compression issue

[e9hack]

If dropbear is executed with option -W xxxx and xxxx is lager than 32768, coping of large files via WinSCP and compression enabled does fail. WinSCP shows the following error message:

Copying file 'D:\Download\carambola\openwrt-ath79-generic-tplink_archer-c7-v2-2-squashfs-sysupgrade-d240224.bin' fatally failed.
Copying files to remote side failed.

Syslog has the following message:
Sat Feb 24 05:02:01 2024 authpriv.info dropbear[5311]: Exit (root) from <192.168.103.2:47725>: Bad packet size 34091

The issue can be fixed by increasing RECV_MAX_PAYLOAD_LEN:

--- src/default_options.h	2024-04-25 16:30:00.000000000 +0200
+++ src/default_options.h	2024-05-24 11:37:25.103361566 +0200
@@ -341,7 +341,7 @@ group1 in Dropbear server too */
 #define DEFAULT_RECV_WINDOW 24576
 /* Maximum size of a received SSH data packet - this _MUST_ be >= 32768
    in order to interoperate with other implementations */
-#define RECV_MAX_PAYLOAD_LEN 32768
+#define RECV_MAX_PAYLOAD_LEN 49152
 /* Maximum size of a transmitted data packet - this can be any value,
    though increasing it may not make a significant difference. */
 #define TRANS_MAX_PAYLOAD_LEN 16384

Regards,
Hartmut