One-way UDP / RTP traffic issue (Debian 13)

    Hello evryone,

    I’m troubleshooting a one-way UDP/RTP issue and would appreciate some advice.

    Environment

    • Server IP: 159.195.85.32

    • Remote IP: 35.146.30.136

    • OS: Debian 13

    • RTP port range: UDP 10000–20000

    • Destination port: UDP 4006

    Issue

    • UDP/RTP packets from 159.195.85.32 → 35.146.30.136 are sent successfully.

    • UDP packets from 35.146.30.136 → 159.195.85.32 do not arrive.

    • This results in one-way audio.

    tcpdump on 159.195.85.32

    Code
    tcpdump -ni any udp portrange 10000-20000

    Observed output shows only outbound traffic:

    Code
    IP 159.195.85.32.11348 > 35.146.30.136.4006: UDP

    No inbound UDP packets are seen from the remote side.

    Checks already done

    • Local firewall allows UDP 10000–20000

    • Routing looks correct

    • Remote side confirms outbound UDP traffic

    • Issue persists even when firewall is temporarily disabled

    Questions

    • Has anyone experienced silent UDP blocking by hosting providers?

    • Could anti-DDoS, rp_filter, or conntrack limits cause this behavior?

    • Any recommended tests to clearly prove provider-side UDP filtering?

    Thanks in advance for any help.