[dhcp4relay] Security and reliability fixes

.gitignore

@@ -8,3 +8,9 @@ dhcp4relay/debian/*
 !dhcp4relay/debian/compat
 !dhcp4relay/debian/control
 !dhcp4relay/debian/rules
+
+# Build artifacts
+*.buildinfo
+*.changes
+dhcp4relay/PcapPlusPlus-*/
+dhcp4relay/pcappp.stamp

dhcp4relay/src/dhcp4_sender.cpp

@@ -20,15 +20,20 @@
  *
  * @return boolean   True if packet successfully sent
  */
+void bootp_pad(uint8_t *out, const uint8_t *buffer, uint32_t *len, bool pad) {
+   if (pad && *len < BOOTP_MIN_LEN) {
+      memcpy(out, buffer, *len);
+      *len = BOOTP_MIN_LEN;
+   }
+}
+
 #ifndef UNIT_TEST
 bool send_udp(int sock, uint8_t *buffer, struct sockaddr_in target, uint32_t len, in_addr src_ip, bool use_src_ip, bool pad) {
-   /* Pad additional bytes if length is lesser than 300
-    * to make DHCP packet length to minimum of 300 bytes */
-   if (pad && len < BOOTP_MIN_LEN) {
-      auto pad_len = BOOTP_MIN_LEN - len;
-      memset(buffer+len, 0, pad_len);
-      len = BOOTP_MIN_LEN;
-   }
+   uint8_t padded[BOOTP_MIN_LEN] = {};
+   uint32_t orig_len = len;
+   bootp_pad(padded, buffer, &len, pad);
+   if (len != orig_len)
+      buffer = padded;
 
    if (use_src_ip && src_ip.s_addr != 0) {
         // Enable IP_PKTINFO on the socket

dhcp4relay/src/dhcp4_sender.h

@@ -6,14 +6,33 @@
 #include <string>
 
 #define BOOTP_MIN_LEN 300
+
+/**
+ * @brief Apply BOOTP minimum-length padding.
+ *
+ * Copies *buffer into out and updates *len to BOOTP_MIN_LEN when pad is true
+ * and *len < BOOTP_MIN_LEN.  out must point to at least BOOTP_MIN_LEN
+ * zero-initialised bytes; the caller is responsible for switching to out as
+ * the send buffer when the returned length differs from the input length.
+ *
+ * Compiled in both production and UNIT_TEST builds so tests can exercise the
+ * padding logic directly without linking the real send_udp().
+ *
+ * @param out     destination buffer (>= BOOTP_MIN_LEN bytes, zero-filled)
+ * @param buffer  source packet bytes
+ * @param len     packet length; updated to BOOTP_MIN_LEN if padding applied
+ * @param pad     enable padding
+ */
+void bootp_pad(uint8_t *out, const uint8_t *buffer, uint32_t *len, bool pad);
+
 /**
  * @code                            bool send_udp(int sock, uint8_t *buffer, struct sockaddr_in target, uint32_t len, const char* src_ip, bool use_src_ip);
  *
  * @brief                           send udp packet and return true if successful
  *
  * @param *buffer                   message buffer
  * @param sockaddr_in  target       target socket
- * @param len                         length of message
+ * @param len                       length of message
  * @param src_ip                    source IP address as string (optional)
  * @param use_src_ip                if true, use src_ip as source address
  * @param pad                       if true, do padding