I have eth0 and wlan0 according to ifconfig and I can ping google.com.
How can I find out (with a normal user, not root) what interface is active, as in, what interface did the ping (or whatever, ping is not mandatory) use?
I am using Ubuntu 11.04 or Fedora 14
You can use route to find your default route:
$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 1 0 0 eth0
link-local * 255.255.0.0 U 1000 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
The Iface column in the line with destination default tells you which interface is used.
route can't be used. @Torgeir have the wright answer.
Commented
Feb 20, 2019 at 18:40
192.168.1.0... is the route that will be used. Of course in this case it refers to the same Iface, but that is just because you have one interface on your machine.
My version which is basically based on this and this:
route | grep '^default' | grep -o '[^ ]*$'
And this, experimentally, for macOS:
route -n get default | grep 'interface:' | grep -o '[^ ]*$'
route |awk '/^default/{print $NF}'
On GNU/Linux systems:
#!/bin/sh
# host we want to "reach"
host=google.com
# get the ip of that host (works with dns and /etc/hosts. In case we get
# multiple IP addresses, we just want one of them
host_ip=$(getent ahosts "$host" | awk '{print $1; exit}')
# only list the interface used to reach a specific host/IP. We only want the part
# between dev and the remainder (use grep for that)
ip route get "$host_ip" | grep -Po '(?<=(dev ))(\S+)'
host is an input argument to the script. As the route (and hence the interface) might be different for different ips.
grep part: grep -Po '(?<= dev ).+?(?= (src|proto))'
One liner:
ip route get 8.8.8.8 | sed -n 's/.*dev \([^\ ]*\).*/\1/p'
print $5 instead of print $3...
print $5 works for me also. Fixing/replacing my initial comment with the one below..
Commented
Jul 16, 2020 at 9:02
ip route get 8.8.8.8, the rest doesn't work on ubuntu 18.04. I used something simpler: ip route get 8.8.8.8 | head -n1 | awk '{print $5}'
Commented
Jul 16, 2020 at 9:02
Get the default network interface typically used to route to the "remaining" internet in opposite to DMZ, private network, VM host etc. which are usually routed explicitly.
$ ip route get 8.8.8.8 | sed -n 's/.* dev \([^\ ]*\) .*/\1/p'
eth0
ip -4 route ls |awk '/^default/{print $5}'
Different distros of Linux, Unix, some versions of Microsoft Windows, and many other Operating Systems are not limited to using just one network interface to reach the internet. Many Operating Systems will detect more than one valid interface, capable of reaching the internet, and set up each of them for carrying traffic to the internet (specifically, gateways that ultimately connect to the internet).
If the OS tries to reach an external network and succeeds using eth0, it will add that interface to the routing table and tie it to that network.
If the OS tries to reach the same external network
and also succeeds using eth1,
it will also add that interface (eth1) into the routing table
as an additional way to reach that same network.
Other posters thus far have not considered the importance of the metric values in a routing table.
My routing table, below, shows two different interfaces, eth0 and wlan0.
Both are up, both have been set automatically by the operating system as default routes to the gateway 192.168.1.1,
and both have had a route built automatically by the operating system
to the 192.168.1.X (i.e., 192.168.1.0/24) network.
This entire routing table was written automatically by the OS.
No edits to it were made by me. The difference shown here is that
the wired ethernet interface (eth0) has a lower metric (202),
and thus more of my traffic to nodes beyond my own network
will be routed through this interface (it is preferred by the lower metric),
while the remainder of my traffic to nodes beyond my network
will be routed through the wireless interface (wlan0)
(it has a higher metric of 303, and thus is less preferred by the OS).
