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@@ -697,7 +697,7 @@ Administrator deleted a lease for a device identified by: duid of 1a:1b:1c:1d:1e
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client classes and other features. The reservation can be based on
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hardware address, DUID, circuit-id or client-id in DHCPv4 and using
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hardware address or DUID in DHCPv6. However, there are sometimes
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- scenario where the reservation is more complex, e.g. uses other
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+ scenarios where the reservation is more complex, e.g. uses other
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options that mentioned above, uses part of specific options or perhaps
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even a combination of several options and fields to uniquely identify
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a client. Those scenarios are addressed by the Flexible Identifiers
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@@ -722,14 +722,16 @@ Administrator deleted a lease for a device identified by: duid of 1a:1b:1c:1d:1e
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<para>
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The library can be loaded in similar way as other hook libraries. It
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- takes one mandatory parameter identifier-expression:
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+ takes a mandatory parameter identifier-expression and optional
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+ boolean parameter replace-client-id:
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<screen>
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"Dhcp6": { <userinput>
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"hooks-libraries": [
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{
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"library": "/path/libdhcp_flex_id.so",
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"parameters": {
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- "identifier-expression": "<userinput>expression</userinput>"
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+ "identifier-expression": "<userinput>expression</userinput>",
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+ "replace-client-id": "<userinput>false</userinput>"
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}
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},
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...
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@@ -818,6 +820,56 @@ Administrator deleted a lease for a device identified by: duid of 1a:1b:1c:1d:1e
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]
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}
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</screen>
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+
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+ <para>
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+ When "replace-client-id" is set to false (which is the default setting),
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+ the flex-id hook library uses evaluated flexible identifier solely for
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+ identifying host reservations, i.e. searching for reservations within a
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+ database. This mode of operation has an implication that replacement of
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+ a client device may cause a conflict between an existing lease (allocated
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+ for old device) and the new lease being allocated for the new device. The
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+ conflict arises because the same flexible identifier is computed for the
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+ replaced device and the server will try to allocate the same lease. The
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+ mismatch between client identifiers sent by new device and old device causes
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+ the server to refuse this new allocation until the old lease expires.
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+ </para>
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+
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+ <para>
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+ The flex-id library offers a way to overcome the problem with lease conflicts
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+ by dynamically replacing client identifier (or DUID in DHCPv6 case) with a
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+ value derived from flexible identifier. The server processes the client's
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+ query as if flexible identifier was sent in the client identifier (or DUID)
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+ option. This guarantees that returning client (for which the same flexible
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+ identifier is evaluated) will be assigned the same lease desplite the client
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+ identifier and/or MAC address change.
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+ </para>
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+
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+ <para>
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+ The following is a stub configuration that enables this behavior:
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+<screen>
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+"Dhcp4": { <userinput>
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+ "hooks-libraries": [
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+ {
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+ "library": "/path/libdhcp_flex_id.so",
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+ "parameters": {
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+ "identifier-expression": "<userinput>expression</userinput>",
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+ "replace-client-id": "<userinput>true</userinput>"
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+ }
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+ },
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+ ...
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+ ] </userinput>
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+}
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+</screen>
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+ </para>
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+
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+ <para>
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+ Note that for this mechanism to take effect, the DHCPv4 server must be configured
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+ to respect the client identifier option value during lease allocation, i.e.
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+ "dhcp4-match-client-id" must be set to true. See
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+ <xref linkend="dhcp4-match-client-id"/> for details. No additional settings
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+ are required for DHCPv6.
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+ </para>
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+
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</section>
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<section id="host-cmds">
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Marcin Siodelski