Old iMac Keeps Taking Port 4500 via UPnP
I have an old iMac running Snow Leopard and it's got a very annoying problem. Every time it boots up, it grabs UDP port 4500 via UPnP from my DD-WRT router. Problem is, I have a Mac Mini running a VPN server and when the iMac takes that port, the VPN no longer works. I think it also grabs the port when it wakes from sleep.
lsof output (Run on the iMac. Note the lack of 4500.)
lsof -Pi
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
SystemUIS 126 roberthsr 6u IPv4 0x07094734 0t0 UDP *:*
SystemUIS 126 roberthsr 10u IPv4 0x08067aa4 0t0 UDP *:*
AppleVNCS 142 roberthsr 8u IPv6 0x06adcf80 0t0 TCP *:5900 (LISTEN)
sshd 191 roberthsr 3u IPv4 0x06adfad8 0t0 TCP imacg5.socamx.net:22->rmbp.socamx.net:62816 (ESTABLISHED)
sshd 191 roberthsr 4u IPv4 0x06adfad8 0t0 TCP imacg5.socamx.net:22->rmbp.socamx.net:62816 (ESTABLISHED)
nmap output (Scanning the iMac from my laptop. Note that it is listening on 4500.):
sudo nmap -sU -T5 -p 500,1701,4500,9999 10.0.10.6
Starting Nmap 6.49BETA4 ( https://nmap.org ) at 2015-07-16 11:20 EDT
Nmap scan report for (10.0.10.6)
Host is up (0.0019s latency).
PORT STATE SERVICE
500/udp open|filtered isakmp
1701/udp closed L2TP
4500/udp open|filtered nat-t-ike
9999/udp closed distinct
MAC Address: 00:1B:63:16:17:0B (Apple)
Nmap done: 1 IP address (1 host up) scanned in 1.27 seconds
How can I make this iMac stop taking port 4500 and thus prevent it from breaking my VPN? This has been driving me nuts for months. I just want the stupid thing to stop grabbing UDP 4500 via UPnP. Turning off UPnP on the router is not an option. Running newer than 10.6 / Snow Leopard isn't an option either, this computer exists for Rosetta functionality.
As a side note, all my other Macs used to do this too but I disabled Back to My Mac on each of them to stop it. This old iMac running Snow Leopard, I can't access that setting because it is pre-iCloud. It's just an inaccessible MobileMe section in System Preferences, which is obviously long outdated.