
To see the version of a currently installed package, use the rpm -q package
command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and
release. Each word is a placeholder, either for text you enter when issuing a command or for text
displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and
important term. For example:
When the Apache HTTP Server accepts requests, it dispatches child processes
or threads to handle them. This group of child processes or threads is known as
a server-pool. Under Apache HTTP Server 2.0, the responsibility for creating and
maintaining these server-pools has been abstracted to a group of modules called
Multi-Processing Modules (MPMs). Unlike other modules, only one module from the
MPM group can be loaded by the Apache HTTP Server.
1.2. Pull-quote Conventions
Two, commonly multi-line, data types are set off visually from the surrounding text.
Introduction
viii
Output sent to a terminal is set in Mono-spaced Roman and presented thus:
books Desktop documentation drafts mss photos stuff svn
books_tests Desktop1 downloads images notes scripts svgs
Source-code listings are also set in Mono-spaced Roman but are presented and highlighted as
follows:
package org.jboss.book.jca.ex1;
import javax.naming.InitialContext;
public class ExClient
{
public static void main(String args[])
throws Exception
{
InitialContext iniCtx = new InitialContext();
Object ref = iniCtx.lookup("EchoBean");
EchoHome home = (EchoHome) ref;
Echo echo = home.create();
System.out.println("Created Echo");
System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
}
}
1.3. Notes and Warnings
Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.
Note
A Note is a tip or shortcut or alternative approach to the task at hand. Ignoring a note
should have no negative consequences, but you might miss out on a trick that makes your
life easier.
Important
Important boxes detail things that are easily missed: configuration changes that only
apply to the current session, or services that need restarting before an update will apply.
Ignoring Important boxes won't cause data loss but may cause irritation and frustration.
Feedback
ix
Warning
A Warning should not be ignored. Ignoring warnings will most likely cause data loss.
2. Feedback
If you spot a typo, or if you have thought of a way to make this manual better, we would love to
hear from you. Please submit a report in Bugzilla (http://bugzilla.redhat.com/bugzilla/) against the
component Documentation-cluster.
Be sure to mention the manual's identifier:
Virtual_Server_Administration(EN)-5 (2009-08-18T15:53)
By mentioning this manual's identifier, we know exactly which version of the guide you have.
If you have a suggestion for improving the documentation, try to be as specific as possible. If you have
found an error, please include the section number and some of the surrounding text so we can find it
easily.
x
Chapter 1.
1
Linux Virtual Server Overview
Linux Virtual Server (LVS) is a set of integrated software components for balancing the IP load across
a set of real servers. LVS runs on a pair of equally configured computers: one that is an active LVS
router and one that is a backup LVS router. The active LVS router serves two roles:
• To balance the load across the real servers.
• To check the integrity of the services on each real server.
The backup LVS router monitors the active LVS router and takes over from it in case the active LVS
router fails.
This chapter provides an overview of LVS components and functions, and consists of the following
sections:
• Section 1.1, “A Basic LVS Configuration”
• Section 1.2, “A Three-Tier LVS Configuration”
• Section 1.3, “LVS Scheduling Overview”
• Section 1.4, “Routing Methods”
• Section 1.5, “Persistence and Firewall Marks”
• Section 1.6, “LVS — A Block Diagram”
1.1. A Basic LVS Configuration
Figure 1.1, “A Basic LVS Configuration” shows a simple LVS configuration consisting of two layers.
On the first layer are two LVS routers — one active and one backup. Each of the LVS routers has two
network interfaces, one interface on the Internet and one on the private network, enabling them to
regulate traffic between the two networks. For this example the active router is using Network Address
Translation or NAT to direct traffic from the Internet to a variable number of real servers on the second
layer, which in turn provide the necessary services. Therefore, the real servers in this example are
connected to a dedicated private network segment and pass all public traffic back and forth through
the active LVS router. To the outside world, the servers appears as one entity.
