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DHCP通信の詳細 – リレーエージェントあり (パケットキャプチャ・デバッグ)

DHCP
DHCP通信の詳細 - リレーエージェントあり (パケットキャプチャ・デバッグ)

リレーエージェントが中継するDHCP通信の詳細について、パケットキャプチャとCiscoルーターのデバッグログを確認しながら説明します。

DHCPの基礎については、下記を参照してください。

関連ページ
DHCP(Dynamic Host Configuration Protocol)とは 【DHCPとは】 PC等の通信機器がネットワークに接続するためには、IPアドレスやデフォルトゲートウェイ等のネットワーク情報が必要です。全ての機器のネットワーク情報を…
目次

ネットワーク構成

下記の4パターンで、DHCP通信の詳細を確認します。DHCPリレーエージェントはCiscoルーターで構築します。

パターンDHCPクライアントDHCPサーバー
パターン1Desktop(Linux)Ciscoルーター
パターン2CiscoルーターCiscoルーター
パターン3Desktop(Linux)Ubuntuサーバー
パターン4CiscoルーターUbuntuサーバー

DHCPサーバー、DHCPクライアントの設定方法は下記を参照してください。

関連ページ
DHCPサーバーの設定方法(Ciscoルーター/L3スイッチ) CiscoCML上のCiscoルーターを利用して、DHCPサーバーの設定を行います。 DHCPの基礎については、下記を参照してください。 【ネットワーク構成】 下記のネットワークを…
関連ページ
DHCPサーバーの構築方法(Ubuntu) CiscoCML上のUbuntuを利用して、DHCPサーバーを構築します。 DHCPの基礎については、下記を参照してください。 【ネットワーク構成】 下記のネットワークを利用して、Ub…
関連ページ
DHCPクライアントの設定方法(Cisco) Ciscoルーターのインターフェース設定を行い、DHCPクライアントとしてネットワーク情報を自動的に取得できるようにします。 【ネットワーク構成】 下記のネットワーク構…

【パターン1】DHCPクライアント：Desktop(Linux)、DHCPサーバー：Ciscoルーター

【パターン1】DHCPクライアント：Desktop(Linux)、DHCPサーバー：Ciscoルーター

【パターン2】DHCPクライアント：Ciscoルーター、DHCPサーバー：Ciscoルーター

【パターン2】DHCPクライアント：Ciscoルーター、DHCPサーバー：Ciscoルーター

【パターン3】DHCPクライアント：Desktop(Linux)、DHCPサーバー：Ubuntuサーバー

【パターン3】DHCPクライアント：Desktop(Linux)、DHCPサーバー：Ubuntuサーバー

【パターン4】DHCPクライアント：Ciscoルーター、DHCPサーバー：Ubuntuサーバー

【パターン4】DHCPクライアント：Ciscoルーター、DHCPサーバー：Ubuntuサーバー

DHCP通信の確認(パケットキャプチャ・デバッグ)

DHCPサーバーとなるCiscoルーターには下記のコマンドで、DHCPのデバッグを有効にします。

debug ip dhcp server events
debug ip dhcp server packet

【パターン1】DHCPクライアント：Desktop(Linux)、DHCPサーバー：Ciscoルーター

パターン1の場合の通信の詳細は下記のとおりです。DHCPリレーエージェントからDHCPクライアントへの通信は、ユニキャスト通信が使用されています。

パターン1の通信の流れ

パケットキャプチャ確認

通信の全体の流れは下記のとおりです。DHCP Discover を受信したDHCPサーバーは、割り当て予定のIPアドレスにPing(ICMP)エコーリクエストを送信し、応答が無いことを確認した上で DHCP Offer を送信しています。また、DHCPリレーエージェントからDHCPサーバーへの通信は、送信元がDHCPクライアント側のIPアドレス(172.16.1.254)となります。

