NFS Services Administrator's Guide (B1031-90072, March 2011)

To configure multiple replicated servers for a directory, follow these steps:
1. Create and configure the /etc/netmasks file. AutoFS requires the /etc/netmasks file
to determine the subnets of local clients in a replicated multiple server environment.
The /etc/netmasks file contains IP address masks with IP network numbers. It supports both
standard subnetting as specified in RFC-950, and variable-length subnetting as specified in
RFC-1519. For standard subnetting, the file must contain a single line for each network. This
provides the network number and the network mask to be used on that network. Specify the
network number and the mask using the conventional IP dot (.) notation. The network number
must be either a class A, B, or C network number. For example:
network number netmask
128.32.0.0 255.255.255.0
For variable length subnetting, the /etc/netmasks file must contain a line for each subnet,
where the first field refers to the subnet and the second field refers to the netmask. The format
for specifying these fields is the same as that of standard subnetting.
Example of using the /etc/netmasks File
The following example shows how AutoFS uses the /etc/netmasks file to determine the local
client subnets in a multiple-server environment. In this example, servers basil and thyme
export/nfs/mount. The IP address for server basil is 15.43.232.30, and the IP address for
the server, thyme is 15.244.10.20:
# /etc/auto_direct file
/nfs_mount basil,thyme:/nfs/mount
#/etc/netmasks file on the client sage:
#network number netmask
15.43.234.210 255.255.248.0
AutoFS uses the /etc/netmasks file to determine that the masked value for the subnet of basil
and the network number is the same (15. 42. 232. 0). This shows that the client is on the same
network as basil. AutoFS then mounts /nfs/mount from basil on the local subnet.
Notes on Configuring Replicated Servers
Directories with multiple servers must be mounted as read-only to ensure that the versions remain
the same on all servers.
The server selected for the mount is the one with the highest preference, based on a sorting order.
The sorting order used gives highest preference to servers on the same local subnet. Servers on
the local network are given the second strongest preference. As a result, if you configure multiple
servers on both sides of a gateway, a server on the same side of the gateway as the NFS client is
always preferred. For multiple servers outside the local network, and with no weighting factors
assigned, the server with the lowest response time is used for the mount.
Multiple servers provide users with reliable access to a mounted directory. If one server is down,
the directory can be mounted from another. Moreover, multiple servers provide some load balancing
across the network; a server that is not busy responds more quickly to an AutoFS poll than one
that is heavily loaded. The directory is mounted from the server that is not busy.
If the list of multiple servers contains a combination of servers that includes all versions of the NFS
protocol, then AutoFS selects a subset of servers with the highest NFS protocol version configured.
For example, a list contains a number of servers configured with the NFSv4 protocol, and a few
servers configured with the NFSv2 protocol. AutoFS will use the subnet of servers configured with
the NFSv4 protocol, unless a server configured with the NFSv2 protocol is closer.
Advanced AutoFS Administration 71