Chapter 1. Linux Virtual Server Overview
2
Figure 1.1. A Basic LVS Configuration
Service requests arriving at the LVS routers are addressed to a virtual IP address, or VIP. This is a
publicly-routable address the administrator of the site associates with a fully-qualified domain name,
such as www.example.com, and is assigned to one or more virtual servers. A virtual server is a service
configured to listen on a specific virtual IP. Refer to Section 4.6, “VIRTUAL SERVERS” for more
information on configuring a virtual server using the Piranha Configuration Tool. A VIP address
migrates from one LVS router to the other during a failover, thus maintaining a presence at that IP
address (also known as floating IP addresses).
VIP addresses may be aliased to the same device which connects the LVS router to the Internet. For
instance, if eth0 is connected to the Internet, than multiple virtual servers can be aliased to eth0:1.
Alternatively, each virtual server can be associated with a separate device per service. For example,
HTTP traffic can be handled on eth0:1, and FTP traffic can be handled on eth0:2.
Only one LVS router is active at a time. The role of the active router is to redirect service requests
from virtual IP addresses to the real servers. The redirection is based on one of eight supported loadbalancing algorithms described further in Section 1.3, “LVS Scheduling Overview”.
The active router also dynamically monitors the overall health of the specific services on the real
servers through simple send/expect scripts. To aid in detecting the health of services that require
dynamic data, such as HTTPS or SSL, the administrator can also call external executables. If a
service on a real server malfunctions, the active router stops sending jobs to that server until it returns
to normal operation.
The backup router performs the role of a standby system. Periodically, the LVS routers exchange
heartbeat messages through the primary external public interface and, in a failover situation, the
private interface. Should the backup node fail to receive a heartbeat message within an expected
interval, it initiates a failover and assumes the role of the active router. During failover, the backup
router takes over the VIP addresses serviced by the failed router using a technique known as ARP
Data Replication and Data Sharing Between Real Servers
3
spoofing — where the backup LVS router announces itself as the destination for IP packets addressed
to the failed node. When the failed node returns to active service, the backup node assumes its hotbackup role again.
The simple, two-layered configuration used in Figure 1.1, “A Basic LVS Configuration” is best for
serving data which does not change very frequently — such as static webpages — because the
individual real servers do not automatically sync data between each node.
1.1.1. Data Replication and Data Sharing Between Real Servers
Since there is no built-in component in LVS to share the same data between the real servers, the
administrator has two basic options:
• Synchronize the data across the real server pool
• Add a third layer to the topology for shared data access
The first option is preferred for servers that do not allow large numbers of users to upload or change
data on the real servers. If the configuration allows large numbers of users to modify data, such as an
e-commerce website, adding a third layer is preferable.
1.1.1.1. Configuring Real Servers to Synchronize Data
There are many ways an administrator can choose to synchronize data across the pool of real servers.
For instance, shell scripts can be employed so that if a Web engineer updates a page, the page is
posted to all of the servers simultaneously. Also, the system administrator can use programs such as
rsync to replicate changed data across all nodes at a set interval.
However, this type of data synchronization does not optimally function if the configuration is
overloaded with users constantly uploading files or issuing database transactions. For a configuration
with a high load, a three-tier topology is the ideal solution.
1.2. A Three-Tier LVS Configuration
Figure 1.2, “A Three-Tier LVS Configuration” shows a typical three-tier LVS topology. In this example,
the active LVS router routes the requests from the Internet to the pool of real servers. Each of the real
servers then accesses a shared data source over the network.
Chapter 1. Linux Virtual Server Overview
4
Figure 1.2. A Three-Tier LVS Configuration
This configuration is ideal for busy FTP servers, where accessible data is stored on a central, highly
available server and accessed by each real server via an exported NFS directory or Samba share.
This topology is also recommended for websites that access a central, highly available database
for transactions. Additionally, using an active-active configuration with Red Hat Cluster Manager,
administrators can configure one high-availability cluster to serve both of these roles simultaneously.
The third tier in the above example does not have to use Red Hat Cluster Manager, but failing to use a
highly available solution would introduce a critical single point of failure.
