Solving a FreeBSD Jails issue: interface already exists

For a long time after I rebuilt my jails host, I could not restart a certain number of jails due to an “interface already exists” error. For the life of me I could not make sense of it, The services running in the jails were useful but not required so I put off tinkering with it. I thought that I would magically stumble into the solution in my sleep or something equally silly.

watermelon# service jail start gitea
Starting jails: cannot start jail  "gitea":
ifconfig: interface epair14 already exists
jail: gitea: ifconfig epair14 create up: failed
.
watermelon# service jail stop gitea
Stopping jails:.

What perplexed me about this issue is that I would run ifconfig epair14a after the failure to start the jail, and the interface would be there. “Surely this must be a FreeBSD bug!”

The “eureka!” happened earlier today, not while I was sleeping, but rather while I was solving other problems. “I bet there’s something fishy in the configuration, I should just rewrite it” I thought to myself. Most esoteric bugs are not bugs with the compiler, libraries, or operating systems. Usually they’re the user doing something slightly stupid and not realizing it.

My jail configuration (/etc/jail.conf) resembled the following:

gitea {
        $id = "14";
        $ip_addr = "10.10.10.${id}";

        vnet.interface = "epair${id}b";

        exec.prestart = "ifconfig epair${id} create up";
        exec.prestart += "ifconfig epair${id}a up descr vnet-${name}";
        exec.prestart += "ifconfig $public_bridge addm epair${id}a up";

        exec.start = "/sbin/ifconfig epair${id}b ${ip_addr}";
        exec.start += "/sbin/route add default ${public_gw}";
        exec.start += "/bin/sh /etc/rc";

        exec.prestop = "ifconfig epair${id}b -vnet ${name}";
        exec.poststop = "ifconfig ${public_bridge} deletem epair${id}a";
        exec.poststop += "ifconfig epair${id}a destroy";
}

Looking at the block and comparing it to other functional jails, I saw something missing: a vnet; declaration:

--- jail.conf   2023-11-12 20:09:03.028010000 -0800
+++ /etc/jail.conf      2023-11-12 19:59:02.867271000 -0800
@@ -230,6 +230,7 @@
        $id = "14";
        $ip_addr = "10.10.10.${id}";

+       vnet;
        vnet.interface = "epair${id}b";

        exec.prestart = "ifconfig epair${id} create up";

Sometimes you have to just walk away from a problem for a bit, but yeesh was that a silly one!