add an api for setting custom DNS records

Works like this:

```curl -d "" --user email:password https://.../admin/dns/set/qname/rtype/value```

where the rtype and value default to "A" and the remote IP address of the request, so that a simple, empty POST to

```https://.../admin/dns/set/desktop.mydomain.com```

will point desktop.mydomain.com to the caller's IPv4 address.

closes #140

conf/nginx.conf

@@ -28,6 +28,7 @@ server {
 	rewrite ^/admin$ /admin/;
 	location /admin/ {
 		proxy_pass http://localhost:10222/;
+		proxy_set_header X-Forwarded-For $remote_addr;
 	}
 
 	# Roundcube Webmail configuration.

management/daemon.py

@@ -172,6 +172,30 @@ def dns_update():
 	except Exception as e:
 		return (str(e), 500)
 
+@app.route('/dns/set/<qname>', methods=['POST'])
+@app.route('/dns/set/<qname>/<rtype>', methods=['POST'])
+@app.route('/dns/set/<qname>/<rtype>/<value>', methods=['POST'])
+@authorized_personnel_only
+def dns_set_record(qname, rtype="A", value=None):
+	from dns_update import do_dns_update, set_custom_dns_record
+	try:
+		# Get the value from the URL, then the POST parameters, or if it is not set then
+		# use the remote IP address of the request --- makes dynamic DNS easy. To clear a
+		# value, '' must be explicitly passed.
+		print(request.environ)
+		if value is None:
+			value = request.form.get("value")
+		if value is None:
+			value = request.environ.get("HTTP_X_FORWARDED_FOR") # normally REMOTE_ADDR but we're behind nginx as a reverse proxy
+		if value == '':
+			# request deletion
+			value = None
+		if set_custom_dns_record(qname, rtype, value, env):
+			return do_dns_update(env)
+		return "OK"
+	except ValueError as e:
+		return (str(e), 400)
+
 @app.route('/dns/dump')
 @authorized_personnel_only
 def dns_get_dump():

management/dns_update.py

@@ -5,6 +5,7 @@
 ########################################################################
 
 import os, os.path, urllib.parse, datetime, re, hashlib
+import ipaddress
 import rtyaml
 
 from mailconfig import get_mail_domains
@@ -551,6 +552,79 @@ def write_opendkim_tables(zonefiles, env):
 
 ########################################################################
 
+def set_custom_dns_record(qname, rtype, value, env):
+	# validate
+	rtype = rtype.upper()
+	if value is not None:
+		if rtype in ("A", "AAAA"):
+			v = ipaddress.ip_address(value)
+			if rtype == "A" and not isinstance(v, ipaddress.IPv4Address): raise ValueError("That's an IPv6 address.")
+			if rtype == "AAAA" and not isinstance(v, ipaddress.IPv6Address): raise ValueError("That's an IPv4 address.")
+		elif rtype in ("CNAME", "TXT"):
+			# anything goes
+			pass
+		else:
+			raise ValueError("Unknown record type '%s'." % rtype)
+
+	# load existing config
+	config = get_custom_dns_config(env)
+
+	# update
+	if qname not in config:
+		if value is None:
+			# Is asking to delete a record that does not exist.
+			return False
+		elif rtype == "A":
+			# Add this record using the short form 'qname: value'.
+			config[qname] = value
+		else:
+			# Add this record. This is the qname's first record.
+			config[qname] = { rtype: value }
+	else:
+		if isinstance(config[qname], str):
+			# This is a short-form 'qname: value' implicit-A record.
+			if value is None and rtype != "A":
+				# Is asking to delete a record that doesn't exist.
+				return False
+			elif value is None and rtype == "A":
+				# Delete record.
+				del config[qname]
+			elif rtype == "A":
+				# Update, keeping short form.
+				if config[qname] == "value":
+					# No change.
+					return False
+				config[qname] = value
+			else:
+				# Expand short form so we can add a new record type.
+				config[qname] = { "A": config[qname], rtype: value }
+		else:
+			# This is the qname: { ... } (dict) format.
+			if value is None:
+				if rtype not in config[qname]:
+					# Is asking to delete a record that doesn't exist.
+					return False
+				else:
+					# Delete the record. If it's the last record, delete the domain.
+					del config[qname][rtype]
+					if len(config[qname]) == 0:
+						del config[qname]
+			else:
+				# Update the record.
+				if config[qname].get(rtype) == "value":
+					# No change.
+					return False
+				config[qname][rtype] = value
+
+	# serialize & save
+	config_yaml = rtyaml.dump(config)
+	with open(os.path.join(env['STORAGE_ROOT'], 'dns/custom.yaml'), "w") as f:
+		f.write(config_yaml)
+
+	return True
+
+########################################################################
+
 def justtestingdotemail(domain, records):
 	# If the domain is a subdomain of justtesting.email, which we own,
 	# automatically populate the zone where it is set up on dns4e.com.