The Tool Desk
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What disabling port 25 actually changes
Sendmail opens an incoming network socket when it runs in daemon mode. The normal daemon configuration uses TCP port 25, the traditional SMTP port for server-to-server delivery. Removing that socket prevents remote SMTP clients from connecting to the host on port 25; it does not remove Sendmail’s local message-delivery command or necessarily stop queue processing.
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Port 25, port 587, and local submission are separate decisions:
- Port 25 (MTA): used for SMTP traffic between mail servers. Removing its listener stops inbound SMTP on that address and port.
- Port 587 (MSA): used for message submission by authorized clients. It can remain available, preferably only on loopback when mail is submitted locally.
- Local Sendmail interface: programs can hand a message to the local Sendmail binary or local queue without an externally reachable SMTP socket.
A firewall rule can block connections, but it does not make Sendmail stop listening. The daemon configuration must be changed when the requirement is that no process listen on port 25.
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Choose the method that matches your Linux packaging
| Configuration model | Primary change | When to use it | Persistence consideration |
|---|---|---|---|
| Oracle Linux 7 service configuration | Set DAEMON=no in /etc/sysconfig/sendmail. |
The package supplies this switch and the service reads the file at startup. | Keep the setting in the package’s supported file; confirm it after package updates. |
| M4-managed Sendmail | Remove the MTA DAEMON_OPTIONS declaration from the .mc source. |
The active sendmail.cf is generated from an M4 source file. |
Edit the source, not only the generated .cf, or regeneration can restore port 25. |
| M4-managed with local submission | Keep an MSA declaration on port 587 bound to 127.0.0.1 and/or ::1; add FEATURE(`no_default_msa') when required. |
Applications on the same host need SMTP submission but the host must not expose SMTP publicly. | Regenerate the configuration from the edited source after every source change. |
Do not apply the Oracle-specific variable blindly to another distribution. First identify which package owns Sendmail, where its service reads startup variables, and whether the active configuration is generated from an M4 file.
Before editing: identify the active configuration
Make a backup of the service configuration and the M4 source, if present. Record current listeners so you can distinguish the intended change from an unrelated service:
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sudo ss -ltnp
Look for Sendmail’s service unit, startup file, and configuration files under the locations used by your distribution. Common names include /etc/sysconfig/sendmail, /etc/mail/sendmail.mc, and /etc/mail/sendmail.cf, but paths are package-dependent. If sendmail.cf is generated, treat it as an output file and locate the corresponding .mc source before making a permanent change.
Save a copy before editing:
sudo cp -a /etc/sysconfig/sendmail /etc/sysconfig/sendmail.before-port25-change
sudo cp -a /etc/mail/sendmail.mc /etc/mail/sendmail.mc.before-port25-change
sudo cp -a /etc/mail/sendmail.cf /etc/mail/sendmail.cf.before-port25-change
If one of those files does not exist on your system, do not create it merely because it appears in an example; use the file supplied by your package.
Method 1: Oracle Linux 7 DAEMON=no
Oracle Linux 7 documents a service switch in /etc/sysconfig/sendmail. Set the value of DAEMON to no:
sudo vi /etc/sysconfig/sendmail
Set or change the line to:
DAEMON=no
This prevents the Sendmail daemon from starting its inbound SMTP listener when the service starts. Save the file, then restart Sendmail using the service manager configured on the host. On a system using systemd, the operation is typically:
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Check the service status and inspect the sockets immediately afterward:
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sudo systemctl status sendmail
sudo ss -ltnp
The Oracle setting is distribution-specific. If a package update replaces the startup file or a local administrator changes the service unit, recheck that DAEMON=no is still being read. If port 25 remains open, inspect the effective service definition and any separately generated Sendmail configuration instead of adding another firewall rule.
Method 2: remove the MTA socket from an M4 configuration
Sendmail’s M4 documentation defines the standard MTA listener with a declaration equivalent to:
DAEMON_OPTIONS(`Port=smtp, Name=MTA')dnl
In an M4-managed setup, remove that MTA declaration from the .mc source when no inbound SMTP socket should exist. The documentation’s important distinction is that omitting the MTA declaration leaves no listener on the standard SMTP port.
Edit the source file used by your package, for example:
sudo vi /etc/mail/sendmail.mc
Find every DAEMON_OPTIONS line and identify its purpose by the Port, Addr, Family, and Name fields. Delete or comment out only the MTA entry for port 25. Do not remove an MSA entry if local applications still need submission on port 587.
Each DAEMON_OPTIONS occurrence can create an incoming socket. Leaving a second declaration that still uses Port=smtp can therefore keep port 25 open even after one line has been removed.
Prevent an unwanted default MSA
Some M4 configurations generate a default Message Submission Agent entry in addition to the MTA entry. If you want neither a public port-25 socket nor an automatically generated MSA, add:
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FEATURE(`no_default_msa')dnl
FEATURE(`no_default_msa') suppresses automatic generation of the default MSA entry. Use it when you intend to define the submission sockets yourself or when no SMTP submission listener should exist at all.
Removing the MTA declaration and disabling the default MSA are separate operations. Removing the MTA stops the standard port-25 listener; the feature controls whether the default MSA is generated. Decide independently whether port 587 is needed.
