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090_applicatifs:165dhcp:40-analyse_protocole [le 16/02/2025 à 14:36] – supprimée - modification externe (Date inconnue) 127.0.0.1090_applicatifs:165dhcp:40-analyse_protocole [le 16/02/2025 à 14:36] (Version actuelle) – ↷ Page déplacée de 050_applicatifs:165dhcp:40-analyse_protocole à 090_applicatifs:165dhcp:40-analyse_protocole prof
Ligne 1: Ligne 1:
  
 +====== Analyse du protocole ======
 +===== Premier démarrage de la station =====
 +==== Résumé de la capture ====
 +<code>
 +No.     Time        Source                Destination           Protocol Info
 +      1 66.901361   0.0.0.0               255.255.255.255       DHCP     DHCP Discover - Transaction ID 0xc0b5592f
 +      2 66.901867   192.168.0.252         192.168.0.67          ICMP     Echo (ping) request
 +      3 67.902846   192.168.0.252         192.168.0.67          DHCP     DHCP Offer    - Transaction ID 0xc0b5592f
 +      4 67.904780   0.0.0.0               255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      5 67.930502   192.168.0.252         192.168.0.67          DHCP     DHCP ACK      - Transaction ID 0xc0b5592f
 +</code>
 +  - Le client effectue une découverte de serveur DHCP. Il n'a pas encore d'adresse IP et adopte donc l'adresse factice 0.0.0.0. En réalité c'est bien sûr au niveau Ethernet que les adresses seront significatives, l'analyse approfondie le montrera.
 +  - Le serveur (192.168.0.252) effectue un ping sur l'adresse 192.168.0.67, parce qu'il a l'intention d'attribuer cette adresse au client. S'il recevait une réponse au ping, cela voudrait dirue que cette adresse est déjà en service sur le réseau, à cause d'une anomalie quelconque. Il n'y a pas de réponse au ping, ce qui est à priori normal sur un réseau normalement géré.
 +  - Le serveur offre une proposition au client.
 +  - Le client fait une contre-proposition. L'analyse détaillée va montrer qu'en principe, elle est identique à la proposition du serveur.
 +  - Le serveur accepte la contre-proposition. Le bail est donc validé par les deux protagonistes.
 +==== Analyse détaillée ====
 +=== Discover ===
 +<html><pre class="code">
 +Frame 1 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 10:13:07.933412000
 +    [Time delta from previous captured frame: 66.901361000 seconds]
 +    [Time delta from previous displayed frame: 66.901361000 seconds]
 +    [Time since reference or first frame: 66.901361000 seconds]
 +    Frame Number: 2
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +<span class="hly">Ethernet II, Src: QuantaCo_51:5d:5a (00:16:36:51:5d:5a), Dst: Broadcast (ff:ff:ff:ff:ff:ff)</span>
 +<span class="txtb">En voilà un joli broadcast ethernet...</span>
 +    Destination: Broadcast (ff:ff:ff:ff:ff:ff)
 +        Address: Broadcast (ff:ff:ff:ff:ff:ff)
 +        .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
 +        .... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
 +    Source: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +<span class="hly">Internet Protocol, Src: 0.0.0.0 (0.0.0.0), Dst: 255.255.255.255 (255.255.255.255)</span>
 +<span class="txtb">Broadcast qui se retrouve sur la couche IP</span>
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
 +        0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x00
 +        0... = Reserved bit: Not set
 +        .0.. = Don't fragment: Not set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 128
 +    Protocol: UDP (0x11)
 +    Header checksum: 0x3996 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 0.0.0.0 (0.0.0.0)
 +    Destination: 255.255.255.255 (255.255.255.255)
 +User Datagram Protocol, Src Port: bootpc (68), Dst Port: bootps (67)
 +    Source port: bootpc (68)
 +    Destination port: bootps (67)
 +    Length: 308
 +    Checksum: 0x746b [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Request (1)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 0
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +    Client IP address: 0.0.0.0 (0.0.0.0)
 +    Your (client) IP address: 0.0.0.0 (0.0.0.0)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +<span class="hly">    Option: (t=53,l=1) DHCP Message Type = DHCP Discover
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 01
 +    Option: (t=55,l=12) Parameter Request List
 +        Option: (55) Parameter Request List
 +        Length: 12
 +        Value: 011C02030F06770C2C2F1A79
 +        1 = Subnet Mask
 +        28 = Broadcast Address
 +        2 = Time Offset
 +        3 = Router
 +        15 = Domain Name
 +        6 = Domain Name Server
 +        119 = Domain Search
 +        12 = Host Name
 +        44 = NetBIOS over TCP/IP Name Server
 +        47 = NetBIOS over TCP/IP Scope
 +        26 = Interface MTU
 +        121 = Classless Static Route
 +    End Option</span>
 +    Padding
 +</pre></html>
 +Nous avons dans cette requête la liste des paramètres que le client souhaite recevoir, en plus bien entendu de son adresse IP.
