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COROSYNC_OVERVIEW(8)       Corosync Cluster Engine Programmer's Manual       COROSYNC_OVERVIEW(8)



NAME
       corosync_overview - Corosync overview

OVERVIEW
       The corosync project is a project to implement a production quality "Revised BSD" licensed
       implementation of the most recent SA Forum's  Application  Interface  Specification.   The
       Application  Interface  Specification  is  a  software  API and policies which are used to
       develop applications that maintain service during faults.  The API consists of  Availabil‐
       ity  Management  Framework  (AMF)  which provides application failover, Cluster Membership
       (CLM), Checkpointing (CKPT), Eventing (EVT),  Messaging  (MSG),  and  Distributed  Locking
       (DLOCK).

       Currently Messaging is unimplemented.

       Faults occur for various reasons:

       * Application Faults

       * Middleware Faults

       * Operating System Faults

       * Hardware Faults

       The  major focus of high availability in the past has been to mask hardware faults. Faults
       in other components of the system have gone unsolved until AIS. AIS can mask many types of
       faults  in  applications,  middleware,  operating systems, or even hardware by providing a
       simple framework for allowing developers to create redundant applications. These redundant
       applications  can  be  distributed  over  multiple nodes such that if any one node faults,
       another node can recover.

       Application programmers develop applications to periodically record their state using  the
       checkpointing  service.  When  an active application fails, a standby application recovers
       the state of the application. This technique, called stateful application  failover,  pro‐
       vides  the fundamental difference between corosync and other systems that have come before
       it.  With stateful application failover, the end-application user doesn't have  to  reload
       the  application or redial a telephone. The full state is recorded, so the end-application
       user sees no interruption in service.

       Because programmers can now distribute applications across multiple processes or nodes,  a
       mechanism must exist for them to communicate.  This mechanism is provided by two services.
       The event service provides a publish/subscribe model for  events.  The  messaging  service
       provides  end  to  end messaging. Finally a mechanism to synchronize access is provided by
       the distributed lock service.

       The corosync project also provides a group messaging toolkit called EVS.  The EVS  service
       implements  a  messaging model known as Extended Virtual Synchrony.  This model allows one
       sender to transmit to many receivers.  Certain guarantees are  provided  for  message  and
       membership delivery which make virtual synchrony ideal for developing distributed applica‐
       tions.


QUICKSTART
       The corosync executive must be configured.  In the directory conf in the source  distribu‐
       tion are several files that must be copied to the /etc/corosync directory.  If corosync is
       packaged by a distro, this may be complete.

       The directory contains the file corosync.conf.  Please read the corosync.conf(5) man  page
       for  details  on the configuration options.  The corosync project will work out of the box
       with the default configuration options, although the administrator  may  desire  different
       options.

       The corosync executive uses cryptographic techniques to ensure authenticity and privacy of
       the messages.  In order for corosync to be secure and operate, a private key must be  gen‐
       erated and shared to all processors.

       First generate the key on one of the nodes:

       unix# corosync-keygen
       Corosync Cluster Engine Authentication key generator.
       Gathering 1024 bits for key from /dev/random.
       Press keys on your keyboard to generate entropy.
       Writing corosync key to /etc/corosync/authkey.

       After  this operation, a private key will be in the file /etc/corosync/authkey.  This pri‐
       vate key must be copied to every processor in the cluster.  If the private key  isn't  the
       same  for  every  node, those nodes with nonmatching private keys will not be able to join
       the same configuration.

       Copy the key to some security transportable storage or use ssh to transmit  the  key  from
       node to node.  Then install the key with the command:

       unix#:    install    -D    --group=0    --owner=0   --mode=0400   /path_to_authkey/authkey
       /etc/corosync/authkey

       If a message "Invalid digest" appears from the corosync executive, the keys are  not  con‐
       sistent between processors.

       Finally  run the corosync executive.  If corosync is packaged from a distro, it may be set
       to start on system start.  It may also be turned off by default in  which  case  the  init
       script for corosync must be enabled.

       After  running  aisexec, a list of all processors IP addresses running the corosync execu‐
       tive and configured on the same multicast address will  appear.   If  they  don't  appear,
       there may be a problem with multicast in the distro or hardware.  If this happens, partic‐
       ipation in the corosync mailing list may help solve the problem.   The  email  address  is
       openais AT lists.org.


USING LIBRARIES
       The  corosync  AIS  libraries  have header files which must be included in the developer's
       application.  Once the header file is included, the developer can reference the AIS inter‐
       faces.

       The    corosync    project   recommends   to   distros   to   place   include   files   in
       /usr/include/corosync.  The following include lines must be added to  the  application  to
       use each of the following services:

       #include <corosync/saClm.h> For the Cluster Membership B.01.01 service.

