Merge pull request #552 from skuhl/systemd-resolved

Intercept DNS requests sent by systemd-resolved.

docs/manpage.rst

@@ -104,10 +104,12 @@ Options
 
     Capture local DNS requests and forward to the remote DNS
     server. All queries to any of the local system's DNS
-    servers (/etc/resolv.conf) will be intercepted and
+    servers (/etc/resolv.conf and, if it exists, 
+    /run/systemd/resolve/resolv.conf) will be intercepted and
     resolved on the remote side of the tunnel instead, there
     using the DNS specified via the :option:`--to-ns` option,
-    if specified.
+    if specified. Only plain DNS traffic sent to these servers
+    on port 53 are captured.
 
 .. option:: --ns-hosts=<server1[,server2[,server3[...]]]>
 

sshuttle/client.py

@@ -597,7 +597,7 @@ def main(listenip_v6, listenip_v4,
     # redirect packets outgoing to this server to the remote host
     # instead.
     if dns:
-        nslist += resolvconf_nameservers()
+        nslist += resolvconf_nameservers(True)
         if to_nameserver is not None:
             to_nameserver = "%s@%s" % tuple(to_nameserver[1:])
     else:

sshuttle/helpers.py

@@ -49,17 +49,64 @@ class Fatal(Exception):
     pass
 
 
-def resolvconf_nameservers():
-    lines = []
-    for line in open('/etc/resolv.conf'):
+def resolvconf_nameservers(systemd_resolved):
+    """Retrieves a list of tuples (address type, address as a string) of
+    the DNS servers used by the system to resolve hostnames.
+
+    If parameter is False, DNS servers are retrieved from only
+    /etc/resolv.conf. This behavior makes sense for the sshuttle
+    server.
+
+    If parameter is True, we retrieve information from both
+    /etc/resolv.conf and /run/systemd/resolve/resolv.conf (if it
+    exists). This behavior makes sense for the sshuttle client.
+
+    """
+
+    # Historically, we just needed to read /etc/resolv.conf.
+    #
+    # If systemd-resolved is active, /etc/resolv.conf will point to
+    # localhost and the actual DNS servers that systemd-resolved uses
+    # are stored in /run/systemd/resolve/resolv.conf. For programs
+    # that use the localhost DNS server, having sshuttle read
+    # /etc/resolv.conf is sufficient. However, resolved provides other
+    # ways of resolving hostnames (such as via dbus) that may not
+    # route requests through localhost. So, we retrieve a list of DNS
+    # servers that resolved uses so we can intercept those as well.
+    #
+    # For more information about systemd-resolved, see:
+    # https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html
+    #
+    # On machines without systemd-resolved, we expect opening the
+    # second file will fail.
+    files = ['/etc/resolv.conf']
+    if systemd_resolved:
+        files += ['/run/systemd/resolve/resolv.conf']
+
+    nsservers = []
+    for f in files:
+        this_file_nsservers = []
+        try:
+            for line in open(f):
                 words = line.lower().split()
                 if len(words) >= 2 and words[0] == 'nameserver':
-            lines.append(family_ip_tuple(words[1]))
-    return lines
+                    this_file_nsservers.append(family_ip_tuple(words[1]))
+            debug2("Found DNS servers in %s: %s\n" %
+                   (f, [n[1] for n in this_file_nsservers]))
+            nsservers += this_file_nsservers
+        except OSError as e:
+            debug3("Failed to read %s when looking for DNS servers: %s\n" %
+                   (f, e.strerror))
+
+    return nsservers
 
 
-def resolvconf_random_nameserver():
-    lines = resolvconf_nameservers()
+def resolvconf_random_nameserver(systemd_resolved):
+    """Return a random nameserver selected from servers produced by
+    resolvconf_nameservers(). See documentation for
+    resolvconf_nameservers() for a description of the parameter.
+    """
+    lines = resolvconf_nameservers(systemd_resolved)
     if lines:
         if len(lines) > 1:
             # don't import this unless we really need it

sshuttle/server.py

@@ -183,7 +183,7 @@ class DnsProxy(Handler):
         self.tries += 1
 
         if self.to_nameserver is None:
-            _, peer = resolvconf_random_nameserver()
+            _, peer = resolvconf_random_nameserver(False)
             port = 53
         else:
             peer = self.to_ns_peer

tests/client/test_helpers.py

@@ -131,7 +131,7 @@ nameserver 2404:6800:4004:80c::3
 nameserver 2404:6800:4004:80c::4
 """)
 
-    ns = sshuttle.helpers.resolvconf_nameservers()
+    ns = sshuttle.helpers.resolvconf_nameservers(False)
     assert ns == [
         (AF_INET, u'192.168.1.1'), (AF_INET, u'192.168.2.1'),
         (AF_INET, u'192.168.3.1'), (AF_INET, u'192.168.4.1'),
@@ -156,7 +156,7 @@ nameserver 2404:6800:4004:80c::2
 nameserver 2404:6800:4004:80c::3
 nameserver 2404:6800:4004:80c::4
 """)
-    ns = sshuttle.helpers.resolvconf_random_nameserver()
+    ns = sshuttle.helpers.resolvconf_random_nameserver(False)
     assert ns in [
         (AF_INET, u'192.168.1.1'), (AF_INET, u'192.168.2.1'),
         (AF_INET, u'192.168.3.1'), (AF_INET, u'192.168.4.1'),