 +
 +Cette capture est également l'occasion de constater que DHCP utilise UDP, sur le port 67 pour le client et le port 68 pour le serveur.
 +
 +=== Ping ===
 +<html><pre class="code">
 +Frame 2 (62 bytes on wire, 62 bytes captured)
 +    Arrival Time: May  8, 2009 10:13:07.933918000
 +    [Time delta from previous captured frame: 0.000506000 seconds]
 +    [Time delta from previous displayed frame: 0.000506000 seconds]
 +    [Time since reference or first frame: 66.901867000 seconds]
 +    Frame Number: 3
 +    Frame Length: 62 bytes
 +    Capture Length: 62 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:icmp:data]
 +    [Coloring Rule Name: ICMP]
 +    [Coloring Rule String: icmp]
 +Ethernet II, Src: D-Link_48:2b:84 (00:05:5d:48:2b:84), Dst: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Destination: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Source: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        Address: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +Internet Protocol, Src: 192.168.0.252 (192.168.0.252), Dst: 192.168.0.67 (192.168.0.67)
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
 +        0000 00.. = Differentiated Services Codepoint: Default (0x00)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 48
 +    Identification: 0x0000 (0)
 +    Flags: 0x04 (Don't Fragment)
 +        0... = Reserved bit: Not set
 +        .1.. = Don't fragment: Set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 64
 +    Protocol: ICMP (0x01)
 +    Header checksum: 0xb83d [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.252 (192.168.0.252)
 +    Destination: 192.168.0.67 (192.168.0.67)
 +Internet Control Message Protocol
 +    Type: 8 (Echo (ping) request)
 +    Code: 0 ()
 +    Checksum: 0xd3c8 [correct]
 +    Identifier: 0x2437
 +    Sequence number: 0 (0x0000)
 +    Data (20 bytes)
 +
 +0000  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
 +0010  00 00 00 00                                       ....
 +        Data: 0000000000000000000000000000000000000000
 +</pre></html>
 +Rien de bien particulier, un ping ICMP classique que le serveur fait sur l'adresse qu'il compte fournir à son client.
 +
 +=== Offer ===
 +<html><pre class="code">
 +Frame 3 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 10:13:08.934897000
 +    [Time delta from previous captured frame: 1.000979000 seconds]
 +    [Time delta from previous displayed frame: 1.000979000 seconds]
 +    [Time since reference or first frame: 67.902846000 seconds]
 +    Frame Number: 4
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +<span class="hly">Ethernet II, Src: D-Link_48:2b:84 (00:05:5d:48:2b:84), Dst: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)</span>
 +<span class="txtb">Ici, ce n'est plus du broadcast</span>
 +    Destination: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Source: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        Address: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +<span class="hly">Internet Protocol, Src: 192.168.0.252 (192.168.0.252), Dst: 192.168.0.67 (192.168.0.67)</span>
 +<span class="txtb">Le serveur répond au client sur sa potentielle future adresse IP.