DHCPクライアント – DHCPリレーエージェント
パターン1のパケットキャプチャ(クライアント - リレーエージェント)
DHCPリレーエージェント – DHCPサーバー
パターン1のパケットキャプチャ(リレーエージェント - サーバー)
DHCP Discover の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x10142107
    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
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000000000000000

0… …. …. …. = Broadcast flag: Unicast
→ この部分のフラグがセットされていない場合、DHCPサーバーからの通信はユニキャストとなります。

Option: (55) Parameter Request List
→ この部分でDHCPにより要求するネットワーク情報を提示しています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x10142107
    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
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000000000000000

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。DHCPサーバーはこのアドレスをもとに割り当てるサブネットを判断します。

※上記以外の情報は、DHCPクライアントからの DHCP Discover と同じ内容です。

DHCP Offer の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x10142107
    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
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

Your (client) IP address: 172.16.1.1
→ 割り当て予定のIPアドレスを通知しています。

Option: (XX)
→ その他のオプションとして割り当てる情報を通知しています。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x10142107
    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
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

DHCP Request の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x10142107
    Seconds elapsed: 2
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000

Option: (50) Requested IP Address (172.16.1.1)
→ 通知されたIPアドレス情報の割り当てをリクエストしています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x10142107
    Seconds elapsed: 2
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。

※上記以外の情報は、DHCPクライアントからの DHCP Request と同じ内容です。

DHCP ACK の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x10142107
    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
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

Option: (53) DHCP Message Type (ACK)
→ メッセージタイプ以外は、DHCP Offer と同じ情報を再送信し、ネットワーク情報の割り当てを完了します。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x10142107
    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
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

デバッグ確認

Router1のデバッグログで、DHCP通信の詳細が確認できます。

Router1のデバッグログ
02:09:08.709: DHCPD: Sending notification of DISCOVER:
02:09:08.709:   DHCPD: htype 1 chaddr 5254.000f.2c5a
02:09:08.710:   DHCPD: remote id 020a0000c0a801fd00000000
02:09:08.710:   DHCPD: circuit id 00000000
02:09:08.710: DHCPD: DHCPDISCOVER received from client 0152.5400.0f2c.5a through relay 172.16.1.254.
02:09:08.710: DHCPD: Option 125 not present in the msg.
02:09:08.710: DHCPD: Seeing if there is an internally specified pool class:
02:09:08.710:   DHCPD: htype 1 chaddr 5254.000f.2c5a
02:09:08.710:   DHCPD: remote id 020a0000c0a801fd00000000
02:09:08.711:   DHCPD: circuit id 00000000
02:09:08.711: DHCPD: Allocate an address without class information (172.16.1.0)
02:09:08.711: DHCPD: Allocated binding E806D78
02:09:08.712: DHCPD: Adding binding to radix tree (172.16.1.1)
02:09:08.712: DHCPD: Adding binding to hash tree
02:09:08.712: DHCPD: assigned IP address 172.16.1.1 to client 0152.5400.0f2c.5a.
02:09:08.713: DHCPD: Saving workspace (ID=0x4F000019)
02:09:10.714: DHCPD: Reprocessing saved workspace (ID=0x4F000019)
02:09:10.714: DHCPD: Option 125 not present in the msg.
02:09:10.714: DHCPD: Sending notification of DISCOVER:
02:09:10.715:   DHCPD: htype 1 chaddr 5254.000f.2c5a
02:09:10.715:   DHCPD: remote id 020a0000c0a801fd00000000
02:09:10.715:   DHCPD: circuit id 00000000
02:09:10.716: DHCPD: DHCPDISCOVER received from client 0152.5400.0f2c.5a through relay 172.16.1.254.
02:09:10.717: DHCPD: Option 125 not present in the msg.
02:09:10.717: DHCPD: Seeing if there is an internally specified pool class:
02:09:10.717:   DHCPD: htype 1 chaddr 5254.000f.2c5a
02:09:10.719:   DHCPD: remote id 020a0000c0a801fd00000000
02:09:10.719:   DHCPD: circuit id 00000000
02:09:10.719: DHCPD: Sending DHCPOFFER to client 0152.5400.0f2c.5a (172.16.1.1).DHCPD: Setting only requested parameters
02:09:10.719: DHCPD: Option 125 not present in the msg.
02:09:10.719: DHCPD: no option 125
02:09:10.720: DHCPD: unicasting BOOTREPLY for client 5254.000f.2c5a to relay 172.16.1.254.
02:09:10.751: DHCPD: New packet workspace 0xEE83C80 (ID=0xDB00001A)
02:09:10.751: DHCPD: client's VPN is .
02:09:10.752: DHCPD: No option 125
02:09:10.752: DHCPD: DHCPREQUEST received from client 0152.5400.0f2c.5a.
02:09:10.753: DHCPD: Option 125 not present in the msg.
02:09:10.753: DHCPD: Sending notification of ASSIGNMENT:
02:09:10.754:  DHCPD: address 172.16.1.1 mask 255.255.255.0
02:09:10.754:   DHCPD: htype 1 chaddr 5254.000f.2c5a
02:09:10.755:   DHCPD: lease time remaining (secs) = 86400
02:09:10.755: DHCPD: Can't find any hostname to update
02:09:10.755: DHCPD: Sending DHCPACK to client 0152.5400.0f2c.5a (172.16.1.1).DHCPD: Setting only requested parameters

