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+title = "gotchas with IPs and Jails"
+date = "2016-06-26T19:00:00+00:00"
+author = "Gibheer"
+draft = false
++++
+
+Through some problems with installing postfix and opensmtpd at the same time, I
+again had the need to invest some time into FreeBSD Jails.
+
+As I had some problems with the IP allocation, I document what I found out here.
+
+First and foremost, I think I could have had it easier using VIMAGE/vnet, but that
+still isn't enabled per default on 10.2 and 10.3, the versions I tested.
+
+The following settings are for the jail.conf system, but can also be used on the
+command line.
+
+## configure an IP
+
+The easiest setup is to define an IP on any interface and tell the jail system
+to use a specific one.
+
+For this example, I use the `prestart` command to define the IP on any interface.
+
+```
+# define on a public interface
+jail1 {
+  exec.prestart = "ifconfig em0 192.168.1.2 alias";
+  ip4.addr = 192.168.1.2;
+}
+
+# define on loopback
+jail2 {
+  exec.prestart = "ifconfig lo0 192.168.1.3 alias";
+  ip4.addr = 192.168.1.3;
+}
+
+# reuse 127.0.0.1 from the host
+jail3 {
+  ip4.addr = 127.0.0.1;
+}
+```
+Using this mechanism, the IP is left alone when starting or stopping the jail.
+
+## configure an IP on an interface
+
+When specifing an IP together with an interface, jails will take over the
+life management. When the jail is started the IP is created and when stopping
+the jail, the IP is removed.
+
+The following will show some ways how to do that:
+
+```
+# set an IP on a public interface
+jail1 {
+  ip4.addr = em0|192.168.1.2;
+}
+
+# define a loopback address
+jail2 {
+  ip4.addr = lo0|192.168.1.3;
+}
+```
+
+There is also the `interface` option, which can be used to pin every IP to that
+specific interface.
+
+```
+# define two addresses on the same interface, maintained by the jail system
+jail1 {
+  interface = em0;
+  ip4.addr = 192.168.1.10, 127.0.0.1;
+}
+```
+
+This should be used when an IP is not used by the host or another jail. The
+following example would destroy the loopback address on shutdown:
+
+```
+# removes localhost at jail shutdown
+jail1 {
+  ip4.addr = lo0|127.0.0.1;
+}
+```
+
+## mixing both options
+
+In the case of poudriere, you have to mix both options. Poudriere wants to put
+`127.0.0.1` and `::1` into the child jails, so that these have to be defined
+in the poudriere jail too.
+
+If other IPs are also needed, this can be done with the `ip4.addr` and `ip6.addr`
+options.
+
+```
+# manage 192.168.1.11 using jails and use localhost unmanaged
+jail1 {
+  ip4.addr = em0|192.168.1.11, 127.0.0.1;
+}
+```
+
+Hope that helps to clarify, what exactly each option does.