Configuring DHCP on RHEL 6

Question

I have a fresh install of RHEL6 and I am unable to find any config file to do advanced configuration for dhclient. I am trying to find something like dhclient.conf on Ubuntu so I can modify options like supersede domains, fqdn, Maybe they should be on "/etc/sysconfig/networking/devices/ifcfg-eth0" in that case what are the settings that matches those dhclient.conf configurations?

Answer 1

The configuration file is one of /etc/dhcp/dhclient-${DEVICE}.conf, /etc/dhclient-${DEVICE}.conf, or /etc/dhcp/dhclient.conf in that order. The first one that exists is used.

From /etc/sysconfig/network-scripts/ifup-eth (Fedora 14; RHEL6 may be similar):

# allow users to use generic '/etc/dhcp/dhclient.conf' (as documented in manpage!)
# if per-device file doesn't exist or is empty
if [ -s /etc/dhcp/dhclient-${DEVICE}.conf ]; then
   DHCLIENTCONF="-cf /etc/dhcp/dhclient-${DEVICE}.conf";
elif [ -s /etc/dhclient-${DEVICE}.conf ]; then
   DHCLIENTCONF="-cf /etc/dhclient-${DEVICE}.conf";
else
   DHCLIENTCONF='';
fi;

From the dhclient man page:

   -cf <config-file>
          Path to the client configuration file.  If unspecified, the default /etc/dhcp/dhclient.conf is used.

Answer 2

Actually you can do a bunch of those things in /etc/sysconfig/network-scripts/ifcfg-eth? -- for Red Hat 5 family, you can find documentation here: http://www.linuxtopia.org/online_books/centos_linux_guides/centos_linux_reference_guide/s1-networkscripts-interfaces.html

I would presume that much, if not all, of that functionality would still work in RH 6.

Answer 3

A few things...

1) NetworkManager dynamically creates the dhclient.conf file for IPv4. It runs dhclient for IPv6 using the dhclient defaults since no configuration file is specified and just specifies the interface device (e.g., eth0). You can check this by doing a "ps aux | grep dhclient". I tried providing the options I wanted in the "correct location" only to find that NetworkManager overwrites the file each time it runs.

2) You can use one of the default locations for the config file to provide whatever options you want to dhclient for IPv6. At least it seems to be working for me.

3) Don't be afraid to edit ifcfg-ethX instead of relying on the GUI network configurator. There are some combinations of options that the configurator doesn't allow that should be legal and may be useful.

Cheers, Dave

Answer 4

/etc/sysconfig/networking/devices/ifcfg-eth0 is just for basic setup, for the options you want go ahead and install the dhclient with running # yum install dhclient in your shell.