5.3 Replies and leases

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5.3.1 Which replies are looked at #

A received message is ignored unless all of these hold:

it is a BOOTREPLY;
its transaction id is the client's current one;
its chaddr is the interface's MAC;
it carries a message type (option 53) the client knows.

5.3.2 The server lock #

Once a server has been chosen — the server of the lease held, or else the server identifier of the offer being requested — an ACK or NAK is accepted only if its option 54 names that server. A stranger who guesses the transaction id cannot take a lease away or substitute one.

In Rebooting, with no lease and no offer, no server has been chosen, so an ACK or NAK from any server is accepted.

5.3.3 What each state accepts #

State	Message	Result
Selecting	OFFER with a non-zero `yiaddr` and a server identifier	taken: Requesting. The first acceptable OFFER wins; there is no comparison between offers.
Selecting	OFFER without either	ignored
Requesting, Rebooting, Renewing, Rebinding	ACK that reads as a lease (below)	Bound, with the lease
Requesting, Rebooting, Renewing, Rebinding	ACK that does not read as a lease	ignored; retransmission continues
Requesting, Rebooting	NAK	Selecting; the previous address and the offer are forgotten
Renewing, Rebinding	NAK	the lease is lost (§5.4); Selecting
any other combination		ignored

An ACK in Renewing or Rebinding that grants a different address replaces the lease. The next reconcile removes the old address and adds the new.

5.3.4 Reading an ACK #

An ACK is a lease only when it has:

a non-zero yiaddr;
a server identifier (option 54);
a lease time (option 51) of exactly 4 bytes, and at least 4 seconds.

A lease time under 4 s is refused: it cannot hold T1 < T2 < lease, and a server asking for it wants the client to spin.

5.3.4.1 The prefix #

From the subnet mask (option 1) when it is a non-zero run of contiguous ones. Otherwise, absent or malformed, from the address's class: /8 for a first octet up to 127, /16 up to 191, /24 above.

5.3.4.2 Routes #

Classless static routes (option 121, RFC 3442) are read as a sequence of prefix length, the significant destination octets, and a 4-byte gateway. In source 23af1f6, a prefix length above 32, or an entry running past the end of the option, makes the whole ACK fail to read as a lease: it is ignored like any other unusable ACK.

The proposed source correction instead discards the entire malformed optional route list, including any valid entries before the malformed one, and keeps an otherwise-valid lease. The existing router-option (3) fallback then applies; without that option, the lease has no routes or gateway. This recovery does not relax whole-message or option framing, the mandatory lease requirements above, or the client and server-identity checks. It describes the proposed source policy, not a released package or installed image, and does not assert an RFC requirement to recover malformed route lists.

When the decoded classless route list is nonempty, the router option (3) is ignored. The lease's gateway is the first router of option 3, or, when classless routes are present, the gateway of the classless route with prefix 0.

5.3.4.3 T1 and T2 #

T1 is option 58 when it is above 0 and below the lease time minus 2. Otherwise it is half the lease time.
T2 is option 59 when it is above T1 and below the lease time. Otherwise it is seven-eighths of the lease time, rounded down, raised to at least T1 + 1 and capped at the lease time minus 1.

So T1 < T2 < the lease time always holds, whatever the server sent.

5.3.4.4 Other options kept #

Option	Kept as
6	DNS servers, every 4-byte entry
15	the domain name: the bytes up to the first NUL; dropped if not UTF-8
119	the domain search list, RFC 1035 names with compression pointers followed, at most 16 jumps per name
12	the hostname offered: the bytes up to the first NUL; dropped if not UTF-8
26	the MTU, when it is at least 68
28	the broadcast address
42	NTP servers, every 4-byte entry

Where an option appears twice, its first occurrence is used. Nothing else in the ACK is kept.

How each of these is used — and whether, by the profile — is in §4.2 (addresses, routes, MTU), §8.3 (DNS and NTP) and §8.4 (the hostname).

5.3.5 The record of a lease #

A lease is logged as interface <name>: lease <address>/<prefix> from <server> for <seconds>s. Binding clears the interface's link-local address and its warning, and marks the interface for re-judgement, because the network might now be identifiable (§7.1).

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