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------------------------------------------
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Link-local IP address auto-configuration
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------------------------------------------
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Negotiate with other link-local clients on the local network
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for an address that doesn't require explicit configuration.
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This is especially useful if a DHCP server cannot be guaranteed
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to exist in all environments that the device must operate.
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This is an implementation of RFC3927.
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----------
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Commands
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----------
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When CONFIG_CMD_LINK_LOCAL is defined in the board config file,
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the "linklocal" command is available. This running this will
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take approximately 5 seconds while the address is negotiated.
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------------------------
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Environment interation
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------------------------
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The "llipaddr" variable is set with the most recently
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negotiated address and is preferred in future negotiations.
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The "ipaddr", "netmask", and "gatewayip" variables are set
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after successful negotiation to enable network access.
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-------------
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Limitations
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-------------
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RFC3927 requires that addresses are continuously checked to
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avoid conflicts, however this can only happen when the net_loop
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is getting called. It is possible for a conflict to go undetected
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until a command that accesses the network is executed.
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Using NetConsole is one way to ensure that net_loop is always
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processing packets and monitoring for conflicts.
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This is also not a concern if the feature is use to connect
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directly to another machine that may not be running a DHCP server.
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----------------
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Example script
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----------------
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This script allows use of DHCP and/or Link-local controlled
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by env variables. It depends on CONFIG_CMD_LINK_LOCAL, CONFIG_CMD_DHCP,
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and CONFIG_BOOTP_MAY_FAIL.
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If both fail or are disabled, static settings are used.
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#define CONFIG_EXTRA_ENV_SETTINGS \
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"ipconfigcmd=if test \\\"$dhcpenabled\\\" -ne 0;" \
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"then " \
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"dhcpfail=0;dhcp || dhcpfail=1;" \
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"else " \
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"dhcpfail=-1;" \
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"fi;" \
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"if test \\\"$linklocalenabled\\\" -ne 0 -a " \
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"\\\"$dhcpfail\\\" -ne 0;" \
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"then " \
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"linklocal;" \
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"llfail=0;" \
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"else " \
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"llfail=-1;" \
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"fi;" \
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"if test \\\"$llfail\\\" -ne 0 -a " \
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"\\\"$dhcpfail\\\" -ne 0; " \
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"then " \
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"setenv ipaddr $sipaddr; " \
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"setenv netmask $snetmask; " \
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"setenv gatewayip $sgatewayip; " \
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"fi;\0" \
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