diff --git a/sdk/include/FreeRTOS-Compat/FreeRTOS_errno.h b/sdk/include/FreeRTOS-Compat/FreeRTOS_errno.h
new file mode 100644
index 000000000..535050dfd
--- /dev/null
+++ b/sdk/include/FreeRTOS-Compat/FreeRTOS_errno.h
@@ -0,0 +1,63 @@
+// Copyright SCI Semiconductor and CHERIoT Contributors.
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <errno.h>
+
+/**
+ * This header defines errno values used by FreeRTOS+TCP.
+ *
+ * In the FreeRTOS core tree, this is included in `projdefs.h`.
+ *
+ * We modified it here to use the same errno codes are the CHERIoT core when
+ * possible.
+ */
+
+#define pdFREERTOS_ERRNO_NONE 0                  // No errors
+#define pdFREERTOS_ERRNO_ENOENT ENOENT           // No such file or directory
+#define pdFREERTOS_ERRNO_EINTR EINTR             // Interrupted system call
+#define pdFREERTOS_ERRNO_EIO EIO                 // I/O error
+#define pdFREERTOS_ERRNO_ENXIO ENXIO             // No such device or address
+#define pdFREERTOS_ERRNO_EBADF EBADF             // Bad file number
+#define pdFREERTOS_ERRNO_EAGAIN EAGAIN           // No more processes
+#define pdFREERTOS_ERRNO_EWOULDBLOCK EWOULDBLOCK // Operation would block
+#define pdFREERTOS_ERRNO_ENOMEM ENOMEM           // Not enough memory
+#define pdFREERTOS_ERRNO_EACCES EACCES           // Permission denied
+#define pdFREERTOS_ERRNO_EFAULT EFAULT           // Bad address
+#define pdFREERTOS_ERRNO_EBUSY EBUSY             // Mount device busy
+#define pdFREERTOS_ERRNO_EEXIST EEXIST           // File exists
+#define pdFREERTOS_ERRNO_EXDEV EXDEV             // Cross-device link
+#define pdFREERTOS_ERRNO_ENODEV ENODEV           // No such device
+#define pdFREERTOS_ERRNO_ENOTDIR ENOTDIR         // Not a directory
+#define pdFREERTOS_ERRNO_EISDIR EISDIR           // Is a directory
+#define pdFREERTOS_ERRNO_EINVAL EINVAL           // Invalid argument
+#define pdFREERTOS_ERRNO_ENOSPC ENOSPC           // No space left on device
+#define pdFREERTOS_ERRNO_ESPIPE ESPIPE           // Illegal seek
+#define pdFREERTOS_ERRNO_EROFS EROFS             // Read only file system
+#define pdFREERTOS_ERRNO_EUNATCH EUNATCH         // Protocol driver not attached
+#define pdFREERTOS_ERRNO_EBADE EBADE             // Invalid exchange
+#define pdFREERTOS_ERRNO_EFTYPE EFTYPE           // Inappropriate file type or format
+#define pdFREERTOS_ERRNO_ENOTEMPTY ENOTEMPTY     // Directory not empty
+#define pdFREERTOS_ERRNO_ENAMETOOLONG ENAMETOOLONG // File or path name too long
+#define pdFREERTOS_ERRNO_EOPNOTSUPP EOPNOTSUPP   // Operation not supported on transport endpoint
+#define pdFREERTOS_ERRNO_EAFNOSUPPORT EAFNOSUPPORT // Address family not supported by protocol
+#define pdFREERTOS_ERRNO_ENOBUFS ENOBUFS         // No buffer space available
+#define pdFREERTOS_ERRNO_ENOPROTOOPT ENOPROTOOPT // Protocol not available
+#define pdFREERTOS_ERRNO_EADDRINUSE EADDRINUSE   // Address already in use
+#define pdFREERTOS_ERRNO_ETIMEDOUT ETIMEDOUT     // Connection timed out
+#define pdFREERTOS_ERRNO_EINPROGRESS EINPROGRESS // Connection already in progress
+#define pdFREERTOS_ERRNO_EALREADY EALREADY       // Socket already connected
+#define pdFREERTOS_ERRNO_EADDRNOTAVAIL EADDRNOTAVAIL // Address not available
+#define pdFREERTOS_ERRNO_EISCONN EISCONN         // Socket is already connected
+#define pdFREERTOS_ERRNO_ENOTCONN ENOTCONN       // Socket is not connected
+#define pdFREERTOS_ERRNO_ENOMEDIUM ENOMEDIUM     // No medium inserted
+#define pdFREERTOS_ERRNO_EILSEQ EILSEQ           // An invalid UTF-16 sequence was encountered
+#define pdFREERTOS_ERRNO_ECANCELED ECANCELED     // Operation canceled
+
+/**
+ * These errno codes are non-standard, assign them a code outside our errno
+ * range.
+ */
+
+#define pdFREERTOS_ERRNO_ENMFILE (__ELASTERROR + 1) // No more files
diff --git a/sdk/include/errno.h b/sdk/include/errno.h
index df7478578..07cf19947 100644
--- a/sdk/include/errno.h
+++ b/sdk/include/errno.h
@@ -46,6 +46,8 @@
 #define ENOMSG 42          // No message of the desired type.
 #define EIDRM 43           // Identifier removed.
 #define EDEADLK 45         // Resource deadlock would occur.
+#define EUNATCH 49         // Protocol driver not attached.
+#define EBADE 52           // Invalid exchange.
 #define ENOSTR 60          // Not a STREAM.
 #define ENODATA 61         // No data available.
 #define ETIME 62           // Timer expired.
@@ -54,6 +56,7 @@
 #define EPROTO 71          // Protocol error.
 #define EMULTIHOP 72       // Reserved.
 #define EBADMSG 74         // Bad message.
+#define EFTYPE 79          // Inappropriate file type or format.
 #define EILSEQ 84          // Illegal byte sequence.
 #define ENOTSOCK 88        // Not a socket.
 #define EDESTADDRREQ 89    // Destination address required.
@@ -80,6 +83,7 @@
 #define EINPROGRESS 115    // Operation in progress.
 #define ESTALE 116         // Reserved.
 #define EDQUOT 122         // Reserved.
+#define ENOMEDIUM 123      // No medium inserted.
 #define ECANCELED 125      // Operation canceled.
 #define EOWNERDEAD 130     // Previous owner died.
 #define ENOTRECOVERABLE 131 // State not recoverable.