 +Notez bien que le client ne la connait pas encore...</span>
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
 +        0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x00
 +        0... = Reserved bit: Not set
 +        .0.. = Don't fragment: Not set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 128
 +    Protocol: UDP (0x11)
 +    Header checksum: 0xb705 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.252 (192.168.0.252)
 +    Destination: 192.168.0.67 (192.168.0.67)
 +User Datagram Protocol, Src Port: bootps (67), Dst Port: bootpc (68)
 +    Source port: bootps (67)
 +    Destination port: bootpc (68)
 +    Length: 308
 +    Checksum: 0x2a4d [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Reply (2)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 0
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +<span class="hly">    Client IP address: 0.0.0.0 (0.0.0.0)
 +    Your (client) IP address: 192.168.0.67 (192.168.0.67)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +    Option: (t=53,l=1) DHCP Message Type = DHCP Offer
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 02
 +    Option: (t=54,l=4) Server Identifier = 192.168.0.252
 +        Option: (54) Server Identifier
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=51,l=4) IP Address Lease Time = 1 hour
 +        Option: (51) IP Address Lease Time
 +        Length: 4
 +        Value: 00000E10
 +    Option: (t=1,l=4) Subnet Mask = 255.255.255.0
 +        Option: (1) Subnet Mask
 +        Length: 4
 +        Value: FFFFFF00
 +    Option: (t=3,l=4) Router = 192.168.0.252
 +        Option: (3) Router
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=15,l=10) Domain Name = "maison.mrs"
 +        Option: (15) Domain Name
 +        Length: 10
 +        Value: 6D6169736F6E2E6D7273
 +    Option: (t=6,l=4) Domain Name Server = 192.168.0.252
 +        Option: (6) Domain Name Server
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=44,l=4) NetBIOS over TCP/IP Name Server = 192.168.0.252
 +        Option: (44) NetBIOS over TCP/IP Name Server
 +        Length: 4
 +        Value: C0A800FC
 +    End Option</span>
 +    Padding
 +</pre></html>
 +Le serveur propose donc à notre client une configuration complète, avec tous les paramètres demandés que le serveur est en état de fournir.
 +
 +=== Request ===
 +<html><pre class="code">
 +Frame 4 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 10:13:08.936831000
 +    [Time delta from previous captured frame: 0.001934000 seconds]
 +    [Time delta from previous displayed frame: 0.001934000 seconds]
 +    [Time since reference or first frame: 67.904780000 seconds]
 +    Frame Number: 5
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +<span class="hly">Ethernet II, Src: QuantaCo_51:5d:5a (00:16:36:51:5d:5a), Dst: Broadcast (ff:ff:ff:ff:ff:ff)</span>
 +    Destination: Broadcast (ff:ff:ff:ff:ff:ff)
 +        Address: Broadcast (ff:ff:ff:ff:ff:ff)
 +        .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
 +        .... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
 +    Source: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +<span class="hly">Internet Protocol, Src: 0.0.0.0 (0.0.0.0), Dst: 255.255.255.255 (255.255.255.255)</span>
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
 +        0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x00
 +        0... = Reserved bit: Not set
 +        .0.. = Don't fragment: Not set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 128
 +    Protocol: UDP (0x11)
 +    Header checksum: 0x3996 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 0.0.0.0 (0.0.0.0)
 +    Destination: 255.255.255.255 (255.255.255.255)
 +User Datagram Protocol, Src Port: bootpc (68), Dst Port: bootps (67)
 +    Source port: bootpc (68)
 +    Destination port: bootps (67)
 +    Length: 308
 +    Checksum: 0xd980 [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Request (1)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 0
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +    Client IP address: 0.0.0.0 (0.0.0.0)
 +    Your (client) IP address: 0.0.0.0 (0.0.0.0)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +<span class="hly">    Option: (t=53,l=1) DHCP Message Type = DHCP Request
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 03
 +<span class="bhly">    Option: (t=54,l=4) Server Identifier = 192.168.0.252</span>
 +        Option: (54) Server Identifier
 +        Length: 4
 +        Value: C0A800FC
 +<span class="bhly">    Option: (t=50,l=4) Requested IP Address = 192.168.0.67</span>
 +        Option: (50) Requested IP Address
 +        Length: 4
 +        Value: C0A80043
 +    Option: (t=55,l=12) Parameter Request List
 +        Option: (55) Parameter Request List
 +        Length: 12
 +        Value: 011C02030F06770C2C2F1A79
 +        1 = Subnet Mask
 +        28 = Broadcast Address
 +        2 = Time Offset
 +        3 = Router
 +        15 = Domain Name
 +        6 = Domain Name Server
 +        119 = Domain Search
 +        12 = Host Name
 +        44 = NetBIOS over TCP/IP Name Server
 +        47 = NetBIOS over TCP/IP Scope
 +        26 = Interface MTU
 +        121 = Classless Static Route
 +    End Option</span>
 +    Padding
 +</pre></html>
 +Notre client effectue sa requête, toujours en broadcast. Il indique cependant :
 +  * l'adresse IP du serveur DHCP auprès duquel il fait la demande, évitant ainsi, s'il y a d'autres serveurs, qu'ils poursuivent le dialogue ;
 +  * l'adresse IP qu'il accepte.
