<p>Is there a way to define in which interface Flannel should be listening? According to <a href="https://github.com/coreos/flannel/blob/master/Documentation/troubleshooting.md#vagrant" rel="noreferrer">his documentation</a> adding FLANNEL_OPTIONS="--iface=enp0s8" in /etc/sysconfig/flanneld should work, but it isn't. </p>

<p>My master node configuration is running in a xenial(ubuntu 16.04) vagrant:</p>

<pre><code>$ sudo kubeadm init --pod-network-cidr 10.244.0.0/16 --apiserver-advertise-address 10.0.0.10 

$ kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml
clusterrole "flannel" created                                                                    
clusterrolebinding "flannel" created                                                                   
serviceaccount "flannel" created                                                                 
configmap "kube-flannel-cfg" created                                                                                                                                                       
daemonset "kube-flannel-ds" created   


ubuntu@master:~$ kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/k8s-manifests/kube-flannel-rbac.yml
clusterrole "flannel" configured                                                          
clusterrolebinding "flannel" configured         
</code></pre>

<p>Host ip addresses:</p>

<pre><code>$ ip addr                      
1: lo: &lt;LOOPBACK,UP,LOWER_UP&gt; 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: enp0s3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1500 qdisc pfifo_fast state UP group default qlen 1000                                                                                    
    link/ether 02:63:8e:2c:ef:cd brd ff:ff:ff:ff:ff:ff                                       
    inet 10.0.2.15/24 brd 10.0.2.255 scope global enp0s3                                     
       valid_lft forever preferred_lft forever                                               
    inet6 fe80::63:8eff:fe2c:efcd/64 scope link                                              
       valid_lft forever preferred_lft forever                                               
3: enp0s8: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1500 qdisc pfifo_fast state UP group default qlen 1000                                                                                    
    link/ether 08:00:27:fb:ad:bb brd ff:ff:ff:ff:ff:ff                                       
    inet 10.0.0.10/24 brd 10.0.0.255 scope global enp0s8                                     
       valid_lft forever preferred_lft forever                                               
4: docker0: &lt;NO-CARRIER,BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc noqueue state DOWN group default                                                                                            
    link/ether 02:42:da:aa:6e:13 brd ff:ff:ff:ff:ff:ff                                       
    inet 172.17.0.1/16 scope global docker0   
       valid_lft forever preferred_lft forever                                               
5: flannel.1: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue state UNKNOWN group default                                                                                         
    link/ether 5e:07:a1:7f:97:53 brd ff:ff:ff:ff:ff:ff                                       
    inet 10.244.0.0/32 scope global flannel.1 
       valid_lft forever preferred_lft forever                                               
    inet6 fe80::5c07:a1ff:fe7f:9753/64 scope link                                            
       valid_lft forever preferred_lft forever                                               
6: cni0: &lt;NO-CARRIER,BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc noqueue state DOWN group default qlen 1000                                                                                     
    link/ether 0a:58:0a:f4:00:01 brd ff:ff:ff:ff:ff:ff                                       
    inet 10.244.0.1/24 scope global cni0      
       valid_lft forever preferred_lft forever                                               
    inet6 fe80::7037:fcff:fe41:b1fb/64 scope link                                            
       valid_lft forever preferred_lft forever                  
</code></pre>

<p>Pods names:</p>

<pre><code>$ kubectl get pods --all-namespaces                                                                                                                                         
NAMESPACE     NAME                             READY     STATUS              RESTARTS   AGE  
kube-system   etcd-master                      1/1       Running             0          2m   
kube-system   kube-apiserver-master            1/1       Running             0          2m   
kube-system   kube-controller-manager-master   1/1       Running             0          2m   
kube-system   kube-dns-545bc4bfd4-gjjth        0/3       ContainerCreating   0          3m   
kube-system   kube-flannel-ds-gdz8f            1/1       Running             0          1m   
kube-system   kube-flannel-ds-h4fd2            1/1       Running             0          33s  
kube-system   kube-flannel-ds-rnlsz            1/1       Running             1          33s  
kube-system   kube-proxy-d4wv9                 1/1       Running             0          33s  
kube-system   kube-proxy-fdkqn                 1/1       Running             0          3m   
kube-system   kube-proxy-kj7tn                 1/1       Running             0          33s  
kube-system   kube-scheduler-master            1/1       Running             0          2m   
</code></pre>

<p>Flannel Logs:</p>

<pre><code>$ kubectl logs -n kube-system kube-flannel-ds-gdz8f -c kube-flannel
I1216 12:00:35.817207       1 main.go:474] Determining IP address of default interface
I1216 12:00:35.822082       1 main.go:487] Using interface with name enp0s3 and address 10.0.2.15
I1216 12:00:35.822335       1 main.go:504] Defaulting external address to interface address (10.0.2.15)
I1216 12:00:35.909906       1 kube.go:130] Waiting 10m0s for node controller to sync
I1216 12:00:35.909950       1 kube.go:283] Starting kube subnet manager
I1216 12:00:36.987719       1 kube.go:137] Node controller sync successful
I1216 12:00:37.087300       1 main.go:234] Created subnet manager: Kubernetes Subnet Manager - master
I1216 12:00:37.087433       1 main.go:237] Installing signal handlers
I1216 12:00:37.088836       1 main.go:352] Found network config - Backend type: vxlan
I1216 12:00:37.089018       1 vxlan.go:119] VXLAN config: VNI=1 Port=0 GBP=false DirectRouting=false
I1216 12:00:37.295988       1 main.go:299] Wrote subnet file to /run/flannel/subnet.env
I1216 12:00:37.296025       1 main.go:303] Running backend.
I1216 12:00:37.296048       1 main.go:321] Waiting for all goroutines to exit
I1216 12:00:37.296084       1 vxlan_network.go:56] watching for new subnet leases
</code></pre>

<p>How do I do to configure flannel in kubernetes <strong>to listen in enp0s8 instead of enp0s3</strong>?</p>

<p>I've the same problem, trying to use k8s and Vagrant.
I've found this note in the <a href="https://github.com/coreos/flannel/blob/master/Documentation/troubleshooting.md#vagrant" rel="noreferrer">documentation of flannel</a>:</p>

<blockquote>
  <p>Vagrant typically assigns two interfaces to all VMs. The first, for
  which all hosts are assigned the IP address 10.0.2.15, is for external
  traffic that gets NATed.</p>
  
  <p>This may lead to problems with flannel. By default, flannel selects
  the first interface on a host. This leads to all hosts thinking they
  have the same public IP address. To prevent this issue, pass the
  --iface eth1 flag to flannel so that the second interface is chosen.</p>
</blockquote>

<p>So I look for it in the flannel's pod configuration.
If you download the <a href="https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml" rel="noreferrer">kube-flannel.yml</a> file, you should look at DaemonSet spec, specifically at the "kube-flannel" container. There, you should add the required "--iface=enp0s8" argument (Don't forget the "="). Part of the code I've used.</p>

<pre><code>  containers:
  - name: kube-flannel
    image: quay.io/coreos/flannel:v0.10.0-amd64
    command:
    - /opt/bin/flanneld
    args:
    - --ip-masq
    - --kube-subnet-mgr
    - --iface=enp0s8
</code></pre>

<p>Then run <code>kubectl apply -f kube-flannel.yml</code></p>

<p>Hope helps.</p>
