NAME
balance 3.34 - A simple TCP proxy with load balancing and failover mechanisms.
SYNOPSIS
balance [ -b addr ] [ -B addr ] [ -t sec ] [ -T sec ] [ -adfpHM ] por t host1[:por t1[:maxc]] [!|%] [ ...
hostn[:por tn[:maxc]]]
balance [ -b addr ] -i [ -d ] [ -M ] por t
balance [ -b addr ] -c cmd [ -d ] [ -M ] por t
DESCRIPTION
Balance is a simple, gener ic "user land" TCP proxy, which allows simple round-robin load balanc-
ing and graceful failover between several destination servers.
Balance is available at http://balance.sourceforge.net.
Definitions: A possible destination consisting of a host address and a port is called a "channel". A
channel is member of a "channel group". Channels are numbered in a group starting with 0.
Groups are numbered starting with 0, which is the initial default group.
Balance accepts connections on the given port and forwards them to the supplied channels. At
least one channel (in the default group) must be specified. If there are two or more channels
specified in a group balance perfor ms a simple round-robin load balancing between the channels.
Balance allows the definition of further channel groups. The connection scheme wor ks as follows:
balance tr ies first to establish a connection to a channel in the first group (0), perfor ming the stan-
dard round-robin load balancing scheme. If no channel in this group is available, balance pro-
ceeds with the next higher channel group. Groups are simply separated with a "!" at the command
line at startup and can be controlled interactively with the "group" command.
A "%" instead of a "!" as a group separator declares the previous group to be of type "hash". This
means that instead of a round-robin algorithm, a hash distribution based on the client ip address
is used to determine the destination channel. This allows connecting one client always to the
same server (e.g. balancing http sessions to a single server).
Hosts may be specified either by hostname or by IP address. Por ts may be specified either by
name (as listed in /etc/services) or numer ically. If no por t is specified in a destination, the desti-
nation port defaults to the source port that balance controls.
Balance allows the specification of the maximum number of connections per channel. This
parameter can be optionally added after the port specification separated by a colon (":"). If a max-
imum number of connections is specified a channel will only be used for this maximum number of
simultaneous connections. A maxc value of 0 denotes an unlimited number of connections. This
is the initial default value of a channel.
The maximum number of groups and channels balance can handle is specified at compile time
and is initially 16 channels in 16 groups.
Failover to another destination (a "channel") occurs if the connection is refused on the current
channel or if the connect timeout is reached trying to establish a connection. If all possible desti-
nations (channels) currently fail, the client connection to balance is closed.
Balance accepts the following options:
a Enable autodisable option: A channel needs to be manually re-enabled after a failure.
b Bindhost: Balance binds to the specified host (or address) for listen() instead to
INADDR_ANY.
B Bindhost: Balance binds to the specified host (or address) for outgoing connections (the
connection will be initiated from this address).
c Command: allows to send a command to the balance master process (see interactive
mode)
d Debug: Balance outputs debugging and tracing infor mation messages on stderr.
H Hashfailover: Balance does failover to next node even if hash is used.
F Foreground: tells balance to stay in foreground. This might be useful for testing and
debugging since
M Use memory mapping for IPC instead of shared memory
balance
can be stopped in that mode using C (or other interrupt character).
i Interactive Control: Balance connects to the running instance defined by local port and
bind address via shared memory and allows to control the behaviour of it using a com-
mand line interface. The access permission using this interface are determined by the
access restrictions of the shared memory segment in effect. help or prints out a short
command overview, create allows to establish a new destination definition (channel) con-
sisting of host and port in the current group, disable disables a channel in the current
group, enable enables a channel again in the current group, group changes the current
group in interactive mode where all following commands are targeted, hash changes the
current group to be of type "Hash", help pr ints out online help infor mations, kill shuts
down the master process and exits interactive mode, maxc <channel> <maxc> sets the
maximum number of connection ot the channel (0 means infinite), mr tg-bytes <group>
<channel> pr ints out the bytes received/sent in MRTG compatible for mat (intended to be
called with -c automatically by MRTG), mr tg-conns <group> <channel> pr ints out the total
connections in MRTG compatible for mat (intended to be called with -c automatically by
MRTG), quit exits the interactive mode, reset resets the byte counters of a channel, rr
changes the current group to be of type "Round Robin", show shows an overview and the
status of all channels including the incoming and outgoing transfer volume in bytes. The
output is sorted by groups. Additionally the current connections (c) and the maximum
allowed connections (maxc) are printed, version pr ints out the version and MAXGROUPS
and MAXCHANNELS constants at compile time.
p Packetdump: Balance shows all incoming and outgoing data on stdout using a simple
always readable exter nal representation of data. This might be useful for debugging and
protocol analysis.
t Connect Timeout: the default timeout trying to establish a connection to any destination
can be changed using this option. The default timeout after which a destination is
regarded to be currently inaccessible is 5 seconds.
T Select Timeout: Timeout for select(), default = 0 (never). This feature is currently
untested.
EXAMPLES
$ balance smtp host1.test.net host2.test.net
Connection to the local SMTP port will be forwarded alterating to the SMTP port on host1
and host2. Balance runs automatically in background.
$ balance -fp imap mailserver
Connections to the local IMAP port will always be forwarded to the host "mailserver". Bal-
ance stays in foreground and all data is printed in readable for mat on stdout.
$ balance -f 8888 host1 10.1.1.1:8000
Connections to the local port 8888 are forwarded alternating to host1, port 8888 and the
host 10.1.1.1, port 8000. Balance stays in foreground connected to the "controlling tty".
$ balance imap mailserver1::16 ! mailserver2
Tw o groups are specified, each containing one channel member. First up to 16 simultane-
ous connections are forwarded to "mailserver1". As soon as they are consumed, balance
proceeds with the next group (1) which will consume all remaining connections forwarding
them to the imap ort on "mailser ver2".
$ balance pop3 host1 host2 host3 ! failover1
Balance does round robin load balancing for the three hosts in the default group 0 for
pop3 services. If all three hosts in group 0 fail, all connections are then forwarded to the
host "failover1".
$ balance telnet target.munich.net::1
Here balance is used to restrict all connections to exactly one at a time forwarding the tel-
net port.
$ balance 8888 localhost::12 ! localhost::4 ! localhost::2 localhost::2 ! localhost:25
This is a simple test, for ming 5 groups where balance is self referencing its own services
20 times. This is simply a test which definitely can be tried at home.
BUGS
In case that balance is not able to forward the connection to any destination the inital connection
to balance is always first accepted and then closed again immediately. This is not in every case
the behaviour that would have been seen directly on the destination host.
AUTHOR
Thomas Obermair, Inlab Software GmbH (obermair@acm.org)
Copyr ight (c) 2000-2005,2006 by Thomas Obermair (obermair@acm.org) and Inlab Software
GmbH (http://www.inlab.de), Gruenwald, Germany. All rights reserved.
Balance is released under the GNU GENERAL PUBLIC LICENSE, see the file COPYING in the
source code distribution.