Module ngx_http_upstream_module

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Module ngx_http_upstream_module

Example Configuration
Directives
upstream
server
zone
state
hash
ip_hash
keepalive
keepalive_requests
keepalive_time
keepalive_timeout
ntlm
least_conn
least_time
queue
random
resolver
resolver_timeout
sticky
Embedded Variables

The ngx_http_upstream_module module
is used to define groups of servers that can be referenced
by the proxy_pass,
fastcgi_pass,
uwsgi_pass,
scgi_pass,
memcached_pass, and
grpc_pass directives.

Example Configuration

upstream backend {
server backend1.example.com weight=5;
server backend2.example.com:8080;
server unix:/tmp/backend3;

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

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

Dynamically configurable group with
periodic health checks is
available as part of our
commercial subscription:

resolver 10.0.0.1;

upstream dynamic {
zone upstream_dynamic 64k;

server backend1.example.com weight=5;
server backend2.example.com:8080 fail_timeout=5s slow_start=30s;
server 192.0.2.1 max_fails=3;
server backend3.example.com resolve;
server backend4.example.com service=http resolve;

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

server {
location / {
proxy_pass http://dynamic;
health_check;
}
}

Directives

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 {
server backend1.example.com weight=5;
server 127.0.0.1:8080 max_fails=3 fail_timeout=30s;
server unix:/tmp/backend3;

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.
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.

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, the port 80 is used.
A domain name that resolves to several IP addresses defines
multiple servers at once.

The following parameters can be defined:

weight=number

sets the weight of the server, by default, 1.

max_conns=number

limits the maximum number of simultaneous active
connections to the proxied server (1.11.5).
Default value is zero, meaning there is no limit.
If the server group does not reside in the shared memory,
the limitation works per each worker process.

If idle keepalive connections,
multiple workers,
and the shared memory are enabled,
the total number of active and idle connections to the proxied server
may exceed the max_conns value.

Since version 1.5.9 and prior to version 1.11.5,
this parameter was available as part of our
commercial subscription.

max_fails=number

sets the number of unsuccessful attempts to communicate with the server
that should happen in the duration set by the fail_timeout
parameter to consider the server unavailable for a duration also set by the
fail_timeout parameter.
By default, the number of unsuccessful attempts is set to 1.
The zero value disables the accounting of attempts.
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.

fail_timeout=time

sets

the time during which the specified number of unsuccessful attempts to
communicate with the server should happen to consider the server unavailable;

and the period of time the server will be considered unavailable.

By default, the parameter is set to 10 seconds.

backup

marks the server as a backup server.
It will be passed requests when the primary servers are unavailable.

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

down

marks the server as permanently unavailable.

resolve

monitors changes of the IP addresses
that correspond to a domain name of the server,
and automatically modifies the upstream configuration
without the need of restarting nginx (1.5.12).
The server group must reside in the shared memory.

In order for this parameter to work,
the resolver directive
must be specified in the
http block
or in the corresponding upstream block.

Prior to version 1.27.3, this parameter was available only as part of our
commercial subscription.

service=name

enables resolving of DNS
SRV
records and sets the service name (1.9.13).
In order for this parameter to work, it is necessary to specify
the resolve parameter for the server
and specify a hostname without a port number.

If the service name does not contain a dot (“.”), then
the RFC-compliant name
is constructed
and the TCP protocol is added to the service prefix.
For example, to look up the
_http._tcp.backend.example.com SRV record,
it is necessary to specify the directive:

server backend.example.com service=http resolve;

If the service name contains one or more dots, then the name is constructed
by joining the service prefix and the server name.
For example, to look up the _http._tcp.backend.example.com
and server1.backend.example.com SRV records,
it is necessary to specify the directives:

server backend.example.com service=_http._tcp resolve;
server example.com service=server1.backend resolve;

Highest-priority SRV records
(records with the same lowest-number priority value)
are resolved as primary servers,
the rest of SRV records are resolved as backup servers.
If the backup parameter is specified for the server,
high-priority SRV records are resolved as backup servers,
the rest of SRV records are ignored.

