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240 Chapter 18. Network File System (NFS)

to the /etc/exports ﬁle to uncover that host’s privileges for the various mounts available. After

granting access, any ﬁle and directory operations are sent to the server using remote procedure calls.

Warning

NFS mount privileges are granted speciﬁcally to a host, not a user. If you grant a host access to a

particular part of your hard drive with NFS, users of that machine will have access to your shared

data.

When conﬁguring the /etc/exports ﬁle, be extremely careful when sharing directories with read-

write permissions (rw) to a remote host. Users of remote systems mounting your export will be able

to modify data in the exported ﬁlesystem.

18.1.1. NFS and portmap

NFS relies upon remote procedure calls (RPC) to function. portmap is required to map RPC requests

to the correct services. RPC processes notify portmap when they start, revealing the port number they

are monitoring and the RPC program numbers they expect to serve. The client system then contacts

portmap on the server with a particular RPC program number. portmap then redirects the client to

the proper port number to communicate with its intended service.

Because RPC-based services rely on portmap to make all connections with incoming client requests,

portmap must be available before any of these services start. If, for some reason, the portmap service

unexpectedly quits, restart portmap and any services running when it was started.

The portmap service can be used with the host access ﬁles (/etc/hosts.allow and

/etc/hosts.deny) to control which remote systems are permitted to use RPC-based services on

your machine. See Chapter 9 for more information. Access control rules for portmap will affect all

RPC-based services. Alternatively, you can specify each of the NFS RPC daemons to be affected

by a particular access control rule. The man pages for rpc.mountd and rpc.statd contain

information regarding the precise syntax of these rules.

18.1.1.1. portmap Status

As portmap provides the coordination between RPC services and the port numbers utilized to com-

municate with them, it is useful to be able to get a picture of the current RPC services using portmap

when troubleshooting. The rpcinfo command shows each RPC-based service with its port number,

RPC program number, version, and IP protocol type (TCP or UDP).

To make sure the proper NFS RPC-based services are enabled for portmap, rpcinfo can be useful:

[root@bleach /]# rpcinfo -p some.machine.com

program vers proto port

100000 2 tcp 111 portmapper

100000 2 udp 111 portmapper

100024 1 udp 1024 status

100024 1 tcp 1024 status

100011 1 udp 819 rquotad

100011 2 udp 819 rquotad

100005 1 udp 1027 mountd

100005 1 tcp 1106 mountd

100005 2 udp 1027 mountd

100005 2 tcp 1106 mountd

100005 3 udp 1027 mountd

100005 3 tcp 1106 mountd

100003 2 udp 2049 nfs

1 2 ... 235 236 237 238 239 240 241 242 243 244 245 ... 281 282

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