主要工具为ethtool来检查，主要关注的字段为"Link detected"，注意如下的输出，其中em4实际物理上并未插上网线，而em1是插上网线的：
# ethtool em4
Settings for em4:
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! ()
Port: Twisted Pair
PHYAD:
Transceiver: internal
Auto-negotiation: on
MDI-X: Unknown
Supports Wake-on: g
Wake-on: d
Current message level: 0x000000ff ()
drv probe link timer ifdown ifup rx_err tx_err
Link detected: no
# ethtool em1
Settings for em1:
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: 100baseT/Full
1000baseT/Full
Link partner advertised pause frame use: Transmit-only
Link partner advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD:
Transceiver: internal
Auto-negotiation: on
MDI-X: on
Supports Wake-on: g
Wake-on: d
Current message level: 0x000000ff ()
drv probe link timer ifdown ifup rx_err tx_err
Link detected: yes