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Linux dhcp client boot process with packets traffic


 
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Operating Systems Linux Linux dhcp client boot process with packets traffic
# 1  
Old 08-12-2010
Linux dhcp client boot process with packets traffic

Hi Everybody,

I would like to know the entire network packets exchange, sequence between a linux dhcp client workstation & dhcp server as client comes up starting from OS booting stage as network service starts to the time client gets dhcp IP address. This would be in a typical LAN environment.

Thanks in advance

Last edited by pludi; 08-12-2010 at 09:09 AM..
# 2  
Old 08-12-2010
The client doesn't really have much at all to do before DHCP happens. Without an IP address, all it can do is broadcast or non-IP communication.

Here's a dump of packets between a client and my DHCP server at work as it boots. There were also some IPv6 packets in there but they're irrelevant since we don't actually use IPv6 at present.
Code:
# The first non-ipv6 packet.
# The client, 00:08:c7:77:9a:a2, broadcasts a DHCP request across the
# entire network segment.
10:14:29.584192 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype IPv4 (0x0800), length 342: 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:08:c7:77:9a:a2 (oui Unknown), length 300

# Our DHCP server, 00:23:54:d9:7f, gives a reply targeted specifically at
# 00:08:c7:77:9a:a2, to tell it to become IP 192.168.0.131.
# There may also be info such as default gateway, DNS servers,
# and PXE boot paths embedded in the reply.
10:14:29.584332 00:23:54:d9:7f:4e (oui Unknown) > 00:08:c7:77:9a:a2 (oui Unknown), ethertype IPv4 (0x0800), length 342: xxxx.xxxx.xxx.bootps > 192.168.0.131.bootpc: BOOTP/DHCP, Reply, length 300

# Here the client requests again for good measure, and gets the same reply.
10:14:29.586406 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype IPv4 (0x0800), length 351: 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:08:c7:77:9a:a2 (oui Unknown), length 309
10:14:29.587720 00:23:54:d9:7f:4e (oui Unknown) > 00:08:c7:77:9a:a2 (oui Unknown), ethertype IPv4 (0x0800), length 342: xxxx.xxxx.xxx.bootps > 192.168.0.131.bootpc: BOOTP/DHCP, Reply, length 300

# Here the client, 00:08:c7:77:9a:a2, broadcasts an ARP who-has request
# across the segment to check if the DHCP server has given us a boneheaded
# occupied IP address.  Some DHCP clients, like windows ones, don't seem to
# check this, causing IP address conflicts should the DHCP server get
# rebooted.
#
# The downside to this doublethink is time wasted.  A process that
# hypothetically could be nearly instant can take many seconds when the
# client is paranoid.
10:14:29.590811 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.0.131 tell 0.0.0.0
10:14:30.748326 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.0.131 tell 0.0.0.0
10:14:32.670568 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.0.131 tell 0.0.0.0

# About 5 seconds after the DHCP server sent its DHCP reply to the client,
# it sends an ARP request directly to the client's MAC address asking what
# IP address it has.
10:14:34.586710 00:23:54:d9:7f:4e (oui Unknown) > 00:08:c7:77:9a:a2 (oui Unknown), ethertype ARP (0x0806), length 42: arp who-has 192.168.0.131 tell xxx.xxx.xxx.xxx

# ...and does so again a second later.  The client missed the last one, busy contemplating the
# mysteries of life or something.
10:14:35.586665 00:23:54:d9:7f:4e (oui Unknown) > 00:08:c7:77:9a:a2 (oui Unknown), ethertype ARP (0x0806), length 42: arp who-has 192.168.0.131 tell xxxx.xxxx.xxx

# The client sends an ARP reply directed to the DHCP server finally.
10:14:35.586921 00:08:c7:77:9a:a2 (oui Unknown) > 00:23:54:d9:7f:4e (oui Unknown), ethertype ARP (0x0806), length 60: arp reply 192.168.0.131 is-at 00:08:c7:77:9a:a2 (oui Unknown)

# This particular interface on the client is configured for two IP addresses:
# one gets set by DHCP, the other is statically set to 192.168.0.2.
#
# To be polite, the client broadcasts an ARP request asking if anyone else
# has 192.168.0.2.
10:14:36.023579 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.0.2 (Broadcast) tell 0.0.0.0
10:14:36.866266 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.0.131 tell 192.168.0.131

# After one second, having received no answer, it asks again.
10:14:37.023771 00:08:c7:77:9a:a2 (oui Unknown) > Broadcast, ethertype ARP (0x0806), length 60: arp who-has 192.168.0.2 (Broadcast) tell 0.0.0.0

# After here ARP requests are still sent to/from it from time to time but the interface is obviously already up.


Last edited by Corona688; 08-12-2010 at 02:04 PM..
# 3  
Old 08-13-2010
Hi,

Thanks for the help, appreciate it.

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