The most direct OpenSSH allowlist is AllowUsers user@address in /etc/ssh/sshd_config. For example, AllowUsers [email protected] permits the admin account only when the connection comes from that public IPv4 address. Use CIDR notation, such as [email protected]/24, for a trusted network.
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For stronger defense, enforce the same source restriction at a firewall layer: UFW on the VPS or a provider network firewall. Keep an existing session or console open, validate the configuration with sudo sshd -t, test a new allowed connection, and only then remove broad SSH rules.
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What an IP-restricted SSH policy does
SSH access has two separate decisions: whether packets can reach the SSH service and whether the SSH daemon will permit a particular account to authenticate. An IP allowlist addresses the source of the connection; it does not replace passwords, keys, multifactor authentication, least-privilege accounts or account-level restrictions.
OpenSSH can match both the login name and the source address. A host firewall can reject unwanted TCP packets before they reach authentication. A VPS provider’s network firewall can apply a similar rule upstream of the virtual machine. These layers solve related but different problems and can be combined.
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- OpenSSH: precise per-user and per-source authorization after a connection reaches
sshd. - Host firewall: blocks unwanted SSH packets before authentication and can reduce log noise.
- Provider firewall: blocks traffic outside the VPS, when the provider offers that feature.
The right policy depends on whether you have one fixed office address, a changing residential address, a VPN exit address, a whole subnet, IPv6 enabled, or several administrators who need different accounts.
Use AllowUsers for a direct user-and-address allowlist
The OpenSSH sshd_config(5) manual defines the AllowUsers syntax as a username followed by a host pattern. The host portion can use an address in CIDR address/mask-length format.
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AllowUsers [email protected]This rule permits only the admin account from 203.0.113.10. Replace the example address with the public source address that the VPS actually sees. An address displayed inside a workstation, private LAN, or VPN tunnel may not be the address presented to the server.
Allow one account from an IPv4 subnet
AllowUsers [email protected]/24A /24 covers the network from 203.0.113.0 through 203.0.113.255. Use the narrowest prefix that includes every legitimate source. A wider prefix is easier to maintain but authorizes more addresses.
Allow multiple accounts or sources
Place multiple AllowUsers entries in the configuration. For example:
AllowUsers [email protected]
AllowUsers [email protected]/24
AllowUsers admin@2001:db8:1234::/48Keep the account and address relationship explicit. Do not assume that allowing one account from an address automatically allows every account from that address.
How AllowUsers interacts with deny rules
When AllowUsers is present, only users matching an entry are permitted. OpenSSH evaluates DenyUsers before AllowUsers, so a matching deny rule still wins. Review both directives, along with any AllowGroups or DenyGroups settings, before deciding that a new allowlist entry should work.
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The username must be the actual account name. Numeric user IDs are not accepted as substitutes for usernames in this rule.
Use Match Address when the policy is conditional
A Match Address block changes selected SSH options only when the source address matches. OpenSSH supports IPv4 and IPv6 CIDR criteria, including formats such as 192.0.2.0/24 and 2001:db8::/32. The mask length must be valid for the address family and network prefix.
Change an authentication setting for one source
Match User admin Address 203.0.113.10
PasswordAuthentication noThis illustrative block disables password authentication for admin when the connection originates at the specified address. Choose the options that match your authentication policy; the address-matching mechanism does not prescribe a universal hardening profile.
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Match criteria can express policies that are awkward to represent as a single global allowlist. You can target a user or group and then match an IPv4 or IPv6 network. This is useful when administrators use keys from a managed office range while a deployment account is allowed only from a separate automation network.
Read the rest of the configuration carefully after a Match block. Options remain in the conditional context until another Match All or a new matching structure resets the scope. Keep blocks near related settings and comment the intended source network so a later edit does not accidentally broaden access.
Restrict packets with UFW on the VPS
Ubuntu describes UFW as a user-friendly frontend for the host firewall. A source-specific IPv4 rule for the standard SSH port is:
sudo ufw allow proto tcp from 192.168.0.2 to any port 22Replace 192.168.0.2 with the public address of the approved administrator. To allow an entire subnet, use its CIDR prefix:
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sudo ufw allow proto tcp from 192.168.0.0/24 to any port 22These examples use port 22. If SSH listens on another port, substitute that port number. A firewall rule for port 22 does not protect a daemon listening elsewhere.
