How to Fix Mastodon Error ‘Instance Not Reachable’ From Federation Check

When you run a federation check on your Mastodon instance, the tool may return the error “Instance Not Reachable” for another server. This error means your instance cannot establish a connection to that remote server. The cause is usually a firewall rule, a DNS misconfiguration, or the remote instance blocking your IP address. This article explains the technical reasons behind the error and provides step-by-step fixes you can apply.

Why the Federation Check Returns “Instance Not Reachable”

The federation check in Mastodon tests whether your instance can connect to a remote instance over HTTPS. The check sends a simple HTTP GET request to the remote instance’s API endpoint /api/v1/instance. If the remote server does not respond within a timeout period, Mastodon marks it as “Not Reachable.”

There are three common technical reasons for this failure:

Firewall or Network Filter Blocks Outbound Traffic

Your server’s firewall, a cloud provider security group, or a corporate network filter may block outbound HTTPS traffic (port 443) to the remote IP address. This is the most frequent cause. The federation check never reaches the remote instance, so it times out.

DNS Resolution Fails for the Remote Domain

If your server cannot resolve the remote instance’s domain name to an IP address, the connection cannot be opened. This can happen if the remote domain has expired, has no A or AAAA record, or if your DNS resolver is misconfigured.

Remote Instance Blocks Your IP Address

The remote instance may have implemented a blocklist that includes your IP address range. This is common when your instance has been flagged for spam or abuse. The remote server drops the connection before responding.

Steps to Diagnose and Fix the Error

Follow these steps in order. Each step isolates a different potential cause.

1. Run the federation check for the exact domain
   In your Mastodon admin panel, go to Preferences > Moderation > Federation. In the search box, type the remote instance domain exactly (for example, mastodon.social). Click the Check button. Note the error message. If it says “Instance Not Reachable,” proceed to the next step.
2. Test DNS resolution from your server
   Open a terminal on your Mastodon server. Run nslookup example.com or dig example.com where example.com is the remote instance domain. If the command returns no IP address or a SERVFAIL error, the domain cannot be resolved. Wait 24 hours for DNS propagation, or contact the remote instance admin to confirm the domain is active.
3. Check outbound connectivity to the remote IP
   Use the IP address from the DNS lookup. Run curl -I https://IP_ADDRESS from your server. If the command hangs or returns “Connection timed out,” your firewall or network is blocking outbound HTTPS traffic. Review your firewall rules (iptables, ufw, or cloud provider security group) and allow outbound TCP on port 443 to all destinations.
4. Verify the remote instance is not blocking your IP
   From a different network (for example, your home computer), run curl -I https://example.com. If it succeeds from the other network but fails from your Mastodon server, the remote instance may have blocked your server IP. Contact the remote instance admin to request removal from their blocklist.
5. Test HTTPS certificate validity
   A valid TLS certificate is required for the connection. Run openssl s_client -connect example.com:443 -servername example.com. If the output shows “Verify return code: 0 (ok)”, the certificate is valid. Any other return code indicates a certificate issue on the remote instance. Wait for the remote admin to fix it, or add the domain to your instance’s allowed domains temporarily.
6. Restart the Mastodon sidekiq and web services
   After applying any network or DNS fix, restart Mastodon services to clear cached connection states. Run systemctl restart mastodon-sidekiq mastodon-web from your server. Wait 30 seconds, then run the federation check again.

If Mastodon Still Shows the Error After the Main Fix

Federation Check Works Intermittently

If the check succeeds sometimes and fails other times, the remote instance may be rate-limiting your requests. Wait at least 5 minutes between checks. Rate limits are usually temporary and reset after a cooldown period.

Multiple Instances Show the Same Error

If many remote instances are unreachable, the problem is likely on your side. Review your server’s firewall rules and ensure outbound HTTPS is not restricted by an upstream provider or a corporate proxy. Also check if your Mastodon instance is running behind a reverse proxy that is misconfigured.

Error Persists After All Steps

If the error remains after DNS, firewall, and certificate checks, the remote instance may be offline entirely. Use a third-party monitoring tool like UpDown.io to check the remote domain’s availability. If the remote instance is down, wait for its admin to restore service.

Item	Local Network Issue	Remote Instance Issue
DNS resolution	Server DNS resolver misconfigured	Remote domain expired or missing A record
Firewall	Outbound port 443 blocked	Remote instance blocks your IP
Certificate	N/A	Expired or invalid TLS certificate
Service status	Mastodon sidekiq or web not restarted	Remote instance offline or overloaded

The federation check is a critical tool for maintaining a healthy Mastodon instance. By following the steps above, you can identify whether the problem is on your network or on the remote side. After fixing the issue, run the check again to confirm the connection succeeds. For persistent cases, consider adding the remote domain to your instance’s whitelist as a temporary workaround while you investigate further.