Prior to version 1.27.3, this parameter was available only as part of our
commercial subscription.

route=string

sets the server route name.

Prior to version 1.29.6,
this parameter was available only as part of our
commercial subscription.

drain

puts the server into the “draining” mode (1.13.6).
In this mode, only requests bound to the server
will be proxied to it.

Prior to version 1.13.6,
the parameter could be changed only with the
API module.

Prior to version 1.29.6,
the parameter was available only as part of our
commercial subscription.

Additionally,
the following parameters are available as part of our
commercial subscription:

slow_start=time

sets the time during which the server will recover its weight
from zero to a nominal value, when unhealthy server becomes
healthy,
or when the server becomes available after a period of time
it was considered unavailable.
Default value is zero, i.e. slow start is disabled.

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

If there is only a single server in a group, max_fails,
fail_timeout and slow_start parameters
are ignored, and such a server will never be considered unavailable.

Syntax:

zone name [size];

Default:

—

Context:

upstream

This directive appeared in version 1.9.0.

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.

Additionally,
as part of our commercial subscription,
such groups allow changing the group membership
or modifying the settings of a particular server
without the need of restarting nginx.
The configuration is accessible via the
API module (1.13.3).

Prior to version 1.13.3,
the configuration was accessible only via a special location
handled by
upstream_conf.

Syntax:

state file;

Default:

—

Context:

upstream

This directive appeared in version 1.9.7.

Specifies a file that keeps the state
of the dynamically configurable group.

Examples:

state /var/lib/nginx/state/servers.conf; # path for Linux
state /var/db/nginx/state/servers.conf; # path for FreeBSD

The state is currently limited to the list of servers with their parameters.
The file is read when parsing the configuration and is updated each time
the upstream configuration is
changed.
Changing the file content directly should be avoided.
The directive cannot be used
along with the server directive.

Changes made during
configuration reload
or binary upgrade
can be lost.

This directive is available as part of our
commercial subscription.

Syntax:

hash key [consistent];

Default:

—

Context:

upstream

This directive appeared in version 1.7.2.

Specifies a load balancing method for a server group
where the client-server mapping is based on the hashed key value.
The key can contain text, variables, and their combinations.
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.

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.

Syntax:

ip_hash;

Default:

—

Context:

upstream

Specifies that a group should use a load balancing method where requests
are distributed between servers based on client IP addresses.
The first three octets of the client 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 the latter case client requests will be passed to another server.
Most probably, it will always be the same server as well.

IPv6 addresses are supported starting from versions 1.3.2 and 1.2.2.

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

Example:

upstream backend {
ip_hash;

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

Until versions 1.3.1 and 1.2.2, it was not possible to specify a weight for
servers using the ip_hash load balancing method.

Syntax:

keepalive connections [local];

Default:

keepalive 32 local;

Context:

upstream

This directive appeared in version 1.1.4.

Enables or disables caching of keepalive connections to upstream servers.
Since 1.29.7, by default the cache for connections is activated
and these connections are not
shared between different
locations.

The local parameter (1.29.7) disables sharing of
cached keepalive connections across different locations
even if the upstream server address matches.
If the local parameter is not specified,
any matching cached connection to the same upstream server
can be reused regardless of a location.

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.
The zero value disables keepalive connections to upstream servers.

Since 1.29.7, keepalive connections are enabled by default, with a
default limit of 32 connections per each worker process.

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

Before 1.29.7, when using load balancing methods other than the default
round-robin method, it was necessary to activate them before
the keepalive directive.

Example configuration of memcached upstream with keepalive connections:

upstream memcached_backend {
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 “1.1” (by default since 1.29.7)
or set to “2”
and the “Connection” header field should be cleared.
This example is for versions prior to 1.29.7:

upstream http_backend {
server 127.0.0.1:8080;

keepalive 16;
}

server {
...

location /http/ {
proxy_pass http://http_backend;
# proxy_http_version 1.1; # before version 1.29.7
# proxy_set_header Connection ""; # before version 1.29.7
...
}
}

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 {
server 127.0.0.1:9000;

keepalive 8;
}

server {
...

