Brillo: 由DHCP server提供hostname

当运行着Brillo系统的RPi 2B连上路由器时,可以自动从路由器端获取IP地址,配置有线网络,接入到互连网。默认情况下,Brillo系统设置的hostname为localhost, 所以当你打开路由器的配置页面,查看接入设备的相关信息时,Brillo设备Hostname那一栏为空。由于client设置的hostname为localhost, DHCP client在请求IP地址时,不会将hostname发送给DHCP server, DHCP server就无法知道client的hostname。

  • DHCP自动分配IP地址过程分析

我们可以在client端通过tcpdump查看使用dhcp server自动分配IP地址的过程:

# tcpdump -i eth0 -s 1500 -vv port 67 or port 68
[  153.095070] device eth0 entered promiscuous mode
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 1500 bytes
[  170.865935] smsc95xx 1-1.1:1.0 eth0: link up, 100Mbps, full-duplex, lpa 0xCDE1
[  170.873395] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
05:34:59.369523 IP (tos 0x0, ttl 64, id 9241, offset 0, flags [none], proto UDP (17), length 351)
    0.0.0.0.bootpc > 255.255.255.255.bootps: [udp sum ok] BOOTP/DHCP, Request from b8:27:eb:b4:d8:bb (oui Unknown), length 323, xid 0xe3b7dd91, Flags [none] (0x0000)
          Client-Ethernet-Address b8:27:eb:b4:d8:bb (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Client-ID Option 61, length 7: ether b8:27:eb:b4:d8:bb
            MSZ Option 57, length 2: 1500
            Vendor-Class Option 60, length 44: "dhcpcd-6.8.2:Linux-4.1.19-v7+:armv7l:BCM2709"
            T145 Option 145, length 1: 1
            Parameter-Request Option 55, length 15: 
              Subnet-Mask, Classless-Static-Route, Static-Route, Default-Gateway
              Domain-Name-Server, Hostname, Domain-Name, MTU
              BR, Lease-Time, Server-ID, RN
              RB, Option 119, Option 252
05:35:01.874523 IP (tos 0x0, ttl 64, id 29112, offset 0, flags [none], proto UDP (17), length 328)
    192.168.5.1.bootps > 192.168.5.127.bootpc: [udp sum ok] BOOTP/DHCP, Reply, length 300, xid 0xe3b7dd91, Flags [none] (0x0000)
          Your-IP 192.168.5.127
          Server-IP 192.168.5.1
          Client-Ethernet-Address b8:27:eb:b4:d8:bb (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.5.1
            Lease-Time Option 51, length 4: 43200
            RN Option 58, length 4: 21600
            RB Option 59, length 4: 37800
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.5.255
            Default-Gateway Option 3, length 4: 192.168.5.1
            Domain-Name-Server Option 6, length 4: 192.168.5.1
            Domain-Name Option 15, length 3: "lan"
05:35:01.877733 IP (tos 0x0, ttl 64, id 46889, offset 0, flags [none], proto UDP (17), length 363)
    0.0.0.0.bootpc > 255.255.255.255.bootps: [udp sum ok] BOOTP/DHCP, Request from b8:27:eb:b4:d8:bb (oui Unknown), length 335, xid 0xe3b7dd91, secs 3, Flags [none] (0x0000)
          Client-Ethernet-Address b8:27:eb:b4:d8:bb (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Request
            Client-ID Option 61, length 7: ether b8:27:eb:b4:d8:bb
            Requested-IP Option 50, length 4: 192.168.5.127
            Server-ID Option 54, length 4: 192.168.5.1
            MSZ Option 57, length 2: 1500
            Vendor-Class Option 60, length 44: "dhcpcd-6.8.2:Linux-4.1.19-v7+:armv7l:BCM2709"
            T145 Option 145, length 1: 1
            Parameter-Request Option 55, length 15: 
              Subnet-Mask, Classless-Static-Route, Static-Route, Default-Gateway
              Domain-Name-Server, Hostname, Domain-Name, MTU
              BR, Lease-Time, Server-ID, RN
              RB, Option 119, Option 252
05:35:01.879320 IP (tos 0x0, ttl 64, id 29113, offset 0, flags [none], proto UDP (17), length 328)
    192.168.5.1.bootps > 192.168.5.127.bootpc: [udp sum ok] BOOTP/DHCP, Reply, length 300, xid 0xe3b7dd91, secs 3, Flags [none] (0x0000)
          Your-IP 192.168.5.127
          Server-IP 192.168.5.1
          Client-Ethernet-Address b8:27:eb:b4:d8:bb (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 192.168.5.1
            Lease-Time Option 51, length 4: 43200
            RN Option 58, length 4: 21600
            RB Option 59, length 4: 37800
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.5.255
            Default-Gateway Option 3, length 4: 192.168.5.1
            Domain-Name-Server Option 6, length 4: 192.168.5.1
            Domain-Name Option 15, length 3: "lan"

