When configuring PostgreSQL for remote access, many administrators encounter the frustrating error:

LOG:  invalid IP mask \"trust\": Name or service not known
CONTEXT:  line 83 of configuration file \"/usr/local/pgsql-9.0.2/data/pg_hba.conf\"
FATAL:  could not load pg_hba.conf

This occurs because the pg_hba.conf file expects a specific format for host-based authentication entries, and "trust" is being incorrectly interpreted as part of the IP address specification.

The proper format for a host entry in pg_hba.conf should be:

# TYPE  DATABASE  USER  ADDRESS  METHOD
host    all       all   192.168.1.100/32  trust

Notice two critical differences from the problematic configuration:

1. The IP address is followed by a CIDR mask (like /32 for single host)
2. The authentication method (trust) is properly separated from the IP specification

In your case, the line:

host    all         all         **.**.***.*         trust

Should be modified to (replace with your actual IP):

host    all         all         192.168.1.100/32    trust

Some variations that would also work:

# For an entire subnet
host    all         all         192.168.1.0/24      md5

# For a specific user and database
host    salesdb     salesuser   203.0.113.45/32     scram-sha-256

After fixing pg_hba.conf, ensure these configurations are set in postgresql.conf:

listen_addresses = '*'
port = 5432

Remember to reload PostgreSQL after changes:

# For systemd systems
sudo systemctl reload postgresql

# For traditional init systems
pg_ctl reload -D /usr/local/pgsql-9.0.2/data/

While "trust" authentication works for testing, in production you should use more secure methods:

# Recommended production setting
host    all         all         192.168.1.100/32    scram-sha-256

Always test connectivity after changes:

psql -h your_server_ip -U username -d dbname

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When working with PostgreSQL 9.0.2 on Slackware 13.1x64, I encountered a critical authentication error when trying to configure remote access:

LOG:  invalid IP mask \"trust\": Name or service not known
CONTEXT:  line 83 of configuration file \"/usr/local/pgsql-9.0.2/data/pg_hba.conf\"
FATAL:  could not load pg_hba.conf

The issue stems from a common misunderstanding of the pg_hba.conf file structure. The correct format for host-based authentication is:

host    DATABASE    USER    ADDRESS    METHOD

The problematic line in my configuration was:

host    all         all         **.**.***.*         trust

Here's what's wrong:
1. The IP address (**.**.***.*) is incomplete without its CIDR mask
2. The authentication method (trust) is being incorrectly interpreted as part of the IP address specification

For single IP access with trust authentication:

host    all    all    192.168.1.100/32    trust

For an entire subnet with md5 authentication:

host    all    all    192.168.1.0/24    md5

Here are frequently used CIDR mask values:

/32 - Single host (exact IP)
/24 - Class C network (254 hosts)
/16 - Class B network (65,534 hosts)
/8  - Class A network (16,777,214 hosts)

After making changes, always reload PostgreSQL:

# For systemd systems:
sudo systemctl reload postgresql

# For init.d systems:
sudo /etc/init.d/postgresql reload

If you still face issues:

1. Check PostgreSQL logs: /var/log/postgresql/postgresql-9.0-main.log
2. Verify file permissions: chmod 600 pg_hba.conf
3. Test connectivity: psql -h hostname -U username -d dbname

For more complex setups using IPv6:

host    all    all    ::1/128    trust
hostssl all    all    ::/0       md5

While 'trust' authentication is simple, consider these more secure alternatives:

md5 - Password authentication
scram-sha-256 - Strong encryption (PostgreSQL 10+)
cert - SSL certificate authentication