feat: enhance network monitoring for UDP packets

[rohan-stepsecurity]

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[step-security-bot]

Please find StepSecurity AI-CodeWise code comments below.

Code Comments

firewall.go

• [High]Handle errors immediately after the Append call to avoid potential nil dereferences or logic errors
  The code appends an iptables rule and then immediately checks for an error. This is good practice as per the Go error handling paradigm described in the official Go blog (https://blog.golang.org/error-handling-and-go). It prevents proceeding with faulty state and improves program stability. No change needed since error is handled immediately; confirm consistent error handling throughout the code.
• [High]Avoid hardcoding nflog group IDs directly in iptables rules; use a constant with a descriptive name
  Hardcoded magic numbers reduce code readability and maintainability. This is a recognized best practice from Clean Code by Robert C. Martin. Using constants improves clarity and eases future changes. Define a constant such as const nflogGroupUDP = "100" at the top of the file and replace "100" with nflogGroupUDP in the Append call.
• [Medium]Validate arguments passed to ipt.Append to prevent command injection or malformed rules
  According to OWASP Secure Coding Practices, input validation is critical before using values in shell or system commands to avoid injection vulnerabilities. Ensure network interface names, protocol strings, and chain names are sanitized. Add validation functions for netInterface, protocol, chain, and direction parameters to ensure they conform to expected values before invoking ipt.Append.
• [Medium]Include comments explaining the purpose of the new UDP NFLOG rule for maintainability
  The Linux iptables NFLOG target is specialized; clear comments help future maintainers understand why this rule is added. This is supported by the standards in the Google engineering documentation on code commenting best practices. Expand the comment above the ipt.Append call to describe why UDP packets are specifically logged using NFLOG and the choice of group ID.
• [Low]Consider combining multiple Append calls into one atomic transaction if supported to reduce partial rule application
  Applying multiple iptables rules atomically can prevent inconsistent firewall states during processing, following recommendations in the iptables man pages for atomic batch operations. If the ipt package supports batch operations or transactions, wrap the Append calls of NFLOG and blocking in a single transaction or batch; otherwise, document the limitation.
• [Low]Check for existing similar rules before adding new ones to avoid duplicates
  Idempotency in firewall rules prevents rule table bloat and potential conflicts, as recommended in the Linux iptables guide and best practices for network security management. Implement a check using ipt.Exists or equivalent before calling Append for the UDP NFLOG rule to ensure no duplicate entries.

netmon.go

• [High]Explicit error handling after type assertion to avoid runtime panics
  The code performs type assertions (e.g., tcpLayer.(*layers.TCP)) without checking for errors. If the type assertion fails, it will assign nil to the variable, possibly leading to runtime nil pointer dereferences. This violates Go best practices as documented in the official Go blog: https://blog.golang.org/errors-are-values. Check the result of the type assertion using the two-value assignment form and handle the failure case gracefully before proceeding to use the asserted type.

Example fix:

if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
  tcp, ok := tcpLayer.(*layers.TCP)
  if !ok {
    // handle error or skip this packet
    return
  }
  port = tcp.DstPort.String()
  isSYN = tcp.SYN
} else if udpLayer := packet.Layer(layers.LayerTypeUDP); udpLayer != nil {
  udp, ok := udpLayer.(*layers.UDP)
  if !ok {
    // handle error or skip this packet
    return
  }
  port = udp.DstPort.String()
  isUDP = true
}

• [High]Avoid redundant boolean flags and improve code clarity
  The code uses multiple boolean flags (isSYN and isUDP) in conditional logic, which can lead to confusion and errors over time. Instead, prefer an enum or distinct states that better represent the protocol and TCP flags. This aligns with clean code principles from Robert C. Martin's "Clean Code" and Go idiomatic patterns. Define an enum or string type to represent the protocol and TCP flag states, then streamline the conditionals accordingly.

Example fix:

type ProtocolType int

const (
  ProtocolUnknown ProtocolType = iota
  ProtocolTCP
  ProtocolUDP
)

// Inside the function:
protocol := ProtocolUnknown
isSYN := false

if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
  tcp, ok := tcpLayer.(*layers.TCP)
  if !ok { return }
  port = tcp.DstPort.String()
  protocol = ProtocolTCP
  isSYN = tcp.SYN
} else if udpLayer := packet.Layer(layers.LayerTypeUDP); udpLayer != nil {
  udp, ok := udpLayer.(*layers.UDP)
  if !ok { return }
  port = udp.DstPort.String()
  protocol = ProtocolUDP
}

// Later use:
if protocol == ProtocolTCP && isSYN || protocol == ProtocolUDP {
  // ...
}

This approach improves readability and reduces the chance of inconsistent flag handling.

• [Medium]Avoid silently ignoring unknown or malformed packets
  Currently, packets that lack TCP or UDP layer are not handled explicitly, potentially leading to missing events or incomplete monitoring. It is a good security practice to log or handle unexpected protocol cases to aid in troubleshooting and monitoring as advised by OWASP Secure Coding Practices. Add handling or logging for packets with neither TCP nor UDP layers. Even a debug log would help identify unexpected packets.

