Using IP address INADDR_ANY
作者: alexclark(http://alexclark.itpub.net)发表于: 2005.06.20 15:51
分类: 所见即所得
出处: http://alexclark.itpub.net/post/670/32777
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The IP address INADDR_ANY
When you wrote your simple FTP server in project 1, you probably bound your listening socket to the special IP address INADDR_ANY. This allowed your program to work without knowing the IP address of the machine it was running on, or, in the case of a machine with multiple network interfaces, it allowed your server to receive packets destined to any of the interfaces. In reality, the semantics of INADDR_ANY are more complex and involved.
In the simulator, INADDR_ANY has the following semantics: When receiving, a socket bound to this address receives packets from allINADDR_ANY and udp port 8000, then the socket will receive all packets for port 8000 that arrive on interfaces 0, 1, or 2. If a second socket attempts to Bind to port 8000 on interface 1, the Bind will fail since the first socket already ``owns'' that port/interface. interfaces. For example, suppose that a host has interfaces 0, 1 and 2. If a UDP socket on this host is bound using
When sending, a socket bound with INADDR_ANY binds to the default IP address, which is that of the lowest-numbered interface.
Using INADDR_ANY
- Some server computers have multiple IP addresses
- A socket bound to one of these will not accept connections to another address
- Frequently you prefer to allow any one of the computer's IP addresses to be used for connections
- Use INADDR_ANY (0L) to allow clients to connect using any one of the host's IP addresses
UDP Checksums
A host MUST implement the facility to generate and validate UDP checksums. An application MAY optionally be able to control whether a UDP checksum will be generated, but it MUST default to checksumming on.
If a UDP datagram is received with a checksum that is non- zero and invalid, UDP MUST silently discard the datagram. An application MAY optionally be able to control whether UDP datagrams without checksums should be discarded or passed to the application.
- DISCUSSION:
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Some applications that normally run only across local area networks have chosen to turn off UDP checksums for efficiency. As a result, numerous cases of undetected errors have been reported. The advisability of ever turning off UDP checksumming is very controversial.
- IMPLEMENTATION:
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There is a common implementation error in UDP checksums. Unlike the TCP checksum, the UDP checksum is optional; the value zero is transmitted in the checksum field of a UDP header to indicate the absence of a checksum. If the transmitter really calculates a UDP checksum of zero, it must transmit the checksum as all 1's (65535). No special action is required at the receiver, since zero and 65535 are equivalent in 1's complement arithmetic.
