topic Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient) in Operating System - Linux

to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

Maaz — Tue, 26 Dec 2006 14:23:28 GMT

dhcp server version: dhcp-3.0.1
Server OS: rhel 4

in /etc/dhcpd.conf
range dynamic-bootp 10.10.10.25 10.10.10.19;

There are two machines say "A" and "B" that have manually configured IP Address, 10.10.10.25 and 10.10.10.20 respectively.
Point here is that I intentionally assigned the IP Address to system A and B that is also include in the IP-Address range of dhcp server.

MY Findings

1, if client is a "rhel ws/es/as" then dhcp server will assign the address as per the avaialability of address in the range defined on the dhcp server, and before assigning the address to the client, the dhcp server never checks for the IP-Address conflict.

2, if client is a "M$ XP(sp2)" then dhcp server will assign the address as per the avaialability of address in the range defined on the dhcp server, BUT before assigning the address to the client, the dhcp server CHECKS for the IP-Address confliction, i.e if the IP-
Address is already in use on the lan, the dhcp server will assign the different IP to the client

Proof:
following is the tcpdump capture, that will proof the above findings

1, tcpdump capture, when client is rhel ws/es/ws, and the address the dhcp server assigned to the client is 10.10.10.20, even though this IP is already in use on the same lan.
tcpdump output:
17:22:21.223815 :: > ff02::16: HBH icmp6: type-#143 [hlim 1]
17:22:21.691671 :: > ff02::1:ff94:7dab: icmp6: neighbor sol: who has
fe80::216:76ff:fe94:7dab
17:22:22.691461 fe80::216:76ff:fe94:7dab > ff02::2: icmp6: router
solicitation
17:22:25.978118 IP 0.0.0.0.bootpc > 255.255.255.255.bootps:
BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 300
17:22:26.037572 IP dhcp1.test.net.bootps > 10.10.10.20.bootpc:
BOOTP/DHCP, Reply, length: 300
17:22:26.691659 fe80::216:76ff:fe94:7dab > ff02::2: icmp6: router
solicitation
17:22:30.690822 fe80::216:76ff:fe94:7dab > ff02::2: icmp6: router
solicitation

2, tcpdump capture, when client is XP, and the address the dhcp server has to assign to the client is 10.10.10.25, but this IP is already in use on the same lan, the dhcp server didnt assigned 10.10.10.25
tcpdump output:
17:14:52.874723 IP 0.0.0.0.bootpc > 255.255.255.255.bootps:
BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 300
17:14:52.875970 arp who-has 10.10.10.25 tell dhcp1.test.net
17:14:52.876171 arp reply 10.10.10.25 is-at 00:11:d8:78:82:11
17:14:52.876177 IP dhcp1.test.net > 10.10.10.25: icmp 28: echo
request seq 0
17:14:52.876389 IP 10.10.10.25 > dhcp1.test.net: icmp 28: echo reply
seq 0
17:14:56.871387 IP 0.0.0.0.bootpc > 255.255.255.255.bootps:
BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 300
17:14:56.872360 arp who-has 10.10.10.24 tell dhcp1.test.net
17:14:57.000453 IP dhcp1.test.net.bootps > 10.10.10.24.bootpc:
BOOTP/DHCP, Reply, length: 300
17:14:57.872208 arp who-has 10.10.10.24 tell dhcp1.test.net
17:14:58.872056 arp who-has 10.10.10.24 tell dhcp1.test.net
17:15:03.996457 IP 0.0.0.0.bootpc > 255.255.255.255.bootps:
BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 300
17:15:03.996728 IP dhcp1.test.net.bootps > 10.10.10.24.bootpc:
BOOTP/DHCP, Reply, length: 300
17:15:03.997681 IP 0.0.0.0.bootpc > 255.255.255.255.bootps:
BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 302
17:15:04.165457 IP smtp4.scorpioon.com.bootps > 10.10.10.24.bootpc:
BOOTP/DHCP, Reply, length: 300
17:15:04.170519 arp who-has 10.10.10.24 tell 10.10.10.24
17:15:04.308601 arp who-has 10.10.10.24 tell 10.10.10.24
17:15:05.308634 arp who-has 10.10.10.24 tell 10.10.10.24

My Question here, when the client is linux.. why dhcp server didnt first checked if the IP-Address is already in use on the lan?
To whom should I credit ... to the dhcp server or to the dhcp client(xp), i.e is it the feature of dhcp server, that it didnt assigned the IP-Add that is already in used ? or does credit goes to the xp(dhcp client) ?

Regards
Maaz

Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

Steven E. Protter — Tue, 26 Dec 2006 15:18:00 GMT

Shalom Maaz,

Your conclusion lacks scientific proof.

I just encountered a pure Windows XP environment where an address had been handed out, the DHCP server handed out the same address again.

