I have obtained (and hand-copied) some additional information that may be
useful.

Like I said, negotiating a lease via `dhclient' seems to work fine, but I
can't seem to send any packets besides that. I've tried 3.0.1r10 (the
included, with NetBSD's custom dhclient-script) and also 3.0.1r12, with the
ISC script for NetBSD. At this point I'm willing to try almost anything to,
if not get it working, then at least make some headway on figuring out what
the problem is.

System: 1.6.1 GENERIC

---dmesg output---
NetBSD 1.6.1 (GENERIC) #0: Tue Apr 8 12:05:52 UTC 2003
autobuild@tgm.daemon.oth:/autobuild/netbsd-1-6/i386/OBJ/autobuild/netbsd-1-6
/src/sys/arch/i386/compile/GENERIC
---snip snip---
sip0 at pci0 dev 11 function 0: NatSemi DP83815 10/100 Ethernet, rev 00
sip0: interrupting at irq 11
sip0: Ethernet address 00:09:5b:20:cd:b1
nsphyter0 at sip0 phy0: DP83815 10/100 media interface, rev. 1
nsphyter0: 10baseT, 10baseT-FDX, 100baseT, 100baseT-FDX, auto
---end---

---ident output--
$NetBSD: if_sip.c,v 1.52.4.7 2002/11/21 18:28:15 he Exp $
$NetBSD: nsphyter.c,v 1.14 2002/03/25 20:51:25 thorpej Exp $
---end---

---the output of `ifconfig sip0', before touching it---
sip0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> mtu 1500
address: 00:09:5b:20:cd:b1
media: Ethernet autoselect (100baseTX full-duplex)
status active
---end---

I ran `dhclient -r', then `dhclient -1 sip0'.

---new output of `ifconfig sip0'---
sip0: flags=8802<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
address: 00:09:5b:20:cd:b1
media: Ethernet autoselect (100baseTX full-duplex)
status active
inet 192.168.1.102 netmask 0xffffff00 broadcast 192.168.1.255
---end---

The kernel routing tables, after the dhclient stuff:

---`route show'---
default    192.168.1.1    UG
loopback    127.0.0.1    UGR
localhost    127.0.0.1    UH
192.168.1.0    link#1    U
192.168.1.1    link#1    UH
192.168.1.102    127.0.0.1    UGH
---end---

Before running the `dhclient -r', I started tcpdump on the interface.

---`tcpdump sip0'---
arp who-has 192.168.1.1 tell 192.168.1.102
192.168.1.100.netbios-dgm > 192.168.1.255.netbios-dgm: udp 201
---`dhclient -r' finished, following stuff is from `dhclient -1 sip0'---
0.0.0.0.bootpc > 255.255.255.255.bootps: xid:0xac015301 [|bootp] [tos 0x10]
192.168.1.1.bootps > 255.255.255.255.bootpc: xid:0xac015301 Y:192.168.1.102
ether:00:09:5b:20:cd:b1 [|bootp]
---the previous two messages were repeated intermittently, until a control-C
(the output, 20, was not erroneous)---
20 packets received by filter
0 packets dropped by kernel
---end---

`ping 192.168.1.1' sends 56-byte packets by default, I don't know much about
IP so I don't know, but should this possibly be 64?