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ABC Transporters in Neurological Disorders: An Important Gateway for Botanical Compounds Mediated Neuro-Therapeutics

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Review

. 2019;19(10):795-811.

doi: 10.2174/1568026619666190412121811.

Niraj Kumar Jha 
¹
, Rohan Kar 
¹
, Rituraj Niranjan 
²

Affiliations

Affiliations

• ¹ Department of Biotechnology, Noida Institute of Engineering & Technology (NIET), Greater Noida, India.
• ² Unit of Microbiology and Molecular Biology, ICMR-Vector Control Research Center, Puducherry-605006, India.

• PMID:

  30977450

• DOI:

  10. 2174/1568026619666190412121811

Review

Niraj Kumar Jha et al.

Curr Top Med Chem.

2019.

. 2019;19(10):795-811.

doi: 10.2174/1568026619666190412121811.

Authors

Niraj Kumar Jha 
¹
, Rohan Kar 
¹
, Rituraj Niranjan 
²

Affiliations

• ¹ Department of Biotechnology, Noida Institute of Engineering & Technology (NIET), Greater Noida, India.
• ² Unit of Microbiology and Molecular Biology, ICMR-Vector Control Research Center, Puducherry-605006, India.

• PMID:

  30977450

• DOI:

  10.2174/1568026619666190412121811

Abstract

Neurodegeneration is a distinguishing feature of many age related disorders and other vector borne neuroinflammatory diseases. There are a number of factors that can modulate the pathology of these disorders. ATP-binding cassette (ABC) transporters are primarily involved in the maintenance of normal brain homeostasis by eliminating toxic peptides and compounds from the brain. Also, ABC transporters protect the brain from the unwanted effects of endogenous and exogenous toxins that can enter the brain parenchyma. Therefore, these transporters have the ability to determine the pathological outcomes of several neurological disorders. For instance, ABC transporters like P-glycoprotein (ABCB1), and BCRP (ABCG2) have been reported to facilitate the clearance of peptides such as amyloid-β (Aβ) that accumulate in the brain during Alzheimer’s disease (AD) progression. Other members such as ABCA1, ABCA2, ABCC8, ABCC9, ABCG1 and ABCG4 also have been reported to be involved in the progression of various brain disorders such as HIV-associated dementia, Multiple sclerosis (MS), Ischemic stroke, Japanese encephalitis (JE) and Epilepsy. However, these defective transporters can be targeted by numerous botanical compounds such as Verapamil, Berberine and Fascalpsyn as a therapeutic target to treat these neurological outcomes. These compounds are already reported to modulate ABC transporter activity in the CNS. Nonetheless, the exact mechanisms involving the ABC transporters role in normal brain functioning, their role in neuronal dysfunction and how these botanical compounds ensure and facilitate their therapeutic action in association with defective transporters still remain elusive. This review therefore, summarizes the role of ABC transporters in neurological disorders, with a special emphasis on its role in AD brains. The prospect of using botanical/natural compounds as modulators of ABC transporters in neurological disorders is discussed in the latter half of the article.

Keywords:

ABC transporter; Alzheimer’s disease; Botanical compounds; Neurodegeneration; Neuroinflammation; Neurotherapeutics..

Copyright© Bentham Science Publishers; For any queries, please email at [email protected].

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Troubleshoot “Invalid Gateway” errors in Azure Application Gateway

• Article
• 05/19/2023

How to troubleshoot “Invalid Gateway” (502) errors received when using Azure Application Gateway.

Note

We recommend that you use the Azure Az PowerShell module to interact with Azure. To get started, see Install Azure PowerShell. For more information, see Migrate Azure PowerShell from AzureRM to Az.

General

After configuring Application Gateway, you may see one of the errors : Server Error: 502 – The web server received an invalid response while acting as a gateway or proxy . Possible reasons for this error:

• Network security group, user-defined route, or custom DNS is blocking access to internal pool members;
• Server VMs or virtual machine scale set instances are not responding to the default health probe.
• Invalid or incorrect custom health probe configuration;
• The Azure Application Gateway backend pool is not configured or is empty.
• There are no healthy instances or virtual machines in the virtual machine scale set;
• User requests timed out or had connection problems.

