documentation/docs/admin-guide/troubleshooting/network.md

Network

DNS / Hostname Resolution

Test hostname resolution with getent, for example getent hosts www.psi.ch. Unlike nslookup or dig, it uses the system resolver.

To flush a systems dns cache you can use:

systemctl restart systemd-resolved.service

Renewal of DHCP Lease

Figure out the connection name with

nmcli connection show

then activate the specific connection again:

nmcli connection up ens192

IP Address / Default Routes

The systems IP addresses and routes can be displayed using ip:

[root@lx ~]# ip address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
        valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
        valid_lft forever preferred_lft forever
2: ens160: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 00:50:56:9d:6d:03 brd ff:ff:ff:ff:ff:ff
    inet 10.129.160.195/24 brd 10.129.160.255 scope global ens160
        valid_lft forever preferred_lft forever
    inet6 fe80::250:56ff:fe9d:6d03/64 scope link 
        valid_lft forever preferred_lft forever


[root@lx ~]# ip route
default via 10.129.160.1 dev ens160
10.129.160.0/24 dev ens160 proto kernel scope link src 10.129.160.195 
169.254.0.0/16 dev ens160 scope link metric 1002    

Link Status / Statistics

The link status and other information of an interface can be displayed using ethtool:

[root@lx ~]# ethtool ens160
Settings for ens160:
    [...]
    Speed: 10000Mb/s
    Duplex: Full
    [...]
    Link detected: yes

Link statistics (driver-specific, but look for errors/discards/dropped) can be acquired as follows:

[root@lx ~]# ethtool -S ens160
NIC statistics:
        Tx Queue#: 0
        TSO pkts tx: 21529
        TSO bytes tx: 91036062
        ucast pkts tx: 1036632
        ucast bytes tx: 235421707
        mcast pkts tx: 8
        mcast bytes tx: 648
        bcast pkts tx: 7
        bcast bytes tx: 294
        pkts tx err: 0
        pkts tx discard: 0
        drv dropped tx total: 0
            too many frags: 0
            giant hdr: 0
            hdr err: 0
            tso: 0
        ring full: 0
        pkts linearized: 0
        hdr cloned: 0
        giant hdr: 0
        Rx Queue#: 0
        LRO pkts rx: 6913
        LRO byte rx: 100534073
        ucast pkts rx: 551554
        ucast bytes rx: 161369441
        mcast pkts rx: 4
        mcast bytes rx: 344
        bcast pkts rx: 753276
        bcast bytes rx: 45787629
        pkts rx OOB: 0
        pkts rx err: 0
        drv dropped rx total: 0
            err: 0
            fcs: 0
        rx buf alloc fail: 0
        tx timeout count: 0