Resource Management and Network Services) (2024)

A multihomed server acts as a host on multiple IP subnets. The servercan sometimes have more than one network interface card and can act as a router.IP packets, including multicast packets, are routed between the interfaces.In some situations, routing between interfaces is disabled. The followingsections describe how to configure SLP for such situations.

Multihoming Configuration

Withoutconfiguration, slpd listens for multicast and for UDP/TCP unicast on the default network interface.If unicast and multicast routingis enabled between interfaces on a multihomed machine, no additional configurationis needed. This is because multicast packets that arrive at another interfaceare properly routed to the default. As a result, multicast requests for DA or other service advertisem*ntsarrive at slpd. If routing is not turned on for some reason,configuration is required.

When to Configure for Nonrouted, Multiple Network Interfaces

If one of the following conditions exist, you might need to configuremultihomed machines.

• Unicast routing is enabled between the interfaces and multicastrouting is disabled.

• Unicast routing and multicast routing are both disabled betweenthe interfaces.

When multicastrouting is disabled between interfaces, it is usually because multicast hasnot been deployed in the network. In that situation, broadcast is normally used for servicediscovery that is notDA-based and for DA discovery on the individual subnets. Broadcast is configuredby setting the net.slp.isBroadcastOnly property to True.

Configuring Nonrouted, Multiple Network Interfaces (Task Map)

Table 20–5 Configuring Nonrouted, Multiple Network Interfaces

Configuring the net.slp.interfaces Property

If the net.slp.interfacesproperty is set, slpd listens for unicast and multicast/broadcastSLP requests on the interfaces that are listed in the property, rather thanon the default interface.

Usually, you set the net.slp.interfaces propertyin conjunction with enabling broadcast by setting the net.slp.isBroadcastOnlyproperty, because multicast has not been deployed in the network. However,if multicast has been deployed, but is not being routed on this particular multihomed host, a multicastrequest can arriveat slpd from more than one interface. This situation canoccur when the routing of packets is handled by another multihomed host orrouter that connects the subnets that are served by the interfaces.

When such a situation occurs, the SA server or the UA that is sendingthe request receives two responses from slpd on the multihomedhost. The responses are then filtered by the client libraries and the clientdoes not see them. The responses are, however, visible in the snoop trace.

Note –

If unicast routing is turned off, services that are advertisedby SA clients on multihomed hosts might not be reachable from all the subnets.If services are unreachable, SA clients can do the following:

• Advertise one service URL for each individual subnet.

• Assure that requests from a particular subnet are answeredwith a reachable URL.

The SA client librarymakes no effort to assure that reachable URLs are advertised. The serviceprogram, which might or might not handle a multihomed host with no routing,is then responsible for assuring that reachable URLs are advertised.

Before you deploy a service on a multihomed host with unicast routing disabled, use snoop to determine whether the service handles requests from multiplesubnets correctly. Furthermore, if you plan to deploy a DA on the multihomedhost, see DA Placement and Scope Name Assignment.

How to Configure the net.slp.interfaces Property

Use the following procedure to change the net.slp.interfaces property in the slp.conf file.

1. Become superuser.

2. Stop slpd and all SLP activity on the host.

   
   

   # /etc/init.d/slpd stop

3. Back up the default /etc/inet/slp.conf file beforeyou change the configuration settings.

4. Change the net.slp.interfaces property in the slpd.conf file:

   
   

   net.slp.interfaces=value

   value

   List of IPv4 addresses or host namesof the network interface cards on which the DA or SA should listen for multicast,unicast UDP, and TCP messages on port 427

   For example, a server with three network cards and multicast routingthat is turned off is connected to three subnets. The IP addresses of thethree network interfaces are 192.147.142.42, 192.147.143.42, and 192.147.144.42. The subnetmask is 255.255.255.0. The following property setting causes slpd to listen on all three interfaces for unicast and multicast/broadcastmessaging:

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   net.slp.interfaces=192.147.142.42,192.147.143.42,192.147.144.42

   Note –

   You can specify IP addresses or resolvable host names for the net.slp.interfaces property.

