Upstream

Provides a context for describing groups of servers that can be used in the proxy_pass, fastcgi_pass, uwsgi_pass, scgi_pass, memcached_pass, and grpc_pass directives.

Configuration Example

upstream backend {
    zone backend 1m;
    server backend1.example.com       weight=5;
    server backend2.example.com:8080;
    server backend3.example.com       service=_example._tcp resolve;
    server unix:/tmp/backend3;

    server backup1.example.com:8080   backup;
    server backup2.example.com:8080   backup;
}

resolver 127.0.0.53 status_zone=resolver;

server {
    location / {
        proxy_pass http://backend;
    }
}

Directives

backup_switch (PRO)

Syntax	backup_switch permanent[=time];
Default	—
Context	upstream

The directive enables the ability to start server selection not from the primary group, but from the active group, i.e., the one where a server was successfully found last time. If a server cannot be found in the active group for the next request, and the search moves to the backup group, then this group becomes active, and subsequent requests are first directed to servers in this group.

If the permanent parameter is defined without a time value, the group remains active after selection, and automatic rechecking of groups with lower levels does not occur. If time is specified, the active status of the group expires after the specified interval, and the balancer again checks groups with lower levels, returning to them if the servers are working normally.

Example:

upstream my_backend {
    zone my_backend 1m;
    server primary1.example.com;
    server primary2.example.com;

    server backup1.example.com backup;
    server backup2.example.com backup;

    backup_switch permanent=2m;
}

If the balancer switches from primary servers to the backup group, all subsequent requests are handled by this backup group for 2 minutes. After 2 minutes elapse, the balancer rechecks the primary servers and makes them active again if they are working normally.

bind_conn (PRO)

Syntax	bind_conn value;
Default	—
Context	upstream

Allows binding a server connection to a client connection when the value, specified as a string of variables, becomes different from "" and "0".

Warning

The bind_conn directive must be used after all directives that set a load balancing method, otherwise it will not work. If it is used together with the sticky directive, then bind_conn must come after sticky.

Warning

When using the directive, the Proxy module settings must allow the use of persistent connections, for example:

proxy_http_version 1.1;
proxy_set_header Connection "";

A typical use case for the directive is proxying connections with NTLM authentication, where it is necessary to ensure client-to-server binding at the beginning of negotiation:

map $http_authorization $ntlm {
    ~*^NTLM       1;
    ~*^Negotiate  1;
}

upstream ntlm_backend {
    zone ntlm_backend 1m;
    server 127.0.0.1:8080;
    bind_conn $ntlm;
}

server {
    # ...
    location / {
        proxy_pass http://ntlm_backend;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        # ...
    }
}

feedback (PRO)

Changed in version 1.12.0: PRO

Syntax	feedback variable [inverse] [factor=number] [account=condition_variable] [last_byte];
Default	—
Context	upstream

Sets up a feedback-based load balancing mechanism in the upstream. It dynamically adjusts balancing decisions by multiplying the weight of each proxied server by the average feedback value, which changes over time depending on the value of the variable and is subject to an optional condition.

The following parameters can be specified:

variable	The variable from which the feedback value is taken. It should represent a performance or health metric; it is assumed that the server provides it in headers or otherwise. The value is evaluated with each response from the server and is factored into the moving average according to the inverse and factor settings.
inverse	If the parameter is set, the feedback value is interpreted inversely: lower values indicate better performance.
factor	The factor by which the feedback value is considered when calculating the average. Valid values are integers from 0 to 99. Default is 90. The average is calculated using the exponential smoothing formula. The larger the factor, the less new values affect the average; if 90 is specified, then 90% of the previous value and only 10% of the new value will be taken.
account	Specifies a condition variable that controls which responses are considered in the calculation. The average value is updated with the feedback value from the response only if the condition variable of that response is not equal to "" or "0". Note By default, responses during active checks are not included in the calculation; combining the $upstream_probe variable with account allows including these responses or even excluding everything else.
last_byte	Allows processing data from the proxied server after receiving the complete response, not just the header.

