Subject: dhcpd(8) _cannot_ be completely disabled on an interface
To: firstname.lastname@example.org, <email@example.com>
From: Jim Wise <firstname.lastname@example.org>
Date: 01/05/2002 10:17:11
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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
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