【パターン2】DHCPクライアント：Ciscoルーター、DHCPサーバー：Ciscoルーター

パターン2の場合の通信の詳細は下記のとおりです。DHCPリレーエージェントからDHCPクライアントへの通信は、ブロードキャスト通信が使用されています。

パターン2の通信の流れ

パケットキャプチャ確認

通信の全体の流れは下記のとおりです。DHCP Discover を受信したDHCPサーバーは、割り当て予定のIPアドレスにPing(ICMP)エコーリクエストを送信し、応答が無いことを確認した上で DHCP Offer を送信しています。また、DHCPリレーエージェントからDHCPサーバーへの通信は、送信元がDHCPクライアント側のIPアドレス(172.16.1.254)となります。

DHCPクライアント – DHCPリレーエージェント
パターン2のパケットキャプチャ(クライアント - リレーエージェント)
DHCPリレーエージェント – DHCPサーバー
パターン2のパケットキャプチャ(リレーエージェント - サーバー)
DHCP Discover の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00001e55
    Seconds elapsed: 3
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

1… …. …. …. = Broadcast flag: Broadcast
→ この部分のフラグがセットされている場合、DHCPサーバーからの通信はブロードキャストとなります。

Option: (55) Parameter Request List
→ この部分でDHCPにより要求するネットワーク情報を提示しています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x00001e55
    Seconds elapsed: 3
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。DHCPサーバーはこのアドレスをもとに割り当てるサブネットを判断します。

※上記以外の情報は、DHCPクライアントからの DHCP Discover と同じ内容です。

DHCP Offer の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00001e55
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

Your (client) IP address: 172.16.1.1
→ 割り当て予定のIPアドレスを通知しています。

Option: (XX)
→ その他のオプションとして割り当てる情報を通知しています。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00001e55
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

DHCP Request の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00001e55
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

Option: (50) Requested IP Address (172.16.1.1)
→ 通知されたIPアドレス情報の割り当てをリクエストしています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x00001e55
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。

※上記以外の情報は、DHCPクライアントからの DHCP Request と同じ内容です。

DHCP ACK の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00001e55
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

Option: (53) DHCP Message Type (ACK)
→ メッセージタイプ以外は、DHCP Offer と同じ情報を再送信し、ネットワーク情報の割り当てを完了します。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00001e55
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (86400s) 1 day
    Option: (58) Renewal Time Value
        Length: 4
        Renewal Time Value: (43200s) 12 hours
    Option: (59) Rebinding Time Value
        Length: 4
        Rebinding Time Value: (75600s) 21 hours
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