 +Il n'a pas d'autres exigences.
 +
 +=== ACK ===
 +<html><pre class="code">
 +Frame 5 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 10:13:08.962553000
 +    [Time delta from previous captured frame: 0.025722000 seconds]
 +    [Time delta from previous displayed frame: 0.025722000 seconds]
 +    [Time since reference or first frame: 67.930502000 seconds]
 +    Frame Number: 6
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +Ethernet II, Src: D-Link_48:2b:84 (00:05:5d:48:2b:84), Dst: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Destination: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Source: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        Address: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +Internet Protocol, Src: 192.168.0.252 (192.168.0.252), Dst: 192.168.0.67 (192.168.0.67)
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
 +        0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x00
 +        0... = Reserved bit: Not set
 +        .0.. = Don't fragment: Not set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 128
 +    Protocol: UDP (0x11)
 +    Header checksum: 0xb705 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.252 (192.168.0.252)
 +    Destination: 192.168.0.67 (192.168.0.67)
 +User Datagram Protocol, Src Port: bootps (67), Dst Port: bootpc (68)
 +    Source port: bootps (67)
 +    Destination port: bootpc (68)
 +    Length: 308
 +    Checksum: 0x274d [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Reply (2)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 0
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +    Client IP address: 0.0.0.0 (0.0.0.0)
 +    Your (client) IP address: 192.168.0.67 (192.168.0.67)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +    Option: (t=53,l=1) DHCP Message Type = DHCP ACK
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 05
 +    Option: (t=54,l=4) Server Identifier = 192.168.0.252
 +        Option: (54) Server Identifier
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=51,l=4) IP Address Lease Time = 1 hour
 +        Option: (51) IP Address Lease Time
 +        Length: 4
 +        Value: 00000E10
 +    Option: (t=1,l=4) Subnet Mask = 255.255.255.0
 +        Option: (1) Subnet Mask
 +        Length: 4
 +        Value: FFFFFF00
 +    Option: (t=3,l=4) Router = 192.168.0.252
 +        Option: (3) Router
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=15,l=10) Domain Name = "maison.mrs"
 +        Option: (15) Domain Name
 +        Length: 10
 +        Value: 6D6169736F6E2E6D7273
 +    Option: (t=6,l=4) Domain Name Server = 192.168.0.252
 +        Option: (6) Domain Name Server
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=44,l=4) NetBIOS over TCP/IP Name Server = 192.168.0.252
 +        Option: (44) NetBIOS over TCP/IP Name Server
 +        Length: 4
 +        Value: C0A800FC
 +    End Option
 +    Padding
 +</pre></html>
 +Le serveur donne donc son accord pour le bail avec ses paramètres définitifs. Il n'y a plus ici de broadcast, le serveur s'adresse en unicast à son client.
 +
 +===== Renouvèlement =====
 +Lorsque nous sommes arrivé à l'heure ''renew'', notre client va contacter le serveur DHCP :
 +<html><pre class="code">
 +No.     Time        Source                Destination           Protocol Info
 +      1 0.000000    192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +
 +Frame 1 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 10:38:01.936555000
 +    [Time delta from previous captured frame: 0.000000000 seconds]
 +    [Time delta from previous displayed frame: 0.000000000 seconds]
 +    [Time since reference or first frame: 0.000000000 seconds]
 +    Frame Number: 1
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +Ethernet II, Src: QuantaCo_51:5d:5a (00:16:36:51:5d:5a), Dst: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +    Destination: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        Address: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Source: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +Internet Protocol, Src: 192.168.0.67 (192.168.0.67), Dst: 192.168.0.252 (192.168.0.252)
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
 +        0000 00.. = Differentiated Services Codepoint: Default (0x00)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x04 (Don't Fragment)
 +        0... = Reserved bit: Not set
 +        .1.. = Don't fragment: Set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 64
 +    Protocol: UDP (0x11)
 +    Header checksum: 0xb715 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.67 (192.168.0.67)
 +    Destination: 192.168.0.252 (192.168.0.252)
 +User Datagram Protocol, Src Port: bootpc (68), Dst Port: bootps (67)
 +    Source port: bootpc (68)
