> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/client-interfaces/libpq/libpq-connect.md).
# 32.1. Database Connection Control Functions \#
[32.1.1. Connection Strings](#LIBPQ-CONNSTRING)
[32.1.2. Parameter Key Words](#LIBPQ-PARAMKEYWORDS)
The following functions deal with making a connection to a PostgreSQL backend server. An application program can have several backend connections open at one time. (One reason to do that is to access more than one database.) Each connection is represented by a `PGconn` object, which is obtained from the function [`PQconnectdb`](#LIBPQ-PQCONNECTDB), [`PQconnectdbParams`](#LIBPQ-PQCONNECTDBPARAMS), or [`PQsetdbLogin`](#LIBPQ-PQSETDBLOGIN). Note that these functions will always return a non-null object pointer, unless perhaps there is too little memory even to allocate the `PGconn` object. The [`PQstatus`](/client-interfaces/libpq/libpq-status.md#LIBPQ-PQSTATUS) function should be called to check the return value for a successful connection before queries are sent via the connection object.
## Warning
If untrusted users have access to a database that has not adopted a [secure schema usage pattern](/the-sql-language/ddl/ddl-schemas.md#DDL-SCHEMAS-PATTERNS), begin each session by removing publicly-writable schemas from `search_path`. One can set parameter key word `options` to value `-csearch_path=`. Alternately, one can issue `PQexec(conn, "SELECT pg_catalog.set_config('search_path', '', false)")` after connecting. This consideration is not specific to libpq; it applies to every interface for executing arbitrary SQL commands.
## Warning
On Unix, forking a process with open libpq connections can lead to unpredictable results because the parent and child processes share the same sockets and operating system resources. For this reason, such usage is not recommended, though doing an `exec` from the child process to load a new executable is safe.
`PQconnectdbParams` [#](#LIBPQ-PQCONNECTDBPARAMS) : Makes a new connection to the database server.
````
```
PGconn *PQconnectdbParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
```
This function opens a new database connection using the parameters taken
from two `NULL`-terminated arrays. The first,
`keywords`, is defined as an array of strings, each one
being a key word. The second, `values`, gives the value
for each key word. Unlike [`PQsetdbLogin`](libpq-connect.md#LIBPQ-PQSETDBLOGIN) below, the parameter
set can be extended without changing the function signature, so use of
this function (or its nonblocking analogs [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS)
and `PQconnectPoll`) is preferred for new application
programming.
The currently recognized parameter key words are listed in
[Section 32.1.2](libpq-connect.md#LIBPQ-PARAMKEYWORDS).
The passed arrays can be empty to use all default parameters, or can
contain one or more parameter settings. They must be matched in length.
Processing will stop at the first `NULL` entry
in the `keywords` array.
Also, if the `values` entry associated with a
non-`NULL` `keywords` entry is
`NULL` or an empty string, that entry is ignored and
processing continues with the next pair of array entries.
When `expand_dbname` is non-zero, the value for
the first *`dbname`* key word is checked to see
if it is a *connection string*. If so, it
is “expanded” into the individual connection
parameters extracted from the string. The value is considered to
be a connection string, rather than just a database name, if it
contains an equal sign (`=`) or it begins with a
URI scheme designator. (More details on connection string formats
appear in [Section 32.1.1](libpq-connect.md#LIBPQ-CONNSTRING).) Only the first
occurrence of *`dbname`* is treated in this way;
any subsequent *`dbname`* parameter is processed
as a plain database name.
In general the parameter arrays are processed from start to end.
If any key word is repeated, the last value (that is
not `NULL` or empty) is used. This rule applies in
particular when a key word found in a connection string conflicts
with one appearing in the `keywords` array. Thus,
the programmer may determine whether array entries can override or
be overridden by values taken from a connection string. Array
entries appearing before an expanded *`dbname`*
entry can be overridden by fields of the connection string, and in
turn those fields are overridden by array entries appearing
after *`dbname`* (but, again, only if those
entries supply non-empty values).
After processing all the array entries and any expanded connection
string, any connection parameters that remain unset are filled with
default values. If an unset parameter's corresponding environment
variable (see [Section 32.15](libpq-envars.md)) is set, its value is
used. If the environment variable is not set either, then the
parameter's built-in default value is used.