デバッグ確認

Router1のデバッグログで、DHCP通信の詳細が確認できます。

Router1のデバッグログ
19 07:37:46.508: DHCPD: Sending notification of DISCOVER:
19 07:37:46.509:   DHCPD: htype 1 chaddr 5254.0017.89b3
19 07:37:46.509:   DHCPD: remote id 020a0000c0a801fd00000000
19 07:37:46.509:   DHCPD: circuit id 00000000
19 07:37:46.510: DHCPD: DHCPDISCOVER received from client 0063.6973.636f.2d35.3235.342e.3030.3137.2e38.3962.332d.4769.302f.30 through relay 172.16.1.254.
19 07:37:46.510: DHCPD: Option 125 not present in the msg.
19 07:37:46.510: DHCPD: Seeing if there is an internally specified pool class:
19 07:37:46.511:   DHCPD: htype 1 chaddr 5254.0017.89b3
19 07:37:46.511:   DHCPD: remote id 020a0000c0a801fd00000000
19 07:37:46.512:   DHCPD: circuit id 00000000
19 07:37:46.512: DHCPD: Allocate an address without class information (172.16.1.0)
19 07:37:46.514: DHCPD: Allocated binding EEBA248
19 07:37:46.514: DHCPD: Adding binding to radix tree (172.16.1.1)
19 07:37:46.514: DHCPD: Adding binding to hash tree
19 07:37:46.514: DHCPD: assigned IP address 172.16.1.1 to client 0063.6973.636f.2d35.3235.342e.3030.3137.2e38.3962.332d.4769.302f.30.
19 07:37:46.514: DHCPD: Saving workspace (ID=0x6D000022)
19 07:37:48.515: DHCPD: Reprocessing saved workspace (ID=0x6D000022)
19 07:37:48.516: DHCPD: Option 125 not present in the msg.
19 07:37:48.516: DHCPD: Sending notification of DISCOVER:
19 07:37:48.517:   DHCPD: htype 1 chaddr 5254.0017.89b3
19 07:37:48.517:   DHCPD: remote id 020a0000c0a801fd00000000
19 07:37:48.518:   DHCPD: circuit id 00000000
19 07:37:48.518: DHCPD: DHCPDISCOVER received from client 0063.6973.636f.2d35.3235.342e.3030.3137.2e38.3962.332d.4769.302f.30 through relay 172.16.1.254.
19 07:37:48.519: DHCPD: Option 125 not present in the msg.
19 07:37:48.519: DHCPD: Seeing if there is an internally specified pool class:
19 07:37:48.519:   DHCPD: htype 1 chaddr 5254.0017.89b3
19 07:37:48.519:   DHCPD: remote id 020a0000c0a801fd00000000
19 07:37:48.519:   DHCPD: circuit id 00000000
19 07:37:48.519: DHCPD: Sending DHCPOFFER to client 0063.6973.636f.2d35.3235.342e.3030.3137.2e38.3962.332d.4769.302f.30 (172.16.1.1).DHCPD: Setting only requested parameters
19 07:37:48.521: DHCPD: Option 125 not present in the msg.
19 07:37:48.522: DHCPD: no option 125
19 07:37:48.522: DHCPD: unicasting BOOTREPLY for client 5254.0017.89b3 to relay 172.16.1.254.
19 07:37:48.539: DHCPD: New packet workspace 0xEE83C80 (ID=0x5000023)
19 07:37:48.540: DHCPD: client's VPN is .
19 07:37:48.540: DHCPD: No option 125
19 07:37:48.540: DHCPD: DHCPREQUEST received from client 0063.6973.636f.2d35.3235.342e.3030.3137.2e38.3962.332d.4769.302f.30.
19 07:37:48.540: DHCPD: Option 125 not present in the msg.
19 07:37:48.540: DHCPD: Sending notification of ASSIGNMENT:
19 07:37:48.540:  DHCPD: address 172.16.1.1 mask 255.255.255.0
19 07:37:48.540:   DHCPD: htype 1 chaddr 5254.0017.89b3
19 07:37:48.540:   DHCPD: lease time remaining (secs) = 86400
19 07:37:48.542: DHCPD: No default domain to append - abort update
19 07:37:48.542: DHCPD: Sending DHCPACK to client 0063.6973.636f.2d35.3235.342e.3030.3137.2e38.3962.332d.4769.302f.30 (172.16.1.1).DHCPD: Setting only requested parameters

