Debian – Can’t bind isc-dhcp-server to interface

debiandhcpdhcp-serverisc-dhcp

OS: Debian Wheezy

DHCP Server: isc-dhcp-server 4.2.2

I need dhcpd to listen to the specific interface on a machine with 2 active NICs.
I added INTERFACES="eth5" to /etc/default/isc-dhcp-server but dhcpd still listens to all interfaces:

root@server:~# netstat -46lpn | grep dhcp
udp        0      0 0.0.0.0:67              0.0.0.0:*                           3229/dhcpd
udp        0      0 0.0.0.0:2174            0.0.0.0:*                           3229/dhcpd
udp6       0      0 :::56980                :::*                                3229/dhcpd
raw        0      0 0.0.0.0:1               0.0.0.0:*               7           3229/dhcpd

Best Answer

The ISC DHCP server will only work when it binds to the all zeros address. Keep in mind that the DHCP server must communicate with clients that have no IP address, so binding to the IP associated with a specific interface doesn't make a lot of sense.

man dhcpd.conf

The local-address statement local-address address; This statement causes the DHCP server to listen for DHCP requests sent to the specified address, rather than requests sent to all addresses. Since serving directly attached DHCP clients implies that the server must respond to requests sent to the all-ones IP address, this option cannot be used if clients are on directly attached networks...it is only realistically useful for a server whose only clients are reached via unicasts, such as via DHCP relay agents.

Note: This statement is only effective if the server was compiled using the USE_SOCKETS #define statement, which is default on a small number of operating systems, and must be explicitly chosen at compile-time for all others. You can be sure if your server is compiled with USE_SOCKETS if you see lines of this format at startup:

Related Solutions

Linux:/sbin/dhclient to bind to a specific interface

DHCP by definition works by the client using the broadcast address to find all responding DHCP servers. It then chooses one of them (based on an algorithm, and AFAIK this is mostly to do with response times) and asks for an address. This address is then bound to the interface that is being configured.

Again, by definition, broadcasts are sent on all network interfaces. So, unless you are willing to write your own DHCP client (the sources are freely available), there is not much you can do about that.

However, you could use a different port. Read the man page for dhclient for details. This would then also require a DHCP server that listens on that port. The standard ISC DHCP server supports this with the -p option. Again, read the man page for dhcpd.

NTPD – How to Prevent Listening on 0.0.0.0:123

Remove all -I or --interface options from /etc/default/ntp and insert the following into your /etc/ntp.conf:

interface ignore wildcard
interface listen 127.0.0.1
interface listen ::1
# NOTE: if you want to update your time using remote machines,
# add at least one remote interface address:
#interface listen 2001:db8::1
#interface listen 192.0.2.1

An excerpt from the ntpd(1) manual page about the -i option:

This option also implies not opening other addresses, except wildcard and localhost. Please consider using the configuration file interface command, which is more versatile.

See also the Debian manual page (I could not find it in Arch Linux one) of ntp.conf(5).

Best Answer

Related Solutions

Linux:/sbin/dhclient to bind to a specific interface

NTPD – How to Prevent Listening on 0.0.0.0:123

Related Topic