````
`PQconnectdb` [#](#LIBPQ-PQCONNECTDB) : Makes a new connection to the database server.
````
```
PGconn *PQconnectdb(const char *conninfo);
```
This function opens a new database connection using the parameters taken
from the string `conninfo`.
The passed string can be empty to use all default parameters, or it can
contain one or more parameter settings separated by whitespace,
or it can contain a URI.
See [Section 32.1.1](libpq-connect.md#LIBPQ-CONNSTRING) for details.
````
`PQsetdbLogin` [#](#LIBPQ-PQSETDBLOGIN) : Makes a new connection to the database server.
````
```
PGconn *PQsetdbLogin(const char *pghost,
const char *pgport,
const char *pgoptions,
const char *pgtty,
const char *dbName,
const char *login,
const char *pwd);
```
This is the predecessor of [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB) with a fixed
set of parameters. It has the same functionality except that the
missing parameters will always take on default values. Write `NULL` or an
empty string for any one of the fixed parameters that is to be defaulted.
If the *`dbName`* contains
an `=` sign or has a valid connection URI prefix, it
is taken as a *`conninfo`* string in exactly the same way as
if it had been passed to [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB), and the remaining
parameters are then applied as specified for [`PQconnectdbParams`](libpq-connect.md#LIBPQ-PQCONNECTDBPARAMS).
`pgtty` is no longer used and any value passed will
be ignored.
````
`PQsetdb` [#](#LIBPQ-PQSETDB) : Makes a new connection to the database server.
````
```
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
```
This is a macro that calls [`PQsetdbLogin`](libpq-connect.md#LIBPQ-PQSETDBLOGIN) with null pointers
for the *`login`* and *`pwd`* parameters. It is provided
for backward compatibility with very old programs.
````
`PQconnectStartParams`\
`PQconnectStart`\
`PQconnectPoll` [#](#LIBPQ-PQCONNECTSTARTPARAMS) :Make a connection to the database server in a nonblocking manner.
````
```
PGconn *PQconnectStartParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
PGconn *PQconnectStart(const char *conninfo);
PostgresPollingStatusType PQconnectPoll(PGconn *conn);
```
These three functions are used to open a connection to a database server such
that your application's thread of execution is not blocked on remote I/O
whilst doing so. The point of this approach is that the waits for I/O to
complete can occur in the application's main loop, rather than down inside
[`PQconnectdbParams`](libpq-connect.md#LIBPQ-PQCONNECTDBPARAMS) or [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB), and so the
application can manage this operation in parallel with other activities.
With [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS), the database connection is made
using the parameters taken from the `keywords` and
`values` arrays, and controlled by `expand_dbname`,
as described above for [`PQconnectdbParams`](libpq-connect.md#LIBPQ-PQCONNECTDBPARAMS).
With `PQconnectStart`, the database connection is made
using the parameters taken from the string `conninfo` as
described above for [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB).
Neither [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS) nor `PQconnectStart`
nor `PQconnectPoll` will block, so long as a number of
restrictions are met:
* The `hostaddr` parameter must be used appropriately
to prevent DNS queries from being made. See the documentation of
this parameter in [Section 32.1.2](libpq-connect.md#LIBPQ-PARAMKEYWORDS) for details.
* If you call [`PQtrace`](libpq-control.md#LIBPQ-PQTRACE), ensure that the stream object
into which you trace will not block.
* You must ensure that the socket is in the appropriate state
before calling `PQconnectPoll`, as described below.
To begin a nonblocking connection request,
call `PQconnectStart`
or [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS). If the result is null,
then libpq has been unable to allocate a
new `PGconn` structure. Otherwise, a
valid `PGconn` pointer is returned (though not
yet representing a valid connection to the database). Next
call `PQstatus(conn)`. If the result
is `CONNECTION_BAD`, the connection attempt has already
failed, typically because of invalid connection parameters.
If `PQconnectStart`
or [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS) succeeds, the next stage
is to poll libpq so that it can proceed with
the connection sequence.
Use `PQsocket(conn)` to obtain the descriptor of the
socket underlying the database connection.
(Caution: do not assume that the socket remains the same
across `PQconnectPoll` calls.)