Network security group, user-defined route, or custom DNS 9 issue0019

Cause

If access to the back end is blocked due to a network security group, a user-defined route, or a custom DNS service, application gateway instances cannot connect to the back-end pool. This issue causes the probe to fail, resulting in 502 errors.

The network security group or user-defined route can be present on the Application Gateway subnet, or on the subnet where the application virtual machines are deployed.

Similarly, having a custom DNS service in a virtual network can also cause problems. The FQDN used for backend pool members may not be correctly resolved by the user-configured DNS server for the virtual network.

Resolution

Verify the NSG, UDR, and DNS configuration by doing the following:

1. Verify the NSGs associated with the Application Gateway subnet. Make sure communication with the backend is not blocked. For more information, see Network Security Groups.

2. Check the user-defined route associated with the Application Gateway subnet. Ensure that the user-defined route does not direct traffic outside the internal subnet. Check the routing to NVAs or the default routes advertised for the Application Gateway subnet through Azure ExpressRoute or VPN.

    $vnet = Get-AzVirtualNetwork -Name vnetName -ResourceGroupName rgName
   Get-AzVirtualNetworkSubnetConfig -Name appGwSubnet -VirtualNetwork $vnet
    

3. Check the valid NSG and route with the server virtual machine.

    Get-AzEffectiveNetworkSecurityGroup -NetworkInterfaceName nic1 -ResourceGroupName testrg
   Get-AzEffectiveRouteTable -NetworkInterfaceName nic1 -ResourceGroupName testrg
    

4. Verify that a custom DNS service exists in the virtual network. You can verify the DNS service by viewing the details of the virtual network properties in the output.

    Get-AzVirtualNetwork -Name vnetName -ResourceGroupName rgName
   dhcpOptions : {
                              "DnsServers": [
                                "x. x.x.x"
                              ]
                            }
    

5. If present, verify that the DNS server can correctly resolve the FQDN of the internal pool member.

Problems with the default health probe

Cause

502 errors can also be common indicators that the default health probe is unable to communicate with the server virtual machines.

When provisioning an application gateway instance, each backend address pool is automatically configured with a standard health probe using the BackendHttpSetting setting. The user does not need to enter anything to set up this probe. In particular, when setting up a load balancing rule, an association is created between the BackendHttpSetting and the backend address pool. A standard sample is configured for each link. Application Gateway periodically checks the health of each instance in the backend address pool. To do this, it connects to the desired instances using the port specified in the BackendHttpSetting parameter.

The table shows the values ​​associated with the standard health probe:

Solution

• The request node value will be 127.0.0.1. Make sure the default site is set up and waiting for data transfer at 127.0.0.1.
• The request protocol is determined by the BackendHttpSetting protocol.
• The URI path will be / .
• If the BackendHttpSetting is set to a port other than 80, the standard site must be configured to wait for data to be sent through this port.
• A call to protocol://127.0.0.1:port must return an HTTP result code of 200. This code must be returned within a 30 second timeout period.
• Verify that the configured port is open and that there are no firewall rules or Azure Network Security Groups blocking traffic to or from the configured port.
• If you use classic Azure VMs or a cloud service with an FQDN or public IP address, make sure the corresponding endpoint is open.
• If the VM is configured with Azure Resource Manager and is outside the virtual network where the application gateway is deployed, configure the network security group to access the correct port.

For more information, see Application Gateway Infrastructure Configuration.

Issues with custom health probes

Cause

Custom health probes are more flexible than standard health probes. When using custom probes, you can configure the probe interval, URL, test path, and number of responses that could not be accepted before marking the backend pool instance as unhealthy.

The following additional properties have been added.

Check properties	Description
Name	Test name. This name is used to refer to the probe in the backend HTTP settings.
Protocol	Protocol used for verification. The probe uses the protocol defined in the backend HTTP settings.
Assembly	Host name to check. Applies only if the application gateway is configured for multi-site connectivity. (That is, it is not the host name of the virtual machine.)
way	Relative check path. The path must start with “/”. The probe is sent to ://:.
Interval	Check interval in seconds. The time interval between two consecutive checks.
Waiting time	Check timeout in seconds. If a health response is not received within this timeout, then the probe is considered a failure.
Fault threshold	Number of retries to check. The backend server is flagged down after the number of consecutive probe failures reaches the outage threshold.

