I normally use DHCP on a LAN at work and home. Everything normally works fine when switching, except when I try to switch to a dial up acct via PPP.

Here's the main issue: after a fresh install and set as DHCP client, resolv.conf is filled with the domain, seach, and nameserver info provided by the DHCP server. If I issue an "ifdown eth0" (or ifconfig eth0 down"), resolv.conf is not wiped out, only the search part is gone. The domain and nameserver (3 or them) entries remain. When I connect w/ PPP, the ISP's nameserver entries are appended to the bottom of resolv.conf. Resolv.conf is only designed to handle 3 nameservers, so it never sees the two added by the dialup ISP, and simply times out on anything I hit by name. This is further complicated by the fact that I am behind a f/w with 1 to 1 NAT'ting... so my internal name server is on the 172.16.*.* network and unreachable when dialed up.

So... aside from changing the ifdown script to cat resolv.conf to /dev/null... anyone know how to have resolv.conf purged when shutting down eth0? It would be nice to have an ip release function in the TCP/IP protocol stack... or something more easily scripted. I'm looking for something I can do enterprise-wide w/o major changes...

Guess I fixed it myself... though I'm still interested in a "cleaner" fix... and preferably a distribution independent one.

Here's my fix:
/etc/sysconfig/network-scripts/ifdown symlinks to /sbin/ifdown. I simply added these two lines to the end of /sbin/ifdown immediately before the final exit:
cp /etc/resolv.conf /etc/resolv.conf.bak
echo "" > /etc/resolv.conf
Testing showed that then issuing "ifdown eth0" resulted in a blank resolv.conf. Dialing up created two nameserver entries in resolv.conf appended with #kppp entry. Diconnecting left a blank resolv.conf (I guess kppp doesn't suffer the same issue as it properly removes its entries). Issuing "ifup eth0" created a resolv.conf with correct information from the DHCP server.

I am using RH 7.2. I'm not sure if ifdown and ifup are RH-centric or Linux standard, but I do know that the rc kill scripts use ifup and ifdown to start and stop network services, so my change should work across reboots.

The way you fixed it is fine. I have also heard of editing the ifup script and editing the PEERDNS line to read the way you want it. This is probably more of a hassle for you, though, since you already have the configuration you want, and wouldn't want to overwrite it. You can also set all the entries the way you want them in your /etc/resolve and then make the file immutable (I forget the exact command, sorry.) Hope that helps.