Apply IPv6 deliberately
An IPv4 rule does not express an IPv6 policy. If the VPS accepts SSH over IPv6, add and audit an equivalent IPv6 source rule. Decide whether the administrator should be allowed from one IPv6 address, a delegated prefix, or no IPv6 address at all. Leaving IPv6 unrestricted while restricting IPv4 creates an unexpected alternate path.
Insert the narrow rule before deleting a broad rule
If UFW currently permits SSH from anywhere, add the source-specific allow rule first. Confirm that it appears in the active rules, test a new session, and only then remove the broad rule. Reversing that order can terminate your only remote path.
Limit which accounts may use SSH with AllowGroups
Source filtering and account authorization are complementary. Ubuntu documents an sshlogin group approach:
AllowGroups sshloginCreate or use the intended group, add each permitted account, and then restart the SSH service according to the distribution. On Ubuntu, the documented example uses:
sudo systemctl restart ssh.serviceA group rule prevents an otherwise valid account from using SSH, regardless of its source address. You can combine it with AllowUsers or a Match Address policy so that both the account and network must be approved.
Choose the enforcement layer
| Approach | Enforcement layer | Best scope | Strength | Important limitation |
|---|---|---|---|---|
AllowUsers |
OpenSSH authorization | Specific account plus host or CIDR | Expresses exactly who may log in from which source | Packets still reach the daemon and may appear in SSH logs |
Match Address |
OpenSSH conditional configuration | Different settings by user, group or IPv4/IPv6 source | Supports conditional authentication policies | Configuration scope and option interactions require careful review |
| UFW source rule | Host firewall | All SSH users from one host or subnet | Rejects unwanted traffic before authentication | Syntax and service names vary by distribution; IPv6 needs separate attention |
| Provider network firewall | Network edge outside the VPS | All traffic to the VPS, often by source and port | Can block traffic before it reaches the virtual machine | Features, IPv6 behavior, logging and pricing are provider-specific |
AllowGroups |
OpenSSH account authorization | Approved set of accounts | Centralizes which users may use SSH | Does not restrict source addresses by itself |
A static office address can usually use a narrow OpenSSH rule plus a matching firewall rule. A changing residential address may be better served by a stable VPN exit, bastion or managed provider rule, provided that the alternate path is maintained and monitored.
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Safe procedure for changing a live VPS
- Preserve recovery access. Keep an existing SSH session open. Also identify a VPS console, out-of-band recovery method, bastion or trusted VPN path that does not depend on the rule you are changing.
- Determine the real source addresses. Record the public IPv4 and, if used, IPv6 addresses visible to the VPS. Do not copy a private LAN address or assume that the address shown before a VPN connection is the one the server receives.
- Write the narrowest policy. Add the exact
AllowUsers user@address, a valid CIDR range, or aMatch User ... Address ...block. If using UFW, add the source-specific allow rule before removing any broad SSH rule. - Check the daemon configuration. Run the distribution’s syntax check before reloading:
sudo sshd -tNo output normally indicates that the syntax check passed. A reported error identifies a line that must be corrected before applying the change. A syntax check does not prove that the chosen address is correct or that another rule will not override the intended result.
- Inspect included configuration. Many distributions load additional files from
/etc/ssh/sshd_config.d/. Look for later or conflictingAllowUsers,DenyUsers,AllowGroups,Matchand authentication settings instead of assuming the main file is the entire policy. - Reload or restart SSH. Use the service operation appropriate to the distribution. Ubuntu’s group example uses
sudo systemctl restart ssh.service. A reload can apply a valid configuration without dropping established sessions, but service behavior varies, so keep the original session open. - Test an allowed connection. Open a separate terminal and connect using the intended account from the approved address. Confirm that key or password authentication behaves as expected.
- Test a denied source. From a deliberately unapproved network, verify that the connection is rejected. If you cannot safely test from a second network, review the effective firewall and daemon rules and retain console access.
- Remove broad access last. Once both tests succeed, delete or narrow any rule that permits SSH from all addresses. Record the final policy, the approved source ranges and the recovery procedure.
IPv4, IPv6 and CIDR details that commonly cause mistakes
Use the address family the server actually accepts
If DNS or client preferences select IPv6 first, an administrator may appear unable to connect even though the IPv4 allowlist is correct. Conversely, a host restricted only in IPv4 may remain reachable through an unrestricted IPv6 listener. Audit listening addresses, firewall rules and OpenSSH criteria for both families.
