The location of this file defaults to /etc/dhcp_probe.cf, but may be overridden by a command-line option to dhcp_probe(8).
The file consists of a series of statements, one per line. Each statement begins with a keyword followed by one or more arguments (depending on the keyword); keywords and arguments are separated by spaces or tabs. Statements may be specified in any order.
Some keywords take an ethernet-address argument. Ethernet address values must be written in a form that ether_aton(3) recognizes; e.g. 1:2:3:4:5:6 or 00:A5:b2:0:BB:c.
Some keywords take an ip-address as a value. IP address values must be written in a form that inet_aton(3) recognizes; e.g. 192.168.1.2.
Blank lines are ignored. Lines for which the first non-blank character is a ’#’ are treated as comments. Trailing comments on statements are not supported.
Because all presently-defined keywords are optional, the file may be empty, however, it must exist.
The keywords are as follows:
The chaddr statement is optional, and is used to specify the value of the chaddr field in the request packets sent by the program. This value is also used to compute the DHCP Client Identifier option in some of the request packets sent by the program (by prepending x’01’).
Specify:
- chaddr
- ethernet-address
If not specified, this value defaults to the Ethernet address corresponding to the interface you specified on the commandline.
You might want to use the chaddr statement if the interface is also a DHCP client, so that sending requests with the interface’s own chaddr/DHCP Client Identifier will not interfere with that functionality.
If you specify a value, be sure to specify a unicast Ethernet address that does not belong to any valid client on your network.
Correctly-functioning BootP and DHCP servers that respond will send any responses to the chaddr address, or in some cases, to the Ethernet broadcast address. Therefore, if you specify a value here (and it differs from your interface’s Ethernet address), the program will have to place the interface into promiscuous mode to be sure it hears unicast responses.
Note that the chaddr value does not affect the Ethernet source address of the Ethernet frames sent by the program.
If you specify this value, you may also wish to specify the same value in the ether_src statement. See the description of that statement for further discussion.
The ether_src statement is optional, and is used to specify the value of the ether_src field in the Ethernet frames sent by the program.
Specify:
- ether_src
- ethernet-address
If not specified, this value defaults to the Ethernet address corresponding to the interface you specified on the commandline.
If you specify a value, be sure to specify a unicast Ethernet address that does not belong to any valid client on your network.
Note that this value does not affect the chaddr field or the DHCP Client Identifier option field in request packets sent by the program.
If you specified a chaddr value, you may also wish to specify an equal ether_src value. While not strictly necessary, doing so will cause any Layer 2 switches on the network to learn that this hardware address is on your leg of the network, so they will not need to flood response packets directed to that hardware address, but instead can direct the response packets only to your leg of the network.
Additionally, specifying the same ether_src value could help you discover any buggy BootP or DHCP servers that mistakenly direct their responses to the sender’s ether_src (instead of to the sender’s bootp_chaddr).
The server_id statement is optional, and is used to specify the value of the DHCP Server Identifer option in some of the request packets sent by the program.
Specify:
- server_id
- ip-address
If not specified, this value defaults to 10.254.254.254.
The DHCP Server Identifer option appears in the packets the program sends when it mimics a DHCP client in the SELECTING state.
It’s best that the DHCP Server Identifier option the program uses not match the IP address of any valid DHCP server on your network, to avoid confusing them. Other than that, any IP address is a reasonable value; you may wish to specify one that could never be a valid address on your network.
The client_ip_address statement is optional, and is used to specify the IP address that the program should request, or claim to have a lease on.
Specify:
- client_ip_address
- ip-address
If not specified, this value defaults to 172.31.254.254.
When the program generates a DHCPREQUEST packet that mimics a DHCP client that is in the INIT-REBOOT or SELECTING state, the packet contains a Requested IP Address option containing this value. When the program generates a DHCPREQUEST packet that mimics a DHCP client that is in the REBINDING state, the packet contains a ciaddr field containing this value.
It’s best that the value the program uses not match the IP address of any valid DHCP client on your network, to avoid confusing valid DHCP servers.
It’s extremely useful if the value the program uses not be valid (topologically speaking) for the physical network on which the program sends the packets. Sending a topologically inappropriate value may stimulate some DHCP servers to respond with a DHCPNAK, which helps the program flush out DHCP servers.
