Hacker News Reader

Comment by tannhaeuser | Hacker News Reader

Isn't the described mismatch between "reality" and what ifconfig reports once again a case of systemd (networkd) not exposing IP aliases in the traditional way, and replacing established network concepts by it's own stuff? Meant as a genuine question rather than trolling (though I know I'd hate to bow to whatever systemd comes up with rather than just using ifconfig which has worked since the begin of times) eg. there might be a material problem here in how network interfaces are presented by ifconfig.

View on HN

No. The Linux kernel has two APIs, and has had since before systemd even existed. ifconfig uses the ioctl() API, which presents the concept of alias IP addresses in one way. ip uses the netlink API, which presents the concept of multiple IP addresses per interface in a different way. M. Siebenmann did include this point.

* https://sourceforge.net/p/net-tools/bugs/12/

* https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=359676

This begs the question of why the kernel doesn't allocate an alias for backwards compatibility when adding IP adresses through the netlink API.

I think they are fundamentally different things. Correct me if I’m wrong, but I think a netlink alias is just an additional address and an old-school alias is more like a virtual interface.

No. systemd has nothing to do with this. the issue has existed long before systemd. it's the kernel that changed to allow interfaces to have alias addresses.

Fortunately, not in this particular case - it's just a question of incompatibility between the two APIs. If you create an alias by ifconfig eth0:0, it will be visible by ifconfig. If you add a NIC:n entry to /etc/network/interfaces, ifconfig will display it. I don't believe this API will ever get obsoleted as millions of systems use it that way.

Not that I know. Systemd itself doesn't manage networking (resolved, networkd and other services do that instead under the SD umbrella).

But those work (to my knowledge) just like before (unless you setup private networking or otherwise container the thing). They just work differently on the surface.

I wouldn't know of a way of exposing IP aliases in a non-traditional way as the kernel needs to know them either way.

Reply on news.ycombinator.com