dhcpd.leases(5) dhcpd.leases(5) NNAAMMEE dhcpd.leases - DHCP client lease database DDEESSCCRRIIPPTTIIOONN The Internet Software Consortium DHCP Server keeps a per- sistent database of leases that it has assigned. This database is a free-form ASCII file containing a series of lease declarations. Every time a lease is acquired, renewed or released, its new value is recorded at the end of the lease file. So if more than one declaration appears for a given lease, the last one in the file is the current one. When dhcpd is first installed, there is no lease database. However, dhcpd requires that a lease database be present before it will start. To make the initial lease database, just create an empty file called /var/db/dhcpd.leases. In order to prevent the lease database from growing with- out bound, the file is rewritten from time to time. First, a temporary lease database is created and all known leases are dumped to it. Then, the old lease database is renamed /var/db/dhcpd.leases~. Finally, the newly writ- ten lease database is moved into place. There is a window of vulnerability where if the dhcpd pro- cess is killed or the system crashes after the old lease database has been renamed but before the new one has been moved into place, there will be no /var/db/dhcpd.leases. In this case, dhcpd will refuse to start, and will require manual intervention. DDOO NNOOTT simply create a new lease file when this happens - if you do, you will lose all your old bindings, and chaos will ensue. Instead, rename /var/db/dhcpd.leases~ to /var/db/dhcpd.leases, restoring the old, valid lease file, and then start dhcpd. This guarantees that a valid lease file will be restored. FFOORRMMAATT Lease descriptions are stored in a format that is parsed by the same recursive descent parser used to read the ddhhccppdd..ccoonnff((55)) and ddhhcclliieenntt..ccoonnff((55)) files. Currently, the only declaration that is used in the dhcpd.leases file is the lleeaassee declaration. lleeaassee _i_p_-_a_d_d_r_e_s_s {{ _s_t_a_t_e_m_e_n_t_s_._._. }} Each lease declaration include the single IP address that has been leased to the client. The statements within the braces define the duration of the lease and to whom it is assigned. The start and end time of a lease are recorded using the ``starts'' and ``ends'' statements: 1 dhcpd.leases(5) dhcpd.leases(5) ssttaarrttss _d_a_t_e;; eennddss _d_a_t_e;; Dates are specified as follows: _w_e_e_k_d_a_y _y_e_a_r//_m_o_n_t_h//_d_a_y _h_o_u_r::_m_i_n_u_t_e::_s_e_c_o_n_d The weekday is present to make it easy for a human to tell when a lease expires - it's specified as a number from zero to six, with zero being Sunday. The day of week is ignored on input. The year is specified with the century, so it should generally be four digits except for really long leases. The month is specified as a number starting with 1 for January. The day of the month is likewise specified starting with 1. The hour is a number between 0 and 23, the minute a number between 0 and 59, and the sec- ond also a number between 0 and 59. Lease times are specified in Greenwich Mean Time (GMT), not in the local time zone. Since Greenwich is actually on Daylight Savings Time part of the year, there is proba- bly nowhere in the world where the times recorded on a lease are always the same as wall clock times. On a unix machine, one can often figure out the current time in GMT by typing ddaattee --uu. The MAC address of the network interface that was used to acquire the lease is recorded with the hhaarrddwwaarree statement: hhaarrddwwaarree _h_a_r_d_w_a_r_e_-_t_y_p_e _m_a_c_-_a_d_d_r_e_s_s;; The MAC address is specified as a series of hexadecimal octets, seperated by colons. If the client used a client identifier to acquire its address, the client identifier is recorded using the uuiidd statement: uuiidd _c_l_i_e_n_t_-_i_d_e_n_t_i_f_i_e_r;; The client identifier is recorded as a series of hexadeci- mal octets, regardless of whether the client specifies an ASCII string or uses the newer hardware type/MAC address format. If the client sends a hostname using the _C_l_i_e_n_t _H_o_s_t_n_a_m_e option, as specified in some versions of the DHCP-DNS Interaction draft, that hostname is recorded using the cclliieenntt--hhoossttnnaammee statement. cclliieenntt--hhoossttnnaammee ""_h_o_s_t_n_a_m_e"";; If the client sends its hostname using the _H_o_s_t_n_a_m_e option, as Windows 95 does, it is recorded using the 2 dhcpd.leases(5) dhcpd.leases(5) hhoossttnnaammee statement. hhoossttnnaammee ""_h_o_s_t_n_a_m_e"";; The DHCP server may determine that a lease has been mis- used in some way, either because a client that has been assigned a lease NAKs it, or because the server's own attempt to see if an address is in use prior to reusing it reveals that the address is in fact already in use. In that case, the aabbaannddoonneedd statement will be used to indi- cate that the lease should not be reassigned. aabbaannddoonneedd;; Abandoned leases are reclaimed automatically. When a client asks for a new address, and the server finds that there are no new addresses, it checks to see if there are any abandoned leases, and allocates the least recently abandoned lease. The standard mechanisms for checking for lease address conflicts are still followed, so if the abandoned lease's IP address is still in use, it will be reabandoned. If a client rreeqquueessttss an abandoned address, the server assumes that the reason the address was abandoned was that the lease file was corrupted, and that the client is the machine that responded when the lease was probed, causing it to be abandoned. In that case, the address is immedi- ately assigned to the client. FFIILLEESS //vvaarr//ddbb//ddhhccppdd..lleeaasseess SSEEEE AALLSSOO dhcpd(8), dhcp-options(5), dhcpd.conf(5), RFC2132, RFC2131. AAUUTTHHOORR ddhhccppdd((88)) was written by Ted Lemon under a contract with Vixie Labs. Funding for this project was provided by the Internet Software Corporation. Informa- tion about the Internet Software Consortium can be found at hhttttpp::////wwwwww..iisscc..oorrgg//iisscc.. 3