Port (TCP) Monitoring

Check that a TCP port accepts connections, with optional protocol banner checks for mail and FTP servers

A port monitor opens a TCP connection to a host and port and succeeds when the connection is accepted. For some protocols it can also check the server’s greeting banner or a response.

When to use it

• Databases: MySQL (3306), PostgreSQL (5432), MongoDB (27017), Redis (6379)
• Mail servers: SMTP (25, 465, 587), POP3 (110, 995), IMAP (143, 993)
• SSH (22), FTP (21) and custom TCP services

To check a web page or HTTP endpoint, use an HTTP monitor. Port monitors do not offer HTTP or HTTPS protocols.

Port checks run from UptimeIO’s probe locations over the public internet, so the target must be publicly reachable. Private and internal targets (private IP ranges such as 10.x, 172.16-31.x and 192.168.x, localhost, and names ending in .local, .internal or .lan) are not accepted. Allow the connection through your firewall or security group.

Create a port monitor

1. Enter the host

   Enter the Hostname or IP Address, a public hostname or IP such as db.yourcompany.com. The port is set separately in Port Configuration.

2. Choose a protocol and port

   In Port Configuration, pick the Protocol Type and enter the Port Number. The protocol decides which optional checks are available.

3. Set interval and locations

   In How often we check, choose the Check Interval. The shortest interval depends on your plan: 5 minutes on Free, 1 minute on Pro and Scale. Pro and Scale can choose probe locations; Free uses automatic selection.

Settings

Setting	What it does	Default / options
Monitor Name	Name shown in your dashboard	Generated from the host, up to 80 characters
Hostname or IP Address	The server to connect to	Required
Protocol Type	The service you expect on the port	SMTP (Email), POP3 (Email), IMAP (Email), FTP (File Transfer), SSH (Secure Shell), Generic TCP
Port Number	TCP port to connect to, 1 to 65535. Choosing a protocol suggests its standard port	Required
Connection Timeout	Longest wait for the connection, 5 to 60 seconds. Must be shorter than the check interval	10 seconds
Check protocol banner/greeting	Verify the server answers with the expected greeting. Only for SMTP, POP3, IMAP and FTP	Off
Check Interval	Time between checks	1 minute to 24 hours. Plan minimum applies

Protocols

Protocol	Standard port	Optional check
Generic TCP	none	None. Connection test only.
SSH	22	None. Connection test only (the SSH banner is not checked).
SMTP	25	Check protocol banner/greeting: expects a greeting containing 220.
POP3	110	Banner check: expects +OK.
IMAP	143	Banner check: expects * OK.
FTP	21	Banner check: expects 220.

Port monitors only open a TCP connection and read a greeting; they never send an HTTP request or negotiate TLS. To check a web page use an HTTP monitor. A port monitor created earlier with HTTP or HTTPS keeps running as before; when you edit and save it, it becomes a connection test.

Example: connection test

• Hostname or IP Address: db.yourcompany.com
• Protocol Type: Generic TCP
• Port Number: 5432
• Check Interval: 5 minutes

Example: SMTP banner check

• Hostname or IP Address: mail.yourcompany.com
• Protocol Type: SMTP (Email)
• Port Number: 25
• Check protocol banner/greeting: on

How a check decides

• Banner check off: succeeds as soon as the TCP connection is established.
• Banner check on: succeeds when the server’s greeting contains the expected text, and fails otherwise.

Common ports

Service	Port	Service	Port
MySQL	3306	SMTP	25 / 587 / 465
PostgreSQL	5432	POP3	110 / 995
MongoDB	27017	IMAP	143 / 993
Redis	6379	SSH / SFTP	22
SQL Server	1433	FTP	21

Best practices

• Use a banner check for mail and FTP servers: an open port does not prove the service answers correctly.
• Keep the timeout around 5-10 seconds.
• Combine with Ping (is the host up?) and HTTP (does the application work?).
• A port monitor proves connectivity only. The service can accept connections while authentication is broken.
• Do not expose databases to the whole internet just to monitor them. Restrict access with a firewall where you can.

Troubleshooting

Connection refused

Nothing is listening on that port, the port is wrong, or the service only listens on localhost. Check with nc -zv hostname port and the service’s bind address.

Connection timeout

A firewall is dropping the traffic, the host is unreachable, or the Connection Timeout is too low. Allow the connection and check firewall logs.

Banner check fails

The server’s greeting did not contain the expected text. The port may be served by a different service or a proxy. Check the banner manually with nc hostname port.

Target not accepted

The target is private, internal, a test domain or one of UptimeIO’s own domains. Use a public hostname or IP.

Next steps

HTTP Monitoring

Monitor web services and APIs

Notifications

Configure alerts for port monitors