docs/173405: Undocumented feature of ifconfig (reading configuration from rc.conf)

[Mateusz Kwiatkowski]

>Number:         173405
>Category:       docs
>Synopsis:       Undocumented feature of ifconfig (reading configuration from rc.conf)
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-doc
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          doc-bug
>Submitter-Id:   current-users
>Arrival-Date:   Mon Nov 05 19:50:00 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator:     Mateusz Kwiatkowski
>Release:        9.0-RELEASE
>Organization:
Atlashost
>Environment:
FreeBSD gw-5 9.0-RELEASE FreeBSD 9.0-RELEASE #1: Wed May 23 23:28:34 CEST 2012     root at gw-5:/usr/obj/usr/src/sys/NPHXGW  amd64
>Description:
If configuration for NIC exists in /etc/rc.conf, ifconfig automatically configures interface when created.
>How-To-Repeat:
1. Create configuration in /etc/rc.conf for non existing NIC:
vlans_em0="181"
ifconfig_em0_181="inet6 2001:123:0:0::e001:121/126"

2. Bring up interface manually:
ifconfig em0.181 create

3. Interface is created with IP from rc.conf:
# ifconfig em0.181
em0.181: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=3<RXCSUM,TXCSUM>
        ether 52:54:00:74:bd:1b
        inet6 2001:123:0:0::e001:121 prefixlen 126 duplicated
        inet6 fe80::5054:ff:fe74:bd1b%em0.181 prefixlen 64 scopeid 0x4
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
        media: Ethernet autoselect (1000baseT <full-duplex>)
        status: active
        vlan: 181 parent interface: em0

This works both for IPv4 and IPv6.
>Fix:
This feature should be documented in ifconfig(8) manual.

>Release-Note:
>Audit-Trail:
>Unformatted: