Feature/pmtu discovery

icmplib/models.py

@@ -70,12 +70,20 @@ class ICMPRequest:
         Intermediate routers must be able to support this feature.
         Only available on Unix systems. Ignored on Windows.
 
+    :type pmtudisc_opt: str, optional
+    :param pmtudisc_opt: The Path MTU Discovery strategy.
+        Options are:
+        do   - prohibit fragmentation, even local one,
+        want - do PMTU discovery and fragment locally when packet size is large), or
+        dont - do not set DF flag.
+
     '''
     __slots__ = '_destination', '_id', '_sequence', '_payload', \
-                '_payload_size', '_ttl', '_traffic_class', '_time'
+                '_payload_size', '_ttl', '_traffic_class', '_time', \
+                '_pmtudisc_opt'
 
     def __init__(self, destination, id, sequence, payload=None,
-            payload_size=56, ttl=64, traffic_class=0):
+            payload_size=56, ttl=64, traffic_class=0, pmtudisc_opt='do'):
 
         if payload:
             payload_size = len(payload)
@@ -88,6 +96,7 @@ def __init__(self, destination, id, sequence, payload=None,
         self._ttl = ttl
         self._traffic_class = traffic_class
         self._time = 0
+        self._pmtudisc_opt = pmtudisc_opt
 
     def __repr__(self):
         return f'<ICMPRequest [{self._destination}]>'
@@ -162,6 +171,19 @@ def time(self):
         '''
         return self._time
 
+    @property
+    def pmtudisc_opt(self):
+        '''
+        The Path MTU Discovery strategy.
+
+        Options are:
+        do   - default; prohibit fragmentation, even local one,
+        want - do PMTU discovery and fragment locally when packet size is large), or
+        dont - do not set DF flag.
+
+        '''
+        return self._pmtudisc_opt
+
 
 class ICMPReply:
     '''

icmplib/ping.py

@@ -101,6 +101,13 @@ def ping(address, count=4, interval=1, timeout=2, id=None, source=None,
         Intermediate routers must be able to support this feature.
         Only available on Unix systems. Ignored on Windows.
 
+    :type pmtudisc_opt: str, optional
+    :param pmtudisc_opt: The Path MTU discovery strategy.
+        Options are:
+        do   - prohibit fragmentation, even local one,
+        want - do PMTU discovery and fragment locally when packet size is large), or
+        dont - do not set DF flag.
+
     :rtype: Host
     :returns: A `Host` object containing statistics about the desired
         destination.
@@ -233,6 +240,13 @@ async def async_ping(address, count=4, interval=1, timeout=2, id=None,
         Intermediate routers must be able to support this feature.
         Only available on Unix systems. Ignored on Windows.
 
+    :type pmtudisc_opt: str, optional
+    :param pmtudisc_opt: The Path MTU discovery strategy.
+        Options are:
+        do   - prohibit fragmentation, even local one,
+        want - do PMTU discovery and fragment locally when packet size is large), or
+        dont - do not set DF flag.
+
     :rtype: Host
     :returns: A `Host` object containing statistics about the desired
         destination.

icmplib/sockets.py

@@ -35,6 +35,28 @@
 from .utils import PLATFORM_LINUX, PLATFORM_MACOS, PLATFORM_WINDOWS
 
 
+socket.IP_MTU_DISCOVER = 10
+socket.IPV6_MTU_DISCOVER = 23
+
+# IP_MTU_DISCOVER values
+socket.IP_PMTUDISC_DONT  = 0  # Never send DF frames
+socket.IP_PMTUDISC_WANT  = 1  # Use per route hints
+socket.IP_PMTUDISC_DO    = 2  # Always DF
+socket.IP_PMTUDISC_PROBE = 3 # Ignore dst pmtu
+
+# IPV6_MTU_DISCOVER values
+socket.IPV6_PMTUDISC_DONT  = 0
+socket.IPV6_PMTUDISC_WANT  = 1
+socket.IPV6_PMTUDISC_DO    = 2
+socket.IPV6_PMTUDISC_PROBE = 3
+
+PMTUDISC_VAL = {
+        'dont' : [socket.IP_PMTUDISC_DONT,  socket.IPV6_PMTUDISC_DONT],
+        'want' : [socket.IP_PMTUDISC_WANT,  socket.IPV6_PMTUDISC_WANT],
+        'do'   : [socket.IP_PMTUDISC_DO,    socket.IPV6_PMTUDISC_DO],
+        'probe': [socket.IP_PMTUDISC_PROBE, socket.IPV6_PMTUDISC_PROBE]
+}
+
 class ICMPSocket:
     '''
     Base class for ICMP sockets.
@@ -146,6 +168,15 @@ def _set_traffic_class(self, traffic_class):
         '''
         raise NotImplementedError
 