       #include <corosync/saCkpt.h> For the Checkpointing B.01.01 service.

       #include <corosync/saEvt.h> For the Eventing B.01.01 service.

       #include <corosync/ais_amf.h> For the AMF A.01.01 service.

       The  corosync  project recommends to distros to place library files in /usr/lib.  The fol‐
       lowing link lines must be added to the LDFLAGS section of the makefile.

       -lsaClm For the Cluster Membership B.01.01 service

       -lsaCkpt For the Checkpointing B.01.01 service

       -lsaEvt For the Eventing B.01.01 service

       -lsaAmf For the AMF A.01.01 service

       -lais Specify this to get access to all AIS  libraries  without  specifying  each  library
       individually.


IPv6
       The  corosync  project  supports both IPv4 and IPv6 network addresses.  The entire cluster
       must use either IPv4 or IPv6 for the cluster communication mechanism.   In  order  to  use
       IPv6, IPv6 addresses must be specified in the bindnetaddr and mcastaddr fields in the con‐
       figuration file.  The nodeid field must also be set.

       An example of this is: nodeid: 2 bindnetaddr: fec0::1:a800:4ff:fe00:20 mcastaddr: ff05::1

       To configure a host for IPv6, use the ifconfig program to add interfaces: box20:  ifconfig
       eth0 add fec0::1:a800:4ff:fe00:20/64 box30: ifconfig eth0 add fec0::1:a800:4ff:fe00:30/64

       If  the  /64  is  not specified, a route for the IPv6 network will not be configured which
       will cause significant problems.  Make sure a route is available for IPv6 traffic.


ARCHITECTURE
       The AIS libraries are a thin IPC interface to the corosync executive.  The corosync execu‐
       tive  provides  services  for  the  SA  Forum  AIS  libraries  as  well as the EVS and CPG
       libraries.

       The corosync executive uses the Totem extended virtual synchrony protocol.  The  advantage
       to the end user is excellent performance characteristics and a proven protocol with excel‐
       lent reliability.  This protocol connects the processors in a  configuration  together  so
       they may communicate.


ENVIRONMENT VARIABLES
       The  corosync  executive process uses four environment variables during startup.  If these
       environment variables are not set, defaults will be used.


       COROSYNC_MAIN_CONFIG_FILE
              This specifies the fully qualified path to the corosync configuration file.

              The default is /etc/corosync/corosync.conf.


       COROSYNC_AMF_CONFIG_FILE
              This specifies the fully qualified path to  the  corosync  Availability  Management
              Framework configuration file.

              The default is /etc/corosync/amf.conf.


       COROSYNC_DEFAULT_CONFIG_IFACE
              This specifies the LCRSO that is used to parse the configuration file.  This allows
              other configuration file parsers to be implemented within the system.

              The default is to use the default corosync configuration file parser  which  parses
              the format specified in corosync.conf (5).


       COROSYNC_TOTEM_AUTHKEY_FILE
              This  specifies the fully qualified path to the shared key used to authenticate and
              encrypt data used within the Totem protocol.

              The default is /etc/corosync/authkey.


SECURITY
       The corosync executive optionally encrypts all messages sent over the  network  using  the
       SOBER-128  stream  cipher.   The corosync executive uses HMAC and SHA1 to authenticate all
       messages.  The corosync executive library uses SOBER-128 as a pseudo random number genera‐
       tor.  The EVS library feeds the PRNG using the /dev/random Linux device.

       If membership messages can be captured by intruders, it is possible to execute a denial of
       service attack on the cluster.  In this scenario, the cluster is  likely  already  compro‐
       mised and a DOS attack is the least of the administration's worries.

       The  security  in  corosync  does  not  offer perfect forward secrecy because the keys are
       reused.  It may be possible for an intruder by capturing packets in an  automated  fashion
       to  determine  the shared key.  No such automated attack has been published as of yet.  In
       this scenario, the cluster is likely already compromised to allow the long-term capture of
       transmitted data.

       For  security  reasons,  the corosync executive binary should NEVER be setuid or setgid in
       the filesystem.


SAFTEST COMPLIANCE
       The corosync libraries are now nearly compliant with every aspect of the  SA  Forum's  AIS
       specification.  The AMF service, however, is not compliant with the B.01.01 specification.
       The remaining services pass most of the tests of the saftest  suite  against  the  B.01.01
       specification.


BUGS
       The  messaging service is partially implemented and not suitable for deployment.  The dis‐
       tributed locking service is buggy and not suitable for deployment.  The Availability  Man‐
       agement Framework is under development and not suitable for deployment..


SEE ALSO
       corosync.conf(5), corosync-keygen(8), evs_overview(8)

corosync Man Page                           2006-05-10                       COROSYNC_OVERVIEW(8)


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