ifup-post: always update nameserver & search entries in /etc/resolv.conf

  This is complete rework of how we generate the /etc/resolv.conf.

  Fixes:
   * always update 'nameserver' & 'search' entries when DNS* & DOMAIN
     options (respectively) were updated in ifcfg-* files
   * always update the order of 'nameserver' entries when the order of
     DNS* options was updated
   * always keep /etc/resolv.conf.save as a proper backup - do not
     overwrite it everytime when updating 'nameserver' or 'search'

  Enhancements:
   * added support for DNS3 option (equals to MAXNS value in GLIBC)
   * added logic to process duplicate DNS* options
   * added logic to process randomly omitted DNS* options (e.g. omitting
     DNS1 while specifying DNS2 and/or DNS3 will still work now)

  This work was based on these two RHEL-7.5 BZs:
  https://bugzilla.redhat.com/show_bug.cgi?id=1364895
  https://bugzilla.redhat.com/show_bug.cgi?id=1357658

1 files changed, 33 insertions, 0 deletions

diff --git a/sysconfig/network-scripts/network-functions b/sysconfig/network-scripts/network-functions
index d7915e31..9d6f9366 100644
--- a/sysconfig/network-scripts/network-functions
+++ b/sysconfig/network-scripts/network-functions
@@ -448,6 +448,10 @@ set_hostname ()
             rsctmp=$(mktemp /tmp/XXXXXX);
             cat /etc/resolv.conf > $rsctmp
             echo "search $domain" >> $rsctmp
+
+            # Backup resolv.conf only if it doesn't exist already:
+            ! [ -f /etc/resolv.conf.save ] && cp -af /etc/resolv.conf /etc/resolv.conf.save
+
             change_resolv_conf $rsctmp
             /bin/rm -f $rsctmp
         fi    
@@ -697,3 +701,32 @@ net_log()
     fi
     return 0
 }
+
+update_DNS_entries()
+{
+    # Remove duplicate values from DNS options if any:
+    if [ -n "${DNS3}" ] && [[ "${DNS3}" == "${DNS2}" || "${DNS3}" == "${DNS1}" ]]; then
+        unset DNS3
+    fi
+
+    if [ -n "${DNS2}" ] && [[ "${DNS2}" == "${DNS1}" ]]; then
+        unset DNS2
+    fi
+
+    # Shift the DNS options if necessary:
+    if [ -z "${DNS1}" ] && [ -n "${DNS2}" ]; then
+        DNS1="${DNS2}"
+        unset DNS2
+    fi
+
+    if [ -z "${DNS2}" ] && [ -n "${DNS3}" ]; then
+        DNS2="${DNS3}"
+        unset DNS3
+    fi
+
+    # We need to check DNS1 again in case only DNS3 was set at all:
+    if [ -z "${DNS1}" ] && [ -n "${DNS2}" ]; then
+        DNS1="${DNS2}"
+        unset DNS2
+    fi
+}