1.3. LVS Scheduling Overview
One of the advantages of using LVS is its ability to perform flexible, IP-level load balancing on the
real server pool. This flexibility is due to the variety of scheduling algorithms an administrator can
choose from when configuring LVS. LVS load balancing is superior to less flexible methods, such as
Round-Robin DNS where the hierarchical nature of DNS and the caching by client machines can lead
to load imbalances. Additionally, the low-level filtering employed by the LVS router has advantages
Scheduling Algorithms
5
over application-level request forwarding because balancing loads at the network packet level causes
minimal computational overhead and allows for greater scalability.
Using scheduling, the active router can take into account the real servers' activity and, optionally, an
administrator-assigned weight factor when routing service requests. Using assigned weights gives
arbitrary priorities to individual machines. Using this form of scheduling, it is possible to create a group
of real servers using a variety of hardware and software combinations and the active router can evenly
load each real server.
The scheduling mechanism for LVS is provided by a collection of kernel patches called IP Virtual
Server or IPVS modules. These modules enable layer 4 (L4) transport layer switching, which is
designed to work well with multiple servers on a single IP address.
To track and route packets to the real servers efficiently, IPVS builds an IPVS table in the kernel.
This table is used by the active LVS router to redirect requests from a virtual server address to and
returning from real servers in the pool. The IPVS table is constantly updated by a utility called ipvsadm
— adding and removing cluster members depending on their availability.
1.3.1. Scheduling Algorithms
The structure that the IPVS table takes depends on the scheduling algorithm that the administrator
chooses for any given virtual server. To allow for maximum flexibility in the types of services you
can cluster and how these services are scheduled, Red Hat Enterprise Linux provides the following
scheduling algorithms listed below. For instructions on how to assign scheduling algorithms refer to
Section 4.6.1, “The VIRTUAL SERVER Subsection”.
Round-Robin Scheduling
Distributes each request sequentially around the pool of real servers. Using this algorithm, all
the real servers are treated as equals without regard to capacity or load. This scheduling model
resembles round-robin DNS but is more granular due to the fact that it is network-connection
based and not host-based. LVS round-robin scheduling also does not suffer the imbalances
caused by cached DNS queries.
Weighted Round-Robin Scheduling
Distributes each request sequentially around the pool of real servers but gives more jobs to
servers with greater capacity. Capacity is indicated by a user-assigned weight factor, which is then
adjusted upward or downward by dynamic load information. Refer to Section 1.3.2, “Server Weight
and Scheduling” for more on weighting real servers.
Weighted round-robin scheduling is a preferred choice if there are significant differences in the
capacity of real servers in the pool. However, if the request load varies dramatically, the more
heavily weighted server may answer more than its share of requests.
Least-Connection
Distributes more requests to real servers with fewer active connections. Because it keeps track of
live connections to the real servers through the IPVS table, least-connection is a type of dynamic
scheduling algorithm, making it a better choice if there is a high degree of variation in the request
load. It is best suited for a real server pool where each member node has roughly the same
capacity. If a group of servers have different capabilities, weighted least-connection scheduling is
a better choice.
Chapter 1. Linux Virtual Server Overview
6
Weighted Least-Connections (default)
Distributes more requests to servers with fewer active connections relative to their capacities.
Capacity is indicated by a user-assigned weight, which is then adjusted upward or downward
by dynamic load information. The addition of weighting makes this algorithm ideal when the real
server pool contains hardware of varying capacity. Refer to Section 1.3.2, “Server Weight and
Scheduling” for more on weighting real servers.
Locality-Based Least-Connection Scheduling
Distributes more requests to servers with fewer active connections relative to their destination IPs.
This algorithm is designed for use in a proxy-cache server cluster. It routes the packets for an IP
address to the server for that address unless that server is above its capacity and has a server in
its half load, in which case it assigns the IP address to the least loaded real server.
Locality-Based Least-Connection Scheduling with Replication Scheduling
Distributes more requests to servers with fewer active connections relative to their destination
IPs. This algorithm is also designed for use in a proxy-cache server cluster. It differs from LocalityBased Least-Connection Scheduling by mapping the target IP address to a subset of real
server nodes. Requests are then routed to the server in this subset with the lowest number of
connections. If all the nodes for the destination IP are above capacity, it replicates a new server
for that destination IP address by adding the real server with the least connections from the overall
pool of real servers to the subset of real servers for that destination IP. The most loaded node is
then dropped from the real server subset to prevent over-replication.
