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物理的にLANケーブルが刺さっているか確認するLinuxコマンド

ethtoolコマンドの Link detected: の欄を見るとそもそもネットに繋がる環境かどうかがわかる

つながるとき Link detected: yesになっている

# ethtool eth0
Settings for eth0:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
                            1000baseT/Half 1000baseT/Full 
    Supported pause frame use: No
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
                            1000baseT/Half 1000baseT/Full 
    Advertised pause frame use: Symmetric
    Advertised auto-negotiation: Yes
    Link partner advertised link modes:  10baseT/Half 10baseT/Full 
                                         100baseT/Half 100baseT/Full 
                                         1000baseT/Full 
    Link partner advertised pause frame use: No
    Link partner advertised auto-negotiation: Yes
    Speed: 1000Mb/s
    Duplex: Full
    Port: Twisted Pair
    PHYAD: 1
    Transceiver: internal
    Auto-negotiation: on
    MDI-X: off
    Supports Wake-on: g
    Wake-on: g
    Current message level: 0x000000ff (255)
                   drv probe link timer ifdown ifup rx_err tx_err
    Link detected: yes

つながらないとき Link detected: noになってる

# ethtool eth1
Settings for eth1:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
                            1000baseT/Half 1000baseT/Full 
    Supported pause frame use: No
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
                            1000baseT/Half 1000baseT/Full 
    Advertised pause frame use: Symmetric
    Advertised auto-negotiation: Yes
    Speed: Unknown!
    Duplex: Unknown! (255)
    Port: Twisted Pair
    PHYAD: 2
    Transceiver: internal
    Auto-negotiation: on
    MDI-X: Unknown
    Supports Wake-on: g
    Wake-on: g
    Current message level: 0x000000ff (255)
                   drv probe link timer ifdown ifup rx_err tx_err
    Link detected: no

参考リンク

@IT:ネットワークインターフェイスの状況を調べるには