Back in April, I explained the new network interface names. Instead of eth0 it’s something like enp0s4 or p7p1.
Let’s see how to check the settings. Run these commands on your system and play along!
First, what are the network class devices? This system has two Ethernet interfaces:
$ ls -l /sys/class/net total 0 lrwxrwxrwx. 1 root root 0 Mar 8 10:19 enp0s18 -> ../../devices/pci0000:00/0000:00:12.0/net/enp0s18 lrwxrwxrwx. 1 root root 0 Mar 8 10:19 enp0s20 -> ../../devices/pci0000:00/0000:00:14.0/net/enp0s20 lrwxrwxrwx. 1 root root 0 Mar 8 10:19 ls -> ../../devices/virtual/net/lo
Here is the Ethernet or link-layer information:
$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enp0s18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
link/ether b8:27:eb:20:9c:a4 brd ff:ff:ff:ff:ff:ff
3: enp0s20: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
link/ether b8:27:eb:20:9c:9a brd ff:ff:ff:ff:ff:ff
Both are plugged into live networks. The interfaces are enabled (the first ”UP”), and both hear Ethernet link beat signals (the second ”UP”).
Now we ask for statistics:
$ ip -s link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
RX: bytes packets errors dropped overrun mcast
56880 689 0 0 0 0
TX: bytes packets errors dropped carrier collsns
56880 689 0 0 0 0
2: enp0s18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
link/ether b8:27:eb:20:9c:a4 brd ff:ff:ff:ff:ff:ff
RX: bytes packets errors dropped overrun mcast
7766356 64823 0 0 0 0
TX: bytes packets errors dropped carrier collsns
434232 2557 0 0 0 0
3: enp0s20: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
link/ether b8:27:eb:20:9c:9a brd ff:ff:ff:ff:ff:ff
RX: bytes packets errors dropped overrun mcast
86377095 1177553 0 0 0 0
TX: bytes packets errors dropped carrier collsns
29380884244 19335457 0 0 0 0
Most of the traffic is over the enp0s20 interface. This server has transmitted a little over 27 GB of data.
Good news: no collisions (which can happen if negotiation with the switch stumbles) and no other errors or drops or carrier problems (which probably mean bad cables, maybe bad Ethernet cards).
Let’s ask for IPv4 and IPv6 individually. We get the link state, the IP information. The enp0s20 device has 3 IPv4 addresses. For IPv6, it has the usual automatically configured link-local fe80::/10 IPv6 address, plus a globally routable 2001:1800:1234:90::/64 address.
$ ip -4 addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
2: enp0s18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
inet 172.16.0.200/16 brd 172.16.255.255 scope global enp0s18
valid_lft forever preferred_lft forever
3: enp0s20: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
inet 10.1.1.200/24 brd 10.1.1.255 scope global enp0s20
valid_lft forever preferred_lft forever
inet 10.1.1.201/24 brd 10.1.1.255 scope global secondary enp0s20
valid_lft forever preferred_lft forever
inet 10.1.1.203/24 brd 10.1.1.255 scope global secondary enp0s20
valid_lft forever preferred_lft forever
The 169.254/16 routes are for link-local autoconfiguration addresses, like Apple’s Bonjour service-discovery protocol.
$ ip -6 addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp0s18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
inet6 fe80::20c:29ff:fe20:9ca4/64 scope link
valid_lft forever preferred_lft forever
3: enp0s20: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
inet6 2001:1800:1234:90::c8/64 scope global
valid_lft forever preferred_lft forever
inet6 fe80::20c:29ff:fe20:9c9a/64 scope link
valid_lft forever preferred_lft forever
These are the ARP and NDP caches. There’s lots of output on this server, with clients, printers, routers, and managed switches chattering away. ”REACHABLE” means it’s in use more or less right now, ”STALE” means it’s been used recently but is due for refreshing.
$ ip neigh fe80::201:5cff:fe79:1446 dev enp0s20 lladdr 02:42:b6:73:57:28 REACHABLE fe80::20c:29ff:fe20:7604 dev enp0s18 lladdr b8:27:eb:20:76:04 STALE fe80::20c:29ff:fe5d:2709 dev enp0s18 lladdr b8:27:eb:5d:27:09 STALE 2001:1800:1234:90:e871:5fb3:1a98:3a7a dev enp0s20 lladdr 2c:27:d7:c5:d3:7b REACHABLE fe80::20c:29ff:fe34:c50d dev enp0s18 lladdr b8:27:eb:34:c5:0d STALE 2001:1800:1234:90:cb5:c013:429f:7c50 dev enp0s20 lladdr 2c:27:d7:c5:d3:7b REACHABLE [... much deleted ...] 10.1.1.254 dev enp0s20 lladdr b8:27:eb:8d:69:01 REACHABLE 10.1.1.1 dev enp0s20 lladdr b8:27:eb:8d:69:01 REACHABLE 10.1.1.16 dev enp0s20 lladdr b8:27:eb:45:e3:10 STALE 10.1.1.15 dev enp0s20 lladdr b8:27:eb:59:66:0f REACHABLE 10.1.1.11 dev enp0s20 lladdr b8:27:eb:43:ec:0b REACHABLE 10.1.1.19 dev enp0s20 lladdr b8:27:eb:c2:f2:13 REACHABLE [... much deleted ...]
How do we reach other IP hosts, possibly several router hops away?
$ ip route default via 10.1.1.254 dev enp0s20 metric 5 10.1.1.0/24 dev enp0s20 proto kernel scope link src 10.1.1.200 169.254.0.0/16 dev enp0s18 scope link metric 1002 169.254.0.0/16 dev enp0s20 scope link metric 1003 172.16.0.0/16 dev enp0s18 proto kernel scope link src 172.16.0.200 $ ip -6 route unreachable ::/96 dev lo metric 1024 error -101 unreachable ::ffff:0.0.0.0/96 dev lo metric 1024 error -101 2001:1800:1234:90::/64 dev enp0s20 proto kernel metric 256 unreachable 2002:a00::/24 dev lo metric 1024 error -101 unreachable 2002:7f00::/24 dev lo metric 1024 error -101 unreachable 2002:a9fe::/32 dev lo metric 1024 error -101 unreachable 2002:ac10::/28 dev lo metric 1024 error -101 unreachable 2002:c0a8::/32 dev lo metric 1024 error -101 unreachable 2002:e000::/19 dev lo metric 1024 error -101 unreachable 3ffe:ffff::/32 dev lo metric 1024 error -101 fe80::/64 dev enp0s18 proto kernel metric 256 fe80::/64 dev enp0s20 proto kernel metric 256 default via fe80::201:5cff:fe79:1446 dev enp0s20 proto ra metric 1024 expires 1797sec pref medium
Don’t worry about the unreachable IPv6 routes! Those are ”black-holed” routes to address blocks for retired experimental projects, for mapping IPv6 to special use and private IPv4 address blocks and other reserved blocks.
The hands-on exercises in Learning Tree’s Linux server administration course fill in the details. Check it out if you want to learn more about Linux networking.