Example:

upstream backend {

    zone backend 1m;

    feedback $feedback_value factor=80 account=$condition_value;

    server backend1.example.com;
    server backend2.example.com;
}

map $upstream_http_custom_score $feedback_value {
    "high"                      100;
    "medium"                    75;
    "low"                       50;
    default                     10;
}

map $upstream_probe $condition_value {
    "high_priority" "1";
    "low_priority"  "0";
    default         "1";
}

This configuration categorizes server responses by feedback levels based on specific scores from response header fields, and also adds a condition on $upstream_probe to consider only responses from the high_priority active check or responses to regular client requests.

hash

Syntax	hash key [consistent];
Default	—
Context	upstream

Specifies a load balancing method for a server group where the client-server mapping is determined using the hashed key value. The key can contain variables. Note that adding or removing a server from the group may result in remapping most of the keys to different servers. The method is compatible with the Cache::Memcached Perl library.

hash $remote_addr;

When using domain names that resolve to multiple IP addresses (for example, with the resolve parameter), the server does not sort the received addresses, so their order may differ across different servers, which affects client distribution. To ensure consistent distribution, use the consistent parameter.

If the consistent parameter is specified, the ketama consistent hashing method will be used instead. The method ensures that only a few keys will be remapped to different servers when a server is added to or removed from the group. This helps to achieve a higher cache hit ratio for caching servers. The method is compatible with the Cache::Memcached::Fast Perl library with the ketama_points parameter set to 160.

ip_hash

Syntax	ip_hash;
Default	—
Context	upstream

Specifies a load balancing method for the group where requests are distributed among servers based on client IP addresses. The first three octets of the client's IPv4 address or the entire IPv6 address are used as a hashing key. The method ensures that requests from the same client will always be passed to the same server except when this server is unavailable. In that case, client requests will be passed to another server. Most likely this will also be the same server.

If one of the servers needs to be temporarily removed, it should be marked with the down parameter to preserve the current hashing of client IP addresses:

upstream backend {
    zone backend 1m;
    ip_hash;

    server backend1.example.com;
    server backend2.example.com;
    server backend3.example.com down;
    server backend4.example.com;
}

keepalive

Syntax	keepalive connections;
Default	—
Context	upstream

Activates the cache of connections to upstream servers.

The connections parameter sets the maximum number of idle keepalive connections to upstream servers that are preserved in the cache of each worker process. When this number is exceeded, the least recently used connections are closed.

Note

It should be particularly noted that the keepalive directive does not limit the total number of connections to upstream servers that Angie worker processes can open. The connections parameter should be set low enough to let upstream servers process new incoming connections as well.

Warning

The keepalive directive must be used after all directives that set a load balancing method, otherwise it will not work.

Example configuration of memcached upstream with keepalive connections:

upstream memcached_backend {
    zone memcached_backend 1m;
    server 127.0.0.1:11211;
    server 10.0.0.2:11211;

    keepalive 32;
}

server {
    #...

    location /memcached/ {
        set $memcached_key $uri;
        memcached_pass memcached_backend;
    }

}

For HTTP, the proxy_http_version directive should be set to "1.1" and the Connection header field should be cleared:

upstream http_backend {
    zone http_backend 1m;
    server 127.0.0.1:8080;

    keepalive 16;
}

server {
    #...

    location /http/ {
        proxy_pass http://http_backend;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
    #    ...
    }
}

Note

Alternatively, HTTP/1.0 persistent connections can be used by passing the "Connection: Keep-Alive" header field to an upstream server, though this method is not recommended.

For FastCGI servers, it is required to set fastcgi_keep_conn for keepalive connections to work:

upstream fastcgi_backend {
    zone fastcgi_backend 1m;
    server 127.0.0.1:9000;

    keepalive 8;
}

server {
    #...

    location /fastcgi/ {
        fastcgi_pass fastcgi_backend;
        fastcgi_keep_conn on;
    #    ...
    }
}

Note

SCGI and uwsgi protocols do not have a concept of keepalive connections.

keepalive_requests

Syntax	keepalive_requests number;
Default	keepalive_requests 1000;
Context	upstream

Sets the maximum number of requests that can be served through one keepalive connection. After the maximum number of requests is made, the connection is closed.

