openstack/openstack-manuals
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Merge "network guide: add dhcp overview"

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Jenkins 2015-06-01 17:25:28 +00:00 committed by Gerrit Code Review
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doc/networking-guide/source/intro_basic_networking.rst

@ -198,8 +198,46 @@ you can view the contents of the ARP cache by using the *arp* command::
DHCP
~~~~
Hosts connected to a network use the Dynamic Host Configuration Protocol (:term:`DHCP`)
to dynamically obtain IP addresses. A DHCP server hands out the IP addresses to
network hosts, which are the DHCP clients.
DHCP clients locate the DHCP server by sending a UDP_ packet from port 68 to
address ``255.255.255.255`` on port 67. Address ``255.255.255.255`` is the local
network broadcast address: all hosts on the local network will see the UDP
packets sent to this address. However, such packets will not be forwarded to
other networks. Consequently, the DHCP server must be on the same local network
as the client, or the server will not receive the broadcast. The DHCP server
will respond by sending a UDP packet from port 67 to port 68 on the client. The
exchange looks like this:
1. The client sends a discover ("I’m a client at MAC address ``08:00:27:b9:88:74``, I need an IP address")
2. The server sends an offer ("OK ``08:00:27:b9:88:74``, I’m offering IP address ``10.10.0.112``")
3. The client sends a request ("Server ``10.10.0.131``, I would like to have IP ``10.10.0.112``")
4. The server sends an acknowledgement ("OK ``08:00:27:b9:88:74``, IP ``10.10.0.112`` is yours")
OpenStack uses a third-party program called dnsmasq_ to implement the DHCP server.
Dnsmasq writes to the syslog (normally found at /var/log/syslog), where you can observe the DHCP
request and replies::
Apr 23 15:53:46 c100-1 dhcpd: DHCPDISCOVER from 08:00:27:b9:88:74 via eth2
Apr 23 15:53:46 c100-1 dhcpd: DHCPOFFER on 10.10.0.112 to 08:00:27:b9:88:74 via eth2
Apr 23 15:53:48 c100-1 dhcpd: DHCPREQUEST for 10.10.0.112 (10.10.0.131) from 08:00:27:b9:88:74 via eth2
Apr 23 15:53:48 c100-1 dhcpd: DHCPACK on 10.10.0.112 to 08:00:27:b9:88:74 via eth2
When troubleshooting an instance that is not reachable over the network, it can
be helpful to examine this log to verify that all four steps of the DHCP
protocol were carried out for the instance in question.
.. _dnsmasq: http://www.thekelleys.org.uk/dnsmasq/doc.html
IP
~~
.. _UDP:
ICMP/TCP/UDP
~~~~~~~~~~~~