dropbear: add option to set receive window size

The default receive window size in dropbear is hardcoded to 24576 byte
to limit memory usage. This value was chosen for 100Mbps networks, and
limits the throughput of scp on faster networks. It also severely limits
scp throughput on high-latency links.

Add an option to set the receive window size so that people can improve
performance without having to recompile dropbear.

Setting the window size to the highest value supported by dropbear
improves throughput from my build machine to an APU2 on the same LAN
from 7MB/s to 7.9MB/s, and to an APU2 over a link with ~65ms latency
from 320KB/s to 7.5MB/s.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

diff --git a/package/network/services/dropbear/files/dropbear.init b/package/network/services/dropbear/files/dropbear.init
index 3d8cb2ca5e372121e3f1c29fa5aca3d49e6c5a10..2225113498a509403300e556a5da690e0b3959fb 100755 (executable)
--- a/package/network/services/dropbear/files/dropbear.init
+++ b/package/network/services/dropbear/files/dropbear.init
@@ -42,6 +42,7 @@ validate_section_dropbear()
                'SSHKeepAlive:uinteger:300' \
                'IdleTimeout:uinteger:0' \
                'MaxAuthTries:uinteger:3' \
+               'RecvWindowSize:uinteger:0' \
                'mdns:bool:1'
 }
 
@@ -50,7 +51,7 @@ dropbear_instance()
        local PasswordAuth enable Interface GatewayPorts \
                RootPasswordAuth RootLogin rsakeyfile \
                BannerFile Port SSHKeepAlive IdleTimeout \
-               MaxAuthTries mdns ipaddrs
+               MaxAuthTries RecvWindowSize mdns ipaddrs
 
        validate_section_dropbear "${1}" || {
                echo "validation failed"
@@ -80,6 +81,8 @@ dropbear_instance()
        [ "${IdleTimeout}" -ne 0 ] && procd_append_param command -I "${IdleTimeout}"
        [ "${SSHKeepAlive}" -ne 0 ] && procd_append_param command -K "${SSHKeepAlive}"
        [ "${MaxAuthTries}" -ne 0 ] && procd_append_param command -T "${MaxAuthTries}"
+       [ "${RecvWindowSize}" -gt 0 -a "${RecvWindowSize}" -le 1048576 ] && \
+               procd_append_param command -W "${RecvWindowSize}"
        [ "${mdns}" -ne 0 ] && procd_add_mdns "ssh" "tcp" "$Port" "daemon=dropbear"
        procd_set_param respawn
        procd_close_instance