The response_wait_time statement is optional, and is used to specify how long the program should wait for responses after sending a single request packet.
Specify
- response_wait_time
- num_milliseconds
If not specified, this value defaults to 5000 milliseconds (5 seconds).
The value is specified in milliseconds, and must fit into an ’int’ on your host. (Values larger than an ’int’ may be silently misinterpreted.) Typical values are on the order of a few thousand milliseconds; i.e. several seconds.
The cycle_time statement is optional, and is used to specify how long the program should sleep between each probe cycle.
Specify
- cycle_time
- num_seconds
If not specified, this value defaults to 300 seconds.
The value is specified in seconds, and must into into an ’unsigned int’ on your host. (Values larger than an ’unsigned int’ may be silently misinterpreted.) Typical valus range from several hundred to several thousand seconds (i.e. several minutes to several hours).
During each probe cycle, the program sends one of the request packet flavors, captures any responses that arrive during the response_wait_time, then repeats this for each of the other request packet flavors. After doing this for each flavor of request packet, the probe cycle is complete, and the program sleeps for the cycle_time.
The legal_server statement is optional, and is used to specify the IP address of a legal BootP or DHCP server on your network. The statement may be specified multiple times.
Specify
- legal_server
- ip-address
If not specified, the program assumes there are no legal BootP and DHCP servers on your network.
When the program receives a response packet, it compares the packet’s IP source address to all the addresses you have specified in legal_server statements. If the IP source address matches one of these values, the response is deemed to have come from a known IP server, and is ignored. If the IP source address does not match any of these values (or you do not specify any legal_server), then the program logs a message that reports the packet’s IP source address (and Ethernet source address). Additionally, if the program was started with the -o commandline option, the packet is also written to a packet capture file.
When relaying a response from a server to a client, some BootP Relay Agents may change the response’s IP source address, replacing the server’s IP address with that of the BootP Relay Agent. If BootP Relay Agents on your network do this, you will need to specify their IP addresses here instead.
The alert_program_name statement is optional, and is used to specify the name of an external program that should be run every time a response packet is received from an unexpected server.
Specify
- alert_program_name
- /absolute/path/name
If not specified, the program does not run an external program when an unexpected response packet is received.
Unexpected response packets are reported as a matter of course, and optionally written to a packet capture file. You may use an alert_program_name to provide additional handling of the event, for example, to alert an appropriate party via mail or paging. The alert_program_name you specify is called with four arguments: the name of the calling program (e.g. dhcp_probe), the name of the interface on which the unexpected response packet was received, the IP source address of the packet, and the Ethernet source address of the packet.
As the alert_program_name is called with the same privileges as dhcp_probe (i.e. root), you should exercise caution to ensure that the alert program is safe for a privileged user to execute.
# dhcp_probe.cf: config file for dhcp_probe # # General syntax: # Comment lines start with ’#’ (trailing comments not permitted). # Blank lines are OK. # Tokens within a line should be separated with spaces and/or tabs. # Entries in the file may be in any order. # Any ’ethernet-address’ must be written in a form that ether_aton(3) recognizes; e.g. # 1:2:3:4:5:6 00:A5:b2:0:BB:c # Any ’ip-address’ must be written in a form that inet_aton(3) recognizes; e.g. # 192.168.1.2 # # ---------------------------------------------------------------------------------- # # CLIENT HARDWARE ADDRESS # # By default, for the ’chaddr’ field in the BootP header, we use the Ethernet # address corresponding to the interface you specified. # We also use this value to compute the DHCP Client Identifier option (by prepending x’01’). # You may optionally override this value. # (Note that this does not override the Ethernet Src address in the Ethernet frame we send.) # # You might want to do this if our interface is also a DHCP client, so # sending requests with the interface’s own chaddr/DHCP Client Identifier would interfere with # that functionality. # # If you specify a value, be sure to specify an Ethernet address that does not belong to # any valid client on your network. Be sure to specify a unicast Ethernet address. # # Syntax: # chaddr enet-addr chaddr 0:0:0:1:2:3 # ---------------------------------------------------------------------------------- # # ETHERNET SOURCE ADDRESS # # By default, for the ’ether_shost’ field in the Ethernet header, we use the