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Introducing DHCP
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DHCP Terminology

You need to know some DHCP terminology if you're going to be able to troubleshoot dhclient or to create your own DHCP server to use on your network.

When a DHCP client receives configuration information from a DHCP server, it is in the form of a "lease". This means that the client configuration remains valid for a limited period of time, configured on the DHCP server. The information contained in that lease was also configured on the server. A DHCP server can give out much more than just a host's IP address, subnet mask, and default gateway. It can inform the client of the IP addresses for the following types of servers in a network:

cookie servers
DNS servers
finger servers
font servers
impress servers
IRC servers
log servers
lpr servers
NDS servers
WINS servers
NIS servers
NIS+ servers
NNTP servers
NTP servers
POP servers
resource location servers
SMTP servers
streettalks servers
swap servers
TFTP server name
time servers
uap servers
www servers

Whew, that's a lot of servers. In addition, DHCP can also set the client's MTU, TTL, hostname, and a few dozen other parameters. To see the complete list, check out man dhcp-options.

Once a DHCP client has a lease, it stores it in /var/db/dhclient.leases. Here is an example lease file:

lease {
  interface "ed0";
  fixed-address 2.2.2.2;
  option subnet-mask 255.255.240.0;
  option routers 2.2.2.1;
  option domain-name-servers 2.2.2.94,2.2.2.93,2.2.2.46;
  option broadcast-address 255.255.255.255;
  option dhcp-server-identifier 2.2.2.21
  option host-name "thishost";
  option domain-name "thisdomain.com";
  renew 3 2003/4/2 00:22:38;
  rebind 6 2003/4/5 02:50:06;
  expire 6 2003/4/5 23:50:06;
}

If you've been running dhclient for a while, you'll note that your lease file will have many such sections which start with lease { and end with the closing curly brace }. The information between the curly braces is the lease itself. Since leases don't last forever, each new lease assigned to the DHCP client is added to the end of the file. Your current lease will be at the very end of the file. If the file itself gets too long, it is copied over to /var/db/dhclient.leases~ and a new /var/db/dhclient.leases file is started.

How does this whole lease process work? Let's assume for a moment that somewhere on your network is a properly configured DHCP server. This server will listen on UDP port 67, waiting for lease requests from DHCP clients. Let's also assume you have a brand new DHCP client with an empty /var/db/dhclient.leases file. Remember, you made that host a DHCP client by adding this line to /etc/rc.conf:

ifconfig_ed0="DHCP"

Nothing in that line indicates the IP address of the DHCP server. This is just as well because at this moment ed0 doesn't even have an IP address; if it did, it wouldn't need to contact a DHCP server.

DHCP Time Periods

The client knows it can't use that configuration forever. A lease expiry is clearly marked in the lease. Take a look at the last 3 lines before the } in my example lease:

renew 3 2003/4/2 00:22:38;
rebind 6 2003/4/5 02:50:06;
expire 6 2003/4/5 23:50:06;

You'll note there are actually three time periods: renew, rebind, and expire. The syntax for each line is

day year/month/day hour:minute:second

where 0 means Sunday. In this example, this lease is ready to "renew" on Wednesday April 2, 2003 at 12:22:38 in the morning.

What is the difference between renew, rebind, and expire? Renew is also known as T1 or when the leased time is at 50%. When T1 occurs, the DHCP client will send a DHCPREQUEST to the DHCP server which assigned the lease. Note that the lease itself indicates the address of the DHCP server in this line:

option dhcp-server-identifier 2.2.2.21

Conclusion

This is all probably more than you thought you'd be learning about DHCP for one day. In the next article, I'll concentrate on the DHCP server and the configuration information it needs in order to assign leases to DHCP clients.

