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 = default gateway =
 * default gateway (default route)の変更方法・扱い

 = 複数設定されているとき =
 == 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
}}}