How to get the hostname from a DHCP server

Oli's answer is demonstrably false ("You don't get your hostname from the DHCP server"), as evidenced by the other answers here, and also by my recent experience on a RHEL7 system. Said system got its host name from the DHCP server.

And, indeed, there are things in the DHCP configuration files that are supposed to make it happen. For example:

host host4 {   # verified                                                                                                                                                                                                                   
  hardware ethernet  41:88:22:11:33:22;
  fixed-address 192.168.0.4;                                                                                                                                                                                 
  option host-name "host4";
}

Is supposed to tell that host that his name is host4.

As it turns out, isc's dhclient DOES NOT APPEAR TO SUPPORT THIS!

However, dhcpcd5 does, out of the box. Stop dhclient, install dhcpcd5, run dhcpcd, renew your lease, and poof, your hostname on your DHCP client is set to the name sent from the DHCP server. No dhclient-exit-hooks.d scripting, no hacks in rc.local, nothing.

As an end-note, I've spent a lot of time trying to get this to work using ISC's dhclient. Absolutely no joy, even when the server sends the host name.

My initial solution to the problem was writing some cute code in rc.local to detect when the network came up and forcing a (in my case) search of /etc/hosts to get the hostname and then running hostname with that host name. It works, but until the network comes up your hostname is probably wrong (when first deploying a host, I remove /etc/hostname, so the host name is localhost until I can run /etc/init.d/hostname.sh start once the network comes up - so when first getting a new name you need to boot twice - once to get your hostname, and once to have that name available when everything starts up...).

There is a way to do it with a little script for a dhcp hook as described here.

Create a new file:

sudoedit /etc/dhcp/dhclient-exit-hooks.d/hostname

and paste the following code:

#!/bin/sh
# Filename:     /etc/dhcp/dhclient-exit-hooks.d/hostname
# Purpose:      Used by dhclient-script to set the hostname of the system
#               to match the DNS information for the host as provided by
#               DHCP.
#


# Do not update hostname for virtual machine IP assignments
if [ "$interface" != "eth0" ] && [ "$interface" != "wlan0" ]
then
    return
fi


if [ "$reason" != BOUND ] && [ "$reason" != RENEW ] \
   && [ "$reason" != REBIND ] && [ "$reason" != REBOOT ]
then
        return
fi

echo dhclient-exit-hooks.d/hostname: Dynamic IP address = $new_ip_address
hostname=$(host $new_ip_address | cut -d ' ' -f 5 | sed -r 's/((.*)[^\.])\.?/\1/g' )
echo $hostname > /etc/hostname
hostname $hostname
echo dhclient-exit-hooks.d/hostname: Dynamic Hostname = $hostname

Replace eth0 and wlan0 with the names of the interfaces from which you want to obtain the hostname. In most cases eth0 and wlan0 should stay the same.

Make sure it is readable...

chmod a+r /etc/dhcp/dhclient-exit-hooks.d/hostname

That's all. On the next dhcp response your hostname will update automatically.

Note that when using Ubuntu 18.04 the tie-in scripts are no longer necessary. If the hostname of the install is set to localhost in /etc/hostname the DHCP client will set the hostname automatically at startup using the name issued by DHCP, if present. When running hostnamectl it will list localhost as the permanent hostname, and whatever DHCP issues as a transient hostname.

testaccount@dhcp-hostname:~$ hostnamectl
   Static hostname: localhost
Transient hostname: dhcp-hostname

You can get your hostname from your DHCP server - it is part of the DHCP specification.

https://www.rfc-editor.org/rfc/rfc1533#section-3.14

"This option specifies the name of the client"

d_inevitable's answer almost solved my problem, but not completely. The problem was that although:

1. The DHCP server was sending a hostname (by adding the

   option host name 'client1' 
   

   in the dhcpd.conf) and I actually verified it by capturing and analyzing the contents of the DHCP offer with wireshark

2. The DHCP client was expecting the hostname from DHCP server (by adding

   request host-name 
   

   in the dhclient.conf)

The client was not getting a new hostname (easily verified by typing

hostname

in terminal and getting the old hostname, or no hostname if I had deleted the contents/file). As a result, the proposed solution by d_inevitable was only copying an empty string.