Loop thus: If `PQconnectPoll(conn)` last returned
`PGRES_POLLING_READING`, wait until the socket is ready to
read (as indicated by `select()`, `poll()`, or
similar system function). Note that `PQsocketPoll`
can help reduce boilerplate by abstracting the setup of
`select(2)` or `poll(2)` if it is
available on your system.
Then call `PQconnectPoll(conn)` again.
Conversely, if `PQconnectPoll(conn)` last returned
`PGRES_POLLING_WRITING`, wait until the socket is ready
to write, then call `PQconnectPoll(conn)` again.
On the first iteration, i.e., if you have yet to call
`PQconnectPoll`, behave as if it last returned
`PGRES_POLLING_WRITING`. Continue this loop until
`PQconnectPoll(conn)` returns
`PGRES_POLLING_FAILED`, indicating the connection procedure
has failed, or `PGRES_POLLING_OK`, indicating the connection
has been successfully made.
At any time during connection, the status of the connection can be
checked by calling [`PQstatus`](libpq-status.md#LIBPQ-PQSTATUS). If this call returns `CONNECTION_BAD`, then the
connection procedure has failed; if the call returns `CONNECTION_OK`, then the
connection is ready. Both of these states are equally detectable
from the return value of `PQconnectPoll`, described above. Other states might also occur
during (and only during) an asynchronous connection procedure. These
indicate the current stage of the connection procedure and might be useful
to provide feedback to the user for example. These statuses are:
`CONNECTION_STARTED` [#](#LIBPQ-CONNECTION-STARTED)
: Waiting for connection to be made.
`CONNECTION_MADE` [#](#LIBPQ-CONNECTION-MADE)
: Connection OK; waiting to send.
`CONNECTION_AWAITING_RESPONSE` [#](#LIBPQ-CONNECTION-AWAITING-RESPONSE)
: Waiting for a response from the server.
`CONNECTION_AUTH_OK` [#](#LIBPQ-CONNECTION-AUTH-OK)
: Received authentication; waiting for backend start-up to finish.
`CONNECTION_SSL_STARTUP` [#](#LIBPQ-CONNECTION-SSL-STARTUP)
: Negotiating SSL encryption.
`CONNECTION_GSS_STARTUP` [#](#LIBPQ-CONNECTION-GSS-STARTUP)
: Negotiating GSS encryption.
`CONNECTION_CHECK_WRITABLE` [#](#LIBPQ-CONNECTION-CHECK-WRITABLE)
: Checking if connection is able to handle write transactions.
`CONNECTION_CHECK_STANDBY` [#](#LIBPQ-CONNECTION-CHECK-STANDBY)
: Checking if connection is to a server in standby mode.
`CONNECTION_CONSUME` [#](#LIBPQ-CONNECTION-CONSUME)
: Consuming any remaining response messages on connection.
Note that, although these constants will remain (in order to maintain
compatibility), an application should never rely upon these occurring in a
particular order, or at all, or on the status always being one of these
documented values. An application might do something like this:
```
switch(PQstatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
.
.
.
default:
feedback = "Connecting...";
}
```
The `connect_timeout` connection parameter is ignored
when using `PQconnectPoll`; it is the application's
responsibility to decide whether an excessive amount of time has elapsed.
Otherwise, `PQconnectStart` followed by a
`PQconnectPoll` loop is equivalent to
[`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB).
Note that when `PQconnectStart`
or [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS) returns a non-null
pointer, you must call [`PQfinish`](libpq-connect.md#LIBPQ-PQFINISH) when you are
finished with it, in order to dispose of the structure and any
associated memory blocks. This must be done even if the connection
attempt fails or is abandoned.
````
`PQsocketPoll` [#](#LIBPQ-PQSOCKETPOLL) :Poll a connection's underlying socket descriptor retrieved with [`PQsocket`](/client-interfaces/libpq/libpq-status.md#LIBPQ-PQSOCKET). The primary use of this function is iterating through the connection sequence described in the documentation of [`PQconnectStartParams`](#LIBPQ-PQCONNECTSTARTPARAMS).
````
```
typedef int64_t pg_usec_time_t;
int PQsocketPoll(int sock, int forRead, int forWrite,
pg_usec_time_t end_time);
```
This function performs polling of a file descriptor, optionally with
a timeout.
