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I have a server which shows following three "secondary" IPv4 addresses in ip addr show output under physical interface eth0 section:

inet 10.10.1.26/32 scope global eth0:vpn
inet 10.10.1.19/32 scope global eth0:webmail
inet 10.10.1.20/32 scope global eth0:wiki

Those eth0:vpn, eth0:webmail and eth0:wiki interfaces are not seen in ip link show output as VLAN-interfaces(for example eth0.123) would. What are those eth0:vpn, eth0:webmail and eth0:wiki interfaces?

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    Those are interface aliases. Basically they allow a single machine to have more than one IP in the same network. Those are useful in many situations, such as running an authoritative DNS resolver on an IP and a recursive one on a different IP, or temporarily moving services to a different machine without reconfiguring an entire LAN when a main server crashes. – lcd047 May 11 '15 at 10:03
  • @lcd047 I see. Am I correct that basically those "interface aliases" are like "secondary addresses" in Cisco IOS terminology, i.e. simply allow one interface to have more than one IP address? And still, what is the reason for those labels eth0:vpn, eth0:webmail and eth0:wiki? – Martin May 11 '15 at 10:57
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    Am I correct that basically those "interface aliases" are like "secondary addresses" in Cisco IOS terminology - Yes, exactly like that. And still, what is the reason for those labels eth0:vpn, eth0:webmail and eth0:wiki? - They are just labels assigned by the sysadmin. I suppose the point is to make it easier to tell which service is associated to which alias. There isn't anything special (AFAICT) about the names themselves. – lcd047 May 11 '15 at 11:05
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Copied from my comments above:

Those are interface aliases. Basically they allow a single machine to have more than one IP in the same network. Those are useful in many situations, such as running an authoritative DNS resolver on an IP and a recursive one on a different IP, or temporarily moving services to a different machine without reconfiguring an entire LAN when a main server crashes.

Am I correct that basically those "interface aliases" are like "secondary addresses" in Cisco IOS terminology

Yes, exactly like that.

And still, what is the reason for those labels eth0:vpn, eth0:webmail and eth0:wiki?

They are just labels assigned by the sysadmin. I suppose the point is to make it easier to tell which service is associated to which alias. There isn't anything special (AFAICT) about the names themselves.

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