/sbin/dhclient-script に対して以下のパッチを当てることで resolv.conf の書き換えを防ぐことができます。
@@ -87,10 +87,6 @@
            fi
        done

-       if [ "${is_in}" = "false" ]; then
-          # Add domain name to search list (#637763)
-          sed -i -e "s/${search}/${search} ${domain}/" /etc/resolv.conf
-       fi
     fi
 }

@@ -135,14 +131,14 @@
             echo "nameserver ${nameserver}" >> ${rscf}
         done

-        change_resolv_conf ${rscf}
+        #change_resolv_conf ${rscf}
         rm -f ${rscf}

         if [ -n "${search}" ]; then
             eventually_add_hosts_domain_to_search "${search}"
         fi

-        fix_context /etc/resolv.conf
+        #fix_context /etc/resolv.conf
     elif [ -n "${new_dhcp6_name_servers}" ] ||
          [ -n "${new_dhcp6_domain_search}" ]; then
         rscf="$(mktemp /tmp/XXXXXX)"
@@ -168,14 +164,14 @@
             echo "nameserver ${nameserver}" >> ${rscf}
         done

-        change_resolv_conf ${rscf}
+        #change_resolv_conf ${rscf}
         rm -f ${rscf}

         if [ -n "${search}" ]; then
             eventually_add_hosts_domain_to_search "${search}"
         fi

-        fix_context /etc/resolv.conf
+        #fix_context /etc/resolv.conf
     fi
 }

@@ -465,7 +461,7 @@
         hostname ${new_host_name}
     fi

-    make_resolv_conf
+    #make_resolv_conf

     if [ -n "${DHCP_TIME_OFFSET_SETS_TIMEZONE}" ] &&
        [[ "${DHCP_TIME_OFFSET_SETS_TIMEZONE}" = [yY1]* ]]; then
@@ -545,7 +541,7 @@

             add_ipv6_addr_with_DAD

-            make_resolv_conf
+            #make_resolv_conf
             ;;

         RENEW6|REBIND6)
@@ -554,11 +550,6 @@
                [  ! "${new_ip6_address}" = "${old_ip6_address}" ]; then
                 add_ipv6_addr_with_DAD
             fi
-
-            if [ ! "${new_dhcp6_name_servers}" = "${old_dhcp6_name_servers}" ] ||
-               [ ! "${new_dhcp6_domain_search}" = "${old_dhcp6_domain_search}" ]; then
-                make_resolv_conf
-            fi
             ;;

         DEPREF6)