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// Other services

Port 1701: l2f

Port 1701 is assigned by IANA to l2f (l2f), l2tp (l2tp).

1701

How l2f works

Other services
client:1701l2fresponse client:1701l2fresponse

The client sends a request to port 1701 and the l2f service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned2 registered services
Range
Registered (1024-49151)
Category
Other services
01 Security

Is port 1701 dangerous?

We have no specific security note for port 1701. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 1701

ServicePortTransportDescriptionReferenceUpdated
l2f 1701 TCP, UDP l2f - -
l2tp 1701 TCP, UDP l2tp - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 1701, and is it reachable?

Linux

sudo ss -ltnup 'sport = :1701'      # process listening on 1701
sudo lsof -nP -i :1701

macOS

sudo lsof -nP -i :1701

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1701 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 1701
netstat -ano | findstr :1701

From another machine

nmap -p 1701 -sV <host>
sudo nmap -sU -p 1701 <host>
nc -zv <host> 1701

Block it in the firewall

sudo ufw deny 1701/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 1701" -Direction Inbound -LocalPort 1701 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 1701 used for?

Port 1701 is assigned by IANA to l2f (l2f), l2tp (l2tp).

Is port 1701 TCP or UDP?

Port 1701 is registered for TCP and UDP.

Is it safe to open port 1701?

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. Close the port if nothing on the host needs it.

How do I check if port 1701 is open?

On Linux run "ss -ltnup 'sport = :1701'", on macOS "lsof -nP -i :1701", and on Windows "netstat -ano | findstr :1701" to see the local process. From another machine, "nmap -p 1701 <host>" shows whether it is reachable.

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