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What you are configuring
A gateway has at least two network interfaces. Clients on a private LAN send packets to the CentOS host, which routes them out through an external interface. Three independent controls must agree:
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- Firewall forwarding: the active firewall must permit the specific traffic in the
FORWARDchain or through its zone policy. - NAT masquerading: packets leaving the external interface need their private source address rewritten to the gateway’s outward-facing address.
Enabling only one or two of these controls does not create a working gateway. Forwarding is disabled by default, and firewall policy can still reject packets after the kernel has been instructed to route them.
Check the topology before changing anything
The example names eth0 and eth1 are placeholders. CentOS 7 installations may use predictable names such as enp0s3 or ens33. Identify the real interfaces and routes first.
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- List interfaces and their addresses:
ip -br address - Display the routing table and default route:
ip route - Confirm which interface owns the default route. That is normally the external interface used with
-oin the NAT rule. - Identify the internal interface or interfaces and the client subnet they serve. For example, an internal network might be
192.168.10.0/24; use your actual subnet instead. - Check which firewall service is active before choosing commands:
systemctl is-active firewalldsystemctl is-active iptables
Run configuration commands as root or through sudo. Record the current firewall rules and sysctl value so you can compare them after the change or roll back deliberately.
Enable IPv4 forwarding immediately
Set the running kernel value with:
sysctl -w net.ipv4.ip_forward=1The Linux kernel documents net.ipv4.ip_forward as the boolean that forwards packets between interfaces: 0 disables forwarding and 1 enables it. The documented default is 0.
Verify the live value:
sysctl net.ipv4.ip_forward
cat /proc/sys/net/ipv4/ip_forwardBoth checks should report 1. This change affects the running system only; a reboot or a later sysctl reload can return it to the configured default unless you persist it in a configuration file.
Make forwarding survive a reboot
Edit /etc/sysctl.conf and add or change this line:
net.ipv4.ip_forward = 1Then load the file immediately:
sysctl -p /etc/sysctl.confsysctl -p is equivalent when the default file is being used. The command prints the values it loads; check that the forwarding assignment is accepted, then confirm with sysctl net.ipv4.ip_forward.
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On systems managed with drop-in files, the same setting can be kept in a file under /etc/sysctl.d/ instead of the main file. Use one deliberate source of truth, document it for the next administrator, and reload the file you changed. A persistent setting controls the kernel; it does not by itself open the firewall or add NAT.
Allow forwarding with iptables
If the host uses direct iptables administration, add rules to the filter table’s FORWARD chain. Red Hat’s basic pattern accepts traffic entering from the internal interface and leaving through it on the return path:
iptables -A FORWARD -i eth1 -j ACCEPT
iptables -A FORWARD -o eth1 -j ACCEPTReplace eth1 with the internal interface. These broad rules are easy to understand but may permit more traffic than your gateway should carry. They also assume that the rest of your policy is appropriate. Review the current chain first:
iptables -L FORWARD -v -n --line-numbersUse a narrower policy when the gateway has a defined LAN
For a single private subnet, constrain forwarding by interface, source network, destination interface, and connection state. The following pattern allows new connections from the LAN to the external interface and permits replies belonging to established connections:
iptables -A FORWARD -i eth1 -o eth0 -s 192.168.10.0/24 -m conntrack --ctstate NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth0 -o eth1 -d 192.168.10.0/24 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPTChange the subnet and interfaces to match your topology. If you have multiple internal interfaces, create a rule for each intended path rather than accepting every interface indiscriminately. If the chain has an earlier DROP or REJECT, rules appended with -A may never be reached; insert or reorder rules only after examining the existing policy and your change-control requirements.
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Do not confuse forwarding with local traffic
The FORWARD chain handles packets routed through the machine. Rules in INPUT and OUTPUT govern traffic addressed to, or generated by, the gateway itself. Opening an SSH or web service on the gateway does not enable transit traffic, and enabling transit traffic does not automatically expose a service listening on the gateway.
Add IPv4 masquerading on the external interface
Append a NAT rule to the POSTROUTING chain:
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADEReplace eth0 with the real outward-facing interface. The command components mean:
| Part | Purpose |
|---|---|
-t nat |
Selects the NAT table. |
-A POSTROUTING |
Appends a rule that runs after the routing decision, as packets are about to leave. |
-o eth0 |
Matches packets whose output interface is the external interface. |
-j MASQUERADE |
Rewrites the private source address to the gateway’s address on that external interface. |
Masquerading is what lets private client addresses communicate with public networks while presenting the gateway’s outward-facing address. It does not decide whether forwarding is allowed; the filter rules still have to accept the traffic.
Inspect the loaded NAT rule and its packet counters:
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Unlike the sysctl value in /etc/sysctl.conf, an iptables rule added at the command line is a running-kernel rule. Plan how your CentOS 7 host restores firewall policy after a reboot. On an iptables-managed installation, the traditional CentOS service workflow is:
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service iptables saveThat saves the currently loaded rules in the service’s configuration location. Confirm that your host actually uses the iptables service and that your change-management process permits this operation. If another firewall manager restores rules at boot, use that manager’s persistence mechanism instead of maintaining two independent copies.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Firewalld-managed hosts: use zones, not parallel iptables administration
CentOS 7 commonly runs firewalld rather than a hand-maintained iptables service. Check with systemctl is-active firewalld. When firewalld is active, make the forwarding sysctl persistent as a drop-in under /etc/sysctl.d/ (or in /etc/sysctl.conf), reload the sysctl configuration, and express interface and NAT policy through firewalld zones.