Ethernet # address corresponding to the interface you specified. # You may optionally override this value. # (Note that this does not override the ’chaddr’ in the BootP header, nor the DHCP Client Identifier.) # # If you are specify the ’chaddr’ statement, you might want to also do this, so you don’t miss buggy # DHCP servers that respond (incorrectly) to ether_src instead of to chaddr. # # If you specify a value, be sure to specify an Ethernet address that does not belong to # any valid client on your network. Be sure to specify a unicast Ethernet address. # # Syntax: # ether_src enet-addr ether_src 0:0:0:1:2:3 # ---------------------------------------------------------------------------------- # # DHCP SERVER IDENTIFIER # # When we generate a DHCPREQUEST packet corresponding to a client that is in the SELECTING # state, the options field must contain a ’DHCP Server Identifier’ option, indicating the # IP address of the DHCP server the client is selecting. It’s best that the value we use # not match the IP address of any valid DHCP server, to avoid confusing them. The program # provides a default value of 10.254.254.254, which you may override here. # # Syntax: # server_id ip-addr server_id 10.1.2.3 # ---------------------------------------------------------------------------------- # # CLIENT IP ADDRESS # # When we generate a DHCPREQUEST packet corresponding to a client that is in the INIT-REBOOT # or SELECTING state, the options field must containg a ’Requested IP Address’ option, indicating # the IP address the client is requesting. When we generate a DHCPREQUEST packet corresponding # to a client that is in the REBINDING state, the ’ciaddr’ field in the BootP header must contain # the IP address that the DHCP client presently has leased and wishes to renew. # # In all these cases, it’s best that the value we use not match the IP address of any valid DHCP client, # to avoid confusing the valid DHCP servers. # # Furthermore, it is extremely useful if the value we use *not* be valid (topologically speaking) for the # physical network on which we send the packets. Sending a topologically inappropriate value # may stimulate some DHCP servers to respond with a DHCPNAK, which helps us flush out DHCP servers. # (This will probably happen only in response to the packets we sending when pretending to be in REBINDING state.) # # The program provides a default value of 172.31.254.254, which you may override here. # # Syntax: # client_ip_address ip-addr # client_ip_address 172.31.254.254 # ---------------------------------------------------------------------------------- # # RESPONSE WAIT TIME # # After sending one packet, we wait for responses. The length of time we wait # is the ’response_wait_time’. The program provides a default value of 5000, which you # may override here. The value is measured in milliseconds, and must fit into # an ’int’ on your host. (Values larger than an ’int’ may be silently misinterpreted.) # Typical values are on the order of a few thousand milliseconds; i.e. several seconds. # # Syntax: # response_wait_time num_milliseconds # response_wait_time 5000 # ---------------------------------------------------------------------------------- # # CYCLE WAIT TIME # # For each flavor packet, we send the packet and listen for responses to that packet. # After doing this for all flavor packets, we go to sleep for the "cycle_time", # then repeat the process. The program provides a default value of 300, which you # may override here. The value is measured in seconds, and must fit into an # ’unsigned int’ on your host. (Values larger than an ’unsigned int’ may be silently # misinterpreted.) Typical valus range from several hundred to several thousand # seconds (i.e. several minutes to several hours). # # Syntax: # cycle_time num_seconds cycle_time 1200 # ---------------------------------------------------------------------------------- # # LEGAL SERVERS # # After sending one packet, we wait for responses. Responses from legal BootP or DHCP # servers are ignored; presumably you aren’t interesting in discovering them. # Specify a legal server’s IP address with the ’legal_server’ statement. # (The value you specify is compared to the IPsrc field in each response’s IP header.) # If you have multiple legal servers, specify each in a separate statement. # Alternatively, do not specify any legal_server statements at all, so *no* responses # are ignored. # # Syntax: # legal_server ip-addr legal_server 192.168.1.2 legal_server 192.168.3.4 # ---------------------------------------------------------------------------------- # # ALERT PROGRAM NAME # # In addition to logging a response received from an unexpected server, we will # call a user-specified ’alert program’ if one is specified here. Optionally # specify the absolute pathname of a program we should execute for each unexpected response. # The program will be called with the name of the calling program, the name of the interface # on which the response was received, the IP source of the response, and the ether_src of the # response. # # Syntax: # alert_program_name /absolute/path/name alert_program_name /usr/local/etc/pageme