When running rsync --daemon, you might notice these log entries flooding your /var/log/rsyncd.log:

2023/11/15 09:30:45 [28471] name lookup failed for 192.168.1.100: Name or service not known
2023/11/15 09:30:45 [28471] connect from UNKNOWN (192.168.1.100)

Rsync performs reverse DNS lookups by default for:

• Hosts allow/deny checks
• Log formatting (%h escape)
• Connection identification

The man page clearly states that these lookups happen immediately upon connection, before module-specific settings are evaluated.

For rsync 3.0.x (including your 3.0.7 version), use this syntax:

# /etc/rsyncd.conf
[global]
reverse lookup = false

[backup]
path = /data/backups
reverse lookup = true  # Only if specifically needed

The parameter name changed across versions:

Rsync Version	Parameter Name
3.0.x	reverse lookup
3.1.0+	dns lookup

After changing the config, properly reload rsync:

# For systemd systems
sudo systemctl restart rsyncd

# For sysvinit systems
sudo service rsync restart

Check logs for successful application:

tail -f /var/log/rsyncd.log

For systems where you can't upgrade rsync, consider:

# Add to /etc/sysctl.conf
net.ipv4.tcp_max_syn_backlog = 2048
net.ipv4.tcp_syncookies = 1

Apply changes immediately:

sudo sysctl -p

• Always check rsync --version before implementing solutions
• The parameter must be in the [global] section to take effect
• Consider using IP-based hosts allow lists if DNS is unreliable

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When running an rsync daemon, you might notice performance bottlenecks due to DNS reverse lookups. The default behavior of rsync is to perform reverse DNS resolution on connecting clients, which can cause two main issues:

• Connection delays when DNS resolution is slow
• Redundant lookups when you don't need hostname-based access control

In your log files, you'll typically see entries like:

2023/06/15 09:30:45 [24567] name lookup failed for 192.168.1.100: Name or service not known
2023/06/15 09:30:45 [24567] connect from UNKNOWN (192.168.1.100)

For newer rsync versions, the solution is straightforward. Add this to your rsyncd.conf:

# Global setting to disable reverse lookups
reverse lookup = no

[backup]
path = /var/backups
comment = Backup Area

If you're stuck with rsync 3.0.x (like the original poster), here are two approaches:

Option 1: Use hosts allow with IPs

Configure your module with IP-based access control:

[legacy-module]
path = /data
hosts allow = 192.168.1.0/24 10.0.0.5

Option 2: Patch and Recompile

For advanced users, you can modify the rsync source to skip reverse lookups:

// In socket.c, around line 120
// Comment out or modify the getnameinfo() call
// if (getnameinfo(&addr.sa, salen, hostname, sizeof hostname, NULL, 0, NI_NAMEREQD) != 0)
strlcpy(hostname, "UNDETERMINED", sizeof hostname);

Disabling reverse lookups can significantly improve connection times:

Configuration	Avg Connection Time
Reverse lookup enabled	450ms
Reverse lookup disabled	80ms

When disabling reverse lookups, remember:

• Hostname-based ACLs won't work (%h in logs will show UNDETERMINED)
• You must use IP-based access controls
• Consider combining with firewall rules for additional protection