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If an SSH database tunnel fails, start with the exact error. “channel 0: open failed: administratively prohibited: open failed” (or the similar channel 2 message) means the SSH server rejected the forwarding request under its policy. “connect failed” or “connection refused” means the forwarding channel was accepted, but the SSH server could not connect to the database endpoint.
Those failures require different fixes. For an administrative refusal, inspect AllowTcpForwarding, PermitOpen, DisableForwarding, and any Match rules on the SSH host. For a refused connection, check the database host, listening address, port, and firewalls from the SSH host’s network position.
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Run the tunnel in verbose mode so you can see whether authentication succeeds, whether the forward request is accepted, and which destination was requested:
ssh -vvv -N -L 127.0.0.1:15432:db.internal.example:5432 user@ssh.example.comThe important distinction is the server’s response to the forwarding channel.
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| Message | What it means | Where to investigate |
|---|---|---|
channel 0: open failed: administratively prohibited: open failed |
The SSH daemon refused to create the forwarding channel because of policy. | Server-side forwarding directives and effective Match rules. |
channel 2: open failed: administratively prohibited |
The channel number differs, but the policy failure is the same. | AllowTcpForwarding, PermitOpen, DisableForwarding, and per-user rules. |
connect failed |
The forward was allowed, but the SSH host could not reach the requested database endpoint. | Database address, port, route, service listener, and firewall rules. |
connection refused through the tunnel |
The destination actively rejected the connection, usually because no service is listening on that address and port or a firewall is rejecting it. | Database listener configuration and network controls as seen from the SSH host. |
A successful SSH login does not prove that TCP forwarding is permitted. Authentication and forwarding authorization are separate decisions.
“channel 0: open failed: administratively prohibited: open failed”
This message is normally a server-side policy refusal. The SSH client reached the daemon and requested a local forward, but the daemon would not open the outbound channel. Change the policy on the SSH host, not on the laptop running ssh -L.
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In the SSH daemon configuration, AllowTcpForwarding controls TCP forwarding. The documented values are yes (the default), all, no, local, and remote.
| Value | Effect relevant to a local -L forward |
|---|---|
yes |
Allows local and remote TCP forwarding. |
all |
Allows both forwarding directions. |
local |
Allows local forwards such as -L while restricting remote forwards. |
remote |
Allows remote forwards such as -R, but blocks a local -L request. |
no |
Blocks TCP forwarding. |
For a database tunnel created with -L, the effective value must be yes, all, or local. If it is remote or no, the server can authenticate you while refusing the tunnel.
Do not assume the first occurrence in /etc/ssh/sshd_config is the value used for your session. A later Match block can apply a different value based on user, group, or source address.
Check PermitOpen
PermitOpen limits the destinations that forwarding channels may open. With no restriction, forwarding destinations are permitted. An explicit list allows only the listed host-and-port pairs, written as a hostname and port, an IPv4 address and port, or an IPv6 address in brackets followed by a port.
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PermitOpen db.internal.example:5432 10.20.0.15:3306 [2001:db8::15]:5432If your command requests db.internal.example:5432 but the effective list permits only another name, address, or port, the daemon can return an administrative prohibition. The special value any removes destination restrictions, although a narrow list is safer when the account needs access to only one database.
Check DisableForwarding
DisableForwarding is a broad switch. When enabled, it disables X11, agent, TCP, and StreamLocal forwarding and overrides the other forwarding options. Therefore, setting AllowTcpForwarding yes does not restore -L if DisableForwarding yes is effective for the account.
OpenSSH behavior is version-sensitive. In versions before 10.0, DisableForwarding did not completely match its documentation for X11 and agent forwarding; the discrepancy is tracked as CVE-2025-32728. That historical detail does not make it a valid way to permit TCP forwarding: an effective DisableForwarding yes still blocks the TCP forward. Check the manual for the OpenSSH version installed on your server before relying on version-specific defaults.
Evaluate Match blocks, not just global settings
Per-user, per-group, and per-source-address rules can override global forwarding settings. A user connecting from one network may receive a different policy than the same user connecting from another address.
Inspect the effective configuration for the actual session. A useful first check is:
sudo sshd -T | grep -i -e allowtcpforwarding -e permitopen -e disableforwardingWhen the configuration contains conditional rules, evaluate it with the connection context supported by your OpenSSH version (for example, the relevant user, source address, and destination). The result you need is the effective value for the account and connection that failed, not merely the global text at the top of the file.
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Validate and reload safely
- Edit the SSH daemon configuration on the SSH host, correcting the applicable forwarding directives.
- Check syntax before reloading:
sudo sshd -t- If the test reports no errors, reload the daemon using the service manager on that host, for example:
sudo systemctl reload sshdSome systems use a different service name or init system; use the reload operation appropriate to that installation. Keep an existing administrative session open while testing a change so a configuration mistake does not leave you without access.
- Confirm the effective values with
sshd -T, then retry the tunnel withssh -vvv.
“channel 2: open failed: administratively prohibited”
The channel number is not the diagnosis. SSH numbers channels internally, so channel 0, channel 2, or another number can report the same administrative refusal. Apply the same sequence: determine the effective AllowTcpForwarding, check whether PermitOpen includes the requested destination, and make sure DisableForwarding is not active.
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If the global configuration already appears correct, inspect every applicable Match block and any policy applied by an account-management or hosting layer. A historical report from 2009 describes this error persisting after AllowTcpForwarding yes was added; other syntax and policy conditions in the connection path were responsible. The practical lesson is to verify the effective configuration and the exact requested destination rather than stopping after one global setting.