Destination Hash Scheduling
Distributes requests to the pool of real servers by looking up the destination IP in a static hash
table. This algorithm is designed for use in a proxy-cache server cluster.
Source Hash Scheduling
Distributes requests to the pool of real servers by looking up the source IP in a static hash table.
This algorithm is designed for LVS routers with multiple firewalls.
1.3.2. Server Weight and Scheduling
The administrator of LVS can assign a weight to each node in the real server pool. This weight is an
integer value which is factored into any weight-aware scheduling algorithms (such as weighted leastconnections) and helps the LVS router more evenly load hardware with different capabilities.
Weights work as a ratio relative to one another. For instance, if one real server has a weight of 1 and
the other server has a weight of 5, then the server with a weight of 5 gets 5 connections for every 1
connection the other server gets. The default value for a real server weight is 1.
Although adding weight to varying hardware configurations in a real server pool can help load-balance
the cluster more efficiently, it can cause temporary imbalances when a real server is introduced to the
real server pool and the virtual server is scheduled using weighted least-connections. For example,
suppose there are three servers in the real server pool. Servers A and B are weighted at 1 and
the third, server C, is weighted at 2. If server C goes down for any reason, servers A and B evenly
distributes the abandoned load. However, once server C comes back online, the LVS router sees it
has zero connections and floods the server with all incoming requests until it is on par with servers A
and B.
To prevent this phenomenon, administrators can make the virtual server a quiesce server — anytime
a new real server node comes online, the least-connections table is reset to zero and the LVS router
routes requests as if all the real servers were newly added to the cluster.
Routing Methods
7
1.4. Routing Methods
Red Hat Enterprise Linux uses Network Address Translation or NAT routing for LVS, which allows the
administrator tremendous flexibility when utilizing available hardware and integrating the LVS into an
existing network.
1.4.1. NAT Routing
Figure 1.3, “LVS Implemented with NAT Routing”, illustrates LVS utilizing NAT routing to move
requests between the Internet and a private network.
Figure 1.3. LVS Implemented with NAT Routing
In the example, there are two NICs in the active LVS router. The NIC for the Internet has a real IP
address on eth0 and has a floating IP address aliased to eth0:1. The NIC for the private network
interface has a real IP address on eth1 and has a floating IP address aliased to eth1:1. In the event of
failover, the virtual interface facing the Internet and the private facing virtual interface are taken-over
by the backup LVS router simultaneously. All of the real servers located on the private network use the
floating IP for the NAT router as their default route to communicate with the active LVS router so that
their abilities to respond to requests from the Internet is not impaired.
In this example, the LVS router's public LVS floating IP address and private NAT floating IP address
are aliased to two physical NICs. While it is possible to associate each floating IP address to its own
physical device on the LVS router nodes, having more than two NICs is not a requirement.
Using this topology, the active LVS router receives the request and routes it to the appropriate server.
The real server then processes the request and returns the packets to the LVS router which uses
network address translation to replace the address of the real server in the packets with the LVS
routers public VIP address. This process is called IP masquerading because the actual IP addresses
of the real servers is hidden from the requesting clients.
Chapter 1. Linux Virtual Server Overview
8
Using this NAT routing, the real servers may be any kind of machine running various operating
systems. The main disadvantage is that the LVS router may become a bottleneck in large cluster
deployments because it must process outgoing as well as incoming requests.
1.4.2. Direct Routing
Building an LVS setup that uses direct routing provides increased performance benefits compared to
other LVS networking topologies. Direct routing allows the real servers to process and route packets
directly to a requesting user rather than passing all outgoing packets through the LVS router. Direct
routing reduces the possibility of network performance issues by relegating the job of the LVS router to
processing incoming packets only.