+    def _set_pmtudisc_opt(self, pmtudisc_opt):
+        '''
+        Set the PMTU Discovery strategy. In IPv4, this is the DF flag.
+        In IPv6, this is a socket option.
+        Must be overridden.
+
+        '''
+        raise NotImplementedError
+
     def _checksum(self, data):
         '''
         Compute the checksum of an ICMP packet. Checksums are used to
@@ -270,6 +301,7 @@ def send(self, request):
 
             self._set_ttl(request.ttl)
             self._set_traffic_class(request.traffic_class)
+            self._set_pmtudisc_opt(request.pmtudisc_opt)
 
             request._time = time()
             self._sock.sendto(packet, sock_destination)
@@ -516,6 +548,20 @@ def _set_traffic_class(self, traffic_class):
             socket.IP_TOS,
             traffic_class)
 
+    def _set_pmtudisc_opt(self, pmtudisc_opt):
+        '''
+        Set the DF flag in the IPv4 header of every packet originating
+        from this socket.
+
+        '''
+        if not PLATFORM_LINUX:
+            raise NotImplementedError
+
+        self._sock.setsockopt(
+            socket.IPPROTO_IP,
+            socket.IP_MTU_DISCOVER,
+            PMTUDISC_VAL[pmtudisc_opt][0])
+
     @property
     def broadcast(self):
         '''
@@ -654,6 +700,25 @@ def _set_traffic_class(self, traffic_class):
             socket.IPV6_TCLASS,
             traffic_class)
 
+    def _set_pmtudisc_opt(self, pmtudisc_opt):
+        '''
+        Set the DONTFRAG socket option for every packet originating
+        from this socket.
+
+        '''
+        if not PLATFORM_LINUX:
+            raise NotImplementedError
+
+        self._sock.setsockopt(
+            socket.IPPROTO_IPV6,
+            socket.IPV6_MTU_DISCOVER,
+            PMTUDISC_VAL[pmtudisc_opt][1])
+
+        self._sock.setsockopt(
+            socket.IPPROTO_IPV6,
+            socket.IPV6_DONTFRAG,
+            1 if pmtudisc_opt in ['do', 'want', 'probe'] else 0)
+
 
 class AsyncSocket:
     '''

test/pmtu.py

@@ -0,0 +1,91 @@
+import argparse
+import ipaddress
+from icmplib import ping
+
+
+def findmtu(host, verbose=False, debug=False):
+    """
+    Find the PMTU to the specified host.
+    Searches the range
+    Host is an IPv4 or IPv6 address.
+    """
+
+    # These headers get added to the ping payload to get to MTU
+    SIZE_ICMP_HDR = 8
+    SIZE_IPV4_HDR = 20
+    SIZE_IPV6_HDR = 40
+
+    # These Ethernet headers are not included in the MTU.
+    # Subtract them from the size-on-wire to get MTU.
+    SIZE_ETH2_HDR = 14
+
+    if isinstance(ipaddress.ip_address(host), ipaddress.IPv6Address):
+        size_ip_hdr = SIZE_IPV6_HDR
+    else:
+        size_ip_hdr = SIZE_IPV4_HDR
+
+    lower = 900
+    upper = 10000
+    MTU = lower
+
+    if verbose:
+        print(f"\nhost: {host}")
+
+    result = ping(host)
+    if not result.is_alive:
+        return 0
+
+    timeout = 1.5 * result.max_rtt / 1E3
+    interval = 0.5 * result.max_rtt / 1E3
+    if verbose:
+        print(result)
+        print(f'PMTU discovery timeout: {timeout}, interval: {interval}')
+
+    test_size = 1700
+    while upper - lower > 1:
+        test_MTU = test_size + SIZE_ICMP_HDR + size_ip_hdr
+        if verbose:
+            if debug:
+                s = f" - MTU search range: {upper}-{lower} - {test_MTU+SIZE_ETH2_HDR} bytes on wire "
+            else:
+                s = ""
+            print(
+                f"checking frame size {test_MTU} {s}",
+                end="",
+                flush=True,
+            )
+        result = ping(
+            host,
+            count=2,
+            interval=interval,
+            timeout=timeout,
+            payload_size=test_size,
+            pmtudisc_opt="do",
+        )
+        if result.is_alive:
+            lower = test_size
+            MTU = test_MTU
+            if verbose:
+                print("✅")
+        else:
+            upper = test_size
+            if verbose:
+                print("❌")
+
+        test_size = lower + int((upper - lower) / 2)
+
+    return MTU
+
+
+parser = argparse.ArgumentParser()
+parser.add_argument("-v", "--verbose", help="verbose output", action="store_true")
+parser.add_argument("-d", "--debug", help="debug output", action="store_true")
+parser.add_argument("hosts", help="one or more hosts to test", nargs='+')
+args = parser.parse_args()
+
+for host in args.hosts:
+    mtu = findmtu(host, verbose=args.verbose, debug=args.debug)
+    if mtu:
+        print(f"host: {host} MTU: {mtu}")
+    else:
+        print("{host} is unreachable")