【パターン3】DHCPクライアント：Desktop(Linux)、DHCPサーバー：Ubuntuサーバー

パターン3の場合の通信の詳細は下記のとおりです。DHCPリレーエージェントからDHCPクライアントへの通信は、ユニキャスト通信が使用されています。

パターン3の通信の流れ

パケットキャプチャ確認

通信の全体の流れは下記のとおりです。DHCP Discover を受信したDHCPサーバーは、割り当て予定のIPアドレスにPing(ICMP)エコーリクエストを送信し、応答が無いことを確認した上で DHCP Offer を送信しています。また、DHCPリレーエージェントからDHCPサーバーへの通信は、送信元がDHCPクライアント側のIPアドレス(172.16.1.254)となります。

DHCPクライアント – DHCPリレーエージェント
パターン3のパケットキャプチャ(クライアント - リレーエージェント)
DHCPリレーエージェント – DHCPサーバー
パターン3のパケットキャプチャ(リレーエージェント - サーバー)
DHCP Discover の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x74fbb017
    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
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000000000000000

0… …. …. …. = Broadcast flag: Unicast
→ この部分のフラグがセットされていない場合、DHCPサーバーからの通信はユニキャストとなります。

Option: (55) Parameter Request List
→ この部分でDHCPにより要求するネットワーク情報を提示しています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x74fbb017
    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
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000000000000000000000000000

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。DHCPサーバーはこのアドレスをもとに割り当てるサブネットを判断します。

※上記以外の情報は、DHCPクライアントからの DHCP Discover と同じ内容です。

DHCP Offer の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x74fbb017
    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
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

Your (client) IP address: 172.16.1.1
→ 割り当て予定のIPアドレスを通知しています。

Option: (XX)
→ その他のオプションとして割り当てる情報を通知しています。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x74fbb017
    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
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

DHCP Request の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x74fbb017
    Seconds elapsed: 1
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000

Option: (50) Requested IP Address (172.16.1.1)
→ 通知されたIPアドレス情報の割り当てをリクエストしています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x74fbb017
    Seconds elapsed: 1
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (61) Client identifier
        Length: 7
        Hardware type: Ethernet (0x01)
        Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 7
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
    Option: (60) Vendor class identifier
        Length: 12
        Vendor class identifier: udhcp 1.32.1
    Option: (255) End
        Option End: 255
    Padding: 0000000000000000

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。

※上記以外の情報は、DHCPクライアントからの DHCP Request と同じ内容です。

DHCP ACK の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x74fbb017
    Seconds elapsed: 1
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

Option: (53) DHCP Message Type (ACK)
→ メッセージタイプ以外は、DHCP Offer と同じ情報を再送信し、ネットワーク情報の割り当てを完了します。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x74fbb017
    Seconds elapsed: 1
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_0f:2c:5a (52:54:00:0f:2c:5a)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

【パターン4】DHCPクライアント：Ciscoルーター、DHCPサーバー：Ubuntuサーバー

パターン4の場合の通信の詳細は下記のとおりです。DHCPリレーエージェントからDHCPクライアントへの通信は、ブロードキャスト通信が使用されています。

パターン4の通信の流れ

パケットキャプチャ確認

通信の全体の流れは下記のとおりです。DHCP Discover を受信したDHCPサーバーは、割り当て予定のIPアドレスにPing(ICMP)エコーリクエストを送信し、応答が無いことを確認した上で DHCP Offer を送信しています。また、DHCPリレーエージェントからDHCPサーバーへの通信は、送信元がDHCPクライアント側のIPアドレス(172.16.1.254)となります。

