Fix dns resolution when a backend container restarts

Readme.md

@@ -110,3 +110,5 @@ podman run -d \
 ```bash
 podman build -t nginx-certbot:latest .
 ```
+## Hostname resolution
+In order to update the IP of a backend container once it has been restarted this container must specify a DNS server. For Podman this defaults to `10.88.0.1`, if you alter your container networking this will have to be amended in `src/skel.conf` and the container will have to be built yourself.

conf.d/site.conf.sample

@@ -1,7 +1,10 @@
 server {
   listen 80;
   server_name contoso.com;
+  resolver @@resolver@@ valid=30s ipv6=off;
+
   location / {
-      proxy_pass http://webserver_container_name/;
+      set $backend http://$webserver_container_name;
+      proxy_pass $backend;
   }
 }

src/nginx_setup.py

@@ -2,6 +2,7 @@
 import json
 import re
 import logging
+import subprocess
 
 logging.basicConfig(format='%(levelname)s: %(message)s', level=logging.INFO)
 
@@ -20,6 +21,19 @@
 email = os.getenv('EMAIL')
 prod = os.getenv('PRODUCTION')
 
+# get resolver
+resolver = None
+try:
+    out = subprocess.check_output(['ip', 'route'], stderr=subprocess.DEVNULL).decode()
+    m = re.search(r'^default via (\d+\.\d+\.\d+\.\d+)', out, re.MULTILINE)
+    if m:
+        resolver = m.group(1)
+except Exception:
+    resolver = os.getenv('10.88.0.1')
+    logging.info("ip route unavailable, obtaining resolver failed. Container IPs most likely won't update after restarting")
+
+logging.info("Using resolver: " + resolver)
+
 # copy in any manual conf files the user made
 os.system("cp -rf /etc/nginx/conf.avail/*.conf /etc/nginx/http.d/ &> /dev/null")
 logging.info("Configurations present: " + str(os.listdir("/etc/nginx/http.d")))
@@ -44,7 +58,11 @@
             with open('/root/skel.conf') as conf_file:
                 conf_contents = conf_file.read()
 
-            conf_complete = re.sub(r"@@(\w+?)@@", lambda match: host[match.group(1)], conf_contents)
+            conf_complete = re.sub(
+                r"@@(\w+?)@@",
+                lambda match, host=host: str(host.get('resolver', resolver)) if match.group(1) == 'resolver' else str(host.get(match.group(1), '')),
+                conf_contents
+            )
 
             logging.info("Writing /etc/nginx/http.d/" + host['hostname'] + ".conf")
 

src/skel.conf

@@ -1,7 +1,10 @@
 server {
   listen 80;
   server_name @@hostname@@;
+  resolver @@resolver@@ valid=30s ipv6=off;
+
   location / {
-      proxy_pass @@proxy_pass@@;
+      set $backend @@proxy_pass@@;
+      proxy_pass $backend;
   }
 }