Simple Network Troubleshooting
Testing Link Status from the Command Line
Both the mii-tool and ethtool commands command will provide reports on the link status and duplex settings for supported NICs.
When used without any switches, the mii-tool gives a very brief report. Use it with the -v switch because it provides more information on the supported autonegotiation speeds of the NIC and this can be useful in troubleshooting speed and duplex issues.
The ethtool command provides much more information than mii-tool and should be your command of choice, especially because mii-tool will be soon deprecated in Linux. In both of the following examples the NICs are operating at 100Mbps, full duplex and the link is ok.
Link Status Output from mii-tool
[root@bigboy tmp]# mii-tool -v eth0: 100 Mbit, full duplex, link ok product info: Intel 82555 rev 4 basic mode: 100 Mbit, full duplex basic status: link ok capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control link partner: 100baseTx-HD [root@bigboy tmp]#
Link Status Output from ethtool
[root@bigboy tmp]# ethtool eth0 Settings for eth0: Supported ports: [ TP MII ] Supported link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full Supports auto-negotiation: Yes Advertised link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full Advertised auto-negotiation: No Speed: 100Mb/s Duplex: Full Port: MII PHYAD: 1 Transceiver: internal Auto-negotiation: off Supports Wake-on: g Wake-on: g Current message level: 0x00000007 (7) Link detected: yes [root@bigboy tmp]#
netstat Error Output
The netstat command is very versatile and can provide a limited report when used with the -i switch. This is useful for systems where mii-tool or ethtool are not available.
[root@bigboy tmp]# netstat -i Kernel Interface table Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg eth0 1500 0 18976655 2 0 0 21343152 142 0 3 BMRU eth1 1500 0 855154 0 0 0 15196620 0 0 0 BMRU lo 16436 0 1784272 0 0 0 1784272 0 0 0 LRU [root@bigboy tmp]#