The initscripts documentation (sysconfig.txt) says you can 
set ARP=yes|no in ifcfg-eth?.  
The line of code which implements this is:

   ifconfig ${DEVICE} ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST}
${ARP:+arp}

This will turn arp on no matter what ARP is set to in the 
configuration file and provides no way of turning it off.

In addition it would be nice to have a similar mechanism
for turning promiscuous mode on and off.  And while we're
at it we might as well throw in the other toggle allmulti.

Here is my suggested patch for ifup to fix arp and add the 
same functionality for promisc and allmulti:

> diff -u ifup.orig ifup
--- ifup.orig   Mon Jun 12 13:18:29 2000
+++ ifup        Mon Jun 12 13:12:33 2000
@@ -4,6 +4,21 @@
 cd /etc/sysconfig/network-scripts
 . network-functions

+# Return the value of a toggle.  $1 is toggle name, $2 is yes|no|null.
+toggle_value ()
+{
+   if [ -z $2 ]
+   then
+      echo ''
+   elif [ $2 = yes  -o $2 = YES ] ; then
+      echo "$1"
+   elif [ $2 = no  -o $2 = NO ] ; then
+      echo "-$1"
+   else
+      echo ''
+   fi
+}
+
 need_hostname

 CONFIG=$1
@@ -108,7 +123,10 @@
        eval `/bin/ipcalc --network ${IPADDR} ${NETMASK}`
     fi

-    ifconfig ${DEVICE} ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST}
${ARP:+arp}
+    ifconfig ${DEVICE} ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST}
\
+       $(toggle_value arp $ARP) \
+       $(toggle_value promisc $PROMISC) \
+       $(toggle_value allmulti $ALLMULTI)
     # don't re-add subnet route on 2.2 kernels, but add a route
     # to a non-local subnet.
     # stupid hack, but it should work

And here is the corresponding patch for the documentation:

> diff -u sysconfig.txt.orig sysconfig.txt
--- sysconfig.txt.orig  Mon Jun 12 13:18:46 2000
+++ sysconfig.txt       Mon Jun 12 13:18:24 2000
@@ -329,6 +329,8 @@
     Managed from /etc/sysconfig/network-scripts/ifup-ipx
     ARP=yes|no (adds 'arp' flag to ifconfig, for use with the
       ethertap device)
+    PROMISC=yes|no (enable or disable promiscuous mode)
+    ALLMULTI=yes|no (enable or disable all-multicast mode)

   PPP/SLIP items:
     PERSIST=yes|no


Ian