可以看到分为四个阶段:

              Discover
1. client ------------------> server
              Offer
2. client <------------------ server
              Request
3. client ------------------> server
              ACK
4. client <------------------ server

在ACK阶段,client就可以从server端获取了IP地址,子网掩码,网关IP,DNS server等信息。

如果在自动获取IP地址之前,我们重新设置了hostname又会怎么样呢?

我们可以通过hostname命令查看与修改hostname:

# hostname
localhost
# hostname RPi2B

NOTE:关于hostname的命名规范请查看:https://en.wikipedia.org/wiki/Hostname

这时在自动获取IP地址时,DHCP client就会发送hostname信息给DHCP server:

    0.0.0.0.bootpc > 255.255.255.255.bootps: [udp sum ok] BOOTP/DHCP, Request from b8:27:eb:b4:d8:bb (oui Unknown), length 330, xid 0x6f097a2b, Flags [none] (0x0000)
          Client-Ethernet-Address b8:27:eb:b4:d8:bb (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Client-ID Option 61, length 7: ether b8:27:eb:b4:d8:bb
            MSZ Option 57, length 2: 1500
            Vendor-Class Option 60, length 44: "dhcpcd-6.8.2:Linux-4.1.19-v7+:armv7l:BCM2709"
            Hostname Option 12, length 5: "RPi2B"
            T145 Option 145, length 1: 1
            Parameter-Request Option 55, length 15: 
              Subnet-Mask, Classless-Static-Route, Static-Route, Default-Gateway
              Domain-Name-Server, Hostname, Domain-Name, MTU
              BR, Lease-Time, Server-ID, RN
              RB, Option 119, Option 252
  • 由DHCP server向client提供hostname

a. 路由器中有相关的配置可以指定MAC地址所对应的Hostname, IP address:

2016_04_16_OpenWrt_DHCP_cfg

这时,DHCP server就会在ACK阶段提供hostname:

    192.168.5.1.bootps > 192.168.5.127.bootpc: [udp sum ok] BOOTP/DHCP, Reply, length 304, xid 0xe10f3a39, Flags [none] (0x0000)
          Your-IP 192.168.5.127
          Server-IP 192.168.5.1
          Client-Ethernet-Address b8:27:eb:b4:d8:bb (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 192.168.5.1
            Lease-Time Option 51, length 4: 43200
            RN Option 58, length 4: 21600
            RB Option 59, length 4: 37800
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.5.255
            Default-Gateway Option 3, length 4: 192.168.5.1
            Domain-Name-Server Option 6, length 4: 192.168.5.1
            Domain-Name Option 15, length 3: "lan"
            Hostname Option 12, length 5: "RPi2B"

b. 修改Brillo系统中shill服务的启动参数(修改system/etc/init/shill.rc)

# For dhcpcd-6.8.2.
on post-fs-data
    mkdir /data/misc/dhcp-6.8.2 0755 dhcp dhcp

service shill /system/bin/shill --device-black-list=sit0,dummy0,p2p0 --accept-hostname-from=eth0
    class late_start
    user root
    group system dbus inet
    disabled

on property:wifi-setup.complete=1
    enable shill

DHCP client接收到hostname的请求之后,会通过shill重新设置hostname。

  • 相关的参考文档:
  1. https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol
  2. https://en.wikipedia.org/wiki/Hostname

 

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