 +    Destination port: bootps (67)
 +    Length: 308
 +    Checksum: 0x2eef [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Request (1)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 0
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +<span class="hly">    Client IP address: 192.168.0.67 (192.168.0.67)</span>
 +    Your (client) IP address: 0.0.0.0 (0.0.0.0)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +<span class="hly">    Option: (t=53,l=1) DHCP Message Type = DHCP Request
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 03
 +    Option: (t=55,l=12) Parameter Request List
 +        Option: (55) Parameter Request List
 +        Length: 12
 +        Value: 011C02030F06770C2C2F1A79
 +        1 = Subnet Mask
 +        28 = Broadcast Address
 +        2 = Time Offset
 +        3 = Router
 +        15 = Domain Name
 +        6 = Domain Name Server
 +        119 = Domain Search
 +        12 = Host Name
 +        44 = NetBIOS over TCP/IP Name Server
 +        47 = NetBIOS over TCP/IP Scope
 +        26 = Interface MTU
 +        121 = Classless Static Route
 +    End Option</span>
 +    Padding
 +</pre></html>
 +Et le serveur répond :
 +<html><pre class="code">
 +No.     Time        Source                Destination           Protocol Info
 +      2 0.027503    192.168.0.252         192.168.0.67          DHCP     DHCP ACK      - Transaction ID 0xc0b5592f
 +
 +Frame 2 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 10:38:01.964058000
 +    [Time delta from previous captured frame: 0.027503000 seconds]
 +    [Time delta from previous displayed frame: 0.027503000 seconds]
 +    [Time since reference or first frame: 0.027503000 seconds]
 +    Frame Number: 2
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +Ethernet II, Src: D-Link_48:2b:84 (00:05:5d:48:2b:84), Dst: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Destination: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Source: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        Address: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +Internet Protocol, Src: 192.168.0.252 (192.168.0.252), Dst: 192.168.0.67 (192.168.0.67)
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
 +        0000 00.. = Differentiated Services Codepoint: Default (0x00)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x04 (Don't Fragment)
 +        0... = Reserved bit: Not set
 +        .1.. = Don't fragment: Set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 64
 +    Protocol: UDP (0x11)
 +    Header checksum: 0xb715 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.252 (192.168.0.252)
 +    Destination: 192.168.0.67 (192.168.0.67)
 +User Datagram Protocol, Src Port: bootps (67), Dst Port: bootpc (68)
 +    Source port: bootps (67)
 +    Destination port: bootpc (68)
 +    Length: 308
 +    Checksum: 0x6661 [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Reply (2)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 0
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +    Client IP address: 192.168.0.67 (192.168.0.67)
 +<span class="hly">    Your (client) IP address: 192.168.0.67 (192.168.0.67)</span>
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +<span class="hly">    Option: (t=53,l=1) DHCP Message Type = DHCP ACK
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 05
 +    Option: (t=54,l=4) Server Identifier = 192.168.0.252
 +        Option: (54) Server Identifier
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=51,l=4) IP Address Lease Time = 1 hour
 +        Option: (51) IP Address Lease Time
 +        Length: 4
 +        Value: 00000E10
 +    Option: (t=1,l=4) Subnet Mask = 255.255.255.0
 +        Option: (1) Subnet Mask
 +        Length: 4
 +        Value: FFFFFF00
 +    Option: (t=3,l=4) Router = 192.168.0.252
 +        Option: (3) Router
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=15,l=10) Domain Name = "maison.mrs"
 +        Option: (15) Domain Name
 +        Length: 10
 +        Value: 6D6169736F6E2E6D7273
 +    Option: (t=6,l=4) Domain Name Server = 192.168.0.252
 +        Option: (6) Domain Name Server
 +        Length: 4
 +        Value: C0A800FC
 +    Option: (t=44,l=4) NetBIOS over TCP/IP Name Server = 192.168.0.252
 +        Option: (44) NetBIOS over TCP/IP Name Server
 +        Length: 4
 +        Value: C0A800FC
 +    End Option</span>
 +    Padding
 +</pre></html>
 +Notez que dans le dialogue, le client annonce cette fois-ci son adresse IP et que le serveur la lui confirme. Toutes les autres options peuvent changer d'un bail à l'autre, ce qui permet, lorsque l'administrateur a planifié par exemple un changement d'adresse de passerelle ou de DNS, de s'arranger pour que la modification se passe en douceur dans un laps de temps que l'on peut estimer.
