Reliably detect all DHCP servers on a network?

deHakkelaar · September 4, 2020, 12:35pm

Yeah I meant instead of.
About DHCPINFORM:

DHCPInform Message

DHCPInform is a new DHCP message type, defined in RFC 2131. DHCPInform is used by DHCP clients to obtain DHCP options.

And I notice just now its doing broadcast and not unicast so no IP arguments:

github.com/pi-hole/FTL

src/dhcp-discover.c

1ff8bdda5


      
          	char sname [MAX_DHCP_SNAME_LENGTH];    // name of DHCP server
          	char file [MAX_DHCP_FILE_LENGTH];      // boot file name (used for diskless booting?)
          	char options[MAX_DHCP_OPTIONS_LENGTH];  // options
          } dhcp_packet;
          
          unsigned int packet_xid;
          
          #define BOOTREQUEST     1
          #define BOOTREPLY       2
          
          // sends a DHCPDISCOVER broadcast message in an attempt to find DHCP servers
          static bool send_dhcp_discover(int sock, const char *iface)
          {
          	dhcp_packet discover_packet;
          	struct sockaddr_in sockaddr_broadcast;
          
          	// clear the packet data structure
          	memset(&discover_packet, 0, sizeof(discover_packet));
          
          	// boot request flag (backward compatible with BOOTP servers)
          	discover_packet.op = BOOTREQUEST;

EDIT:

Your right:

DHCPINFORM - Client to server, asking only for local configuration
parameters; client already has externally configured
network address.

https://www.ietf.org/rfc/rfc2131.txt