Dru Lavigne is a network and systems administrator, IT instructor, author and international speaker. She has over a decade of experience administering and teaching Netware, Microsoft, Cisco, Checkpoint, SCO, Solaris, Linux, and BSD systems. A prolific author, she pens the popular FreeBSD Basics column for O'Reilly and is author of BSD Hacks and The Best of FreeBSD Basics.

Read more FreeBSD Basics columns.

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Showing messages 1 through 7 of 7.

Broadcast of 255.255.255.255 ?
2003-05-22 19:54:41 anonymous2 [Reply | View]

I noticed that there is a broadcast of 255.255.255.255. Why? My isp (adelphia) hands me a broadcast address like that too. I thought the broadcast needed to be the network address with all bits on for the host portion.

Is it a misconfiguration by my isp, or is there some reason why they are doing this?

Broadcast of 255.255.255.255 ?
2003-05-23 03:56:07 anonymous2 [Reply | View]

This is the right way to specify it. If you mean by "all bits on" the binary representation, then you are right. 255 in Decimal can be interpreted 11111111 in binary representation. So if you take a closer look at the network address, we have
255.255.255.255 4 times 8 bit.
that is:
11111111 11111111 11111111 11111111
in binary or
FF FF FF FF in hex.

DHCP client inquire tool?
2003-05-01 10:59:57 anonymous2 [Reply | View]

Thanks for the informative DHCP article 4/17/03.
Our DHCP server has short time-leases, and does not nameserv. Our clients often reboot Monday and get different IP's. Is there a way to ask the server what IP a particular machine "down the hall" has today?

Host name lookup failure
2003-04-22 20:17:41 anonymous2 [Reply | View]

Greetings. Enjoyed the DHCP article.

Running FreeBSD 4.6.

I looked at my /var/db/dhclient.leases file and saw that I have (what looks like) a valid lease. I can ping IP addresses all day and I get a response. But when I try to ping "yahoo.com" or "google.com" I get "ping: cannot resolve yahoo.com: host name lookup failure."

I can see in the leases file that the correct domain name server addresses are being picked up but there seems to be a disconnect somewhere. Also, I do NOT have an /etc/resolv.conf file but don't know why I'd need to create one if I'm using DHCP.

Any ideas?

Thanks,

Jeff

Host name lookup failure
2003-12-15 18:04:31 anonymous2 [Reply | View]

Usually a freebsd machine will create a resolv.conf file from the DHCP option in the DHCP offer:

option domain-name-servers #.#.#.#, #.#.#.#;

1) make sure that the dns declaration in the dhcp lease is spelt correctly (i.e. is plural.)

Also you can test the dns server itself:

2) this will work if the dns server is working:
-> dnsquery -n #.#.#.# www.yahoo.com
#[replace #.#.#.# with the dns server given]
#also try dig ( dig(0) )

Production Environments
2003-04-18 04:02:20 anonymous2 [Reply | View]

It would be nice if you said a few things (since you're plannng for this to be a series) about DHCP in production environments. Typically I've seen DHCP used to handle desktops, but a lot of people are a bit wary of using it to assign IPs to production servers.

This leaves the problem of how to administer large production environments using DHCP. On the one hand, it's a hassle to assign IPs manually and on the other hand if the DHCP server goes down and someone tries to renew a lease, you lose a production system temporarily, and perhaps you lose more than one.

I know that failover and redundancy or on the horizon for ISC's dhcpd, but that's only part of the solution...

Production Environments
2003-04-24 08:41:56 Dustin Puryear | [Reply | View]

A common solution when using DHCP with servers is to have a reasonable renew time but a long expire time. That way if you lose DHCP service you won't find half of your servers without IP addresses.

You can also easily setup a backup DHCP server that comes online if it detects the primary DHCP server has died. This can be done manually and can be quite effective.

Perhaps an article re: DHCP and servers is not a bad idea.

I personally much prefer to use DHCP when assigning IPs to servers. However, there are some obvious security issues that must be considered.

DHCP Terminology

DHCP Time Periods

Conclusion

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