When migrating applications or restructuring endpoints, we often need to modify URL paths while preserving the original query parameters. The standard approach using path_dir and set-path might not work as expected due to HAProxy's path processing behavior.

Here's what typically doesn't work:

acl old_name path_dir -i /old_name
http-request set-path /new_name/%[query] if old_name

This would transform /old_name/Default.aspx?Id=123 into /new_name/?Id=123, losing the actual file path component.

We need to capture both the base path and subsequent path components. Here's the correct approach:

acl is_old_path path_beg -i /old_name/
http-request set-path /new_name%[path,regsub(^/old_name/,/)] if is_old_path

This solution:

• Uses path_beg for more precise matching
• Preserves the query parameters automatically
• Maintains the file path after the base directory

For a production-ready setup:

frontend http_in
    bind *:80
    acl is_old_path path_beg -i /old_name/
    http-request redirect code 301 location /new_name%[path,regsub(^/old_name/,/)] unless { path_beg -i /new_name/ }
    http-request set-path /new_name%[path,regsub(^/old_name/,/)] if is_old_path
    use_backend app_servers

backend app_servers
    server s1 192.168.1.10:8080 check

For complex rewrites with multiple path components:

http-request set-path /new_name/v2%[path,regsub(^/old_name/v1/,/)] if { path_beg -i /old_name/v1/ }

This handles versioned API endpoints while maintaining all URL components.

Always verify your configuration with:

haproxy -c -f /etc/haproxy/haproxy.cfg

And test with curl:

curl -v "http://localhost/old_name/Default.aspx?Id=123"

---

When migrating applications with renamed endpoints in HAProxy, we often need to modify just part of the URL path while preserving all query parameters. A common case is when an application changes its base path from /old_name to /new_name but maintains the same file structure and query parameters.

The configuration you tried:

acl old_name path_dir -i /old_name
http-request set-path /new_name/%[query] if old_name

has several issues:

• %[query] only includes the query string (everything after ?)
• It completely replaces the path instead of modifying part of it
• The original path components after /old_name/ are lost

Here's the correct approach using HAProxy 1.8+ syntax:

acl is_old_path path_beg -i /old_name/
http-request replace-path /old_name/(.*) /new_name/\1 if is_old_path

For HAProxy 2.0+, you can use more advanced regex:

http-request replace-path ^/old_name(/.*)?$ /new_name\1 if { path_beg /old_name/ }

Here's a full frontend configuration that handles the redirection:

frontend http-in
    bind *:80
    acl is_old_path path_beg -i /old_name/
    
    # Option 1: Simple path replacement
    http-request replace-path /old_name/(.*) /new_name/\1 if is_old_path
    
    # Option 2: Alternative using regex capture (HAProxy 2.2+)
    # http-request replace-path ^/old_name(/.*)?$ /new_name\1 if is_old_path
    
    # Send to backend
    use_backend app_servers if { path_beg /new_name/ }
    default_backend app_servers

The above configuration will transform:

http://www.site.com/old_name/Default.aspx?Id=123

into:

http://www.site.com/new_name/Default.aspx?Id=123

while preserving all path components after /old_name/ and all query parameters.

For more complex scenarios:

1. Multiple path changes: Chain multiple replace-path rules
2. Host-based routing: Combine with req.hdr(host) conditions
3. 301 Redirects: Use 'redirect' instead for permanent changes