wiki:TipAndDoc/network/defaultgw

Version 4 (modified by mitty, 15 years ago) (diff)

--

  1. default gateway
  2. CentOS
    1. ifcfg-eth?
    2. sysconfig/network
    3. 結論
      1. 例
    4. route コマンド
    5. default が複数あるときの挙動
      1. 例1
      2. 例2
      3. 例3

default gateway

  • default gateway (default route)の変更方法・扱い

CentOS

ifcfg-eth?

  • ifcfg-eth? に書いた場合
  • /etc/sysconfig/network-scripts/ifcfg-eth0 
    DEVICE=eth0
BOOTPROTO=static
IPADDR=192.168.40.200
NETMASK=255.255.255.0
GATEWAY=192.168.40.1
ONBOOT=yes
  • /etc/sysconfig/network-scripts/ifcfg-eth1 
    DEVICE=eth1
BOOTPROTO=static
IPADDR=192.168.50.200
NETMASK=255.255.255.0
GATEWAY=192.168.50.1
ONBOOT=yes
  • service network restart 
    Bringing up interface eth0:                                [  OK  ]
Bringing up interface eth1:                                [  OK  ]
  • 結果 => route 
    Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.50.0    *               255.255.255.0   U     0      0        0 eth1
192.168.40.0    *               255.255.255.0   U     0      0        0 eth0
169.254.0.0     *               255.255.0.0     U     0      0        0 eth1
default         192.168.50.1    0.0.0.0         UG    0      0        0 eth1
    • この状態で、実際にWANへpingを送るとeth1を通る
    • tcpdump icmp -n -i eth1 
      listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
01:32:42.314543 IP 192.168.50.200 > 219.94.129.108: ICMP echo request, id 16947, seq 1, length 64
01:32:42.330849 IP 219.94.129.108 > 192.168.50.200: ICMP echo reply, id 16947, seq 1, length 64
  • eth0とeth1を逆にしてみる
    • eth0 
      IPADDR=192.168.50.200
NETMASK=255.255.255.0
GATEWAY=192.168.50.1
    • eth1 
      IPADDR=192.168.40.200
NETMASK=255.255.255.0
GATEWAY=192.168.40.1
    • service network restart 
      Bringing up interface eth0:                                [  OK  ]
Bringing up interface eth1:                                [  OK  ]
    • 結果 => route 
      Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.50.0    *               255.255.255.0   U     0      0        0 eth0
192.168.40.0    *               255.255.255.0   U     0      0        0 eth1
169.254.0.0     *               255.255.0.0     U     0      0        0 eth1
default         192.168.40.1    0.0.0.0         UG    0      0        0 eth1
      • 今度もeth1を通る
      • tcpdump icmp -n -i eth1 
        listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
01:39:05.380029 IP 192.168.40.200 > 219.94.129.108: ICMP echo request, id 26423, seq 1, length 64
01:39:05.392541 IP 219.94.129.108 > 192.168.40.200: ICMP echo reply, id 26423, seq 1, length 64
  • LinkUPの順番を変える
    • ifup eth1; ifup eth0 
      Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.50.0    *               255.255.255.0   U     0      0        0 eth0
192.168.40.0    *               255.255.255.0   U     0      0        0 eth1
169.254.0.0     *               255.255.0.0     U     0      0        0 eth0
default         192.168.50.1    0.0.0.0         UG    0      0        0 eth0
    • tcpdump icmp -n -i eth0 
      listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
01:42:55.952191 IP 192.168.50.200 > 219.94.129.108: ICMP echo request, id 52536, seq 1, length 64
01:42:55.965580 IP 219.94.129.108 > 192.168.50.200: ICMP echo reply, id 52536, seq 1, length 64

sysconfig/network

  • /etc/sysconfig/network 
    GATEWAY=192.168.50.2
  • ifcfg-eth? に GATEWAY= があるとき
    • そちらが優先される
  • 無いとき
    • /etc/sysconfig/network の GATEWAY= が設定される 
      Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.50.0    *               255.255.255.0   U     0      0        0 eth0
192.168.40.0    *               255.255.255.0   U     0      0        0 eth1
169.254.0.0     *               255.255.0.0     U     0      0        0 eth1
default         192.168.50.2    0.0.0.0         UG    0      0        0 eth0

