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Port 691 (msexch-routing)

IANA assigns port 691 to msexch-routing: MS Exchange Routing.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
msexch-routing TCP MS Exchange Routing David Lemson -
msexch-routing UDP MS Exchange Routing David Lemson -

Check what is listening on port 691

Linux: is anything listening on TCP 691?
sudo ss -tlnp 'sport = :691'
Linux: UDP 691
sudo ss -ulnp 'sport = :691'
Linux / macOS: which process owns the port
sudo lsof -nP -i :691
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':691 '
Windows (Command Prompt)
netstat -ano | findstr :691
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 691 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 691
From Windows
Test-NetConnection HOST -Port 691
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 691

Block port 691 in the firewall

ufw
sudo ufw deny 691/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 691 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 691 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 691' -Direction Inbound -Protocol TCP -LocalPort 691 -Action Block
ufw
sudo ufw deny 691/udp
iptables
sudo iptables -A INPUT -p udp --dport 691 -j DROP
nftables
sudo nft add rule inet filter input udp dport 691 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 691' -Direction Inbound -Protocol UDP -LocalPort 691 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.