Solution

Verify that the custom health probe is configured correctly, as shown in the previous table. In addition to the troubleshooting steps above, the following conditions must be met:

• the sample is correct according to the manual;
• If Application Gateway is configured for a single site, the default host name should be 127.0.0.1 unless configured otherwise in the custom probe.
• Verify that a call to http://: returns an HTTP result code of 200.
• Verify that the Interval, Timeout, and Health Threshold values ​​are within the allowed range.
• When using an HTTPS probe, ensure that the internal server does not require SNI by configuring a fallback certificate.

Request timed out

Cause

When a user request is received, Application Gateway applies the configured rules to the request and routes it to the backend pool instance. It then waits for a specified interval for a response from the server instance. By default, this interval is 20 seconds. If Application Gateway version 1 does not receive a response from the server application within this time, a 502 error is displayed for the user request. In Application Gateway version 2, if Application Gateway does not receive a response from the server application within this interval, the request will be tried to the second member of the backend pool . If the second request fails, the user’s request will receive a 502 error.0013

Solution

Application Gateway allows you to configure this setting using the BackendHttpSetting setting, which can then be applied to different pools. Different backend pools may have a different BackendHttpSetting value and a different request timeout.

 New-AzApplicationGatewayBackendHttpSettings -Name 'Setting01' -Port 80 -Protocol Http -CookieBasedAffinity Enabled -RequestTimeout 60
 

Empty server address pool

Cause

If Application Gateway does not have VMs or VM Scale Set configured in the internal address pool, it cannot redirect client requests and sends a bad gateway error.

Resolution

Verify that the internal address pool is not empty. You can do this using PowerShell, the CLI, or the portal.

 Get-AzApplicationGateway -Name "SampleGateway" -ResourceGroupName "ExampleResourceGroup"
 

The output of the previous cmdlet must contain a non-empty server address pool. The following example shows two returned pools configured with FQDN or IP addresses for server VMs. The provisioning state of the backend address pool should be “Completed”.

BackendAddressPoolsText:

 [{
    "BackendAddresses": [{
        "ipAddress": "10.0.0.10",
        "ipAddress": "10.0.0.11"
    }],
    "BackendIpConfigurations": [],
    "ProvisioningState": "Succeeded",
    "Name": "Pool01",
    "Etag": "W/\"00000000-0000-0000-0000-000000000000\"",
    "Id": "/subscriptions//resourceGroups//providers/Microsoft.Network/applicationGateways//backendAddressPools/pool01"
}, {
    "BackendAddresses": [{
        "Fqdn": "xyx. cloudapp.net",
        "Fqdn": "abc.cloudapp.net"
    }],
    "BackendIpConfigurations": [],
    "ProvisioningState": "Succeeded",
    "Name": "Pool02",
    "Etag": "W/\"00000000-0000-0000-0000-000000000000\"",
    "Id": "/subscriptions//resourceGroups//providers/Microsoft.Network/applicationGateways//backendAddressPools/pool02"
}]
 

Unhealthy instances in the backend address pool

Cause

If all BackendAddressPool instances are unhealthy, the Application Gateway does not have a back end to route the user request. This can also be the case if the server instances are healthy but the necessary applications are not deployed.

Solution

Make the instances healthy and properly configure the application. Check if the server instances can respond to validation requests from another virtual machine in the same virtual network. If a public endpoint is configured, ensure that the browser request to the web application is serviceable.

The upstream SSL certificate does not match

Cause

The Tls certificate installed on the internal servers does not match the host name received in the host request header.

In scenarios where end-to-end TLS is enabled, a configuration that is achieved by changing the corresponding backend HTTP settings and changing the Internal Protocol configuration to HTTPS, you need to ensure that the CNAME of the TLS certificate installed on the backend servers matches the hostname, coming to the back end in an HTTP host header request.

Recall that if you enable HTTPS instead of HTTP in the HTTP settings for the server side, the second part of the communication that occurs between the Application Gateway instances and the backend servers will be encrypted using TLS.

