The value of the object iesIfDefaultSecondaryDnsIPv4Addr
specifies the IPv4 address of the default secondary DNS
server for the subscribers using this interface.
Subscribers that cannot obtain an IPv4 DNS server addres
by other means, can use this for DNS name resolution.
The object iesIfDefaultSecondaryDnsIPv4Addr can only be set
to a nonzero value if the value of iesIfType is
'subscriber'.