Yay I fixed a bug, that I made!

_posts/2023-11-12-interface-already-exists.md

@@ -0,0 +1,77 @@
+---
+layout: post
+title: "Solving a FreeBSD Jails issue: interface already exists"
+tags:
+- freebsd
+- jails
+---
+
+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:
+
+```conf
+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:
+
+```diff
+--- 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!
+
+