 +
 +Ce renouvèlement se fait entièrement en mode unicast.
 +
 +===== Le grain de sable =====
 +Tout ceci est parfait, mais imaginons que notre serveur DHCP tombe en panne. Que va-t-il se produire ? Faisons la manip. Nous jouons un sale tour à notre client en posant sur le serveur DHCP la règle IPtables :
 +  iptables -A OUTPUT -d 192.168.0.67  -j DROP
 +Alors...
 +
 +==== Renew ====
 +A l'heure dite, le client va lancer un ''renew'', mais le serveur ne répond pas...
 +
 +Le client insiste :
 +<code>
 +No.     Time        Source                Destination           Protocol Info
 +      1 0.000000    192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      2 4.995823    192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      3 14.995826   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      4 29.995825   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      5 44.995826   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      6 51.995826   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      7 62.995841   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      8 74.995824   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      9 95.995830   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     10 116.995824  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     11 129.995829  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     12 148.995836  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     13 163.995835  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     14 177.995833  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     15 193.995844  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     16 211.995839  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     17 225.995830  192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +...
 +</code>
 +Admirez la patience (obstination ?) de notre client qui va sans relâche insister jusqu'à :
 +<html><pre class="code">
 +No.     Time        Source                Destination           Protocol Info
 +      1 0.000000    192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      2 15.000000   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      3 23.999995   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      4 39.999999   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      5 58.000005   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      6 78.999998   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +<span class="hly">      7 89.999997   192.168.0.67          192.168.0.252         DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      8 111.000002  192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f</span>
 +      9 131.999996  192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     10 151.999988  192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +     11 159.999990  192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +</pre></html>
 +Notre client change de tactique. Il n'interroge plus 192.168.0.252. Il a fini par se rendre à l'évidence : ce serveur est hors service. Il va alors, tout en conservant son adresse IP actuelle, commencer une série de requêtes en broadcast, des fois qu'un bon admin aurait mis en place un autre DHCP, mais avec une autre adresse IP.
 +
 +Voyons le détail des paquets 7 et 8 :
 +<html><pre class="code">
 +Frame 7 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 16:25:50.932358000
 +    [Time delta from previous captured frame: 10.999999000 seconds]
 +    [Time delta from previous displayed frame: 10.999999000 seconds]
 +    [Time since reference or first frame: 89.999997000 seconds]
 +    Frame Number: 7
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +<span class="hly">Ethernet II, Src: QuantaCo_51:5d:5a (00:16:36:51:5d:5a), Dst: D-Link_48:2b:84 (00:05:5d:48:2b:84)</span>
 +    Destination: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        Address: D-Link_48:2b:84 (00:05:5d:48:2b:84)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Source: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +<span class="hly">Internet Protocol, Src: 192.168.0.67 (192.168.0.67), Dst: 192.168.0.252 (192.168.0.252)</span>
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
 +        0000 00.. = Differentiated Services Codepoint: Default (0x00)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x04 (Don't Fragment)
 +        0... = Reserved bit: Not set
 +        .1.. = Don't fragment: Set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 64
 +    Protocol: UDP (0x11)
 +    Header checksum: 0xb715 [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.67 (192.168.0.67)
 +    Destination: 192.168.0.252 (192.168.0.252)
 +User Datagram Protocol, Src Port: bootpc (68), Dst Port: bootps (67)
 +    Source port: bootpc (68)
 +    Destination port: bootps (67)
 +    Length: 308
