Created attachment 1314403 [details]
Example script

Description of problem:
PyDNS randomly binds to TCP or UDP source ports from the range 1024-65535 as a defense against cache poisoning.  Some ports in this range are defined as SELinux ports.  Someone who is writing an SELinux policy to confine a python script that uses PyDNS will either have to:

* Accept that ~1% of TCP queries will fail
* Make the SELinux policy allow more than needed (i.e. corenet_tcp_bind_all_ports)


Version-Release number of selected component (if applicable):
2.3.6-2.el7


How reproducible:
Basic SELinux policy files and python script to demonstrate the issue are attached.


Steps to Reproduce:
1. Build and install SELinux policy
Place pydnstest.te and pydnstest.fc in ~/pydnstest/
$ cd ~/pydnstest
$ make -f /usr/share/selinux/devel/Makefile
# semodule -i ~/pydnstest/pydnstest.pp
# semanage dontaudit off

2. Install script
Place pydnstest.py in /usr/local/bin/
# restorecon /usr/local/bin/pydnstest.py

3. Run script
$ runcon system_u:system_r:pydnstest_t:s0 /usr/local/bin/pydnstest.py


Actual results:
SELinux denials occur during approx. 1% of lookups.

$ runcon system_u:system_r:pydnstest_t:s0 /usr/local/bin/pydnstest.py 
Lookup 107 failed: SocketError(error(13, 'Permission denied'),)
Lookup 176 failed: SocketError(error(13, 'Permission denied'),)

# ausearch -ts today 20:21 -i -m avc -su pydnstest_t | audit2allow


#============= pydnstest_t ==============
allow pydnstest_t i18n_input_port_t:tcp_socket name_bind;
allow pydnstest_t munin_port_t:tcp_socket name_bind;

In this case, ports 4949 and 9010 were among the 200 randomly selected source ports.  Since they were already defined as SELinux ports (i18n_input_port_t and munin_port_t), it resulted in denials.


Expected results:
DNS lookups use source ports in the ephemeral range, avoiding conflicts with defined SELinux ports.


Additional info:
Changing source_port to 0 makes the kernel assign a random-looking port in the ephemeral range.  Making this change resulted in no SELinux denials with the attached script/policy:

--- /usr/lib/python2.7/site-packages/DNS/Base.py.orig	2017-05-18 16:17:50.000000000 +0000
+++ /usr/lib/python2.7/site-packages/DNS/Base.py	2017-08-16 20:30:05.717033232 +0000
@@ -181,7 +181,7 @@
         "Pick random source port to avoid DNS cache poisoning attack."
         while True:
             try:
-                source_port = random.randint(1024,65535)
+                source_port = 0
                 self.s.bind(('', source_port))
                 break
             except socket.error, msg:

This reduces the pool of random source ports from 64512 to 28232.  This shouldn't matter for TCP, but will weaken this defense against DNS cache poisoning attacks for UDP queries.