Windows threw up the duplicate IP address warning but used the address just the same as Linux did when I poppped open a DHCP machnine in vmware.

The client software for the OS is stupid and relies on the DHCP server to hand out the address correctly.

The DHCP server assumes it is the master of the universe and when a lease is expired the address can be handed out again. The designers of DHCP didn't account for an address in the DCHP address range being in deep sleep or being handed out manually. They apparently assumed good networking practice.

Good networking practice precludes your testing conditions and apparently neither microsoft or the Linux distributions bother to check if the address thats been handed out is in use. Admittedly Windows is a little better about wrning you.

SEP

Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

George Liu_4 — Tue, 26 Dec 2006 15:26:20 GMT

you don't need to make the things complicated. Have you checked the dhcpd.lease?

Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

George Liu_4 — Tue, 26 Dec 2006 15:28:50 GMT

and dhclient.conf?

Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

Vitaly Karasik_1 — Wed, 27 Dec 2006 02:27:00 GMT

Please send us relevant parts of /var/log/messages.

Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

Maaz — Wed, 27 Dec 2006 07:16:57 GMT

>range dynamic-bootp 10.10.10.25 10.10.10.19;
please read the above line as
range dynamic-bootp 10.10.10.19 10.10.10.25;

Dear SEP, George Liu, and Vitaly Karasik thanks for reply.
some points i should clear first are:
1, By "available IP-Addr" i mean that the IP-Addr has not been leased by the dhcpd, and thus not written to the dhcpd.leases file.
2, I know that dhcpd maintains the dhcpd.leases file, and never assigns/offers the IP-Addr that has already assigned to any dhcpclient.
3, I know that dhcpd always CHECKS the dhcpd.leases before assigning the IP-Addr, so that duplication never occurs.

as you can see the tcpdump output, when the client was a xp(sp2) machine, the dhcpd first checks that if the "next available IP-Addr(IP-Addr that hasnt assigned by the dhcpd to any dhcclient)" is already in use by some machine(not a dhcpclient), by "arp" and "icmp echo" request.

tcpdump output:
17:14:56.871387 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: ##request from client
BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 300
17:14:52.875970 arp who-has 10.10.10.25 tell dhcp1.test.net ##dhcpd is checking if the IP is in use
17:14:52.876171 arp reply 10.10.10.25 is-at 00:11:d8:78:82:11 ##yes the IP is already in use
17:14:52.876177 IP dhcp1.test.net > 10.10.10.25: icmp 28: echo request seq 0 ##dhcpd pings to double check the use of the IP-Address by some machine(not a dhcclient)
17:14:52.876389 IP 10.10.10.25 > dhcp1.test.net: icmp 28: echo reply seq 0 ##reply ... confirmed the IP is already in use...now offer the ANOTHER/NEXT available IP to the client.

17:14:56.872360 arp who-has 10.10.10.24 tell dhcp1.test.net##dhcpd is checking if the ANOTHER/NEXT available IP is in use.
17:14:57.000453 IP dhcp1.test.net.bootps > 10.10.10.24.bootpc: BOOTP/DHCP, Reply, length: 300 ##since arp return no result so offer the NEXT available IP to the client

but when the client was linux machine ... simply dhcpd offers the available IP-Addr(by available IP-Addr, i means that the IP has not been leased by the dhcpd, and thus not written to the dhcpd.leases file), without checking that if a machine(NOT a dhcclient) is already using the same IP-Addr(manually configured IP-Addr).

17:22:25.978118 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:16:76:94:7d:ab, length: 300
17:22:26.037572 IP dhcp1.test.net.bootps > 10.10.10.20.bootpc: BOOTP/DHCP, Reply, length: 300

if the dhcpclient is a linux machine..its clear from the tcpdump output that dhcpd didnt check if some other machine(not a dhcpclient) is already using the same IP-Addr.I have machine with a manually configured IP-Addr of 10.10.10.20, but since dhcpd neither "arp", nor "icmp echo"(as in case of xp), but only relies on the "dhcpd.leases", thats why dhcpd offered/assigned the 10.10.10.20 to the linux client.

so question remained.. to whom should I give credit.. to the dhcpd or to the win xp?

Re: to whom credit goes .. to the dhcpd or to the xp(dhcpclient)

Alexander Chuzhoy — Wed, 27 Dec 2006 08:17:06 GMT

There are 4 steps involved in DHCP communication:
1. DHCPDISCOVER - a client broadcasts for a server.
2. DHCPOFFER - a server offers an address.
3. DHCPREQUEST - A client requests that address.
4. DHCPACK - acknowledgement.

When a client renews it's IP only steps 3 and 4 are taken.

The server doesn't check anything besides it's own databases. Does it's your responsibility to make sure that there aren't any duplicates, nor other DHCP servers.
Best regards.