 +    Checksum: 0x27f7 [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Request (1)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 1784
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +    Client IP address: 192.168.0.67 (192.168.0.67)
 +    Your (client) IP address: 0.0.0.0 (0.0.0.0)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +    Option: (t=53,l=1) DHCP Message Type = DHCP Request
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 03
 +    Option: (t=55,l=12) Parameter Request List
 +        Option: (55) Parameter Request List
 +        Length: 12
 +        Value: 011C02030F06770C2C2F1A79
 +        1 = Subnet Mask
 +        28 = Broadcast Address
 +        2 = Time Offset
 +        3 = Router
 +        15 = Domain Name
 +        6 = Domain Name Server
 +        119 = Domain Search
 +        12 = Host Name
 +        44 = NetBIOS over TCP/IP Name Server
 +        47 = NetBIOS over TCP/IP Scope
 +        26 = Interface MTU
 +        121 = Classless Static Route
 +    End Option
 +    Padding
 +</pre></html>
 +Paquet 7, la requête est bien encore unicast. Dans la suivante :
 +<html><pre class="code">
 +Frame 8 (342 bytes on wire, 342 bytes captured)
 +    Arrival Time: May  8, 2009 16:26:11.932363000
 +    [Time delta from previous captured frame: 21.000005000 seconds]
 +    [Time delta from previous displayed frame: 21.000005000 seconds]
 +    [Time since reference or first frame: 111.000002000 seconds]
 +    Frame Number: 8
 +    Frame Length: 342 bytes
 +    Capture Length: 342 bytes
 +    [Frame is marked: False]
 +    [Protocols in frame: eth:ip:udp:bootp]
 +    [Coloring Rule Name: UDP]
 +    [Coloring Rule String: udp]
 +<span class="hly">Ethernet II, Src: QuantaCo_51:5d:5a (00:16:36:51:5d:5a), Dst: Broadcast (ff:ff:ff:ff:ff:ff)</span>
 +    Destination: Broadcast (ff:ff:ff:ff:ff:ff)
 +        Address: Broadcast (ff:ff:ff:ff:ff:ff)
 +        .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
 +        .... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
 +    Source: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        Address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
 +        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
 +    Type: IP (0x0800)
 +<span class="hly">Internet Protocol, Src: 192.168.0.67 (192.168.0.67), Dst: 255.255.255.255 (255.255.255.255)</span>
 +    Version: 4
 +    Header length: 20 bytes
 +    Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
 +        0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
 +        .... ..0. = ECN-Capable Transport (ECT): 0
 +        .... ...0 = ECN-CE: 0
 +    Total Length: 328
 +    Identification: 0x0000 (0)
 +    Flags: 0x00
 +        0... = Reserved bit: Not set
 +        .0.. = Don't fragment: Not set
 +        ..0. = More fragments: Not set
 +    Fragment offset: 0
 +    Time to live: 128
 +    Protocol: UDP (0x11)
 +    Header checksum: 0x78aa [correct]
 +        [Good: True]
 +        [Bad : False]
 +    Source: 192.168.0.67 (192.168.0.67)
 +    Destination: 255.255.255.255 (255.255.255.255)
 +User Datagram Protocol, Src Port: bootpc (68), Dst Port: bootps (67)
 +    Source port: bootpc (68)
 +    Destination port: bootps (67)
 +    Length: 308
 +    Checksum: 0xe986 [correct]
 +        [Good Checksum: True]
 +        [Bad Checksum: False]
 +Bootstrap Protocol
 +    Message type: Boot Request (1)
 +    Hardware type: Ethernet
 +    Hardware address length: 6
 +    Hops: 0
 +    Transaction ID: 0xc0b5592f
 +    Seconds elapsed: 1805
 +    Bootp flags: 0x0000 (Unicast)
 +        0... .... .... .... = Broadcast flag: Unicast
 +        .000 0000 0000 0000 = Reserved flags: 0x0000
 +    Client IP address: 192.168.0.67 (192.168.0.67)
 +    Your (client) IP address: 0.0.0.0 (0.0.0.0)
 +    Next server IP address: 0.0.0.0 (0.0.0.0)
 +    Relay agent IP address: 0.0.0.0 (0.0.0.0)
 +    Client MAC address: QuantaCo_51:5d:5a (00:16:36:51:5d:5a)
 +    Server host name not given
 +    Boot file name not given
 +    Magic cookie: (OK)
 +    Option: (t=53,l=1) DHCP Message Type = DHCP Request
 +        Option: (53) DHCP Message Type
 +        Length: 1
 +        Value: 03
 +    Option: (t=55,l=12) Parameter Request List
 +        Option: (55) Parameter Request List
 +        Length: 12
 +        Value: 011C02030F06770C2C2F1A79
 +        1 = Subnet Mask
 +        28 = Broadcast Address
 +        2 = Time Offset
 +        3 = Router
 +        15 = Domain Name
 +        6 = Domain Name Server
 +        119 = Domain Search
 +        12 = Host Name
 +        44 = NetBIOS over TCP/IP Name Server
 +        47 = NetBIOS over TCP/IP Scope
 +        26 = Interface MTU
 +        121 = Classless Static Route
 +    End Option
 +    Padding
 +</pre></html>
 +Nous avons bien ici du broadcast, mais le reste de la requête reste inchangé. Le client conserve l'espoir de trouver un autre serveur DHCP qui lui renouvellera son bail actuel. Notre client a changé de tactique à l'heure ''rebind'' indiquée dans le bail précédent.
