Randomize initial UA's local CSeq selection.

Author: sobomax

sippy/SipCSeq.py

@@ -25,11 +25,15 @@
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 from sippy.SipGenericHF import SipGenericHF
+from secrets import randbelow
 
 class SipCSeq(SipGenericHF):
     hf_names = ('cseq',)
     cseq = None
     method = None
+    initial_min = 1
+    initial_max = (2 ** 31) - 1
+    initial_headroom = 1024
 
     def __init__(self, body = None, cseq = None, method = None):
         SipGenericHF.__init__(self, body)
@@ -71,3 +75,9 @@ def getCanName(self, name, compact = False):
     def incCSeqNum(self):
         self.cseq += 1
         return self.cseq
+
+    @classmethod
+    def genRandCSeq(cls):
+        cseq_max = cls.initial_max - cls.initial_headroom
+        cseq_span = cseq_max - cls.initial_min + 1
+        return cls.initial_min + randbelow(cseq_span)

sippy/UacStateIdle.py

@@ -36,6 +36,7 @@
 from sippy.SipTo import SipTo
 from sippy.SipFrom import SipFrom
 from sippy.SipCallId import SipCallId
+from sippy.SipCSeq import SipCSeq
 from sippy.SipHeader import SipHeader
 
 class UacStateIdle(UaStateGeneric):
@@ -66,7 +67,7 @@ def recvEvent(self, event):
             lurl.port = lurl.transport = None
             self.ua.lUri = SipFrom(address = SipAddress(url = lurl, hadbrace = True, name = callingName))
             self.ua.lUri.setTag(self.ua.lTag)
-            self.ua.lCSeq = 200
+            self.ua.lCSeq = SipCSeq.genRandCSeq()
             if self.ua.lContact == None:
                 self.ua.lContact = SipContact()
             curl = self.ua.lContact.getUrl()

sippy/UasStateIdle.py

@@ -32,6 +32,7 @@
 from sippy.UaStateGeneric import UaStateGeneric
 from sippy.CCEvents import CCEventTry
 from sippy.SipContact import SipContact
+from sippy.SipCSeq import SipCSeq
 from sippy.SipFrom import SipFrom
 from sippy.SipTo import SipTo
 
@@ -45,7 +46,7 @@ def recvRequest(self, req):
         self.ua.origin = 'caller'
         #print('INVITE received in the Idle state, going to the Trying state')
         self.ua.uasResp = req.genResponse(100, 'Trying', server = self.ua.local_ua)
-        self.ua.lCSeq = 100 # XXX: 100 for debugging so that incorrect CSeq generation will be easily spotted
+        self.ua.lCSeq = SipCSeq.genRandCSeq()
         if self.ua.lContact == None:
             self.ua.lContact = SipContact()
         self.ua.rTarget = req.getHFBody('contact').getUrl().getCopy()