If *`forRead`* is nonzero, the
function will terminate when the socket is ready for
reading. If *`forWrite`* is nonzero,
the function will terminate when the
socket is ready for writing.
The timeout is specified by *`end_time`*, which
is the time to stop waiting expressed as a number of microseconds since
the Unix epoch (that is, `time_t` times 1 million).
Timeout is infinite if *`end_time`*
is `-1`. Timeout is immediate (no blocking) if
*`end_time`* is `0` (or indeed, any
time before now). Timeout values can be calculated conveniently by
adding the desired number of microseconds to the result of
[`PQgetCurrentTimeUSec`](libpq-misc.md#LIBPQ-PQGETCURRENTTIMEUSEC).
Note that the underlying system calls may have less than microsecond
precision, so that the actual delay may be imprecise.
The function returns a value greater than `0` if the
specified condition is met, `0` if a timeout occurred,
or `-1` if an error occurred. The error can be
retrieved by checking the `errno(3)` value. In the
event both *`forRead`*
and *`forWrite`* are zero, the function immediately
returns a timeout indication.
`PQsocketPoll` is implemented using either
`poll(2)` or `select(2)`,
depending on platform. See `POLLIN`
and `POLLOUT` from `poll(2)`,
or *`readfds`* and
*`writefds`* from `select(2)`,
for more information.
````
`PQconndefaults` [#](#LIBPQ-PQCONNDEFAULTS) : Returns the default connection options.
````
```
PQconninfoOption *PQconndefaults(void);
typedef struct
{
char *keyword; /* The keyword of the option */
char *envvar; /* Fallback environment variable name */
char *compiled; /* Fallback compiled in default value */
char *val; /* Option's current value, or NULL */
char *label; /* Label for field in connect dialog */
char *dispchar; /* Indicates how to display this field
in a connect dialog. Values are:
"" Display entered value as is
"*" Password field - hide value
"D" Debug option - don't show by default */
int dispsize; /* Field size in characters for dialog */
} PQconninfoOption;
```
Returns a connection options array. This can be used to determine
all possible [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB) options and their
current default values. The return value points to an array of
`PQconninfoOption` structures, which ends
with an entry having a null `keyword` pointer. The
null pointer is returned if memory could not be allocated. Note that
the current default values (`val` fields)
will depend on environment variables and other context. A
missing or invalid service file will be silently ignored. Callers
must treat the connection options data as read-only.
After processing the options array, free it by passing it to
[`PQconninfoFree`](libpq-misc.md#LIBPQ-PQCONNINFOFREE). If this is not done, a small amount of memory
is leaked for each call to [`PQconndefaults`](libpq-connect.md#LIBPQ-PQCONNDEFAULTS).
````
`PQconninfo` [#](#LIBPQ-PQCONNINFO) : Returns the connection options used by a live connection.
````
```
PQconninfoOption *PQconninfo(PGconn *conn);
```
Returns a connection options array. This can be used to determine
all possible [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB) options and the
values that were used to connect to the server. The return
value points to an array of `PQconninfoOption`
structures, which ends with an entry having a null `keyword`
pointer. All notes above for [`PQconndefaults`](libpq-connect.md#LIBPQ-PQCONNDEFAULTS) also
apply to the result of [`PQconninfo`](libpq-connect.md#LIBPQ-PQCONNINFO).
````
`PQconninfoParse` [#](#LIBPQ-PQCONNINFOPARSE) : Returns parsed connection options from the provided connection string.
````
```
PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);
```
Parses a connection string and returns the resulting options as an
array; or returns `NULL` if there is a problem with the connection
string. This function can be used to extract
the [`PQconnectdb`](libpq-connect.md#LIBPQ-PQCONNECTDB) options in the provided
connection string. The return value points to an array of
`PQconninfoOption` structures, which ends
with an entry having a null `keyword` pointer.
All legal options will be present in the result array, but the
`PQconninfoOption` for any option not present
in the connection string will have `val` set to
`NULL`; default values are not inserted.
If `errmsg` is not `NULL`, then `*errmsg` is set
to `NULL` on success, else to a `malloc`'d error string explaining
the problem. (It is also possible for `*errmsg` to be
set to `NULL` and the function to return `NULL`;
this indicates an out-of-memory condition.)
