Thank you, Gordon. Yes, there is a sleep proxy:
~ garysmith$ dns-sd -B _sleep-proxy._udp Browsing for _sleep-proxy._udp DATE: ---Mon 17 Sep 2012--- 9:03:08.447 ...STARTING...
Timestamp.....A/R..Flags.if Domain.....Service Type.......Instance Name
9:03:08.448..Add.....2..4 local......._sleep-proxy._udp.....50-yy-zz-xx SomeoneElses-Lab (name and machine ID changed to protect the innocent).
It seems to be on someone else's Bonjour computer. But I know them, and the computer is always on.
I looked at Stuart Cheshire's page, and set up SomeoneElses-Lab (the never-sleeper). It seemed to work.
Then, I made things worse. I edited the com.apple.mDNSResponder.plist on the sleeper, as instructed (using TextEditor). But the dns-sd -B failed...and I find that the sleeper Mac no longer can browse the web or receive email. All the TCP-IP info is correct, the ethernet connection is working, and the Apache server IS ON and working as long as the cpu is awake. So the Mac can send but not receive. I replaced the modified .plist with the original, and restarted the computer. The server serves, but the clients do not receive. Firefox can't find the DNS, and can only display the pages that it itself hosts if I give the IP address instead of the domain.
I can ping it from another computer via either its domain name or IP address. If I try ping from the sleeper, it can reach IP addresses, but can't resolve domain names.
Where did I go wrong? Did the killall command stop something that needs to be restarted?