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Port 646 (LDP (MPLS))

Label Distribution Protocol for MPLS networks.

Well-known (system) port, 0-1023 TCPUDP Network infrastructure Do not expose to the internet

What port 646 is used for

Label Distribution Protocol for MPLS networks.

Security: should port 646 be open?

Do not expose to the internet. Core network only.

IANA assignment

Service nameProtoDescriptionAssigneeReference
ldp TCP LDP Bob Thomas -
ldp UDP LDP Bob Thomas -

Check what is listening on port 646

Linux: is anything listening on TCP 646?
sudo ss -tlnp 'sport = :646'
Linux: UDP 646
sudo ss -ulnp 'sport = :646'
Linux / macOS: which process owns the port
sudo lsof -nP -i :646
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':646 '
Windows (Command Prompt)
netstat -ano | findstr :646
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 646 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 646
From Windows
Test-NetConnection HOST -Port 646
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 646

Block port 646 in the firewall

ufw
sudo ufw deny 646/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 646 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 646 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 646' -Direction Inbound -Protocol TCP -LocalPort 646 -Action Block
ufw
sudo ufw deny 646/udp
iptables
sudo iptables -A INPUT -p udp --dport 646 -j DROP
nftables
sudo nft add rule inet filter input udp dport 646 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 646' -Direction Inbound -Protocol UDP -LocalPort 646 -Action Block

Related ports

179BGP644dwr645pssc647dhcp-failover648rrp

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.