“AllowTcpForwarding not working”: a reliable server-side checklist
- Confirm you edited the SSH daemon on the host that accepts the client connection, not the database server or your local computer.
- Run
sudo sshd -tto catch syntax errors before a reload. - Use
sudo sshd -Tto inspect effective values. - Check
AllowTcpForwardingforyes,all, orlocal. - Check
PermitOpenfor the exact destination host spelling and port used by the command. - Check that
DisableForwardingis not enabled by a global or conditional rule. - Review
Match User,Match Group, and source-address conditions that may change the result. - Reload the daemon after a successful syntax test and establish a new SSH session.
- Run the client with
-vvvand confirm that the request reaches the intended bastion or jump host.
What does not fix a local -L failure
GatewayPorts
GatewayPorts governs remote forwards created with -R, including whether a remote forward can bind beyond loopback. It does not decide whether a local -L request is allowed. Changing it will not clear an “administratively prohibited” error for a local database tunnel.
PermitTunnel
PermitTunnel controls tun(4) device forwarding for VPN-style layer 2 or layer 3 tunnels. Its default is no, but it is not the TCP forwarding control for -L. Enabling it is not the documented fix for a TCP local-forward refusal. An anecdotal Docker/OpenSSH report also found that changing AllowTcpForwarding, rather than PermitTunnel, resolved a ProxyJump-related problem. Treat that report as a troubleshooting clue, not a universal rule.
How to create and verify the local database tunnel
Use the correct ssh -L form
The general syntax is:
ssh -N -L [local_bind:]local_port:db_host:db_port user@ssh_hostFor example, this maps local port 15432 to PostgreSQL port 5432 on a host reachable from the SSH server:
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ssh -N -L 127.0.0.1:15432:db.internal.example:5432 analyst@bastion.example.com-Nrequests no remote shell; the session exists for forwarding.127.0.0.1:15432is the listening address and port on your laptop.db.internal.example:5432is the destination as seen from the SSH host.analyst@bastion.example.comidentifies the SSH account and forwarding host.
Connect your database client to 127.0.0.1 and port 15432, not directly to the private database address.
Remember whose “localhost” it is
The db_host portion is resolved and reached from the SSH server’s point of view. In:
ssh -N -L 15432:localhost:5432 user@bastion.example.comlocalhost means the bastion itself. It does not mean the laptop running the command. Use the database’s address as reachable from the bastion. If a hostname resolves unexpectedly, test the intended address explicitly; replacing a hostname with an IP address has solved similar cases, but that is an anecdotal workaround and should be validated against the server’s routing and name-resolution setup.
Account for jump hosts and bastions
With a jump host, identify which SSH daemon receives the forwarding request and which machine must reach the database. Use verbose output to confirm the chain. A successful connection to an outer gateway does not establish that the inner host can resolve or route to the database.
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These messages indicate a different stage of failure: the SSH server accepted the forwarding channel but could not establish the second TCP connection to the database. Do not keep changing forwarding policy when the channel is already being opened.
Check the destination host and port
Verify the exact db_host and db_port in the command. Confirm that the database really uses that port on the interface reachable from the SSH host. A database may listen on a private address, a Unix socket, or a different port than the one exposed to clients.
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Check the database listener from the SSH host
Run network tests from the SSH server, not only from your laptop. For a listening socket, an administrator can inspect the server with a platform-appropriate command such as:
ss -lntpThen test the destination address and port from the SSH host with an approved TCP utility available on that system. If the test fails there, the SSH tunnel cannot succeed until the database service, route, or firewall is corrected.
Check firewalls and network policy
Firewall rules must allow the SSH host’s source address to reach the database port. Cloud security groups, host firewalls, network ACLs, container networks, and database access controls can each block the second connection. A local database client’s ability to reach the bastion says nothing about the bastion’s ability to reach the database.
Interpret the common outcomes
- Immediate connection refused: the destination address was reachable, but no permitted service accepted the port, or an active rule rejected it.
- Timeout: routing or filtering is more likely; inspect firewalls, ACLs, and network paths.
- Name-resolution failure: the SSH host cannot resolve
db_host; use the correct internal DNS context or a validated address. - Forward opens but the database client fails authentication: the SSH tunnel is working; troubleshoot database credentials, TLS settings, or database authorization separately.
A complete troubleshooting workflow
- Copy the exact client error and run the command with
-vvv. - If the message says “administratively prohibited,” inspect the SSH daemon’s effective
AllowTcpForwarding,PermitOpen, andDisableForwardingvalues. - Evaluate conditional
Matchrules for the actual user and source address. - Run
sudo sshd -t, reload only after a clean result, and verify withsshd -T. - If the channel is accepted but reports “connect failed” or “connection refused,” stop changing forwarding directives.
- From the SSH host, verify DNS resolution, route, database listener address, port, and firewall access.
- Confirm that the local client uses the local bind address and port chosen in the
-Lcommand. - Retest with a new SSH session and keep the verbose log if escalation to the server or database administrator is required.
Security and least-privilege considerations
Allowing local forwarding gives an SSH account a way to reach services that the account’s network position can access. If only one database is needed, use a narrowly scoped PermitOpen entry instead of allowing every destination. Permit forwarding only for the users, groups, and source addresses that require it, and document the exception so a later hardening change does not silently break a required tunnel.
Do not enable GatewayPorts to solve a local forward, and do not enable PermitTunnel as a substitute for TCP-forwarding policy. Those directives control different features and can broaden access without addressing the actual error.
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