how to determine primary (source) IP address in jail

[Miroslav Lachman]

Is there some easy way to determine the primary (source) address which 
is used in jail with multiple IP addresses?

I came to this problem with running local_unbound in jail. Unbound 
refuses queries originating in this jail because the do not come from 
real 127.0.0.1 (which is the only one allowed by default). Unbound in 
jail see requests come from jails IP. It is easy to determine (in shell 
script) if jail has only one IP.
But what in case where jail has multiple IPs? Is there some sysctl or 
some call to ifconfig or any other util to get the IP which will be used 
as source address for queries on local services in jail?

I know I can allow all IPs of jail in
access-control: a.b.c.d/32 allow
access-control: e.f.g.h/32 allow

I am just curios if there is some way to get "primary" IP in jail 
without calling anything from the host environment.

Kind regards
Miroslav Lachman