Subject: Re: dhcpd(8) _cannot_ be completely disabled on an interface
To: None <>
From: Christos Zoulas <>
List: tech-net
Date: 01/06/2002 01:00:56
In article <>,
Jim Wise <> wrote:

Don't run dhcpd or rarpd on the external interfaces; use a list of interfaces
for them to listen.


>Hash: SHA1
>OK, so I'm setting up a replacement border NAT router for the network
>here.  This machine has a few real outside addresses, and NATs for two
>internal LANs.  It provides DHCP service on the two internal LANs.
>As one would expect, I have used ipf(8) to block access to all ports
>which are inadvertently opened on the outside interface due to daemons
>listening on INADDR_ANY (see previous post on this subject).  On the tcp
>side, with the help of ipf's `block ... return-rst' capability, this is
>completely transparent to a scanning host.  On the udp side, with the
>help of ipf's `block ... return-icmp-as-dest(port-unr)' this is likewise
>completely transparent -- with one exception.
>port 68.  dhcpd(8).
>The problem is, unlike the other udp ports which dhcpd(8) uses (67,
>111), dhcpd does _not_ listen on port 68.  It appears that it is using
>bpf to snatch packets directly from the wire.  As bpf does (and should)
>get a shot at packets before ipf does its magic, this port is de facto
>open regardless of ipfilter rules stating otherwise (test this.  no
>really.  run dhcpd on a host, block all udp packets to port 68, and nmap
>- -sU scan the host.  you may be surprised).
>And yes, this occurs even though dhcpd(8) is explicitly _not_ started on
>the outside interface.
>To review:
>Inside interfaces are ray0 and le0 (yes, dhcp is limited to a specific
>set of hardware addresses on ray0.  that's another discussion).  Outside
>interface is vr0.
>from rc.conf:
>ipfilter=YES                                    # uses /etc/ipf.conf
>ipnat=YES                                       # uses /etc/ipnat.conf
>dhcpd=YES               dhcpd_flags="-q le0 ray0"
>from ipf.conf:
>block return-icmp-as-dest(port-unr) in log quick on vr0 from any to any
>port = 68
>from nmap from an outside host:
>68/udp     open        bootpc
>- -- 
>				Jim Wise
>Version: GnuPG v1.0.6 (NetBSD)
>Comment: For info see