Record and summarize network-related DHCP options

ethernet/dhcpv4/dhcp.go

@@ -43,6 +43,7 @@ func HandleDHCPv4Packet(packet gopacket.Packet) error {
 
 	// Check for Hostname DHCP option (12)
 	checkForHostname(dhcppacket)
+	checkForNetworkInfos(dhcppacket)
 
 	return nil
 }
@@ -50,6 +51,8 @@ func HandleDHCPv4Packet(packet gopacket.Packet) error {
 // Print summary after all packets are processed
 func PrintDHCPv4Summary() {
 	headline := color.New(color.FgRed, color.Bold)
+	headline.Println("DHCP Network Overview")
+	printNetworkSummary()
 	headline.Println("DHCP Requests")
 	printRequestSummary()
 	headline.Println("DHCP Responses/Offers")

ethernet/dhcpv4/network.go

@@ -0,0 +1,153 @@
+package dhcpv4
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"github.com/fatih/color"
+	"github.com/google/gopacket/layers"
+	"log"
+	"net"
+)
+
+var (
+	networkSetup = make(map[layers.DHCPOpt][]byte)
+	watchedOpts = []layers.DHCPOpt{
+		layers.DHCPOptSubnetMask, // Option 1
+		layers.DHCPOptRouter, // Option 3
+		layers.DHCPOptDNS, // Option 6
+		layers.DHCPOptBroadcastAddr, // Option 28
+		layers.DHCPOptNTPServers, // Option 42
+		layers.DHCPOptLeaseTime, // Option 51
+		layers.DHCPOptT1, // Option 58
+	}
+)
+
+// Looks for information specifying the setup of the network. This includes
+//  - Option  1: Subnet Mask
+//  - Option  3: Router address
+//  - Option  6: Domain Name Server address
+//  - Option 28: Broadcast address
+//  - Option 42: NTP Server address
+//  - Option 51: IP Address Lease time
+//  - Option 58: IP Renewal time
+func checkForNetworkInfos(dhcppacket layers.DHCPv4) {
+	// Check if it is a DHCP request
+	if dhcppacket.Operation == layers.DHCPOpRequest {
+		// We can ignore requests, they won't help us here
+		return
+	}
+
+	// Search for different options (1, 3, 6, 28, 42, 51, 58) in DHCP Packet Options
+	for _, o := range dhcppacket.Options {
+		if isRelevantOption(o) {
+			// Found DHCP option to be watched, let's watch it
+			saveOption(o)
+		}
+	}
+
+}
+
+// Checks if the given DHCPOption is part of the watchlist
+func isRelevantOption(opt layers.DHCPOption) bool {
+	// Iterate over all DHCP options in our watchlist
+	for _, o := range watchedOpts {
+		if o == opt.Type {
+			// Found.
+			return true
+		}
+	}
+
+	// This option is not on our watchlist.
+	return false
+}
+
+// Saves the given option in the networkSetup map, and informs the user if the value changes
+func saveOption(opt layers.DHCPOption) {
+	// check if we already stored this value
+	if networkSetup[opt.Type] != nil {
+		// We already stored a value, let's check if it's the same as the new one
+		if !bytes.Equal(networkSetup[opt.Type], opt.Data) {
+			// Already stored a value and it's different from our new value - inform user and overwrite value later
+			log.Printf("Received different values for DHCP Option %s (ID %d). (Old: %s, New. %s)", opt.Type.String(), opt.Type, networkSetup[opt.Type], opt.Data)
+		} else {
+			// Exactly this value was already stored, no need to overwrite it
+			return
+		}
+	}
+
+	networkSetup[opt.Type] = opt.Data
+}
+
+// Formats the given byte array as string representing the IP address, or returns an error (as string)
+func formatIP(rawIP []byte) string {
+	// Check if we even have an IP
+	if rawIP == nil {
+		// We don't have an IP, construct an error message (as string)
+		error := color.New(color.FgRed)
+		return error.Sprint("(not found)")
+	}
+
+	// Return formatted IP
+	return net.IP(rawIP).String()
+}
+
+func formatDate(rawDate []byte) string {
+	// Check if we even have a date
+	if rawDate == nil {
+		// We don't have a date, construct an error message (as string)
+		error := color.New(color.FgRed)
+		return error.Sprint("(not found)")
+	}
+
+	// Actually format date
+	intDate := binary.LittleEndian.Uint32(rawDate)
+	seconds := intDate % 60
+	minutes := intDate / 60 % 60
+	hours   := intDate / 60 / 60 % 60
+	formattedDate := ""
+
+	// Check which words we need to pick
+	// ... regarding hours
+	if hours > 0 {
+		formattedDate = fmt.Sprintf("%d hours", hours)
+	}
+
+	// ... regarding minutes
+	if minutes > 0 {
+		// check if we got a previous string we need to take care of
+		if len(formattedDate) > 0 {
+			// yes, append our information to existing string
+			formattedDate = fmt.Sprintf("%s, %d minutes", formattedDate, minutes)
+		} else {
+			// no, use our string
+			formattedDate = fmt.Sprintf("%d minutes", minutes)
+		}
+	}
+
+	// ... regarding seconds
+	if seconds > 0 {
+		// check if we got a previous string we need to take care of
+		if len(formattedDate) > 0 {
+			// yes, append our information to existing string
+			formattedDate = fmt.Sprintf("%s, %d seconds", formattedDate, seconds)
+		} else {
+			// no, use our string
+			formattedDate = fmt.Sprintf("%d seconds", seconds)
+		}
+	}
+
+	return formattedDate
+}
+
+// Prints the summary of relevant DHCP options
+func printNetworkSummary() {
+	fmt.Printf("Subnet Mask: %s\n", formatIP(networkSetup[layers.DHCPOptSubnetMask]))
+	fmt.Printf("Broadcast: %s\n", formatIP(networkSetup[layers.DHCPOptBroadcastAddr]))
+	fmt.Printf("Router: %s\n", formatIP(networkSetup[layers.DHCPOptRouter]))
+	fmt.Printf("DNS Server: %s\n", formatIP(networkSetup[layers.DHCPOptDNS]))
+	fmt.Printf("NTP Server: %s\n", formatIP(networkSetup[layers.DHCPOptNTPServers]))
+	fmt.Printf("Lease Time: %s\n", formatDate(networkSetup[layers.DHCPOptLeaseTime]))
+	fmt.Printf("Renewal Time: %s\n", formatDate(networkSetup[layers.DHCPOptT1]))
+}
+