[Feature] Allow pass ip instead of container to loadbalancer

Author: grachevko

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.7.4-alpine3.10
+FROM python:3.8.0-alpine3.10
 
 WORKDIR /usr/local/app
 

README.md

@@ -5,8 +5,9 @@ Automatic update `/etc/hosts` on start/stop containers by labels.
 Requirements
 -----
 * **Native linux**  
-_This tool has no effect on macOS or windows, because docker on these OS running in 
-VM and you can't directly access from host to each container via ip._  
+_This tool has no effect on macOS or windows, because docker on these OS run in 
+VM and you can't directly access from host to each container via ip.
+Yet you can pass traffic through loadbalancer, see section above._  
 
 Usage
 -----
@@ -64,12 +65,12 @@ If priority is the same then early created container will be used.
 
 Load Balancer
 ----
-To pass the traffic through the loadbalancer you should define container's name in order to use it's ip for recording in hosts. 
+In order to pass the traffic through the loadbalancer you should define container's name or valid IPv4. 
 Just add one more colon and container name after it.
 ```bash
 $ docker run -d --name lb nginx
 $ docker run -d --label ru.grachevko.dhu="nginx1.local:0:lb" nginx
-$ docker run -d --label ru.grachevko.dhu="nginx2.local:0:lb" nginx
+$ docker run -d --label ru.grachevko.dhu="nginx2.local:0:127.0.0.1" nginx
 $ ping nginx1.local // ip of lb
 $ ping nginx2.local // ip of lb
 ```

docker-compose.yml

@@ -38,3 +38,8 @@ services:
         image: nginx:alpine
         labels:
             ru.grachevko.dhu: 'nginx5.local:0:lb'
+
+    nginx6:
+        image: nginx:alpine
+        labels:
+            ru.grachevko.dhu: 'nginx6.local:0:127.0.0.1'

main.py

@@ -1,5 +1,6 @@
 import docker
 import re
+from netaddr import valid_ipv4
 
 LABEL = 'ru.grachevko.dhu'
 MARKER = '#### DOCKER HOSTS UPDATER ####'
@@ -22,17 +23,24 @@ def scan():
         for string in label.split(';'):
             priority = 0
             lb = container
+            ip = False
 
             if ':' in string:
                 parts = string.split(':')
                 string = parts[0]
                 priority = int(parts[1]) if len(parts) >= 2 else priority
-                lb = docker.containers.get(parts[2]) if len(parts) == 3 else lb
 
-            if not lb:
-                continue
+                if len(parts) == 3:
+                    lbString = parts[2]
+
+                    if valid_ipv4(lbString):
+                        ip = lbString
+                    else:
+                        lb = docker.containers.get(lbString)
+
+            if ip == False:
+                ip = next(iter(lb.attrs.get('NetworkSettings').get('Networks').values())).get('IPAddress')
 
-            ip = next(iter(lb.attrs.get('NetworkSettings').get('Networks').values())).get('IPAddress')
             if ip:
                 containers.append({
                     'ip': ip,

requirements.txt

@@ -1 +1,2 @@
 docker>=4.0.2
+netaddr