Configure ForwardHeadersMiddleware from appsettings? - asp.net-core

Question

I'm trying to configure ASP.NET Core 5's ForwardedHeadersMiddleware from appsettings.config. I'm having trouble to set KnownProxies (IList<IPAddress> KnownProxies { get; }) and it keeps reverting back to the default value. I assume it has to do with the options machinery not knowing how to convert the string to an IPAddress, or KnownProxies only having a getter.
{
"ForwardedHeaders": {
"ForwardedHeaders": "All"
"KnownProxies": ["10.0.0.1"]
}
}
services.Configure<ForwardedHeadersOptions>(Configuration.GetSection("ForwardedHeaders"));
How can I achieve what I want, without doing the parsing manually?
Can I specify the mapping somewhere generic?
Why doesn't this throw an exception that some of my configuration could not be parsed / is invalid?

Answer 1

I may propose my recipe for this:
Define your own options class like the following:
public class ForwardedForKnownNetworks
{
public class Network
{
public string Prefix { get; set; }
public int PrefixLength { get; set; }
}
public List<Network> Networks { get; set; } = new List<Network>();
public List<string> Proxies { get; set; } = new List<string>();
}
To make your code shorter and more agile you may need kind of helper method to resolve hostnames and/or convert parse IP addressed strings:
public static class Extensions
{
public static IPAddress[] ResolveIP(this string? host)
{
return (!string.IsNullOrWhiteSpace(host))
? Dns.GetHostAddresses(host)
: new IPAddress[0];
}
}
When configuring your services, you can add something similar to the following:
var forwardedForKnownNetworks = builder.Configuration.GetSection(nameof(ForwardedForKnownNetworks)).Get<ForwardedForKnownNetworks>();
_ = builder.Services.Configure<ForwardedHeadersOptions>(options => {
options.ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto;
forwardedForKnownNetworks?.Networks?.ForEach((network) => options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse(network.Prefix), network.PrefixLength)));
forwardedForKnownNetworks?.Proxies?.ForEach((proxy) => proxy.ResolveIP().ToList().ForEach((ip) => options.KnownProxies.Add(ip)));
});
And finally your appsettings.json may appear like the following:
{
"ForwardedForKnownNetworks": {
"Networks": [
{
"Prefix": "172.16.0.0",
"PrefixLength": 12
},
{
"Prefix": "192.168.0.0",
"PrefixLength": 16
},
{
"Prefix": "10.0.0.0",
"PrefixLength": 8
}
],
"Proxies": ["123.234.32.21", "my.proxy.local"] // Here you can mention IPs and/or hostnames as the ResolveIP() helper will take care of that and resolve any hostname to its IP(s)
}
}