Example fix:

if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
  // ...
} else if udpLayer := packet.Layer(layers.LayerTypeUDP); udpLayer != nil {
  // ...
} else {
  // log unknown packet type or increment counter
  log.Printf("Unknown packet protocol: %v", packet)
  return
}

• [Medium]Avoid redundant setting of boolean flag 'isUDP' when only used once in conditional
  The boolean flag 'isUDP' is set but only used once in the condition 'if isSYN || isUDP'. This flag could be replaced by directly checking the protocol presence to simplify the code, reducing state complexity as per Go idioms. Replace 'isUDP' flag with direct check on the presence of the UDP Layer or protocol type variable as proposed above, removing the 'isUDP' variable altogether.

Example:

protocol := ProtocolUnknown
// set protocol accordingly

if (protocol == ProtocolTCP && isSYN) || protocol == ProtocolUDP {
  // ...
}

• [Low]Use consistent naming conventions and comments
  While the code has some comments, some variable names like 'port', which is stringified DstPort, could be clearer (e.g., 'dstPort'). Also, comments should clarify intent rather than restate obvious code as recommended by Google's Go Style Guide. Rename 'port' to 'dstPort' for clarity. Improve comments to explain why certain decisions are made rather than what is done.

Example:

// Extract destination port from TCP or UDP layer
var dstPort string

if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
  tcp, ok := tcpLayer.(*layers.TCP)
  if !ok { return }
  dstPort = tcp.DstPort.String()
  protocol = ProtocolTCP
  isSYN = tcp.SYN
} else if udpLayer := packet.Layer(layers.LayerTypeUDP); udpLayer != nil {
  udp, ok := udpLayer.(*layers.UDP)
  if !ok { return }
  dstPort = udp.DstPort.String()
  protocol = ProtocolUDP
}

• [Low]Avoid magic string literals for statuses, use constants
  The code compares 'netMonitor.Status' to literal string "Dropped". Using constants avoids typos and makes maintenance easier, as detailed in Go best practices. Define constants for status strings.

Example:

const (
  StatusDropped = "Dropped"
  // other statuses...
)

// usage:
if netMonitor.Status == StatusDropped {
  // ...
}

procmon_linux.go

• [High]Check and handle errors returned by flags.Parse and rule.Build functions
  The code calls flags.Parse and rule.Build but ignores any returned errors. This can lead to unpredictable behavior or runtime panics if these functions fail. According to Go best practices and the official Go error handling guidelines (https://go.dev/doc/effective_go#error_handling), all errors must be checked and handled appropriately. Modify the calls to flags.Parse and rule.Build to capture and handle errors. For example:

r, err = flags.Parse(fmt.Sprintf("-a exit,always -S sendto -S sendmsg -k %s", netMonitorTag))
if err != nil {
errc <- errors.Wrap(err, "failed to parse audit rule for sendto and sendmsg")
return
}

actualBytes, err = rule.Build(r)
if err != nil {
errc <- errors.Wrap(err, "failed to build audit rule for sendto and sendmsg")
return
}

• [High]Propagate errors properly and stop further execution when critical errors occur
  The function writes logs about failure but continues execution even after a failed call to client.AddRule. This could cause invalid state or unexpected behavior. Per effective error handling principles (https://blog.golang.org/error-handling-and-go), the function should stop further execution or return after a critical error occurrence. After sending the wrapped error to errc channel, return from the function to prevent further processing:

if err = client.AddRule(actualBytes); err != nil {
WriteLog(fmt.Sprintf("failed to add audit rule for sendto %v", err))
errc <- errors.Wrap(err, "failed to add audit rule for syscall sendto")
return
}

• [Medium]Improve log clarity by specifying exact syscalls monitored
  Logging the message 'Net monitor added' is vague, leading to ambiguity about which syscalls are being monitored. Clear and specific log messages improve maintainability and debugging ease (https://12factor.net/logs). Change the log message to 'Net monitor added for TCP (connect)' before adding the TCP connect syscall, and log 'Net monitor added for UDP (sendto & sendmsg)' after adding the UDP syscalls as already done in the patch.
• [Medium]Use consistent variable declaration and initialization patterns for clarity
  The patch mixes shadowing and reassigning existing variables (r, actualBytes, err) without clear intent, making it harder to follow the variable lifecycle (https://golang.org/ref/spec#Short_variable_declarations). Use explicit assignment instead of short variable declarations when variables are already declared in the outer scope:

var err error
r, err = flags.Parse(...)

actualBytes, err = rule.Build(...)

This avoids variable shadowing and clarifies error handling.

• [Low]Avoid redundant error messages with duplicate information
  Both WriteLog and errc sending wrap relate to the same error. This results in redundant logs. According to logging best practices (https://www.loggly.com/ultimate-guide/docker-logging-best-practices/), it's better to unify error reporting to reduce noise. Consider removing or consolidating the WriteLog on error to a single reporting channel or standardize error propagation so errors are only logged once in the consuming component.
• [Low]Add comments explaining the rationale behind adding multiple syscalls in the net monitor
  The code adds multiple syscalls for auditing but does not explain why these particular syscalls (sendto, sendmsg) are grouped for UDP monitoring. Clear comments improve code understandability and maintenance (https://google.github.io/styleguide/go/commentary.html). Add a comment like:

// Add audit rule for UDP-related syscalls: sendto and sendmsg to monitor outgoing UDP traffic.

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