Q. In Windows XP / 2000 / Vista ipconfig /all command will show me DNS server and DHCP server ip address. But, how to know DHCP server address in Linux?
A. The DHCP protocol allows a host to contact a central server which maintains a list of IP addresses which may be assigned on one or more subnets. A DHCP client may request an address from this pool, and then use it on a temporary basis for communication on network.
dhclient.leases file
Under Linux you use command dhclient to obtain and managing dhcp tasks. In order to keep track of leases across system reboots and server restarts, dhclient keeps a list of leases it has been assigned in the dhclient.leases file. On startup, after reading the dhclient.conf file, dhclient reads the dhclient.leases file to refresh its memory about what leases it has been assigned.
When a new lease is acquired, it is appended to the end of the dhclient.leases file. In order to prevent the file from becoming arbitrarily large, from time to time dhclient creates a new dhclient.leases file from its in-core lease database. The old version of the dhclient.leases file is retained under the name dhclient.leases~ until the next time dhclient rewrites the database.
Find out DHCP server address
Usually dhclient.leases file is located at /var/lib/dhcp3/dhclient.leases, type the following command:
less /var/lib/dhcp3/dhclient.leases
OR
cat /var/lib/dhcp3/dhclient.leases
Alternatively, you can just use grep command to get DHCP server address, enter:
grep dhcp-server-identifier /var/lib/dhcp3/dhclient.leases
Output:
lease {
interface "ra0";
fixed-address 192.168.1.106;
option subnet-mask 255.255.255.0;
option dhcp-lease-time 86400;
option routers 192.168.1.1;
option dhcp-message-type 5;
option dhcp-server-identifier 192.168.1.1;
option domain-name-servers 208.67.222.222,208.67.220.220;
option dhcp-renewal-time 43200;
option dhcp-rebinding-time 75600;
option host-name "vivek-desktop";
renew 0 2007/12/9 05:17:36;
rebind 0 2007/12/9 15:06:37;
expire 0 2007/12/9 18:06:37;
}
lease {
interface "ra0";
fixed-address 192.168.1.106;
option subnet-mask 255.255.255.0;
option routers 192.168.1.1;
option dhcp-lease-time 86400;
option dhcp-message-type 5;
option domain-name-servers 208.67.222.222,208.67.220.220;
option dhcp-server-identifier 192.168.1.1;
option dhcp-renewal-time 43200;
option dhcp-rebinding-time 75600;
option host-name "vivek-desktop";
renew 0 2007/12/9 06:11:22;
rebind 0 2007/12/9 16:13:50;
expire 0 2007/12/9 19:13:50;
}
A note about RHEL / CentOS / Fedora Linux user
Redhat and friends uses /var/lib/dhcp/dhclient.leases file:
less /var/lib/dhcp/dhclient.leases
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{ 8 comments… read them below or add one }
In case you don’t have a lease, you could (but it’s more work) request a lease with dhclient(8) and use tcpdump(8) to see what server answered. Use something like this in one terminal:
# tcpdump -i eth0 port 67 or port 68
And something like this in a second terminal:
# dhclient eth0
If you are lucky, dhclient will even tell you what the DHCP server is.
Regards, Robert.
Robert,
When you run dhclient eth0, it always show DHCP address on screen, just look for DHCPACK (no need to run tcpdump)
How can I get just DHCPACK value from code(C,C++) ?
I need to know just my DHCP server address value from code, so if there is some script or a neath way to do it, without parsing the dhclient.leasing file?
Run this to get just your DHCP Server name:
cat dhclient.leases | grep dhcp-server | uniq | cut -d” ” -f5 | cut -d”;” -f1
Mine is at
/var/lib/dhclient/dhclient-eth0.leases
so the server would be
grep dhcp-server /var/lib/dhclient/dhclient-eth0.leases
Thanks for the good post admin
and thanks to robert for telling how to get to know the dhcp server ip when their is no leases
my case was also same as robert explain
thanks
sir
iam kotesh pleces data to dhcp and dns server