I finally solved a problem that I've had for the past 36 hours or so, in which my laptop has been able to open SOME web pages consistently, while other pages could be opened sometimes, but at other times "www.suchandsuch could not be found", and a few pages could not be opened at all.

I finally found that the nameservers listed in /etc/resolv.conf on the laptop were completely different from the ones listed in that file on my desktop machines, so I edited resolv.conf on the laptop to agree with the desktops, and voila, everything seems to work.

But I'm wondering how I got those apparently bogus entries in the laptop, which, except for these past 2 days, has always worked OK. I'm guessing that it has something to do with the fact that the last time I used it before this episode, it wasn't plugged into the router in my home network, but instead into a cable modem at another location.

Can anybody explain how, and when, and how often, that file is generated and/or modified? On the first line it says:
; generated by /sbin/dhclient-script

Are changes in this file a natural consequence of moving to a different internet connection? Does the problem suggest that someone hacked into it and changed those entries?

yeah, when you plugged in at the other location your dhcp client downloaded the new ip configuration... among it were the different dns servers...

the weird thing is that your dhcp client didn't overwrite these when you came back to the original location...

/sbin/dhclient and /sbin/dhclient-script appear to be unchanged since I installed RH9 on this machine a year and a half ago.

Does /sbin/dhclient run each time linux boots up?

Where do the nameserver entries come from? Does dhclient download them from the ISP?

the dns server ip addresses are part of the information your dhcp client downloads from your dhcp server... your dhcp client will write the dns server addresses to /etc/resolv.conf...

the dhcp client is usually set to run at startup time...