Managing NFS and NIS, 2nd Edition - Mike Eisler [269]
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[5] The ability to forcibly unmount a filesystem was introduced in Solaris 8. This feature is supported by the Linux kernel 2.1.116 or later. Previously, you would have had to reboot the NFS client to clear the stale filehandles.
Appendix A. IP Packet Routing
Routers and gateways join multiple IP networks, forwarding packets between the networks. A single organization may have multiple IP networks because it has multiple buildings, multiple sites, or multiple subgroups that require their own networks. For example, the history and math departments at a university are likely to each have their own IP networks, just as an engineering and manufacturing facility separated by several miles will have independent networks. Section 17.2 discussed network partitioning using routers, and some of the performance considerations when running NFS and NIS in an internetworked environment. This appendix explores the mechanics of IP packet routing in greater detail.
Routers and their routing tables
A router has a unique IP address on each network interface; associated with each IP address is also a unique hostname. A common convention is to add a suffix associated with the network number to the name of the host used on the each network interface as shown in this /etc/hosts fragment:[1]
#
# local network hosts
#
192.9.200.1 fred fred-200
192.9.200.2 barney
192.9.200.3 wilma
#
# remote network gateway
192.9.201.1 fred-201
Host fred is on both the 192.9.200.0 and 192.9.201.0 networks, and has a distinct name and address on each. netstat -i shows both interfaces and their associated networks and hostnames:
% netstat -i
Name Mtu Net/Dest Address Ipkts Ierrs Opkts Oerrs Collis Queue
hem0 1500 192.9.200.0 fred 349175 104 542039 363 816 0
hme1 1500 192.9.201.0 fred-201 108635 1 4020 22 301 0
lo0 8232 loopback localhost 74347 0 74347 0 0 0
To send a packet to another network, the local host needs some picture of the network and its connections to other networks. Ideally, this picture presents other networks as a "black box" outside of some local gateway, rather than an itemization of a route to every host on every attached network. This paradigm is how we view the U.S. Post Office. Once you drop a letter in the mailbox, the route it takes may involve trucks, planes, or people, and the decisions about routing vehicles are left up to the people doing the delivery.
A host's picture of the local network's connections to other IP networks is contained in the kernel's routing table. This table may be modified in three ways:
Dynamic routing information is sent periodically by routers that advertise themselves using some well-known protocols, and daemons such as in.routed send and interpret route announcements and update the routing table.[2]
Static routing involves hand-crafting a route table. Static routing is typically used when there is only one router on a network, so a single route suffices for all outbound traffic. Client machines often set up static routing to avoid having to listen to the regular route information broadcasts (see Section A.2 later in this appendix).
Route redirection requests are sent by routers that are asked to forward packets to networks for which the chosen router is not the best choice. These route table updates are sent in ICMP redirect messages.
The routing table determines how to get to foreign IP networks. You can examine the current routing table using netstat -r :
% netstat -r
Routing tables
Destination Gateway Flags Refcnt Use Interface
131.40.191.1 gatehost UGH 0 0 hme0
131.40.56.0 gatehost UG 0 0 hme0
131.40.208.0