After processing the options array, free it by passing it to
[`PQconninfoFree`](libpq-misc.md#LIBPQ-PQCONNINFOFREE). If this is not done, some memory
is leaked for each call to [`PQconninfoParse`](libpq-connect.md#LIBPQ-PQCONNINFOPARSE).
Conversely, if an error occurs and `errmsg` is not `NULL`,
be sure to free the error string using [`PQfreemem`](libpq-misc.md#LIBPQ-PQFREEMEM).
````
`PQfinish` [#](#LIBPQ-PQFINISH) : Closes the connection to the server. Also frees memory used by the `PGconn` object.
````
```
void PQfinish(PGconn *conn);
```
Note that even if the server connection attempt fails (as
indicated by [`PQstatus`](libpq-status.md#LIBPQ-PQSTATUS)), the application should call [`PQfinish`](libpq-connect.md#LIBPQ-PQFINISH)
to free the memory used by the `PGconn` object.
The `PGconn` pointer must not be used again after
[`PQfinish`](libpq-connect.md#LIBPQ-PQFINISH) has been called.
````
`PQreset` [#](#LIBPQ-PQRESET) : Resets the communication channel to the server.
````
```
void PQreset(PGconn *conn);
```
This function will close the connection
to the server and attempt to establish a new
connection, using all the same
parameters previously used. This might be useful for
error recovery if a working connection is lost.
````
`PQresetStart`\
`PQresetPoll` [#](#LIBPQ-PQRESETSTART) : Reset the communication channel to the server, in a nonblocking manner.
````
```
int PQresetStart(PGconn *conn);
PostgresPollingStatusType PQresetPoll(PGconn *conn);
```
These functions will close the connection to the server and attempt to
establish a new connection, using all the same
parameters previously used. This can be useful for error recovery if a
working connection is lost. They differ from [`PQreset`](libpq-connect.md#LIBPQ-PQRESET) (above) in that they
act in a nonblocking manner. These functions suffer from the same
restrictions as [`PQconnectStartParams`](libpq-connect.md#LIBPQ-PQCONNECTSTARTPARAMS), `PQconnectStart`
and `PQconnectPoll`.
To initiate a connection reset, call
[`PQresetStart`](libpq-connect.md#LIBPQ-PQRESETSTART). If it returns 0, the reset has
failed. If it returns 1, poll the reset using
`PQresetPoll` in exactly the same way as you
would create the connection using `PQconnectPoll`.
````
`PQpingParams` [#](#LIBPQ-PQPINGPARAMS) : [`PQpingParams`](#LIBPQ-PQPINGPARAMS) reports the status of the server. It accepts connection parameters identical to those of [`PQconnectdbParams`](#LIBPQ-PQCONNECTDBPARAMS), described above. It is not necessary to supply correct user name, password, or database name values to obtain the server status; however, if incorrect values are provided, the server will log a failed connection attempt.
````
```
PGPing PQpingParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
```
The function returns one of the following values:
`PQPING_OK` [#](#LIBPQ-PQPINGPARAMS-PQPING_OK)
: The server is running and appears to be accepting connections.
`PQPING_REJECT` [#](#LIBPQ-PQPINGPARAMS-PQPING_REJECT)
: The server is running but is in a state that disallows connections
(startup, shutdown, or crash recovery).
`PQPING_NO_RESPONSE` [#](#LIBPQ-PQPINGPARAMS-PQPING_NO_RESPONSE)
: The server could not be contacted. This might indicate that the
server is not running, or that there is something wrong with the
given connection parameters (for example, wrong port number), or
that there is a network connectivity problem (for example, a
firewall blocking the connection request).
`PQPING_NO_ATTEMPT` [#](#LIBPQ-PQPINGPARAMS-PQPING_NO_ATTEMPT)
: No attempt was made to contact the server, because the supplied
parameters were obviously incorrect or there was some client-side
problem (for example, out of memory).
````
`PQping` [#](#LIBPQ-PQPING) : [`PQping`](#LIBPQ-PQPING) reports the status of the server. It accepts connection parameters identical to those of [`PQconnectdb`](#LIBPQ-PQCONNECTDB), described above. It is not necessary to supply correct user name, password, or database name values to obtain the server status; however, if incorrect values are provided, the server will log a failed connection attempt.
