There appear to be two problems with a small home network that puts two
computers behind a Linksys router, which connects to a DSL modem that is
in bridge mode.

First, what works:
* The Internet more-or-less works. Both machines can surf. The router
can authenticate and establish a PPPoE connection to the ISP; both
computers can see google.com and the like. Name service works, HTTP
works, etc.
* The machines can ping each other.
* A shared folder on one of the machines can be browsed from the other.

Now, what doesn't:
* A shared folder on the other machine doesn't show up on the first.
* Applications like eMule seem to have difficulties, but the nature of
the difficulties is odd. In the particular case of eMule, for
instance, it's configured to use port 13130 for TCP and 13141 for UDP.
The router is set up to forward the range 13130-13141, both TCP and
UDP, to the IP 192.161.1.101, which is the IP of the machine running
eMule. eMule on that machine doesn't *seem* to get lowID, but it
behaves in some respects as if it is (e.g. search only seems to find
files on the same server -- global search finds nothing that local
search doesn't find; also, nobody seems to upload).
eMule also seems only able to connect to one server, Razorback 3.1. I
don't know what this signifies though I have some experience with
eDonkey/eMule.
The port forwarding is definitely doing *something*, however, as
without it eMule will attempt to connect to Razorback 3.1 and complain
that it got lowID. So it actually gets lowID without port forwarding,
and it technically doesn't with port forwarding, but the search
horizon still seems to be the Razorback 3.1 server.
I'm tempted to class this as an eMule problem rather than a firewall
or router problem given this.
* One of the computers has a PPPoE connectoid for the DSL service which
was used prior to establishing the network with the router and second
computer. This connectoid sometimes seems to try to connect, which
hopefully *won't* annoy the ISP with the apparent attempt to log in
two simultaneous sessions. And Explorer sometimes seems to try to open
a listen port (I don't know which one) on the same machine, as
indicated by ZoneAlarm.

Detailed configuration details:
* The physical setup is as described above.
* The router logs into the ISP using PPPoE and this seems to work
correctly.
* One machine has IP 192.168.1.100, the other 192.168.1.101. The latter
is the one with eMule.
* Ports 13130-13141 are forwarded to 192.168.1.101, TCP and UDP both.
* Neither machine is in the router's DMZ.
* There are no other port forward type settings set on the router.
* Both computers run WinXP; one Home (192.168.1.101) and one MCE
(192.168.1.100).
* Both have Local Area Connection bound to Client for Microsoft
Networks, File and Printer Sharing for Microsoft Networks, TCP/IP, and
in case it matters QoS Packet Scheduler and Network Monitor Driver.
* Both have the same workgroup name.
* They have different computer names.
* All of the relevant services seem to be running on both machines, and
there are no obvious failure-related events.
* There's a share configured on each machine, but only 192.168.1.101
shows up in the "Microsoft Windows Network" workgroup. It shows up
here on both boxes, and the other shows up there on neither. Browsing
it on both boxes shows the contents of the share on 192.168.1.101.
* MSN Messenger works.
* Surfing (HTTP) works.
* Email (POP and SMTP, dunno about IMAP) works.
* If you're reading this, NNTP works.
* eMule partially works.
* Most likely, everything that doesn't involve opening listen ports
works.
* That old connectoid has been made non-default. It doesn't look like it
can be completely disabled without actually deleting it, which I
suppose I may as well do now. I could always recreate it later anyway.
* The cabling is all Ethernet, and I think all Cat-5 (the cable that
came with the router may not be; it connects to 192.168.1.100; one
bought separately and one that came with the DSL modem definitely are)
but it's fairly unlikely that any problems exist at the physical layer
given that 99% of what should work does work.
* Both machines test clean with all of AVG, Spybot S&D, and Ad-Aware.
* Both have the XP firewall disabled and ZoneAlarm installed and
running. ZoneAlarm appears properly configured. On 192.168.1.101,
eMule is granted full permissions in ZoneAlarm. The issue with
"semi-lowID" behavior persists when that ZoneAlarm is lowered to
medium security in the Internet Zone. Both have Trusted Zone at medium
security. 192.168.1.101 is in the Trusted Zone in 192.168.1.100's copy
of ZoneAlarm and vice versa. This should according to ZoneAlarm's
instructions enable file sharing, and file sharing does at least half
work. I'm inclined to clear ZoneAlarm from the list of suspects,
though I haven't tried outright disabling it. Since that would be like
having sex with a pro without a condom, though, I'm loath to risk it.
(It could be safely tested in connection with file and print
sharing, by powering off the DSL modem, but this won't work to test if
it's affecting eMule, for obvious reasons.)
(The router does have some firewall functionality. I'd still rather
not have machines with network shares not running firewall software if
I can avoid it! The users sure won't like it if I set up their network
and everything, only for some virus to eat it alive inside of a week.)
* The users of this small network consider getting port forwarding to
behave as expected to be moderately urgent and getting file and print
sharing to work symmetrically to be not very urgent, in case this
matters to anyone, which I doubt.

So, does anyone here have any ideas about any of the three problems
being observed?

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