Networking – Understanding BOOTPROTO and /etc/resolv.conf in RHEL

networkingresolvconfrhel

What does it mean when it says BOOTPROTO=none in the /etc/sysconfig/network-scripts/ifcfg-eth0 file.

I remember there used to be BOOTPROTO=static and it was very clear and straight-forward in telling us that if an IPADDR=<x.x.x.x> is given, the server will come up with the IP address specified. Likewise, BOOTPROTO=dhcp will look for a DHCP server to obtain a dynamic IP address. Redhat says:

 BOOTPROTO=protocol
    where protocol is one of the following:

        none — No boot-time protocol should be used.
        bootp — The BOOTP protocol should be used.
        dhcp — The DHCP protocol should be used.

  • Does it mean if we don't specify the IP in ifcfg-eth0 file, it will look for a DHCP server and if an IP is specified, it will pick up that static IP?

  • What are the chances that it will look for a DHCP server and modify /etc/resolv.conf even if an IP address is specified with IPADDR= when BOOTPROTO is set to none?

Context:- We moved datacenters and had to change IP addresses in many of the servers. We had modified /etc/resolv.conf with the IP addresses of the new DNS servers, but for some reason, in some of the servers the /etc/resolv.conf got blanked out, or came up with old DNS IP addresses. In the /etc/init.d/network script I see it is calling /etc/sysconfig/network-scripts/network-functions which has this function. Is this the culprit?

# Invoke this when /etc/resolv.conf has changed:
change_resolv_conf ()
{
    s=$(/bin/grep '^[\ \        ]*option' /etc/resolv.conf 2>/dev/null);
    if [ "x$s" != "x" ]; then
       s="$s"$'\n';
    fi;
    if [ $# -gt 1 ]; then
       n_args=$#;
       while [ $n_args -gt 0 ];
         do
            if [[ "$s" = *$1* ]]; then
               shift;
               n_args=$(($n_args-1));
               continue;
            fi;
            s="$s$1";
            shift;
            if [ $# -gt 0 ]; then
                s="$s"$'\n';
            fi;
            n_args=$(($n_args-1));
         done;
    elif [ $# -eq 1 ]; then
       if [ "x$s" != "x" ]; then
          s="$s"$(/bin/grep -vF "$s" $1);
       else
          s=$(cat $1);
       fi;
    fi;
    (echo "$s" > /etc/resolv.conf;) >/dev/null 2>&1;
    r=$?
    if [ $r -eq 0 ]; then
        [ -x /sbin/restorecon ] && /sbin/restorecon /etc/resolv.conf >/dev/null 2>&1 # reset the correct context
        /usr/bin/logger -p local7.notice -t "NET" -i "$0 : updated /etc/resolv.conf";
        [ -e /var/lock/subsys/nscd ] && /usr/sbin/nscd -i hosts; # invalidate cache
    fi;
    return $r;
}

Under what cicumstances are this function called?

I know that setting PEERDNS to no will prevent the /etc/resolv.conf from changing, however, I would like to know whether our server had started looking for a DHCP server even though BOOTPROTO was set to none and an IP address was specified? if yes, why?

I rebooted the servers issues with this issue a few times to replicate the issue again, but the contents of /etc/resolv.conf is not changing now. What could've caused the /etc/resolv.conf to be changed in the first reboot?

Can we use BOOTPROTO=static? I read its deprecated. Our machines are all RHEL 6.5

Best Answer

If you read /etc/sysconfig/network-scripts/ifup-eth you'll see that networking uses DHCP if BOOTPROTO is set to dhcp or bootp, otherwise it's not used:

if ["${BOOTPROTO}" = "bootp" -o "${BOOTPROTO}" = "dhcp" ]; then DYNCONFIG=true

Further down, if DYNCONFIG is not null (and dhclient is available) then the scripts attempts to use DHCP otherwise static IP addressing is attempted.

Using grep -r BOOTPROTO * within /etc doesn't show anything other than the above snippet, suggesting you could use anything in BOOTPROTO as long as it isn't one of the two above.

You could use BOOTPROTO=static, but if we're told it's unsupported then you cannot guarantee that it will work like this in the future. Additionally, it won't make a difference to your issue - static or none will cause the script not to use DHCP.

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