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

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

Syntax:

keepalive_requests number;

Default:

keepalive_requests 1000;

Context:

upstream

This directive appeared in version 1.15.3.

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.

Prior to version 1.19.10, the default value was 100.

Syntax:

keepalive_time time;

Default:

keepalive_time 1h;

Context:

upstream

This directive appeared in version 1.19.10.

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.

Syntax:

keepalive_timeout timeout;

Default:

keepalive_timeout 60s;

Context:

upstream

This directive appeared in version 1.15.3.

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

Syntax:

ntlm;

Default:

—

Context:

upstream

This directive appeared in version 1.9.2.

Allows proxying requests with
NTLM
Authentication.
The upstream connection is bound to the client connection
once the client sends a request with the “Authorization”
header field value
starting with “Negotiate” or “NTLM”.
Further client requests will be proxied through the same upstream connection,
keeping the authentication context.

In order for NTLM authentication to work,
it is necessary to enable keepalive connections to upstream servers.
The proxy_http_version
directive should be “1.1” (by default since 1.29.7)
or set to “2”
and the “Connection” header field should be cleared.
This example is for versions prior to 1.29.7:

upstream http_backend {
server 127.0.0.1:8080;

ntlm;
}

server {
...

location /http/ {
proxy_pass http://http_backend;
# proxy_http_version 1.1; # before version 1.29.7
# proxy_set_header Connection ""; # before version 1.29.7
...
}
}

When using load balancer methods other than the default
round-robin method, it is necessary to activate them before
the ntlm directive.

This directive is available as part of our
commercial subscription.

Syntax:

least_conn;

Default:

—

Context:

upstream

This directive appeared in versions 1.3.1 and 1.2.2.

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.

Syntax:

least_time
header |
last_byte
[inflight];

Default:

—

Context:

upstream

This directive appeared in version 1.7.10.

Specifies that a group should use a load balancing method where a request
is passed to the server with the least average response time and
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.

If the header parameter is specified,
time to receive the
response header is used.
If the last_byte parameter is specified,
time to receive the full response
is used.
If the inflight parameter is specified (1.11.6),
incomplete requests are also taken into account.

Prior to version 1.11.6, incomplete requests were taken into account by default.

Prior to version 1.31.0,
this directive was available only as part of our
commercial subscription.

Syntax:

queue
number
[timeout=time];

Default:

—

Context:

upstream

This directive appeared in version 1.5.12.

If an upstream server cannot be selected immediately
while processing a request,
the request will be placed into the queue.
The directive specifies the maximum number of requests
that can be in the queue at the same time.
If the queue is filled up,
or the server to pass the request to cannot be selected within
the time period specified in the timeout parameter,
the 502 (Bad Gateway)
error will be returned to the client.

The default value of the timeout parameter is 60 seconds.

When using load balancer methods other than the default
round-robin method, it is necessary to activate them before
the queue directive.

This directive is available as part of our
commercial subscription.

Syntax:

random [two [method]];

Default:

—

Context:

upstream

This directive appeared in version 1.15.1.

Specifies that a group should use a load balancing method where a request
is passed to a randomly selected server, taking into account weights
of servers.

The optional two parameter
instructs nginx to randomly select
two
servers and then choose a server
using the specified method.
The default method is least_conn
which passes a request to a server
with the least number of active connections.

The least_time method passes a request to a server
with the least average response time and least number of active connections.
If least_time=header is specified, the time to receive the
response header is used.
If least_time=last_byte is specified, the time to receive the
full response is used.

The least_time method is available as a part of our
commercial subscription.

Syntax:

resolver
address ...
[valid=time]
[ipv4=on|off]
[ipv6=on|off]
[status_zone=zone];

Default:

—

Context:

upstream

This directive appeared in version 1.27.3.