To solve that, I applied a crud solution, that generally should not be followed unless you are desperate to make it work, like I was.

First, open with edit capability the DHCP client control script:

sudo vi /sbin/dhclient-script

There, you will have to locate the function

set_hostname()

Just use the search and it should come right up. Now, at least on my computer, this function has three if-then-else conditions, encapsulated to each other:

# set host name set_hostname() { local current_hostname

if [ -n "$new_host_name" ]; then
    current_hostname=$(hostname)

    # current host name is empty, '(none)' or 'localhost' or differs from new one from DHCP
    if [ -z "$current_hostname" ] ||
       [ "$current_hostname" = '(none)' ] ||
       [ "$current_hostname" = 'localhost' ] ||
       [ "$current_hostname" = "$old_host_name" ]; then
       if [ "$new_host_name" != "$old_host_name" ]; then
           hostname "$new_host_name"
       fi
    fi
fi }

Now, what you need is to force the assignment of the new hostname to your host, no matter what. Therefore you want to comment out the two encapsulated if-then-else. The result should look something like:

# set host name set_hostname() { local current_hostname

if [ -n "$new_host_name" ]; then
    current_hostname=$(hostname)

    # current host name is empty, '(none)' or 'localhost' or differs from new one from DHCP
    #if [ -z "$current_hostname" ] ||
    #   [ "$current_hostname" = '(none)' ] ||
    #   [ "$current_hostname" = 'localhost' ] ||
    #   [ "$current_hostname" = "$old_host_name" ]; then
    #   if [ "$new_host_name" != "$old_host_name" ]; then
           hostname "$new_host_name"
    #   fi
    #fi
fi }

Now the d_inevitable's or this should work as expected. Hope that helps if you are in a similar desperate frustration as I was.

You don't get your hostname from the DHCP server.

You can send your hostname to the server, which may change the IP you're assigned. You can change what name is sent either by editing your Network Manager connection (the field is called DHCP Client ID) or you can edit (as root) /etc/dhcp/dhclient.conf. Look for the line that says:

send host-name "<hostname>";

... and change <hostname> to whatever you like.

By default Ubuntu will get its DNS settings from the router (if it sends them) but I suspect you're talking about local DNS/mDNS where you can access other computers by their hostname. This is called Ahavi or Zeroconf in Ubuntu and it's installed by default.

You should be able to access your computer by <hostname>.local

The answer depends on whether or not you are using static leases on your DHCP server. If you are, it is unnecessary to get the hostname from DNS. You can change this line in d_inevitable's solution

hostname=$(host $new_ip_address | cut -d ' ' -f 5)

to

hostname=${new_host_name}

But this should happen automatically if your hostname is originally set to localhost.localdomain, so you don't have to write a script. However, if you want to set the hostname to the FQDN, you'll need to change d_inevitable's script to

hostname=${new_host_name}.${new_domain_name}

Again, all this only works if you're using static leases.

If found that can be a dhcpclient scripts bug. http://blog.schlomo.schapiro.org/2013/11/setting-hostname-from-dhcp-in-debian.html

Try to clean $old_host_name on ip renew

echo unset old_host_name > /etc/dhcp/dhclient-enter-hooks.d/unset_old_hostname

Also static /etc/hostname seems to has prority over dhcp answer so leave it empty

> /etc/hostname

Tested on ubuntu 14.04 and dnsmasq server.

Don't have enough reputation to comment, but I'd like to piggy-back on the previous answer as it almost solved the problem for me using a dhclient hook.

I've found that using the standard ISC DHCP Server for some reason, the aforementioned hook outputs a host name with a '.' period character at the end of the hostname for some reason.

So, in the previous answer you might need "cut out" the extraneous period with a sed:

hostname=$(host $new_ip_address | cut -d ' ' -f 5)

Would become:

hostname=$(host $new_ip_address | cut -d ' ' -f 5 | sed -e "s/\.$//g")

First get an IP address for your client from your DHCP server, then you'll find the client hostname in /var/lib/dhcp/dhcpd.leases looks for the client-hostname entries. HTH