Subject: Re: dhcpd(8) _cannot_ be completely disabled on an interface
To: None <firstname.lastname@example.org>
From: Christos Zoulas <email@example.com>
Date: 01/06/2002 01:00:56
In article <Pine.NEB.firstname.lastname@example.org>,
Jim Wise <email@example.com> wrote:
Don't run dhcpd or rarpd on the external interfaces; use a list of interfaces
for them to listen.
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>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.
>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.
>ipfilter=YES # uses /etc/ipf.conf
>ipnat=YES # uses /etc/ipnat.conf
>dhcpd=YES dhcpd_flags="-q le0 ray0"
>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
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