Configures name servers used to resolve names of upstream servers
into addresses, for example:

resolver 127.0.0.1 [::1]:5353;

The address can be specified as a domain name or IP address,
with an optional port.
If port is not specified, the port 53 is used.
Name servers are queried in a round-robin fashion.

By default, nginx will look up both IPv4 and IPv6 addresses while resolving.
If looking up of IPv4 or IPv6 addresses is not desired,
the ipv4=off (1.23.1) or
the ipv6=off parameter can be specified.

By default, nginx caches answers using the TTL value of a response.
An optional valid parameter allows overriding it:

resolver 127.0.0.1 [::1]:5353 valid=30s;

To prevent DNS spoofing, it is recommended
configuring DNS servers in a properly secured trusted local network.

The optional status_zone parameter (1.17.5)
enables
collection
of DNS server statistics of requests and responses
in the specified zone.
The parameter is available as part of our
commercial subscription.

Since version 1.17.5 and prior to version 1.27.3,
this directive was available only as part of our
commercial subscription.

Syntax:

resolver_timeout time;

Default:

resolver_timeout 30s;

Context:

upstream

This directive appeared in version 1.27.3.

Sets a timeout for name resolution, for example:

resolver_timeout 5s;

Since version 1.17.5 and prior to version 1.27.3,
this directive was available only as part of our
commercial subscription.

Syntax:

sticky
cookie name
[expires=time]
[domain=domain]
[httponly]
[samesite=strict|lax|none|$variable]
[secure]
[path=path];
sticky
route $variable ...;
sticky
learn
create=$variable
lookup=$variable
zone=name:size
[timeout=time]
[header]
[sync];

Default:

—

Context:

upstream

This directive appeared in version 1.5.7.

Enables session affinity, which causes requests from the same client to be
passed to the same server in a group of servers.
Three methods are available:

cookie

When the cookie method is used, information about the
designated server is passed in an HTTP cookie generated by nginx:

upstream backend {
server backend1.example.com;
server backend2.example.com;

sticky cookie srv_id expires=1h domain=.example.com path=/;
}

A request that comes from a client not yet bound to a particular server
is passed to the server selected by the configured balancing method.
Further requests with this cookie will be passed to the designated server.
If the designated server cannot process a request, the new server is
selected as if the client has not been bound yet.

As a load balancing method always tries to evenly distribute the load
considering already bound requests,
the server with a higher number of active bound requests
has less possibility of getting new unbound requests.

The first parameter sets the name of the cookie to be set or inspected.
The cookie value is
a hexadecimal representation of the MD5 hash of the IP address and port,
or of the UNIX-domain socket path.
However, if the “route” parameter of the
server directive is specified, the cookie value will be
the value of the “route” parameter:

upstream backend {
server backend1.example.com route=a;
server backend2.example.com route=b;

sticky cookie srv_id expires=1h domain=.example.com path=/;
}

In this case, the value of the “srv_id” cookie will be
either a or b.

Additional parameters may be as follows:

expires=time

Sets the time for which a browser should keep the cookie.
The special value max will cause the cookie to expire on
“31 Dec 2037 23:55:55 GMT”.
If the parameter is not specified, it will cause the cookie to expire at
the end of a browser session.

domain=domain

Defines the domain for which the cookie is set.
Parameter value can contain variables (1.11.5).

httponly

Adds the HttpOnly attribute to the cookie (1.7.11).

samesite=strict |
lax | none | $variable

Adds the SameSite (1.19.4) attribute to the cookie
with one of the following values:
Strict,
Lax,
None, or
using variables (1.23.3).
In the latter case, if the variable value is empty,
the SameSite attribute will not be added to the cookie,
if the value is resolved to
Strict,
Lax, or
None,
the corresponding value will be assigned,
otherwise the Strict value will be assigned.

secure

Adds the Secure attribute to the cookie (1.7.11).

path=path

Defines the path for which the cookie is set.

