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netmask on aliases overriden by netmask on interface

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=25
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?

Comments

Re: netmask on aliases overriden by netmask on interfac

By Ulf Volmer at 02/08/2019 - 12:48

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

#ifcfg-eth2
[...]
IPADDR=192.168.200.1
NETMASK=255.255.255.0
IPADDR2=192.168.201.1
NETMASK2=255.255.255.192

ip addr show dev eth2
4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> 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

Re: netmask on aliases overriden by netmask on interfac

By Leroy Tennison at 02/08/2019 - 13:30

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.

Leroy Tennison
Network Information/Cyber Security Specialist
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Interface aliases are evil from my point of view. I recommend to
configure the ip directly to the interface.

#ifcfg-eth2
[...]
IPADDR=192.168.200.1
NETMASK=255.255.255.0
IPADDR2=192.168.201.1
NETMASK2=255.255.255.192

ip addr show dev eth2
4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> 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