It seems, that the avove problem stems from the usage of transmission?

transmission itself gives me the following "hints" at startup:
Code:
$ EVENT_NOEPOLL=1 transmission-daemon -f --log-debug
[20:35:34.294] Transmission 2.50 (13228) started (session.c:718)
[20:35:34.296] Cache Maximum cache size set to 2.00 MiB (128 blocks) (cache.c:249)
[20:35:34.297] RPC Server Adding address to whitelist: *.*.*.* (rpc-server.c:803)
[20:35:34.298] RPC Server Serving RPC and Web requests on port 127.0.0.1:9091/transmission/ (rpc-server.c:997)
[20:35:34.299] RPC Server Whitelist enabled (rpc-server.c:1001)
[20:35:34.300] Bound socket 10 to port 51413 on 0.0.0.0 (net.c:371)
[20:35:34.300] Bound socket 11 to port 51413 on :: (net.c:371)
[20:35:34.301] UDP Failed to set receive buffer: requested 4194304, got 221184 (tr-udp.c:75)
[20:35:34.304] UDP Please add the line "net.core.rmem_max = 4194304" to /etc/sysctl.conf (tr-udp.c:80)
[20:35:34.305] UDP Failed to set send buffer: requested 1048576, got 221184 (tr-udp.c:86)
[20:35:34.306] UDP Please add the line "net.core.wmem_max = 1048576" to /etc/sysctl.conf (tr-udp.c:91)
.
.
.
[20:35:34.313] Port Forwarding (NAT-PMP) initnatpmp succeeded (0) (natpmp.c:72)
[20:35:34.314] Port Forwarding (NAT-PMP) sendpublicaddressrequest succeeded (2) (natpmp.c:72)
[20:35:34.315] Port Forwarding (UPnP) UPNP_GetValidIGD failed (errno 0 - Success) (upnp.c:214)
[20:35:34.316] Port Forwarding (UPnP) If your router supports UPnP, please make sure UPnP is enabled! (upnp.c:217)
[20:35:34.317] Port Forwarding Starting (port-forwarding.c:93)
.
.
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May this be related?
Does the Asus provide a "/etc/sysctl.conf" (I couldn't find one)?
Should I create one?

Regarding the ports-problems --> what is the preferred way to enable UPnP?

Thanks for help,
pheidrias

PS:
my transmission config:
Code:
cat .config/transmission-daemon/settings.json 
{
    "alt-speed-down": 50, 
    "alt-speed-enabled": false, 
    "alt-speed-time-begin": 540, 
    "alt-speed-time-day": 127, 
    "alt-speed-time-enabled": false, 
    "alt-speed-time-end": 1020, 
    "alt-speed-up": 50, 
    "bind-address-ipv4": "0.0.0.0", 
    "bind-address-ipv6": "::", 
    "blocklist-enabled": false, 
    "blocklist-url": "http://www.example.com/blocklist", 
    "cache-size-mb": 2, 
    "dht-enabled": true, 
    "download-dir": "/tmp/harddisk/transmission/download", 
    "download-queue-enabled": true, 
    "download-queue-size": 5, 
    "encryption": 0, 
    "idle-seeding-limit": 30, 
    "idle-seeding-limit-enabled": false, 
    "incomplete-dir": "/usr/local/root/Downloads", 
    "incomplete-dir-enabled": false, 
    "lpd-enabled": false, 
    "message-level": 3, 
    "peer-congestion-algorithm": "", 
    "peer-limit-global": 240, 
    "peer-limit-per-torrent": 60, 
    "peer-port": 51413, 
    "peer-port-random-high": 65535, 
    "peer-port-random-low": 49152, 
    "peer-port-random-on-start": false, 
    "peer-socket-tos": "default", 
    "pex-enabled": true, 
    "port-forwarding-enabled": true, 
    "preallocation": 1, 
    "prefetch-enabled": 0, 
    "queue-stalled-enabled": true, 
    "queue-stalled-minutes": 30, 
    "ratio-limit": 2, 
    "ratio-limit-enabled": false, 
    "rename-partial-files": true, 
    "rpc-authentication-required": false, 
    "rpc-bind-address": "0.0.0.0", 
    "rpc-enabled": true, 
    "rpc-password": "whoknows", 
    "rpc-port": 9091, 
    "rpc-url": "/transmission/", 
    "rpc-username": "", 
    "rpc-whitelist": "*.*.*.*", 
    "rpc-whitelist-enabled": true, 
    "scrape-paused-torrents-enabled": true, 
    "script-torrent-done-enabled": false, 
    "script-torrent-done-filename": "", 
    "seed-queue-enabled": false, 
    "seed-queue-size": 10, 
    "speed-limit-down": 100, 
    "speed-limit-down-enabled": false, 
    "speed-limit-up": 100, 
    "speed-limit-up-enabled": false, 
    "start-added-torrents": true, 
    "trash-original-torrent-files": false, 
    "umask": 18, 
    "upload-slots-per-torrent": 14, 
    "utp-enabled": true
}