5. Save your changes and close the file.

6. Restart slpd to activate your changes.

   
   

   # /etc/init.d/slpd start

Proxy Advertising on Multihomed Hosts

If a host with multiple interfacesadvertises services by using slpd and proxy registration,the service URLs that are advertised by slpd must containreachable host names or addresses. If unicast routing is enabled between theinterfaces, hosts on all subnets can reach hosts on other subnets. Proxy registrationscan also be made for a service on any subnet. If, however, unicast routingis disabled, service clients on one subnet cannot reach services on anothersubnet through the multihomed host. However, those clients might be able toreach the services through another router.

For example, suppose the host with default host name bigguy has three interface cards on three different unrouted subnets.The host names on these subnets are bigguy, with IP address 192.147.142.42, bigguy1, with IP address 192.147.143.42, and bigguy2, with IP address 192.147.144.42. Now suppose that a legacy printer, oldprinter, is connected to the 143 subnet and that theURL service:printing:lpr://oldprinter/queue1 is configuredwith the net.slp.interfaces to listen on all interfaces. The oldprinter URL is proxy-advertised on all interfaces. The machineson the 142 and 144 subnets receive theURL in response to service requests, but are unable to access the oldprinter service.

The solution to this problem is to perform the proxy advertisem*nt with slpd running on a machine that is connected to the 143 subnet only, rather than on the multihomed host. Only hosts onthe 143 subnet can obtain the advertisem*nt in responseto a service request.

DA Placement and Scope Name Assignment

The placement of DAs and assignmentof scope names on a network with a multihomed host must be done carefullyto assure that clients obtain accessible services. Be particularly cautiouswhen routing is disabled and the net.slp.interfaces propertyis configured. Again, if unicast routing is enabled between the interfaceson a multihomed machine, no special DA and scope configuration is necessary.The advertisem*nts are cached with the DA identify services that are accessiblefrom any of the subnets. However, if unicast routing is disabled, poor placementof DAs can result in problems.

To see what problems can result in the previous example, consider whatwould happen if bigguy runs a DA, and clients on all subnetshave the same scopes. SAs on the 143 subnet register theirservice advertisem*nts with the DA. UAs on the 144 subnetcan obtain those service advertisem*nts, even though hosts on the 143 subnet are unreachable.

One solution to this problem is to run a DA on each subnet and not onthe multihomed host. In this situation, the net.slp.interfaces propertyon the multihomed hosts should be configured with a single interface hostname or address, or it should be left unconfigured, forcing the default interfaceto be used. A disadvantage of this solution is that multihomed hosts are oftenlarge machines that could better handle the computational load of a DA.

Another solution is to run a DA on the multihomed host, but configurescopes so that the SAs and UAs on each subnet have a different scope. Forexample, in the previous situation, UAs and SAs on the 142subnet might have a scope that is called scope142. UAsand SAs on the 143 subnet might have another scope thatis called scope143 and UAs and SAs on the 144 subnet could have third scope that is called scope144. You can configure the net.slp.interfaces propertyon bigguy with the three interfaces so that the DA servesthree scopes on the three subnets.

Considerations When Configuring for Nonrouted, Multiple Network Interfaces

Configuring the net.slp.interfacesproperty enables a DA on the multihomed host to bridge service advertisem*ntsbetween the subnets. Such configuration is useful if multicast routing isturned off in the network, but unicast routing between interfaces on a multihomedhost is enabled. Because unicast is routed between the interfaces, hostson a subnet different from the subnet on which the service is located cancontact the service when they receive the service URL. Without the DA, SAservers on a particular subnet receive only broadcasts that were made on thesame subnet, so they cannot locate services off of their subnet.

The most common situation that necessitates configuration of the net.slp.interfaces property occurs when multicast is not deployedon the network and broadcast is used instead. Other situations require carefulthought and planning to avoid unnecessary duplicate responses or unreachableservices.