- Require TLS and authenticated submission.
- Make sure the server is not an open relay.
- Publish SPF, DKIM and DMARC for the domain.
Port 691: MS Exchange Routing
Port 691 is assigned by IANA to msexch-routing (MS Exchange Routing).
691
How msexch-routing works
EmailMail servers hand messages to each other over port 691, one hop at a time, until the recipient's server.
- Transport
- TCP · UDP
- Exposure risk
- not rated depends on the program listening
- Encryption
- Unknownnot stated by the registry
- IANA status
- Assigned1 registered service
- Range
- Well-known (0-1023)
- Category
Is port 691 dangerous?
We have no specific security note for port 691. Its risk depends on the program actually listening on it. General advice for email services:
IANA registry entries for port 691
| Service | Port | Transport | Description | Reference | Updated |
|---|---|---|---|---|---|
| msexch-routing | 691 | TCP, UDP | MS Exchange Routing | - | - |
// source: IANA service name and port number registry
Which program uses port 691, and is it reachable?
Linux
sudo ss -ltnup 'sport = :691' # process listening on 691
sudo lsof -nP -i :691macOS
sudo lsof -nP -i :691
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 691 -State Listen | Select LocalAddress, OwningProcess Get-NetUDPEndpoint -LocalPort 691 netstat -ano | findstr :691
From another machine
nmap -p 691 -sV <host> sudo nmap -sU -p 691 <host> nc -zv <host> 691
Block it in the firewall
sudo ufw deny 691/tcp # Linux (ufw) New-NetFirewallRule -DisplayName "Block 691" -Direction Inbound -LocalPort 691 -Protocol TCP -Action Block # Windows
What is port 691 used for?
Port 691 is assigned by IANA to msexch-routing (MS Exchange Routing).
Is port 691 TCP or UDP?
Port 691 is registered for TCP and UDP.
Is it safe to open port 691?
It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Require TLS and authenticated submission.
How do I check if port 691 is open?
On Linux run "ss -ltnup 'sport = :691'", on macOS "lsof -nP -i :691", and on Windows "netstat -ano | findstr :691" to see the local process. From another machine, "nmap -p 691 <host>" shows whether it is reachable.