 +
 +Cependant notre règle IPtables est encore plus obstinée que notre client, il n'y a pas de nouveau serveur DHCP sur le réseau et finalement, le bail expire à l'heure ''expire'' :
 +
 +==== Mort (et résurection) ====
 +<html><pre class="code">
 +No.     Time        Source                Destination           Protocol Info
 +      1 0.000000    192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      2 14.999984   192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      3 24.999987   192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      4 38.999972   192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      5 47.999964   192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +      6 64.999972   192.168.0.67          255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xc0b5592f
 +<span class="hly">      7 71.045835   0.0.0.0               255.255.255.255       DHCP     DHCP Discover - Transaction ID 0xf4b2bf16</span>
 +<span class="bhlo">      8 71.046413   192.168.0.252         192.168.0.67          DHCP     DHCP Offer    - Transaction ID 0xf4b2bf16
 +      9 71.046704   0.0.0.0               255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xf4b2bf16
 +     10 71.115898   192.168.0.252         192.168.0.67          DHCP     DHCP ACK      - Transaction ID 0xf4b2bf16</span>
 +</pre></html>
 +La trame 7 montre que notre client a perdu son adresse IP, mais il ne renonce toujours pas. Il fait maintenant une recherche de DHCP (Discover) et là, le miracle se produit, il redécouvre notre DHCP qui lui attribue alors un bail tout neuf. Contre toute attente, l'histoire finit bien.
 +
 +Mais est-ce vraiment un miracle ?
 +
 +En réalité, l'obstination de notre client a eu raison de celle de la règle IPtables. En effet, notre Netfilter ne laisse rien sortir vers 192.168.0.67, mais notre client a repris l'adresse factice 0.0.0.0 et Netfilter, leurré, laisse tomber son fromage. Notre client ne manque alors pas de s'en saisir.
 +
 +S'il s'était agi d'une vraie panne de DHCP, il n'y aurait pas eu de fromage et notre client serait resté le bec dans l'eau.
 +===== Conclusion =====
 +Nous avons pu constater ici que DHCP est un protocole extrêmement opiniâtre et prudent. Le client se laisse de la marge en cas d'accident et commence à demander un renouvèlement bien avant l'heure d'expiration, en cas d'accident, il essaye de retrouver un autre serveur qui lui renouvèlerait son bail, et même mort, il essaye encore.
 +
 +Il n'aura pas échappé au lecteur attentif que, bien que le bail ait expiré, le nouveau bail récupéré par tromperie de Netfilter propose la même adresse IP que la précédente. Est-ce un hasard ?
 +
 +Pas du tout. En réalité le serveur garde en mémoire toutes les informations concernant les baux qu'il distribue et dans toute la mesure du possible, cherchera à attribuer la même adresse IP à une adresse MAC donnée. C'est généralement le cas, sauf lorsqu'il y a pénurie d'adresses IP. Le serveur est alors obligé de donner des adresses déjà attribuées, mais libérées, à de nouveaux clients.
 +
 +Nous n'avons pas vu en détail toutes les possibilités de DHCP, mais ce chapitre a pu montrer le principe de base. Un client peut avoir quelques exigences sur divers paramètres, qu'il va alors annoncer dans sa requête. Le serveur pourra ou non satisfaire à ces exigences une négociation plus ou moins serrée pourra s'en suivre terminée par une entente cordiale ou non. Ce genre de situation reste cependant assez rare.
 +
 +Pour tout savoir sur DHCP, le mieux est de poursuivre par la lecture des [[http://www.ietf.org/rfc/rfc2131.txt|RFC 2131 : « Dynamic Host Configuration Protocol »]] et aussi pourquoi pas, des [[http://www.ietf.org/rfc/rfc2132.txt|RFC 2132 : « DHCP Options and BOOTP Vendor Extensions »
 +]].
Analyse du protocole: Dernière modification le: 01/01/1970 à 00:00 par