When creating user accounts on Linux systems, you'll encounter restrictions defined by the NAME_REGEX configuration. The default regex pattern ^[a-z][-a-z0-9]*\$ enforces:

1. Must start with lowercase letter [a-z]
2. Can contain only lowercase letters, numbers, and hyphens [-a-z0-9]
3. No uppercase letters, underscores, or dots permitted

While useradd may accept non-compliant usernames, various system components expect POSIX-compliant names:

• Dot (.): Causes issues with configuration files (e.g., .bashrc) and path resolution
• Underscore (_): May conflict with environment variable naming conventions
• Mixed case: Breaks scripts assuming lowercase-only usernames

For domain-based usernames, consider these transformation approaches:

# Python conversion example
def domain_to_username(domain):
    return domain.replace('.', '-').replace('_', '-').lower()

# Example conversions:
domain_to_username("example.com")  # → "example-com"
domain_to_username("api_v2.test") # → "api-v2-test"

If you must support additional characters, edit /etc/adduser.conf:

# Extended pattern allowing dots and underscores
NAME_REGEX="^[a-z][-a-z0-9._]*\$"

# Verify with:
sudo adduser --force-badname test.user

Important caveats when modifying:

• Test all automation scripts that process usernames
• Ensure compatibility with NFS, Samba, and LDAP if used
• Document the non-standard convention for other admins

These areas may require additional configuration:

# Samba example for dotted usernames
[global]
username map = /etc/samba/usermap

# /etc/samba/usermap content:
example.user = EXAMPLEUSER

Remember that while technically possible to use special characters, maintaining compatibility with POSIX standards prevents countless subtle issues across the Linux ecosystem.

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When creating user accounts on Linux systems, particularly when naming accounts after domain names, you'll encounter strict character restrictions. The default NAME_REGEX pattern in most Linux distributions is:

NAME_REGEX="^[a-z][-a-z0-9_]*\$"

This means usernames must:

• Start with a lowercase letter (a-z)
• Contain only lowercase letters, digits (0-9), hyphens (-), and underscores (_)
• Not begin or end with special characters

Certain characters can cause significant issues in usernames:

# These will cause problems:
@, ., /, \, :, *, ?, ", ', <, >, |, $, %, #, !, ~, , +, =, {, }, [, ], (, ), &, ;, space

Specific issues with common domain-related characters:

• Dots (.): Can interfere with path resolution and email handling
• Hyphens (-): Generally safe but can cause issues if at start/end
• Underscores (_): Mostly safe but may conflict with some scripts

To allow domain-style usernames while maintaining security, you could modify NAME_REGEX in /etc/adduser.conf:

# Example modified regex for domain-style names
NAME_REGEX="^[a-z][-a-z0-9_.]*\$"

Important considerations when modifying:

• Never allow spaces or shell metacharacters
• Avoid allowing usernames to start/end with special chars
• Test thoroughly with your specific applications

For domain-based usernames, consider these transformations:

Original domain: example.com
Bad username: example.com
Good alternatives:
- example_com
- example-com
- example.com (only if regex modified and tested)

Loose username restrictions can lead to:

• Shell injection vulnerabilities
• Path traversal issues
• Script parsing problems
• UID/GID resolution conflicts

Best practice is to stick with the default regex unless you have specific, tested requirements for alternative formats.

If you must use non-standard characters, bypass adduser checks:

sudo useradd --force-badname 'user.name'

Or modify the regex temporarily:

sudo NAME_REGEX="^[a-z][-a-z0-9_.]*\$" adduser example.com