DHCPクライアント – DHCPリレーエージェント
パターン4のパケットキャプチャ(クライアント - リレーエージェント)
DHCPリレーエージェント – DHCPサーバー
パターン4のパケットキャプチャ(リレーエージェント - サーバー)
DHCP Discover の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

1… …. …. …. = Broadcast flag: Broadcast
→ この部分のフラグがセットされている場合、DHCPサーバーからの通信はブロードキャストとなります。

Option: (55) Parameter Request List
→ この部分でDHCPにより要求するネットワーク情報を提示しています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
        Length: 1
        DHCP: Discover (1)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。DHCPサーバーはこのアドレスをもとに割り当てるサブネットを判断します。

※上記以外の情報は、DHCPクライアントからの DHCP Discover と同じ内容です。

DHCP Offer の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

Your (client) IP address: 172.16.1.1
→ 割り当て予定のIPアドレスを通知しています。

Option: (XX)
→ その他のオプションとして割り当てる情報を通知しています。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
        Length: 1
        DHCP: Offer (2)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

DHCP Request の詳細
DHCPクライアント → DHCPリレーエージェント
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

Option: (50) Requested IP Address (172.16.1.1)
→ 通知されたIPアドレス情報の割り当てをリクエストしています。

DHCPリレーエージェント → DHCPサーバー
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
        Length: 1
        DHCP: Request (3)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 1200
    Option: (61) Client identifier
        Length: 27
        Type: 0
        Client Identifier: cisco-5254.0017.89b3-Gi0/0
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (50) Requested IP Address (172.16.1.1)
        Length: 4
        Requested IP Address: 172.16.1.1
    Option: (12) Host Name
        Length: 7
        Host Name: Router1
    Option: (55) Parameter Request List
        Length: 8
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (33) Static Route
        Parameter Request List Item: (150) TFTP Server Address
        Parameter Request List Item: (43) Vendor-Specific Information
    Option: (60) Vendor class identifier
        Length: 8
        Vendor class identifier: ciscopnp
    Option: (255) End
        Option End: 255

Hops: 1
→ DHCPパケットがリレーエージェントを経由するとカウントアップされます。

Relay agent IP address: 172.16.1.254
→ DHCPリレーエージェントのIPアドレスが入ります。

※上記以外の情報は、DHCPクライアントからの DHCP Request と同じ内容です。

DHCP ACK の詳細
DHCPサーバー → DHCPリレーエージェント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

Option: (53) DHCP Message Type (ACK)
→ メッセージタイプ以外は、DHCP Offer と同じ情報を再送信し、ネットワーク情報の割り当てを完了します。

DHCPリレーエージェント → DHCPクライアント
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x0000245d
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 172.16.1.1
    Next server IP address: 192.168.1.253
    Relay agent IP address: 172.16.1.254
    Client MAC address: RealtekU_17:89:b3 (52:54:00:17:89:b3)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (54) DHCP Server Identifier (192.168.1.253)
        Length: 4
        DHCP Server Identifier: 192.168.1.253
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (600s) 10 minutes
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 8.8.8.8
    Option: (15) Domain Name
        Length: 11
        Domain Name: example.org
    Option: (3) Router
        Length: 4
        Router: 172.16.1.254
    Option: (255) End
        Option End: 255
    Padding: 00000000000000000000000000

DHCPサーバー → DHCPリレーエージェントへの通信と同じ内容です。

以上で、DHCP通信の詳細 – リレーエージェントあり (パケットキャプチャ・デバッグ)の説明は完了です！

DHCPリレーエージェントを経由しない場合のDHCP通信の詳細は、下記を参照してください。

関連ページ
DHCP通信の詳細(パケットキャプチャ・デバッグ) DHCP通信の詳細について、パケットキャプチャとCiscoルーターのデバッグログを確認しながら説明します。 DHCPの基礎については、下記を参照してください。 【ネットワー…
DHCP
Cisco IP関連技術 TCP/IP基礎 ネットワークエンジニア
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