Keep local submission without exposing SMTP publicly
For a host whose applications need SMTP submission, bind the MSA only to loopback addresses. Debian’s Sendmail guidance illustrates loopback bindings on IPv4 and IPv6. A configuration can contain entries of this form, adjusted to the syntax and naming conventions of your package:
FEATURE(`no_default_msa')dnl
DAEMON_OPTIONS(`Port=587, Addr=127.0.0.1, Name=MSA')dnl
DAEMON_OPTIONS(`Port=587, Addr=::1, Family=inet6, Name=MSA')dnl
The first line prevents an automatic MSA from being added in addition to your explicit entries. The IPv4 entry accepts submissions only through 127.0.0.1; the IPv6 entry accepts them only through ::1. Neither address is reachable from another machine through a normal network interface.
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If the application uses the local Sendmail command rather than SMTP, no port-587 listener is required. Conversely, if an application is configured to connect to localhost:587, retain the loopback MSA and make sure the application’s address family matches the socket you created.
Do not bind the MSA to 0.0.0.0 or :: when the goal is local-only submission. Those wildcard addresses expose the service on every matching interface and can create a new externally reachable submission endpoint.
Regenerate and install the active configuration
Editing a .mc file does not change the running daemon by itself. Build a new sendmail.cf using the regeneration mechanism supplied by your distribution, install it in the location used by the Sendmail service, and then restart Sendmail.
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A commonly used M4 workflow is conceptually:
sudo m4 /etc/mail/sendmail.mc > /tmp/sendmail.cf.new
sudo install -o root -g root -m 0644 /tmp/sendmail.cf.new /etc/mail/sendmail.cf
sudo systemctl restart sendmail
The exact source path, ownership, permissions, and helper command vary by package. Some distributions provide a make target or a configuration script that performs these steps. Prefer that package-supported method when available. The essential order is:
- Edit the authoritative
.mcsource. - Generate a new
sendmail.cf. - Install the generated file where the service reads it.
- Restart Sendmail.
- Inspect listeners and logs for errors.
If generation fails, do not restart with a partially written file. Restore the previous working configuration, correct the M4 syntax, regenerate to a temporary file, and compare the result before installing it.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Verify that port 25 is closed on both IPv4 and IPv6
Use ss after the restart:
sudo ss -ltnp | grep -E '(:25b|:smtpb)'
No output means this command found no matching listening TCP socket. To inspect all Sendmail sockets without filtering:
sudo ss -ltnp
Check the address as well as the port. A successful port-25 shutdown must not show public bindings such as:
0.0.0.0:25
[::]:25
When local submission is retained, the expected remaining MSA sockets are limited to the addresses you configured, for example:
127.0.0.1:587
[::1]:587
An entry on 0.0.0.0:587, a server’s real IPv4 address, or [::]:587 indicates that the MSA is not loopback-only. An entry on [::]:25 matters even when no IPv4 port-25 socket appears; IPv6 can provide a separate public path.
On systems without ss, use the platform’s socket-listing equivalent, such as netstat -ltnp where that utility is installed. The verification requirement is the same: inspect listening TCP sockets for both address families and identify the owning process.
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When port 25 still appears
A second daemon declaration remains
Search the active M4 source and generated configuration for additional DAEMON_OPTIONS entries. Sendmail creates an incoming socket for each daemon-port declaration, so one leftover MTA or SMTP declaration can recreate port 25.
The generated file was not rebuilt
If the source is correct but the running service still listens, compare the modification time of sendmail.cf with the time of regeneration. Rebuild and install the file through the package’s supported method, then restart the service.
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Multiple Sendmail installations or custom service units can use different configuration paths. Inspect the service definition and the process command line to determine which sendmail.cf and startup variables are actually active.
IPv6 was overlooked
A check that looks only for an IPv4 address can miss [::]:25. Always inspect both families, or review the complete unfiltered ss -ltnp output.
Another process owns port 25
The socket listing identifies the owning process. If it is not Sendmail, changing Sendmail’s M4 source cannot close that listener; investigate the other service separately.
The service failed after the change
Check the service status and system logs for M4 syntax errors, invalid address-family options, or permission problems. Restore the saved configuration if necessary, then regenerate a corrected file before attempting another restart.
Quick Recap
Operational effects and trade-offs
- Inbound mail to this host: remote SMTP servers cannot deliver directly over port 25 after the listener is removed. If the host is intended to receive mail, provide another designated mail exchanger or forwarding arrangement before making the change.
- Local mail generation: applications that invoke the local Sendmail interface can continue to hand messages to the local service, subject to the rest of the configuration.
- Queue processing: inbound daemon sockets and queue work are separate Sendmail functions. Disabling the port-25 listener is not the same as disabling all message delivery or queue runs.
- Authenticated submission: retaining port 587 on loopback supports local SMTP clients, but it does not create a public submission service. A client on another host will not reach a loopback-only binding.
- Configuration regeneration: direct edits to a generated
.cffile are fragile. Future M4 regeneration or package maintenance can overwrite them, so keep the change in the authoritative source or supported service configuration.
Port-25 shutdown checklist
- Determine whether the host uses Oracle Linux 7’s
/etc/sysconfig/sendmailswitch, an M4 source file, or another package-specific mechanism. - Back up the active startup and configuration files.
- Set
DAEMON=noon Oracle Linux 7, or remove the MTADAEMON_OPTIONSentry from the M4 source. - Add
FEATURE(`no_default_msa')if an automatically generated MSA must also be suppressed. - If local SMTP submission is required, define port 587 on
127.0.0.1and/or::1only. - Regenerate and install
sendmail.cfusing the distribution’s supported procedure. - Restart Sendmail and confirm the service is healthy.
- Use
ss -ltnpto verify that neither public IPv4 nor public IPv6 has a TCP port-25 listener. - Confirm that any remaining port-587 sockets are exactly the loopback addresses intended for local clients.
- Recheck the configuration after package upgrades or any process that regenerates Sendmail’s
.cffile.
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