Figure 1.4. LVS Implemented with Direct Routing
In the typical direct routing LVS setup, the LVS router receives incoming server requests through
the virtual IP (VIP) and uses a scheduling algorithm to route the request to the real servers. The
real server processes the request and sends the response directly to the client, bypassing the LVS
routers. This method of routing allows for scalability in that real servers can be added without the
added burden on the LVS router to route outgoing packets from the real server to the client, which can
become a bottleneck under heavy network load.
Persistence and Firewall Marks
9
1.4.2.1. Direct Routing and the ARP Limitation
While there are many advantages to using direct routing in LVS, there are limitations as well. The most
common issue with LVS via direct routing is with Address Resolution Protocol (ARP).
In typical situations, a client on the Internet sends a request to an IP address. Network routers typically
send requests to their destination by relating IP addresses to a machine's MAC address with ARP.
ARP requests are broadcast to all connected machines on a network, and the machine with the
correct IP/MAC address combination receives the packet. The IP/MAC associations are stored in
an ARP cache, which is cleared periodically (usually every 15 minutes) and refilled with IP/MAC
associations.
The issue with ARP requests in a direct routing LVS setup is that because a client request to an IP
address must be associated with a MAC address for the request to be handled, the virtual IP address
of the LVS system must also be associated to a MAC as well. However, since both the LVS router
and the real servers all have the same VIP, the ARP request will be broadcast ed to all the machines
associated with the VIP. This can cause several problems, such as the VIP being associated directly
to one of the real servers and processing requests directly, bypassing the LVS router completely and
defeating the purpose of the LVS setup.
To solve this issue, ensure that the incoming requests are always sent to the LVS router rather than
one of the real servers. This can be done by using either the arptables_jf or the iptables packet
filtering tool for the following reasons:
• The arptables_jf prevents ARP from associating VIPs with real servers.
• The iptables method completely sidesteps the ARP problem by not configuring VIPs on real
servers in the first place.
For more information on using arptables or iptables in a direct routing LVS environment, refer to
Section 3.2.1, “Direct Routing and arptables_jf” or Section 3.2.2, “Direct Routing and iptables”.
1.5. Persistence and Firewall Marks
In certain situations, it may be desirable for a client to reconnect repeatedly to the same real server,
rather than have an LVS load balancing algorithm send that request to the best available server.
Examples of such situations include multi-screen web forms, cookies, SSL, and FTP connections. In
these cases, a client may not work properly unless the transactions are being handled by the same
server to retain context. LVS provides two different features to handle this: persistence and firewall
marks.
1.5.1. Persistence
When enabled, persistence acts like a timer. When a client connects to a service, LVS remembers
the last connection for a specified period of time. If that same client IP address connects again within
that period, it is sent to the same server it connected to previously — bypassing the load-balancing
mechanisms. When a connection occurs outside the time window, it is handled according to the
scheduling rules in place.
Persistence also allows the administrator to specify a subnet mask to apply to the client IP address
test as a tool for controlling what addresses have a higher level of persistence, thereby grouping
connections to that subnet.
Grouping connections destined for different ports can be important for protocols which use more than
one port to communicate, such as FTP. However, persistence is not the most efficient way to deal with
Chapter 1. Linux Virtual Server Overview
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the problem of grouping together connections destined for different ports. For these situations, it is
best to use firewall marks.
1.5.2. Firewall Marks
Firewall marks are an easy and efficient way to a group ports used for a protocol or group of related
protocols. For instance, if LVS is deployed to run an e-commerce site, firewall marks can be used to
bundle HTTP connections on port 80 and secure, HTTPS connections on port 443. By assigning the
same firewall mark to the virtual server for each protocol, state information for the transaction can be
preserved because the LVS router forwards all requests to the same real server after a connection is
opened.
Because of its efficiency and ease-of-use, administrators of LVS should use firewall marks instead
of persistence whenever possible for grouping connections. However, administrators should still
add persistence to the virtual servers in conjunction with firewall marks to ensure the clients are
reconnected to the same server for an adequate period of time.
1.6. LVS — A Block Diagram
LVS routers use a collection of programs to monitor cluster members and cluster services. Figure 1.5,
“LVS Components” illustrates how these various programs on both the active and backup LVS routers
work together to manage the cluster.
Figure 1.5. LVS Components
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