pi@raspberrypi:~ $ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 192.168.1.1 0.0.0.0 UG 202 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 303 0 0 wlan0
192.168.1.0 0.0.0.0 255.255.255.0 U 202 0 0 eth0
192.168.1.0 0.0.0.0 255.255.255.0 U 303 0 0 wlan0
Both interfaces are being used to reach "the internet", and their use is weighted by the "Metric" values, as can be seen in the bytes and packets statistics for eth0 and wlan0 below:
pi@raspberrypi:~ $ ip -s address
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
valid_lft forever preferred_lft forever
RX: bytes packets errors dropped overrun mcast
0 0 0 0 0 0
TX: bytes packets errors dropped carrier collsns
0 0 0 0 0 0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether dc:a6:32:31:a2:c7 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.195/24 brd 192.168.1.255 scope global dynamic noprefixroute eth0
valid_lft 80787sec preferred_lft 69987sec
inet6 fe80::2f3f:3f1d:8c35:a05e/64 scope link
valid_lft forever preferred_lft forever
RX: bytes packets errors dropped overrun mcast
14341060 22393 0 0 0 971 ←—— high
TX: bytes packets errors dropped carrier collsns
1190274 10745 0 0 0 0 ←—— high
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether dc:a6:32:31:a2:c8 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.193/24 brd 192.168.1.255 scope global dynamic noprefixroute wlan0
valid_lft 80787sec preferred_lft 69987sec
inet6 fe80::4f31:5fcf:8f70:b5ca/64 scope link
valid_lft forever preferred_lft forever
RX: bytes packets errors dropped overrun mcast
4963408 7954 0 0 0 929 ←—— lower
TX: bytes packets errors dropped carrier collsns
49371 235 0 0 0 0 ←—— lower
As the annotations indicate, eth0 is being given more traffic by the OS.
Many Operating Systems will give a lower metric to a wired interface that has a faster connection,
and a higher metric to a wired interface with a slower connection.
For example, if eth0 and eth1 both connect to the same node,
but eth0 negotiated a 100 Mbs connection,
and eth1 negotiated only a 10 Mbs connection,
eth0 would be given a lower metric than eth1.
Similarly, many Operating Systems will give a lower metric to a wired interface, and a higher metric to a wireless interface.
More than one interface can be set up automatically (depending on the OS), or manually, to reach the same external node by making/editing entries to the routing table.
A minimally tested script to find the interface with the lowest metric
as reported by route: [Added by G-Man]
route | awk '
BEGIN { min = -1 }
$1 == "default" {
if (min < 0 || $5 < min) {
min = $5
iface = $8
}
}
END {
if (iface == "") {
print "No \"default\" route found!" > "/dev/stderr"
exit 1
} else {
print iface
exit 0
}
}
'
You might want to extend this to look at the "Flags"; i.e., $4.
Running ifconfig will give you the information you need.
The active interface will have an inet addr and will show a record of transmitted data, like so:
RX bytes:1930741 (1.8 Mb) TX bytes:204768 (199.9 Kb)You can also use the ip addr command and any inactive interfaces will be designated as having: NO-CARRIER.
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:
Commented
Jun 14, 2011 at 18:32
route command though that wouldn't necessarily be foolproof either due to routing. The default route does not have to be the one to get to the Internet though it may often be.
Commented
Aug 10, 2016 at 18:36
The command ip route ls will give a list of active routes and their sources:
caleburn: ~/ >ip route ls
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.7
default via 192.168.10.254 dev eth0
Use this command:
$ route | grep default | awk '{print $8}'
enp0s3
Debian: apt-get install net-tools
Alpine: apk add net-tools
Arch Linux: pacman -S net-tools
CentOS: yum install net-tools
Fedora: dnf install net-tools
route | head -3 | tail -1 | tr -s " " | cut -d " " -f8
Debian: apt-get install iproute2
Alpine: apk add iproute2
Arch Linux: pacman -S iproute2
CentOS: yum install iproute2
Fedora: dnf install iproute2
OS X: brew install iproute2
ip route | head -1 | cut -d " " -f5
If your machine has multiple interfaces (which I assume), there is no one interface that will be used for connecting to the internet.
Depending on the destination you are connecting to, your system will consult the ip table (the one shown when you do route command) to find the next hope/router, when it does find one, it will use the interface associated with it.
That said, please refer to @torgeir's answer as it does exactly that:
Even shorter one liner:
ip route show default | cut -d " " -f 5
or
ip route show default | sed -n 's/.* dev \([^\ ]*\) .*/\1/p'
