diff --git a/doc/sphinx/man/perfdhcp.8.rst b/doc/sphinx/man/perfdhcp.8.rst
index 222486d3fc..e3867bac3e 100644
--- a/doc/sphinx/man/perfdhcp.8.rst
+++ b/doc/sphinx/man/perfdhcp.8.rst
@@ -225,6 +225,10 @@ Options
    integer up to 65535. A value of 0 (the default) allows ``perfdhcp``
    to choose its own port.
 
+   Note that ``perfdhcp -4`` behaves as a relay agent, and the server may send
+   replies to the BOOTPS port (67) irrespective of what port ``perfdhcp`` is
+   listening on.
+
 ``-M mac-list-file``
    Specifies a text file containing a list of MAC addresses, one per line. If
    provided, a MAC address is chosen randomly from this list for
diff --git a/src/bin/perfdhcp/perf_socket.cc b/src/bin/perfdhcp/perf_socket.cc
index aa67796a1d..4cf1a9388f 100644
--- a/src/bin/perfdhcp/perf_socket.cc
+++ b/src/bin/perfdhcp/perf_socket.cc
@@ -13,6 +13,7 @@
 
 #include <dhcp/iface_mgr.h>
 #include <asiolink/io_address.h>
+#include <iostream>
 
 using namespace isc::dhcp;
 using namespace isc::asiolink;
@@ -53,8 +54,16 @@ PerfSocket::openSocket(CommandOptions& options) const {
               port = DHCP6_SERVER_PORT;
             }
         } else if (options.getIpVersion() == 4) {
-            port = 67; /// @todo: find out why port 68 is wrong here.
+            // perfdhcp sets giaddr to the bound socket address, so kea always
+            // responds to the server port. perfdhcp doesn't currently have a
+            // client behavior for DHCPv4; it either sets giaddr to the bound
+            // socket address or to a random address in the multi_subnet_ case.
+            port = DHCP4_SERVER_PORT;
         }
+    } else if ((options.getIpVersion() == 4) && (port != DHCP4_SERVER_PORT)) {
+        std::cerr << "WARNING: Port " << port << " specified, but server will "
+                  << "respond to port " << DHCP4_SERVER_PORT << "."
+                  << std::endl;
     }
 
     // Local name is specified along with '-l' option.