jails thinjails nullfs and additional mount points in jails

toddg · 2026-08-10T18:47:23+0100

I'm following the handbook instructions for nullfs thin jails here.
I'd like to mount additional directories in my jails like so:

Code:

Jail1 :
/host/A -> /a

Jail2 :
/host/A -> /a
/host/B -> /b

But these mounts are particular to the jails, e.g. Jail2 doesn't want or need /host/B. So I don't want to define the mount points /a and /b in the base template. Esp as I will have many different types of jails, and they will each need different mount directories.

Is this possible? As I understand it, the mount points must exist inside the jail before the system will mount a directory to them. But it seems that 'mkdir -p /jails/nullfs/loadbalancer_lb_10_0_0_2/opt/config/caddy' (or 'mkdir -p /jails/containers/loadbalancer_lb_10_0_0_2/opt/config/caddy') is not sufficient or correct...

Here's an example jail conf that's failing:

/etc/jail.conf.d/loadbalancer_lb_10_0_0_2.jail

Code:

loadbalancer_lb_10_0_0_2 {

    # ----------------------------------------
    # networking
    # ----------------------------------------
    $ipaddr =  "10.0.0.2";
    $gw     =  "10.0.0.1";

    # ----------------------------------------
    # thinjail mounts
    # ----------------------------------------
    mount += "/jails/templates/RELEASE-15.0-base /jails/nullfs/loadbalancer_lb_10_0_0_2 nullfs ro 0 0";
    mount += "/jails/containers/loadbalancer_lb_10_0_0_2 /jails/nullfs/loadbalancer_lb_10_0_0_2/skeleton nullfs rw 0 0";

    # ----------------------------------------
    # jail specific mounts
    # ----------------------------------------
    # these are failing to mount. I've verified both source and target dirs exist. I have also tried creating these directories and
    # mounting them from the /jails/containers directory. Those also failed to mount.
    mount += "/opt/config/global/caddy /jails/nullfs/loadbalancer_lb_10_0_0_2/opt/config/caddy nullfs rw 0 0";
    mount += "/opt/data/global/caddy/db /jails/nullfs/loadbalancer_lb_10_0_0_2/var/db/caddy nullfs rw 0 0";
    mount += "/opt/config/global/caddy/www /jails/nullfs/loadbalancer_lb_10_0_0_2/var/www nullfs rw 0 0";
 
    # ----------------------------------------
    # jail dependencies
    # ----------------------------------------
    #depend="";
}

Errors

Code:

toddg@fbsdlocal ~/repos/biz/freebsd/cloud $ sudo service jail start
Starting jails:jail: loadbalancer_lb_10_0_0_2: mount: /jails/nullfs/loadbalancer_lb_10_0_0_2/opt: No such file or directory

System

Code:

toddg@fbsdlocal ~/repos/biz/freebsd/cloud $ tree /jails/nullfs/loadbalancer_lb_10_0_0_2/
/jails/nullfs/loadbalancer_lb_10_0_0_2/
├── opt
│   └── config
│       └── caddy
└── var
    ├── db
    │   └── caddy
    └── www

8 directories, 0 files

So the directories exist.

I've also tried the same thing but instead using the /jails/containers/loadbalancer_lb_10_0_0_2 directory instead of /jails/nullfs/loadbalancer_lb_10_0_0_2

I had everything working using fat jails, so I could always go back to that.