Netmask On Aliases Overriden By Netmask On Interface

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CentOS 2 Comments

CentOS-6.10

We have a host with the following ifcfg file contents:

BOOTPROTO=none BROADCAST=””
DEFROUTE=yes DEVICE=eth1
. . . GATEWAY=X.Y.Z.234
IPADDR=A.B.C.2
IPV4_FAILURE_FATAL=yes NAME=”LAN Link – eth1″
NETMASK=”255.255.255.128″
NETWORK=”A.B.C.0″
NM_CONTROLLED=no ONBOOT=yes PREFIX%
TYPE=Ethernet USERCTL=no

And an aliased ifcfg containing this:

BOOTPROTO=”none”
BROADCAST=”192.168.8.255″
DEVICE=”eth1:192008001″
IPADDR=”192.168.8.1″
IPV6INIT=”no”
NETMASK=”255.255.255.0″
NETWORK=”192.168.8.0″
ONPARENT=”yes”

However, ifconfig shows this:

# ifconfig eth1:192008001
eth1:192008001 Link encap:Ethernet HWaddr 00:25:90:61:74:C1
inet addr:192.168.8.1 Bcast:192.168.8.255
Mask:255.255.255.128
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:17 Memory:feae0000-feb00000

Which shows that the network mask is determined by the interface mask and is not overridden by the alias definition.

Is this expected behaviour? Does this mean that a particular physical interface cannot belong to more than one network, or at least not to networks having differing cidr masks?

2 thoughts on - Netmask On Aliases Overriden By Netmask On Interface

  • Interface aliases are evil from my point of view. I recommend to configure the ip directly to the interface.

    #ifcfg-eth2
    […]
    IPADDR2.168.200.1
    NETMASK%5.255.255.0
    IPADDR22.168.201.1
    NETMASK2%5.255.255.192

    ip addr show dev eth2
    4: eth2: mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:b0:c5:7c brd ff:ff:ff:ff:ff:ff
    inet 192.168.200.1/24 brd 192.168.200.255 scope global eth2
    inet 192.168.201.1/26 brd 192.168.201.63 scope global eth2
    inet6 fe80::a00:27ff:feb0:c57c/64 scope link
    valid_lft forever preferred_lft forever

    Best regards Ulf

  • Just a cautionary note, if you use snmpd you may start seeing regular “ia_addr insert” errors using this approach depending on your version of snmpd and how reporting is configured.

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