nss-mymachines

systemd
nss-mymachines 8
nss-mymachines libnss_mymachines.so.2 Provide hostname resolution for local container instances.

`libnss_mymachines.so.2`

## Description

`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.

The module also provides name resolution for user and group identifiers mapped to containers. All names from the range allocated to a given container container are exposed on the host as `vu-container-uid` and `vg-container-gid` (see example below). This functionality only applies to containers using user namespacing (see the description of `--private-users` in systemd-nspawn 1).

To activate the NSS module, add `mymachines` to the lines starting with `hosts:`, `passwd:` and `group:` in `/etc/nsswitch.conf`.

It is recommended to place `mymachines` after the `files` or `compat` entry of the `/etc/nsswitch.conf` lines to make sure that its mappings are preferred over other resolvers such as DNS, but so that `/etc/hosts`, `/etc/passwd` and `/etc/group` based mappings take precedence.

## Configuration in `/etc/nsswitch.conf`

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

    passwd:         compat mymachines systemd
    group:          compat mymachines systemd
    shadow:         compat

    hosts:          files mymachines resolve [!UNAVAIL=return] dns myhostname
    networks:       files

    protocols:      db files
    services:       db files
    ethers:         db files
    rpc:            db files

    netgroup:       nis

## Mappings provided by `nss-mymachines`

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
    MACHINE CLASS     SERVICE        OS     VERSION ADDRESSES
    rawhide container systemd-nspawn fedora 30      169.254.40.164 fe80::94aa:3aff:fe7b:d4b9

    $ getent passwd vu-rawhide-0 vu-rawhide-81
    vu-rawhide-0:*:20119552:65534:vu-rawhide-0:/:/usr/sbin/nologin
    vu-rawhide-81:*:20119633:65534:vu-rawhide-81:/:/usr/sbin/nologin

    $ getent group vg-rawhide-0 vg-rawhide-81
    vg-rawhide-0:*:20119552:
    vg-rawhide-81:*:20119633:

    $ ps -o user:15,pid,tty,command -e|grep '^vu-rawhide'
    vu-rawhide-0      692 ?        /usr/lib/systemd/systemd
    vu-rawhide-0      731 ?        /usr/lib/systemd/systemd-journald
    vu-rawhide-192    734 ?        /usr/lib/systemd/systemd-networkd
    vu-rawhide-193    738 ?        /usr/lib/systemd/systemd-resolved
    vu-rawhide-0      742 ?        /usr/lib/systemd/systemd-logind
    vu-rawhide-81     744 ?        /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
    vu-rawhide-0      746 ?        /usr/sbin/sshd -D ...
    vu-rawhide-0      752 ?        /usr/lib/systemd/systemd --user
    vu-rawhide-0      753 ?        (sd-pam)
    vu-rawhide-0     1628 ?        login -- zbyszek
    vu-rawhide-1000  1630 ?        /usr/lib/systemd/systemd --user
    vu-rawhide-1000  1631 ?        (sd-pam)
    vu-rawhide-1000  1637 pts/8    -zsh

    $ 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 (169.254.40.164) 56(84) bytes of data.
    64 bytes from 169.254.40.164 (169.254.40.164): 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 169.254.40.164/16 brd 169.254.255.255 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.

## See Also

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