Assign the internal and external interfaces to the appropriate zones, then enable masquerading on the external zone. A typical command shape is:
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firewall-cmd --permanent --zone=internal --add-interface=eth1
firewall-cmd --permanent --zone=external --add-interface=eth0
firewall-cmd --permanent --zone=external --add-masquerade
firewall-cmd --reloadUse the zones and interface names that match your design. Zone assignment alone is not a substitute for a forwarding policy: ensure the active zone configuration permits the intended inter-interface traffic, and limit sources and destinations where your policy requires it. Inspect the result with:
firewall-cmd --get-active-zones
firewall-cmd --zone=external --query-masquerade
firewall-cmd --zone=external --list-all
firewall-cmd --zone=internal --list-allDo not independently append unmanaged iptables rules while firewalld is controlling the host. Choose one policy owner, because competing services can overwrite rules, produce misleading tests, or leave NAT and forwarding inconsistent after a reload.
Verify the gateway in three layers
1. Confirm the kernel switch
sysctl net.ipv4.ip_forwardThe result must be net.ipv4.ip_forward = 1. If it is 0, reload the file containing the setting and look for syntax errors or a later configuration file overriding it.
2. Confirm filter and NAT rules
For iptables, check both the forwarding chain and the NAT chain:
iptables -L FORWARD -v -n --line-numbers
iptables -t nat -L POSTROUTING -v -n --line-numbersPacket and byte counters should increase when an allowed client generates traffic. A zero counter usually means the packet is taking a different interface path, is being rejected earlier, or the client is not using this machine as its gateway.
3. Test from an actual client
Configure a permitted LAN client to use the CentOS host as its default gateway, then test a destination outside the private network. Test an IP address first to separate routing and NAT problems from DNS problems. Repeat while watching the firewall counters. If the client can reach the gateway but not an external address, inspect the FORWARD policy and masquerade rule; if counters remain at zero, revisit interface names, routes, and the client’s default gateway.
Troubleshoot common failures
| Symptom | Likely cause | What to check |
|---|---|---|
| Forwarding value is 0 | The setting was applied only temporarily, the wrong file was edited, or another sysctl file overrides it. | Run sysctl -p /etc/sysctl.conf, inspect the exact file containing net.ipv4.ip_forward = 1, and verify again. |
| Client reaches the gateway but not the Internet | FORWARD traffic is blocked or the NAT rule uses the wrong output interface. | Inspect iptables -L FORWARD -v -n, ip route, and iptables -t nat -L POSTROUTING -v -n. |
| NAT counters stay at zero | The rule’s -o interface does not match the route actually used. |
Compare the default route’s device with the interface named after -o. |
| Only some connections work | Rules are too broad in one direction, omit return traffic, or are shadowed by an earlier rule. | Review rule order and connection-state matches; permit established and related replies on the return path. |
| Rules disappear after reboot or reload | Runtime iptables commands were never saved, or firewalld replaced unmanaged rules. | Identify the active firewall service and persist policy through that service only. |
| Firewalld commands appear ineffective | The interface is in an unexpected zone or the zone policy does not allow forwarding. | Run firewall-cmd --get-active-zones, inspect each zone, and correct assignments and forwarding policy. |
Limit the gateway to the traffic you intend to route
- Prefer interface and source-subnet matches over an unrestricted
FORWARDaccept rule. - Allow return traffic with connection-state rules rather than opening every direction equally.
- Use the actual external interface in the MASQUERADE rule; do not leave the example name unchanged.
- Review counters after deployment and during incident response. They show whether packets reached the rule, not whether the remote service replied.
- Document the internal subnet, interface roles, firewall owner, sysctl file, and rollback procedure.
To remove a test iptables rule, repeat its command with -D instead of -A, or delete it by the line number shown by --line-numbers. Remove the corresponding NAT rule separately; filter and NAT tables are independent.
CentOS 7 lifecycle consideration
CentOS 7 is end-of-life. It no longer receives the normal maintenance and security coverage expected of a supported server platform. Keep any remaining gateway tightly controlled, record the legacy configuration, and plan a migration to a maintained distribution. Recreate and test the forwarding, firewall, and NAT policy on the target system rather than assuming that interface names, firewall defaults, or service ownership will be identical.
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Quick Recap
Quick deployment checklist
- Identify the internal and external interfaces with
ip -br addressandip route. - Check whether iptables or firewalld owns the firewall.
- Run
sysctl -w net.ipv4.ip_forward=1. - Set
net.ipv4.ip_forward = 1in/etc/sysctl.confor a documented/etc/sysctl.d/drop-in, then reload it. - Permit only the required internal-to-external and established return traffic.
- Add MASQUERADE to
POSTROUTINGon the real external interface, or enable masquerading in the external firewalld zone. - Persist the policy through the firewall service that is actually active.
- Verify sysctl output, rule counters, and an end-to-end test from an authorized client.
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