結論

  • /etc/sysconfig/network-scripts/ifcfg-eth? に GATEWAY= の指定がない場合は、/etc/sysconfig/network が参照される
  • ifcfg-eth? に GATEWAY= の指定がある際は、そちらが参照される
    • NICが複数ある場合は、最後にLinkUPしたデバイスのgatewayが有効になる

例

  • eth0 
    GATEWAY=192.168.50.1
  • eth1
    • 指定なし
  • network 
    GATEWAY=192.168.50.2
  • => route 
    default         192.168.50.1    0.0.0.0         UG    0      0        0 eth0

route コマンド

  • 以下の状態でroute addする 
    Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.50.0    *               255.255.255.0   U     0      0        0 eth0
192.168.40.0    *               255.255.255.0   U     0      0        0 eth1
169.254.0.0     *               255.255.0.0     U     0      0        0 eth1
default         192.168.50.1    0.0.0.0         UG    0      0        0 eth0
  • route add default gw 192.168.40.1 
    default         192.168.40.1    0.0.0.0         UG    0      0        0 eth1
default         192.168.50.1    0.0.0.0         UG    0      0        0 eth0
    • この場合、実際にはeth1を通る 
      listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
02:03:27.195895 IP 192.168.40.200 > 219.94.129.108: ICMP echo request, id 51265, seq 1, length 64
02:03:27.212349 IP 219.94.129.108 > 192.168.40.200: ICMP echo reply, id 51265, seq 1, length 64
  • route add default gw 192.168.0.1 
    SIOCADDRT: Network is unreachable
    • routing table は変化せず

default が複数あるときの挙動

  • 挙動法則が不明
    • route コマンドで、先に表示された行のgwが必ず使われるとも限らない模様

  • ifconfig 
    eth0 => 192.168.50.100
eth1 => 192.168.60.100
eth2 => 192.168.70.100
eth3 => 192.168.40.100

例1

  • route 
    default         192.168.70.200  0.0.0.0         UG    0      0        0 eth2
default         192.168.60.200  0.0.0.0         UG    0      0        0 eth1
default         192.168.50.200  0.0.0.0         UG    0      0        0 eth0
default         192.168.40.200  0.0.0.0         UG    0      0        0 eth3
  • ping 219.94.129.108 -c1
    • client:eth2 (192.168.70.100) 
      03:20:58.808400 IP 192.168.70.100 > 219.94.129.108: ICMP echo request, id 33555, seq 1, length 64
    • gw:eth2 (192.168.70.200) 
      03:20:58.834723 IP 192.168.70.100 > 219.94.129.108: ICMP echo request, id 33555, seq 1, length 64

例2

  • route 
    default         192.168.40.200  0.0.0.0         UG    0      0        0 eth3
default         192.168.50.200  0.0.0.0         UG    0      0        0 eth0
default         192.168.60.200  0.0.0.0         UG    0      0        0 eth1
default         192.168.70.200  0.0.0.0         UG    0      0        0 eth2
  • ping 219.94.129.108 -c1
    • client:eth1 (192.168.60.100) 
      03:25:08.077550 IP 192.168.60.100 > 219.94.129.108: ICMP echo request, id 37651, seq 1, length 64
    • gw:eth1 (192.168.60.200) 
      03:25:08.106974 IP 192.168.60.100 > 219.94.129.108: ICMP echo request, id 37651, seq 1, length 64

例3

  • route 
    default         192.168.50.200  0.0.0.0         UG    0      0        0 eth0
default         192.168.60.200  0.0.0.0         UG    0      0        0 eth1
default         192.168.70.200  0.0.0.0         UG    0      0        0 eth2
default         192.168.40.200  0.0.0.0         UG    0      0        0 eth3
  • ping 219.94.129.108 -c1
    • client:eth3 (192.168.40.100) 
      03:29:45.739584 IP 192.168.40.100 > 219.94.129.108: ICMP echo request, id 42515, seq 1, length 64
    • gw:eth3 (192.168.40.200) 
      03:29:45.773401 IP 192.168.40.100 > 219.94.129.108: ICMP echo request, id 42515, seq 1, length 64