Because, by default, Application Gateway sends the same back-end HTTP host header as from the client, you must ensure that the TLS certificate installed on the back-end server is issued with a CNAME that matches the host name received by the server, in the HTTP host header.
Note that, unless otherwise specified, this hostname will be the same as the hostname received from the client.

Example:

Imagine you have an Application Gateway to service HTTPS requests for the www.contoso.com domain. You can delegate the contoso.com domain to the Azure DNS public zone and a DNS entry in that zone that points www.contoso.com to the public IP address of the specific Application Gateway that will service requests.

On this Application Gateway, you must have a listener for the www.contoso.com host with the rule set to “Backed HTTP” to force HTTPS (ensuring end-to-end TLS). The same rule could have configured an internal pool with two virtual machines running IIS as web servers.

As we know, enabling HTTPS in the fallback http parameter of the rule will make the second part of the communication that happens between the Application Gateway instances and the servers in the backend to use TLS.

If the internal servers do not have a TLS certificate issued for the www. contoso.com or *.contoso.com CNAME, the request will fail with server error : 502 – The web server received an invalid response while acting as a gateway or proxy because the upstream SSL certificate (the certificate installed on the server servers) will not match the hostname in the host header. and hence the TLS negotiation will fail.

www.contoso.com –> APP GW INTERFACE IP –> listener with a rule that configures “backend HTTP settings” to use HTTPS instead of HTTP .> Internal pool –> web server (for www. contoso.com must have a TLS certificate installed)

Resolution

requires that the CNAME of the tls certificate installed on the internal server matches the hostname configured in the HTTP backend settings, otherwise the second part of the end-to-end communication that occurs between Application Gateway instances and the back end will fail with the error “Upstream SSL certificate does not match” and call back callServerError: 502 – The web server received an invalid response while acting as a gateway or proxy server

What to do next

If the above steps do not resolve the issue, submit a support ticket.

HTTP response codes – Azure Application Gateway

• Article
• 07/07/2023

This article explains why Azure Application Gateway returns certain HTTP response codes. Common causes and troubleshooting steps are provided to help you determine the root cause of an error in the HTTP response code. HTTP response codes can be returned in a client request regardless of whether or not a connection to a backend target was initiated.

3XX response codes (redirect)

Responses 300-399 are displayed when a client request matches an application gateway rule that has redirects configured. Redirects can be set up for the “as is” rule or with a path map rule. For more information about redirection, see Application Gateway redirection overview.

301 permanent redirect;

HTTP 301 responses are displayed when you specify a redirect rule with a value of Permanent .

302 – object found

HTTP 302 responses are displayed when you specify a redirect rule with a value of Found .

303 – other object;

HTTP 302 responses are displayed when you specify a redirect rule with a value of See other .

307 – temporary redirect.

HTTP 307 responses are displayed when you specify a redirect rule with a value of Temporary .

Response codes 4xx (client error)

Response codes 400-499 indicate a problem initiated by the client. These issues can range from a client initiating requests to a mismatched hostname, request timeout, unauthenticated request, malicious request, and more.

Application Gateway collects metrics that capture the distribution of 4xx/5xx status codes, has a logging mechanism that collects information such as the IP address of the client URI with the response code. Metrics and logging allow for additional troubleshooting. Clients may also receive a 4xx response from other proxies between the client device and Application Gateway. For example, CDN and other authentication providers. See the articles below for more information.

Metrics supported by diagnostic logs Application Gateway SKU version 2

400 – Bad request

HTTP 400 response codes are commonly seen when: HTTP or HTTPS listener.

The following are some common reasons why a request does not comply with the RFC.

