Changing The Network Parameters Of The Head Node

6.1 Introduction

After a cluster physically arrives at its site, the administrator often has to change the network settings to suit the site. Details on this are given in section 3.2.1 of the Administrator Manual. However, it relies on understanding the material leading up to that section.

This chapter is therefore a quickstart document—conveniently a mere 3 pages—explaining how to change basic IPv4 network settings while assuming no prior knowledge of NVIDIA Base Command Manager and its network configuration interface.

6.2 Method

A cluster consists of a head node, say basecm10 and one or more regular nodes. The head node of the cluster is assumed to face the internal network (the network of regular nodes) on one interface, say eth0. The external network leading to the internet is then on another interface, say eth1. This is referred to as a type 1 configuration in this manual (section 3.3.9).

Typically, an administrator gives the head node a static external IP address before actually connecting it up to the external network. This requires logging into the physical head node with the vendor-supplied root password. The original network parameters of the head node can then be viewed and set. For example for eth1:

# cmsh -c "device interfaces basecm10; get eth1 dhcp"
yes

Here, yes means the interface accepts DHCP server-supplied values.

Disabling DHCP acceptance allows a static IP address, for example 192.168.1.176, to be set:

# cmsh -c "device interfaces basecm10; set eth1 dhcp no"
# cmsh -c "device interfaces basecm10; set eth1 ip 192.168.1.176; commit"
# cmsh -c "device interfaces basecm10; get eth1 ip"
192.168.1.176

Other external network parameters can be viewed and set in a similar way, as shown in table 6.1. A reboot implements the networking changes.

Table 6.1: External Network Parameters And How To Change Them On The Head Node

* If address is set to 0.0.0.0 then the value offered by the DHCP server on the external network is accepted
** Space-separated multiple values are also accepted for these parameters when setting the value for address or hostname.

6.3 Terminology

A reminder about the less well-known terminology in the table:

• netmaskbits is the netmask size, or prefix-length, in bits. In IPv4’s 32-bit addressing, this can be up to 31 bits, so it is a number between 1 and 31. For example: networks with 256 (2⁸) addresses (i.e. with host addresses specified with the last 8 bits) have a netmask size of 24 bits. They are written in CIDR notation with a trailing “/24”, and are commonly spoken of as “slash 24” networks.

• baseaddress is the IP address of the network the head node is on, rather than the IP address of the head node itself. The baseaddress is specified by taking netmaskbits number of bits from the IP address of the head node. Examples:

  • A network with 256 (2⁸) host addresses: This implies the first 24 bits of the head node’s IP address are the network address, and the remaining 8 bits are zeroed. This is specified by using “0” as the last value in the dotted-quad notation (i.e. zeroing the last 8 bits). For example: 192.168.3.0

  • A network with 128 (2⁷) host addresses: Here netmaskbits is 25 bits in size, and only the last 7 bits are zeroed. In dotted-quad notation this implies “128” as the last quad value (i.e. zeroing the last 7 bits). For example: 192.168.3.128.

When in doubt, or if the preceding terminology is not understood, then the values to use can be calculated using the head node’s sipcalc utility. To use it, the IP address in CIDR format for the head node must be known.

When run using a CIDR address value of 192.168.3.130/25, the output is (some output removed for clarity):

# sipcalc 192.168.3.130/25
Host address            - 192.168.3.130
Network address         - 192.168.3.128
Network mask            - 255.255.255.128
Network mask (bits)     - 25
Broadcast address       - 192.168.3.255
Addresses in network    - 128
Network range           - 192.168.3.128 - 192.168.3.255

Running it with the -b (binary) option may aid comprehension:

# sipcalc -b 192.168.3.130/25
Host address            - 11000000.10101000.00000011.10000010
Network address         - 11000000.10101000.00000011.10000000
Network mask            - 11111111.11111111.11111111.10000000
Broadcast address       - 11000000.10101000.00000011.11111111
Network range           - 11000000.10101000.00000011.10000000 -
                          11000000.10101000.00000011.11111111