Soft phone on linux server does not work due to udp:50000?

Hi everyone,

great forum. I'm running since more than 10 years a linux server behind my router (fritzbox which connects to the internet / dsl) and since a few year a home automation but I stumbled across a problem I need some advice.

My linux server has the ip 192.168.1.1 and is connected via ethernet to my fritzbox router on 192.168.1.2. My linux router has wifi and dhcp and does all the wireless stuff. My fritzbox is responsible for the ethernet and the dhcp on wired devices. Everything runs smoothly and I can get internet access with my smartphones connecting to my linux server which forwards it to the router. Also my self built home automation (fhem) runs on the server without any issues:

smartphone (192.168.2.xxx) wlp2s0 <--> linux server wlp2s0 (192.168.2.1) <--> linux server enp0s10 (192.168.1.1) <--> fritzbox (192.168.1.2) <--> dsl/internet

With one exception: on my smartphones I got an softphone app for my fritzbox. Basically it is a SIP to call others on a landline. When I connect my smartphones to my linux server wlp2s0 (192.168.2.1) I can ring other phones but voice is not transmitted in any direction. Affected ports by the sip are tcp 5060 and udp 50000:50100.

Now if I instead use a xiaomi wifi mini router instead the linux server which connects also via ethernet to my fritzbox and I connect my smartphones to this access point it works.

Additionally I installed an app on my smartphone to check which apps use which ips and ports. I found in the case of my smartpphone connecting to my linux server the sip:5060 is used. In the case where I connect to the xiaomi mini router the sip:5060 and additionally udp:50000.

Looks like the udp:50000 is missing on my linux server. Any idea how I could solve this?

Thanks Marc

My config is
> ifconfig
enp0s10: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.1 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 xxxxxxxxxxxxxxxxxx prefixlen 64 scopeid 0x20<link>
ether xx:xx:xx:xx:xx:xx txqueuelen 1000 (Ethernet)
RX packets 529503 bytes 533861226 (509.1 MiB)
RX errors 0 dropped 83480 overruns 0 frame 0
TX packets 266939 bytes 34459918 (32.8 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1 (Local Loopback)
RX packets 24732 bytes 2011483 (1.9 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 24732 bytes 2011483 (1.9 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
tun0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1500
inet 172.16.0.1 netmask 255.255.255.255 destination 172.16.0.2
inet6 xxxxxxxxxxxxxx:933d prefixlen 64 scopeid 0x20<link>
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 100 (UNSPEC)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 8 bytes 544 (544.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlp2s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.2.1 netmask 255.255.255.0 broadcast 192.168.2.255
inet6 xxxxxxxxxxxxxxxx prefixlen 64 scopeid 0x20<link>
ether xx:xx:xx:xx:xx:xx txqueuelen 1000 (Ethernet)
RX packets 217970 bytes 28356969 (27.0 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 372911 bytes 479375138 (457.1 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

> iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate NEW
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

> ip route
default via 192.168.1.2 dev enp0s10 onlink
172.16.0.0/24 via 172.16.0.2 dev tun0
172.16.0.2 dev tun0 proto kernel scope link src 172.16.0.1
192.168.1.0/24 dev enp0s10 proto kernel scope link src 192.168.1.1
192.168.2.0/24 dev wlp2s0 proto kernel scope link src 192.168.2.1