Microsoft KB Archive/138268

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BUG: Winsock Sends IP Packets with TTL 0

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Q138268

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The information in this article applies to:


 * Microsoft Win32 Software Development Kit (SDK), versions 3.5, 3.51, 4.0

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SYMPTOMS
If an application is using IP multicasting on Windows NT version 3.5 or version 3.51 or on Windows 95, then it is possible to send packets with Time to Live (TTL) set to 0.

STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are reasearching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION
Please see the following article in the Microsoft Knowledge Base:

Q131978 Receiving, Sending Multicasts in Windows NT Using WinSock

It is possible to change the TTL for an IP datagram. For example:

  int ttl  = 0; int sock = socket( .... );

err = setsockopt( sock, IPPROTO_IP, IP_MULTICAST_TTL, (char *)&ttl,  sizeof(ttl));

However, as per the requirements of RFC 1122, a host must not send an IP datagram with TTL = 0. Here is a quote from the relavent section of RFC 1122:

  3.2.1.7  Time-to-Live: RFC-791 Section 3.2

A host must not send a datagram with a Time-to-Live (TTL) value of zero.

A host must not discard a datagram just because it was received with TTL less than 2.