Category	Examples
Invalid host in query string	Node containing two colons (example.com: 8090:8080 )
Host header missing	Request does not have host header
Misformed or illegal nature	Reserved characters ,!. & A workaround is the percentage code. For example: %&
Invalid HTTP Version	Get /content.css HTTP/ 0.3
The header field name and Uniform Resource Identifier (URI) contain a non-ASCII character	GET / “úü”,” .doc HTTP/1.1
Missing Content Length header for POST request	Clear
Invalid HTTP Method	GET123 /index.html HTTP/1.1
Duplicate headings	Authorization: , Authorization:
Invalid value in content-length	Content-Length: abc, Content-Length : -10

In cases where mutual authentication is configured, several scenarios can result in an HTTP 400 response being returned to the client, for example:

• The client certificate is not displayed, but mutual authentication is enabled.
• DN checking is enabled and the DN of the client certificate does not match the DN of the specified certificate chain.
• The client certificate chain does not match the certificate chain configured in the specific SSL policy.
• The client certificate has expired.
• OCSP client revocation checking is enabled and the certificate has been revoked.
• OCSP Client Revocation check is enabled but cannot be contacted.
• OCSP client revocation feature validation is enabled, but no OCSP responder is specified in the certificate.

For more information about troubleshooting mutual authentication, see Troubleshooting Error Code.

401 – not authorized;

If the client is not authorized to access the resource, an unauthorized HTTP 401 response is returned to the client. There are several reasons for returning a 401. Below are several reasons for possible fixes.

• If the client has access, it may have an outdated browser cache. Clear your browser cache and try accessing the app again.

Unauthorized HTTP 401 response can be returned in an AppGW probe request if the backend pool is configured with NTLM authentication. In this scenario, the backend is marked as healthy. There are several ways to fix this problem.

• Allow anonymous access in the internal pool.
• Configure a probe to send a request to another “fake” site that does not require NTLM.
• Not recommended as it won’t tell us if the actual site behind the Application Gateway is active.
• Configure Application Gateway to allow 401 responses as valid for probes: probe matching conditions.

403 – prohibited;

Http 403 Forbidden is displayed when clients are using WAF SKU and configured in prevent mode. If the included WAF rule sets or custom WAF deny rules match the characteristics of the incoming request, a 403 Forbidden response is displayed to the client.0013

Other reasons why clients receive 403 responses:

• You are using App Service as a backend and only allow access from Application Gateway. App Services may return a 403 error. This is usually due to redirects or hrefs that point directly to App Services and not to the Application Gateway IP address.
• If you access the Storage Blog and the Application Gateway and storage endpoint is in a different region, a 403 error is returned if the Application Gateway public IP address is not in the allowed list. Cm . See Allow access from an Internet IP range.

404 – Page not found

An HTTP 404 response can be returned if a request is sent to Application Gateway that is:

• Using a version 2 SKU.
• Does not match the hostname defined in multisite listeners.
• No base listener configured.

408 – Request timed out

HTTP response 408 can be observed when client requests to the Application Gateway front-end listener are not responded for 60 seconds. This error can be observed due to traffic congestion between on-premises and Azure when the virtual (pod) inspects traffic or the client itself becomes congested.

499 – Client closed connection

An HTTP 499 response is displayed if a client request sent to Application Gateways using the v2 SKU is closed before a response is received from the server. This error can be observed in two scenarios. The first scenario is that a large response is returned to the client, and the client may close or refresh the application before the server has finished sending the large response. The second scenario is that the client side timeout is short and doesn’t wait long enough to receive a response from the server. In this case, it is better to increase the client’s wait time. Application gateways using version 1 SKUs can cause an HTTP response code of 0 for a client to close the connection before the server also completes the response.

Response Codes 5XX (Server Error)

Response codes 500-599 indicate a problem with the Application Gateway or backend server while making the request.

500 – Internal Server Error

Azure Application Gateway must not contain 500 response codes. Open a support ticket if you see this code as this issue is an internal service error. For information on how to open a support case, see Create an Azure support request.

502 – Invalid Gateway

HTTP 502 errors can have multiple root causes, for example:

• NSG, UDR, or custom DNS is blocking access to members of the backend pool.
• Server VMs or virtual machine scale set instances are not responding to the default health probe.
• Invalid or incorrect custom health probe configuration;
• The Azure Application Gateway backend pool is not configured or is empty.
• There are no healthy instances or virtual machines in the virtual machine scale set;
• User requests timed out or had connection problems.

For information about scenarios in which 502 errors occur and how to resolve them, see Resolve Wrong Gateway Errors.

504 – Gateway timeout

Azure Application Gateway v2 SKU sends HTTP 504 errors if the backend response time exceeds the timeout configured in the backend setting.