Closing connections periodically is necessary to free per-connection memory allocations. Therefore, using too high maximum number of requests could result in excessive memory usage and not recommended.

keepalive_time

Syntax	keepalive_time time;
Default	keepalive_time 1h;
Context	upstream

Limits the maximum time during which requests can be processed through one keepalive connection. After this time is reached, the connection is closed following the subsequent request processing.

keepalive_timeout

Syntax	keepalive_timeout timeout;
Default	keepalive_timeout 60s;
Context	upstream

Sets a timeout during which an idle keepalive connection to an upstream server will stay open.

least_conn

Syntax	least_conn;
Default	—
Context	upstream

Specifies that a group should use a load balancing method where a request is passed to the server with the least number of active connections, taking into account weights of servers. If there are several such servers, they are tried in turn using a weighted round-robin balancing method.

least_time

Syntax	least_time header | last_byte [factor=number] [account=condition_variable];
Default	—
Context	upstream

Specifies that the group should use a load balancing method where an active server's chance of receiving the request is inversely proportional to its average response time; the less it is, the more requests a server gets.

header	The directive accounts for the average time to receive response headers.
last_byte	The directive uses the average time to receive the entire response.

factor	Serves the same purpose as response_time_factor and overrides it if the parameter is set.
account	Specifies a condition variable that controls which responses are included in the calculation. The average is updated only if the condition variable for the response is not "" or "0". Note By default, probes (PRO) are not included in the calculation; combining the $upstream_probe variable with account allows including these responses or even excluding everything else.

Current values are presented as header_time (headers only) and response_time (entire responses) in the server's health object among the upstream metrics in the API.

queue (PRO)

Syntax	queue number [timeout=time];
Default	—
Context	upstream

If it is not possible to assign a proxied server to a request on the first attempt (for example, during a brief service interruption or when there is a surge in load reaching the max_conns limit), the request is not rejected; instead, Angie attempts to enqueue it for processing.

The number parameter of the directive sets the maximum number of requests in the queue for a worker process. If the queue is full, a 502 (Bad Gateway) error is returned to the client.

Note

The logic of the proxy_next_upstream directive also applies to queued requests. Specifically, if a server was selected for a request but it cannot be handed over to it, the request may be returned to the queue.

If a server is not selected to process a queued request within the time set by timeout (default is 60 seconds), a 502 (Bad Gateway) error is returned to the client. Requests from clients that prematurely close the connection are also removed from the queue; there are counters for the states of requests passing through the queue in the API.

Warning

The queue directive must be used after all directives that set the load balancing method; otherwise, it won't work.

random

Syntax	random [two];
Default	—
Context	upstream

Specifies a load balancing method for the group where a request is passed to a randomly selected server, taking into account server weights.

If the optional two parameter is specified, Angie randomly selects two servers, then selects one of them using the least_conn method, where a request is passed to the server with the least number of active connections.

response_time_factor

Syntax	response_time_factor number;
Default	response_time_factor 90;
Context	upstream

Sets the smoothing factor for the previous value when calculating the average response time for the least_time load balancing method using the exponentially weighted moving average formula.

The higher the specified number, the less new values affect the average; if 90 is specified, 90% of the previous value and only 10% of the new value are taken. Valid values range from 0 to 99 inclusive.

Current calculation results are presented as header_time (headers only) and response_time (entire responses) in the server's health object among the upstream metrics in the API.

Note

Only successful responses are included in the calculation; what is considered an unsuccessful response is determined by the proxy_next_upstream, fastcgi_next_upstream, uwsgi_next_upstream, scgi_next_upstream, memcached_next_upstream, and grpc_next_upstream directives. Additionally, the header_time value is recalculated only if all headers are received and processed, and response_time — only if the entire response is received.

server

Syntax	server address [parameters];
Default	—
Context	upstream

Defines the address and other parameters of a server. The address can be specified as a domain name or IP address, with an optional port, or as a UNIX-domain socket path specified after the unix: prefix. If a port is not specified, port 80 is used. A domain name that resolves to multiple IP addresses defines multiple servers at once.

The following parameters can be defined:

weight=number	Sets the weight of the server. Default is 1.
max_conns=number	Limits the maximum number of simultaneous active connections to the proxied server. The default value is 0, meaning there is no limit. If the group does not reside in the shared memory zone, the limitation works per worker process.

Note

With idle keepalive connections enabled, multiple worker processes, and a shared memory zone, the total number of active and idle connections to the proxied server may exceed the max_conns value.

max_fails=number — sets the number of unsuccessful attempts to communicate with the server that should occur during the specified fail_timeout period for the server to be considered unavailable; after this, it will be checked again after the same period.

