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nss-mymachines, - Hostname resolution for local container instances



nss-mymachines is a plug-in module for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc), providing hostname resolution for the names of containers running locally that are registered with systemd-machined.service(8). The container names are resolved to the IP addresses of the specific container, ordered by their scope. This functionality only applies to containers using network namespacing (see the description of --private-network in systemd-nspawn(1)). Note that the name that is resolved is the one registered with systemd-machined, which may be different than the hostname configured inside of the container.

Note that this NSS module only makes available names of the containers running immediately below the current system context. It does not provide host name resolution for containers running side-by-side with the invoking system context, or containers further up or down the container hierarchy. Or in other words, on the host system it provides host name resolution for the containers running immediately below the host environment. When used inside a container environment however, it will not be able to provide name resolution for containers running on the host (as those are siblings and not children of the current container environment), but instead only for nested containers running immediately below its own container environment.

To activate the NSS module, add "mymachines" to the line starting with "hosts:" in /etc/nsswitch.conf.

It is recommended to place "mymachines" before the "resolve" or "dns" entry of the "hosts:" line of /etc/nsswitch.conf in order to make sure that its mappings are preferred over other resolvers such as DNS.


Here is an example /etc/nsswitch.conf file that enables nss-mymachines correctly:

passwd:         compat systemd
group:          compat [SUCCESS=merge] systemd
shadow:         compat systemd
gshadow:        files systemd
hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns
networks:       files
protocols:      db files
services:       db files
ethers:         db files
rpc:            db files
netgroup:       nis


The container "rawhide" is spawned using systemd-nspawn(1):

# systemd-nspawn -M rawhide --boot --network-veth --private-users=pick
Spawning container rawhide on /var/lib/machines/rawhide.
Selected user namespace base 20119552 and range 65536.
$ machinectl --max-addresses=3
rawhide container systemd-nspawn fedora 30 fe80::94aa:3aff:fe7b:d4b9
$ ping -c1 rawhide
PING rawhide(fe80::94aa:3aff:fe7b:d4b9%ve-rawhide (fe80::94aa:3aff:fe7b:d4b9%ve-rawhide)) 56 data bytes
64 bytes from fe80::94aa:3aff:fe7b:d4b9%ve-rawhide (fe80::94aa:3aff:fe7b:d4b9%ve-rawhide): icmp_seq=1 ttl=64 time=0.045 ms
$ ping -c1 -4 rawhide
PING rawhide ( 56(84) bytes of data.
64 bytes from ( icmp_seq=1 ttl=64 time=0.064 ms
# machinectl shell rawhide /sbin/ip a
Connected to machine rawhide. Press ^] three times within 1s to exit session.
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
2: host0@if21: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 96:aa:3a:7b:d4:b9 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet brd scope link host0
       valid_lft forever preferred_lft forever
    inet6 fe80::94aa:3aff:fe7b:d4b9/64 scope link
       valid_lft forever preferred_lft forever
Connection to machine rawhide terminated.


systemd(1), systemd-machined.service(8), machinectl(1), nss-systemd(8), nss-resolve(8), nss-myhostname(8), nsswitch.conf(5), getent(1)
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