docs: document ifstate

Makefile

@@ -138,6 +138,7 @@ install: all
 	${SCDOC} < $< > $@
 
 MANPAGES_5 = \
+	doc/ifstate.5 \
 	doc/ifupdown-ng.conf.5 \
 	doc/interfaces.5 \
 	doc/interfaces-bond.5 \

doc/ifstate.scd

@@ -0,0 +1,57 @@
+ifstate(5)
+
+# NAME
+
+*/run/ifstate* - interface state database
+
+# DESCRIPTION
+
+The */run/ifstate* file describes the present state of the interface
+configuration -- namely, how many interfaces are up, how many
+dependencies each interface has which are up (the refcount), and
+whether or not an interface was explicitly brought up due to request
+or configuration.
+
+# FILE SYNTAX
+
+At a minimum, the */run/ifstate* file contains at least one column,
+the physical to logical interface mapping.  This column is formatted
+as such:
+
+```
+lo=lo
+eth0=eth0
+wlan0=work
+```
+
+The left side of the mapping is the physical interface, while the right
+side is the logical interface.  This field is required to be present.
+
+The next field is the reference count.  This is a number that reflects
+the number of *active* dependencies an interface has.  As interfaces
+are brought up and down, the refcount may change.  This field is
+optional.
+
+The final field denotes whether or not an interface was brought up
+explicitly -- either by being marked as _auto_ or brought up manually
+using *ifup*(8).  The contents of this field if present is the
+_explicit_ keyword.
+
+# EXAMPLES
+
+An example from a typical system with localhost, eth0 and a wireguard
+VPN:
+
+```
+lo=lo 1 explicit
+eth0=eth0 2 explicit
+wg0=wg0 1 explicit
+```
+
+# SEE ALSO
+
+*interfaces*(5)
+
+# AUTHORS
+
+Ariadne Conill <ariadne@dereferenced.org>

doc/interfaces.scd

@@ -216,6 +216,7 @@ iface eth0
 
 # SEE ALSO
 
+*ifstate*(5)
 *ifupdown-ng.conf*(5)
 *ifup*(8)
 *ifdown*(8)