What is considered an unsuccessful attempt is defined by the proxy_next_upstream, fastcgi_next_upstream, uwsgi_next_upstream, scgi_next_upstream, memcached_next_upstream, and grpc_next_upstream directives.

When max_fails is exceeded, the server is also considered unavailable from the perspective of upstream_probe (PRO); client requests will not be directed to it until checks determine it is available.

Note

If a server directive in a group resolves to multiple servers, its max_fails setting applies to each server separately.

If after resolving all server directives only one server remains in the upstream, the max_fails setting has no effect and will be ignored.

max_fails=1	Default number of attempts;
max_fails=0	Disables attempt accounting.

fail_timeout=time — sets the period of time during which a specified number of unsuccessful attempts to communicate with the server (max_fails) must occur for the server to be considered unavailable. The server then remains unavailable for the same period of time before being checked again.

Default value is 10 seconds.

Note

If a server directive in a group resolves to multiple servers, its fail_timeout setting applies to each server separately.

If after resolving all server directives only one server remains in the upstream, the fail_timeout setting has no effect and will be ignored.

backup	Marks the server as a backup server. It will be passed requests when the primary servers are unavailable. If the backup_switch (PRO) directive is specified, its active backup logic also applies.
down	Marks the server as permanently unavailable.
drain	Marks the server as draining; this means it only receives requests from sessions previously bound via sticky. Otherwise, the behavior is the same as in down mode.

Warning

The backup parameter cannot be used together with the hash, ip_hash, and random load balancing methods.

The down and drain parameters are mutually exclusive.

resolve

Allows monitoring changes to the list of IP addresses corresponding to a domain name and updating it without reloading the configuration. The group must reside in shared memory; a resolver must also be defined.

service=name

Enables resolution of DNS SRV records and sets the service name. For the parameter to work, the resolve parameter must be specified for the server, without specifying a port in the hostname. If the service name does not contain a dot, a name is formed according to the RFC standard: the service name is prefixed with _, then _tcp is appended after a dot. Thus, the service name http results in _http._tcp. Angie resolves SRV records by combining the normalized service name and hostname and obtaining a list of servers for the resulting combination via DNS, along with their priorities and weights. SRV records with the highest priority (those with the lowest priority value) are resolved as primary servers, while other records become backup servers. If backup is set with server, SRV records with the highest priority are resolved as backup servers, and other records are ignored. Weight is analogous to the weight parameter of the server directive. If weight is specified both in the directive itself and in the SRV record, the weight set in the directive is used.

In this example, a lookup is performed for the record _http._tcp.backend.example.com:

server backend.example.com service=http resolve;

sid=id

Sets the server ID in the group. If the parameter is not specified, the ID is set as a hexadecimal MD5 hash of the IP address and port or UNIX socket path.

slow_start=time

Sets the time for gradual recovery of a returning server's weight when load balancing with the round-robin or least_conn method. If the parameter is set and the server is considered operational again after a failure from the perspective of max_fails and upstream_probe (PRO), such a server gradually increases to its specified weight over the given time period. If the parameter is not set, in a similar situation the server immediately starts operating with its specified weight.

Note

If only one server is specified in the upstream, slow_start does not work and will be ignored.

state (PRO)

Syntax	state file;
Default	—
Context	upstream

Specifies a file where the list of upstream servers is persistently stored. When installing from our packages, the directory /var/lib/angie/state/ (/var/db/angie/state/ on FreeBSD) is created specifically for storing such files with appropriate access permissions, and in the configuration you only need to add the file name:

upstream backend {

    zone backend 1m;
    state /var/lib/angie/state/<FILE NAME>;
}

The server list here has a format similar to server. The file contents are modified whenever servers are changed in the /config/http/upstreams/ section via the configuration API. The file is read when Angie starts or when the configuration is reloaded.

Warning

To use the state directive in an upstream block, there must be no server directives in it, but a shared memory zone (zone) is required.

sticky

Changed in version 1.10.0: PRO

Changed in version 1.11.0.