Keep network prefixes valid
OpenSSH requires a mask length consistent with the address. A malformed network prefix is an error, not a permissive fallback. Use a host address with the appropriate host prefix when you mean one address, and use a network address with the intended prefix when you mean a subnet.
Understand NAT and VPN exits
Several workstations behind one public NAT address all appear to SSH from that address. Conversely, a VPN can make every administrator appear to come from one exit address. Whitelist the address presented by the VPS, then control membership and authentication inside that network. If an ISP changes a residential address, a fixed /32 entry will eventually stop matching.
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Common failure modes and recovery
The rule was added to the wrong effective file
Included files under /etc/ssh/sshd_config.d/ may contribute to the active configuration. Search the main file and included directory for every allow and deny directive. A later edit, an existing deny rule or a conditional block may explain behavior that is not visible near the line you changed.
The administrator’s address changed
Dynamic ISP addresses, VPN reconnects and cellular networks can change the source address without warning. Keep console access, use a stable VPN exit or bastion, or update the allowlist through a controlled management path. Do not respond by opening SSH to every address permanently.
IPv6 remains open
Check whether the daemon listens on an IPv6 address and whether the host or provider firewall has an IPv6 rule. Add the intended IPv6 CIDR rule or disable the unused exposure according to the VPS operating system’s documented procedure.
A CIDR prefix is malformed
Check the address family, network boundary and mask length. Correct the prefix, run sudo sshd -t again, and only then reload the service.
The account name does not match
AllowUsers matches usernames and source hosts. Confirm the account spelling and ensure that it is not excluded by DenyUsers or group restrictions. Numeric user IDs are not valid replacements for account names in this directive.
A hostname assumption produces unexpected results
For predictable control, prefer explicit addresses or CIDR ranges. With UseDNS no, OpenSSH uses addresses rather than host names for the from patterns in authorized_keys and for Match Host rules. Reverse DNS is not a substitute for an IP allowlist.
The firewall caused a lockout
Do not remove a broad port-22 rule first. Use the preserved SSH session or VPS console to restore access, insert the correct source rule, validate it, and then narrow the policy again. If no recovery path exists, contact the VPS provider’s documented console or rescue service.
Recommended policy patterns
One administrator with a fixed public address
Use AllowUsers [email protected], add a matching TCP source rule at the host or provider firewall, and include the administrator’s IPv6 address only if IPv6 is intentionally used. This gives precise account-and-source control.
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Use a subnet CIDR only when every address in that range is trusted. Pair the network restriction with AllowGroups sshlogin so that only designated accounts can authenticate. A subnet allowlist should not be treated as proof that every device on the subnet is secure.
Administrators with changing home addresses
A permanent single-address rule is brittle. Route administration through a stable VPN exit, bastion or managed provider rule, then allow only that stable source to reach SSH. Keep individual accounts and keys restricted even when they share the same exit address.
Separate deployment automation from human administration
Give automation its own account and source network, then express the relationship with separate AllowUsers entries or conditional blocks. Avoid granting a deployment account the same interactive privileges as an administrator merely because both originate from an approved network.
What to monitor after the change
- SSH daemon authentication logs for rejected users, source addresses and repeated attempts.
- Host-firewall logs showing blocked packets and unexpected IPv4 or IPv6 sources.
- Provider firewall flow logs, when available, for traffic rejected before it reaches the VPS.
- Changes to the administrator’s public address, VPN exit or office network prefix.
- Membership changes in the group allowed by
AllowGroups.
Review logs after the first change and after any network migration. Observability differs by layer: daemon logs explain account decisions, while firewall and provider logs explain packet filtering.
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Practical checklist
- Identify the VPS-visible public IPv4 and IPv6 source addresses.
- Choose the smallest host or subnet range that meets the access requirement.
- Use
AllowUsersfor a direct user-and-source rule. - Use
Match Addresswhen settings vary by source, user or group. - Use
AllowGroupsto limit the set of SSH-capable accounts. - Add UFW or provider firewall rules at the correct port and address family.
- Inspect included SSH configuration files and existing deny rules.
- Run
sudo sshd -tbefore applying changes. - Keep an existing session or console path open.
- Test one allowed and one denied source before removing broad access.
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