If any parameters are omitted, the corresponding cookie fields are not set.

route

When the route method is used, proxied server assigns
client a route on receipt of the first request.
All subsequent requests from this client will carry routing information
in a cookie or URI.
This information is compared with the “route” parameter
of the server directive to identify the server to which the
request should be proxied.
If the “route” parameter is not specified, the route name
will be a hexadecimal representation of the MD5 hash of the IP address and port,
or of the UNIX-domain socket path.
If the designated server cannot process a request, the new server is
selected by the configured balancing method as if there is no routing
information in the request.

The parameters of the route method specify variables that
may contain routing information.
The first non-empty variable is used to find the matching server.

Example:

map $cookie_jsessionid $route_cookie {
~.+\.(?P<route>\w+)$ $route;
}

map $request_uri $route_uri {
~jsessionid=.+\.(?P<route>\w+)$ $route;
}

upstream backend {
server backend1.example.com route=a;
server backend2.example.com route=b;

sticky route $route_cookie $route_uri;
}

Here, the route is taken from the “JSESSIONID” cookie
if present in a request.
Otherwise, the route from the URI is used.

learn

When the learn method (1.7.1) is used, nginx
analyzes upstream server responses and learns server-initiated sessions
usually passed in an HTTP cookie.

upstream backend {
server backend1.example.com:8080;
server backend2.example.com:8081;

sticky learn
create=$upstream_cookie_examplecookie
lookup=$cookie_examplecookie
zone=client_sessions:1m;
}

In the example, the upstream server creates a session by setting the
cookie “EXAMPLECOOKIE” in the response.
Further requests with this cookie will be passed to the same server.
If the server cannot process the request, the new server is
selected as if the client has not been bound yet.

The parameters create and lookup
specify variables that indicate how new sessions are created and existing
sessions are searched, respectively.
Both parameters may be specified more than once, in which case the first
non-empty variable is used.

Sessions are stored in a shared memory zone, whose name and
size are configured by the zone parameter.
One megabyte zone can store about 4000 sessions on the 64-bit platform.
The sessions that are not accessed during the time specified by the
timeout parameter get removed from the zone.
By default, timeout is set to 10 minutes.

The header parameter (1.13.1) allows creating a session
right after receiving response headers from the upstream server.

The sync parameter (1.13.8) enables
synchronization
of the shared memory zone.
The parameter is available as part of our
commercial subscription.

Prior to version 1.29.6,
this directive was available only as part of our
commercial subscription.

Embedded Variables

The ngx_http_upstream_module module
supports the following embedded variables:

$upstream_addr

keeps 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 (1.11.4).
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 (1.15.8).
Values from several connections
are separated by commas and colons like addresses in the
$upstream_addr variable.

$upstream_cache_status

keeps the status of accessing a response cache (0.8.3).
The status can be either “MISS”,
“BYPASS”, “EXPIRED”,
“STALE”, “UPDATING”,
“REVALIDATED”, or “HIT”.

$upstream_connect_time

keeps time spent on establishing a connection with the upstream server (1.9.1);
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 (1.7.1).
Only the cookies from the response of the last server are saved.

$upstream_header_time

keeps time
spent on receiving the response header from the upstream server (1.7.10);
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_last_addr

keeps the IP address or the path to the UNIX-domain socket
of the last selected upstream server (1.29.3).

This variable is available as part of our
commercial subscription.

$upstream_last_server_name

keeps the name of last selected upstream server (1.25.3);
allows passing it
through SNI:

proxy_ssl_server_name on;
proxy_ssl_name $upstream_last_server_name;

This variable is available as part of our
commercial subscription.

$upstream_queue_time

keeps time the request spent in the upstream queue
(1.13.9);
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_response_length

keeps the length of the response obtained from the upstream server (0.7.27);
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 status code of the response obtained from the upstream server.
Status codes of several responses
are separated by commas and colons like addresses in the
$upstream_addr variable.
If a server cannot be selected,
the variable keeps the 502 (Bad Gateway) status code.

$upstream_trailer_name

keeps fields from the end of the response
obtained from the upstream server (1.13.10).