Changed in version 1.11.0: PRO

Syntax	sticky cookie name [attr=value]...; sticky route value...; sticky learn zone=zone create=$create_var1... lookup=$lookup_var1... [header] [norefresh] [timeout=time]; sticky learn [zone=zone] lookup=$lookup_var1... remote_action=uri remote_result=$remote_var [norefresh] [timeout=time]; (PRO)
Default	—
Context	upstream

Configures session affinity to bind client sessions to proxied servers in the mode specified by the first parameter; to drain servers that have the sticky directive configured, you can use the drain option in the server block.

Warning

The sticky directive must be used after all directives that specify a particular load balancing method, otherwise it will not work. If it is used alongside the bind_conn (PRO) directive, then bind_conn must come after sticky.

cookie moderoute modelearn modelearn with remote_action (PRO)

This mode uses cookies to manage sessions. It's suitable when cookies are already used for session tracking.

The first client request, before any stickiness applies, is sent to a backend server according to the configured balancing method. Angie then sets a cookie identifying the chosen server.

The cookie name (name) is defined by the sticky directive, and its value corresponds to the sid from the server directive. This value is further hashed if sticky_secret is set.

Subsequent requests with this cookie are routed to the server specified by its sid. If the server is unavailable or can't process the request, another one is chosen via the configured balancing method.

You can assign cookie attributes in the directive; by default, only path=/ is set. Attribute values can contain variables. To remove an attribute, set it to an empty value: attr=. For example, sticky cookie path= omits the path attribute.

This example sets a cookie srv_id for 1 hour, using a domain from a variable:

upstream backend {
    zone backend 1m;
    server backend1.example.com:8080;
    server backend2.example.com:8080;

    sticky cookie srv_id domain=$my_domain max-age=3600;
}

The sticky directive respects upstream server states:

• Servers marked down or failing are excluded.

• Servers over max_conns limit are skipped.

• drain servers may still be selected for new sessions in sticky mode when identifiers match.

• Recovered servers are reused automatically.

You can further adjust behavior using sticky_secret and sticky_strict. If stickiness fails and sticky_strict is off, fallback balancing is used; if on, the request is rejected.

Each zone used in sticky must be exclusive to a single upstream. Zones cannot be shared across multiple upstream blocks.

sticky_secret

Syntax	sticky_secret string;
Default	—
Context	upstream

Adds the string as the salt value to the MD5 hashing function for the sticky directive in cookie and route modes. The string may contain variables, for example, $remote_addr:

upstream backend {
    zone backend 1m;
    server backend1.example.com:8080;
    server backend2.example.com:8080;

    sticky cookie cookie_name;
    sticky_secret my_secret.$remote_addr;
}

The salt is appended to the value being hashed; to verify the hashing mechanism independently:

$ echo -n "<VALUE><SALT>" | md5sum

sticky_strict

Syntax	sticky_strict on | off;
Default	sticky_strict off;
Context	upstream

When enabled, causes Angie to return an HTTP 502 error to the client if the desired server is unavailable, rather than using any other available server as it would when no servers in the upstream are available.

upstream

Syntax	upstream name { ... }
Default	—
Context	http

Defines a group of servers. Servers can listen on different ports. In addition, servers listening on TCP and UNIX domain sockets can be mixed.

Example:

upstream backend {
    zone backend 1m;
    server backend1.example.com weight=5;
    server 127.0.0.1:8080       max_fails=3 fail_timeout=30s;
    server backup1.example.com  backup;
}

By default, requests are distributed between the servers using a weighted round-robin balancing method. In the above example, each 7 requests will be distributed as follows: 5 requests go to backend1.example.com and one request to each of the second and third servers. The distribution is smooth: a higher-weight server's requests are spread across the rotation rather than sent in a single burst.

If an error occurs during communication with a server, the request will be passed to the next server, and so on until all of the functioning servers will be tried. If a successful response could not be obtained from any of the servers, the client will receive the result of the communication with the last server.

Note

By default, a server that fails an occasional attempt without reaching max_fails temporarily receives a smaller share of requests, regaining its full share over the requests that follow. This differs from slow_start, which ramps a server back up only after it has been marked unavailable and has recovered.

zone

Syntax	zone name [size];
Default	—
Context	upstream

Defines the name and size of the shared memory zone that keeps the group's configuration and run-time state that are shared between worker processes. Several groups may share the same zone. In this case, it is enough to specify the size only once.

Note

The zone's content is only preserved on reload when the configured size is unchanged. Any size change — increase or decrease — causes the zone to be re-created empty.

