Get local IP address/addresses on Arch Linux

Question

Till now, my network scripts used hostname -i, but after a recent update, the command started throwing an error: hostname: gethostbyname: Unknown host.

Now I don't know... should I be looking into a new command like ip? Or is this a common issue? I can't seem to find it mentioned anywhere. I would use ip, but I can't figure out a way, to display two IP's at the same time. For example when both of my interfaces are connected (eth and wlp).

Any suggestions?

Answer 1

Same thing happened to me today. I checked/checksummed that /usr/bin/hostname file hasn't changed recently.

I grepped the strace output for opened files (excluding "No Such Files" in alternative $PATH's):

$ strace hostname -i |& grep open | grep -v "No such file"
open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/etc/host.conf", O_RDONLY|O_CLOEXEC) = 3
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 3
open("/usr/lib/libnss_mymachines.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libcap.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/librt.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_resolve.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 4

One-by-one inspection and comparison with the incremental backups for the last few days showed that /etc/nsswitch.conf changed from its previous version as of Sep 30, 2015, on Dec 10, 2016.

A side-by-side diff shows the differences (left side previous and restored, right side latest and backed-up):

$ diff -y /etc/nsswitch.conf /etc/nsswitch.conf.bck | grep \|
passwd: files                                                 | passwd: compat mymachines systemd
group: files                                                  | group: compat mymachines systemd
shadow: files                                                 | shadow: compat
hosts: files dns myhostname                                   | hosts: files mymachines resolve [!UNAVAIL=return] dns myhostn

Restoring the previous /etc/nsswitch.conf solves the problem and "hostname -i" returns the IPs given by interfaces as usual. You should backup the latest one just in case.

Update: After the bug I filed (https://bugs.archlinux.org/task/52133), systemd package maintainer Dave Reisner pointed out that the problem only occurs when the system-resolved daemon does not work. I checked and it is correct. In order for the new /etc/nsswitch.conf to work:

systemctl enable systemd-resolved

is needed.

According to the "top" output this adds a resident size of around 4m. Alternatively, one can go on with the previous line without systemd-resolved.

Answer 2

If you plan to use ip, this is an example on how to get the correct ip address from this ip address show output:

$ ip a

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    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 noprefixroute 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:92:55:ff brd ff:ff:ff:ff:ff:ff
    altname enp0s5
    altname ens5
    inet 192.168.124.29/24 metric 1024 brd 192.168.124.255 scope global dynamic eth0
       valid_lft 2742sec preferred_lft 2742sec
    inet6 fe80::5054:ff:fe92:55ff/64 scope link proto kernel_ll 
       valid_lft forever preferred_lft forever

Run:

ip a s dev eth0 | grep "inet " | grep -oE "[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}" | head -1

Explanation:

• ip a s dev eth0, which is the same as ip address show dev eth0, will output only information regarding device eth0. This is the device with the ip address that I need. Output is:

$ ip a s dev eth0

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:92:55:ff brd ff:ff:ff:ff:ff:ff
    altname enp0s5
    altname ens5
    inet 192.168.124.29/24 metric 1024 brd 192.168.124.255 scope global dynamic eth0
       valid_lft 2331sec preferred_lft 2331sec
    inet6 fe80::5054:ff:fe92:55ff/64 scope link proto kernel_ll 
       valid_lft forever preferred_lft forever

• grep "inet " will match the line containing inet . The whitespace is quite important to avoid matching inet6 (you can also run ip -4 a s dev eth0). Output:

$ ip a s dev eth0 | grep "inet "

    inet 192.168.124.29/24 metric 1024 brd 192.168.124.255 scope global dynamic eth0

• grep -oE "[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}" is a REGEX that will match ONLY strings that have: "(1 to 3 digits)DOT(1 to 3 digits)DOT(1 to 3 digits)DOT(1 to 3 digits)".
  The regex pattern can also be stored in a variable and used later:
  REGEX="[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}", Output:

$ REGEX="[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}"
$ ip a s dev eth0 | grep "inet " | grep -oE $REGEX

192.168.124.29
192.168.124.255

• head -1 to output only the first match (the first line), which is the one just after init (the second one is the broadcast address). Output:

$ REGEX="[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}"
$ ip a s dev eth0 | grep "inet " | grep -oE $REGEX | head -1

192.168.124.29

---

If you have access to hostname -I, just use this:

hostname -I | cut -d' ' -f1