Note

Upstream metrics are collected only when this zone is configured. Without it, the group does not appear in /status/http/upstreams/<upstream>, the HTTP Upstreams Widget, or Prometheus output, and no warning is logged. See Example configuration.

Built-in Variables

The http_upstream module supports the following built-in variables:

$sticky_sessid

Used with remote_action in sticky; stores the initial session ID taken from lookup.

$sticky_sid

Used with remote_action in sticky; stores the server ID previously associated with the session.

$upstream_addr

stores the IP address and port, or the path to the UNIX domain socket of the upstream server. If several servers were contacted during request processing, their addresses are separated by commas, e.g.:

192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock

If an internal redirect from one server group to another happens, initiated by X-Accel-Redirect or error_page, then the server addresses from different groups are separated by colons, e.g.:

192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock : 192.168.10.1:80, 192.168.10.2:80

If a server cannot be selected, the variable keeps the name of the server group.

$upstream_bytes_received

number of bytes received from an upstream server. Values from several connections are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_bytes_sent

number of bytes sent to an upstream server. Values from several connections are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_cache_key

Added in version 1.11.0.

contains the cache key used for the request.

$upstream_cache_status

keeps the status of accessing a response cache. The status can be either MISS, BYPASS, EXPIRED, STALE, UPDATING, REVALIDATED, or HIT:

• MISS: The response is not found in the cache, and the request is passed to the upstream server.

• BYPASS: The cache is bypassed, and the request is passed directly to the upstream server.

• EXPIRED: The cached response is stale, and a new request is passed to the upstream server to update the content.

• STALE: The cached response is stale, but is still served to clients until the content is eventually updated from the upstream server.

• UPDATING: The cached response is stale, but is still served to clients while the currently ongoing update from the upstream server is in progress.

• REVALIDATED: The cached response is stale, but was successfully revalidated and does not need to be updated from the upstream server.

• HIT: The response was taken from the cache.

If the request bypassed the cache without accessing it, the variable is not set.

$upstream_connect_time

keeps time spent on establishing a connection with the upstream server; the time is kept in seconds with millisecond resolution. In case of SSL, includes time spent on handshake. Times of several connections are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_cookie_<name>

cookie with the specified name sent by the upstream server in the Set-Cookie response header field. Only the cookies from the response of the last server are saved.

$upstream_current_addr (PRO)

Added in version 1.12.0: PRO

address of the proxied server selected for the current connection attempt; unlike $upstream_addr, which accumulates the addresses of every server tried across retries, this variable always holds a single address and is available immediately after the server is selected, including while proxy_bind variables are evaluated.

$upstream_header_time

keeps time spent on receiving the response header from the upstream server; the time is kept in seconds with millisecond resolution. Times of several responses are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_http_<name>

keep server response header fields. For example, the Server response header field is available through the $upstream_http_server variable. The rules of converting header field names to variable names are the same as for the variables that start with the $http_ prefix. Only the header fields from the response of the last server are saved.

$upstream_queue_time

keeps time the request spent in the queue before the next server selection; the time is kept in seconds with millisecond resolution. Times of several attempts are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_request_method

Added in version 1.11.0.

request method used for the upstream request. It can differ from the client request method when caching converts HEAD to GET or when proxy_method is set.

$upstream_response_length

keeps the length of the response obtained from the upstream server; the length is kept in bytes. Lengths of several responses are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_response_time

keeps time spent on receiving the response from the upstream server; the time is kept in seconds with millisecond resolution. Times of several responses are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_status

keeps the status code recorded by Angie for each upstream attempt. For DoH requests, a truncated UDP response is recorded as 426 and an accepted DNS response as 200. Status codes from several attempts are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_sticky_status

Status of sticky requests.

""	Request sent to an upstream where sticky is not enabled.
NEW	Request does not contain sticky information.
HIT	Request with sticky information sent to the desired server.
MISS	Request with sticky information sent to a server selected by the load balancing algorithm.

Statuses from several connections are separated by commas and colons like addresses in the $upstream_addr variable.

$upstream_trailer_<name>

keeps fields from the end of the response obtained from the upstream server.

$upstream_transport

Added in version 1.12.0.

keeps the transport protocol, TCP or UDP, used for each upstream connection attempt. Values from several attempts are separated by commas and colons like addresses in the $upstream_addr variable.