````
```
PGPing PQping(const char *conninfo);
```
The return values are the same as for [`PQpingParams`](libpq-connect.md#LIBPQ-PQPINGPARAMS).
````
`PQsetSSLKeyPassHook_OpenSSL` [#](#LIBPQ-PQSETSSLKEYPASSHOOK-OPENSSL) : `PQsetSSLKeyPassHook_OpenSSL` lets an application override libpq's [default handling of encrypted client certificate key files](/client-interfaces/libpq/libpq-ssl.md#LIBPQ-SSL-CLIENTCERT) using [sslpassword](#LIBPQ-CONNECT-SSLPASSWORD) or interactive prompting.
````
```
void PQsetSSLKeyPassHook_OpenSSL(PQsslKeyPassHook_OpenSSL_type hook);
```
The application passes a pointer to a callback function with signature:
```
int callback_fn(char *buf, int size, PGconn *conn);
```
which libpq will then call
*instead of* its default
`PQdefaultSSLKeyPassHook_OpenSSL` handler. The
callback should determine the password for the key and copy it to
result-buffer *`buf`* of size
*`size`*. The string in *`buf`*
must be null-terminated. The callback must return the length of the
password stored in *`buf`* excluding the null
terminator. On failure, the callback should set
`buf[0] = '\0'` and return 0. See
`PQdefaultSSLKeyPassHook_OpenSSL` in
libpq's source code for an example.
If the user specified an explicit key location,
its path will be in `conn->sslkey` when the callback
is invoked. This will be empty if the default key path is being used.
For keys that are engine specifiers, it is up to engine implementations
whether they use the OpenSSL password
callback or define their own handling.
The app callback may choose to delegate unhandled cases to
`PQdefaultSSLKeyPassHook_OpenSSL`,
or call it first and try something else if it returns 0, or completely override it.
The callback *must not* escape normal flow control with exceptions,
`longjmp(...)`, etc. It must return normally.
````
`PQgetSSLKeyPassHook_OpenSSL` [#](#LIBPQ-PQGETSSLKEYPASSHOOK-OPENSSL) : `PQgetSSLKeyPassHook_OpenSSL` returns the current client certificate key password hook, or `NULL` if none has been set.
````
```
PQsslKeyPassHook_OpenSSL_type PQgetSSLKeyPassHook_OpenSSL(void);
```
````
## 32.1.1. Connection Strings [#](#LIBPQ-CONNSTRING)
Several libpq functions parse a user-specified string to obtain connection parameters. There are two accepted formats for these strings: plain keyword/value strings and URIs. URIs generally follow [RFC 3986](https://datatracker.ietf.org/doc/html/rfc3986), except that multi-host connection strings are allowed as further described below.
### 32.1.1.1. Keyword/Value Connection Strings [#](#LIBPQ-CONNSTRING-KEYWORD-VALUE)
In the keyword/value format, each parameter setting is in the form *`keyword`* `=` *`value`*, with space(s) between settings. Spaces around a setting's equal sign are optional. To write an empty value, or a value containing spaces, surround it with single quotes, for example `keyword = 'a value'`. Single quotes and backslashes within a value must be escaped with a backslash, i.e., `\'` and `\\`.
Example:
```
host=localhost port=5432 dbname=mydb connect_timeout=10
```
The recognized parameter key words are listed in [Section 32.1.2](#LIBPQ-PARAMKEYWORDS).
### 32.1.1.2. Connection URIs [#](#LIBPQ-CONNSTRING-URIS)
The general form for a connection URI is:
```
postgresql://[userspec@][hostspec][/dbname][?paramspec]
where userspec is:
user[:password]
and hostspec is:
[host][:port][,...]
and paramspec is:
name=value[&...]
```
The URI scheme designator can be either `postgresql://` or `postgres://`. Each of the remaining URI parts is optional. The following examples illustrate valid URI syntax:
```
postgresql://
postgresql://localhost
postgresql://localhost:5433
postgresql://localhost/mydb
postgresql://user@localhost
postgresql://user:secret@localhost
postgresql://other@localhost/otherdb?connect_timeout=10&application_name=myapp
postgresql://host1:123,host2:456/somedb?target_session_attrs=any&application_name=myapp
```
Values that would normally appear in the hierarchical part of the URI can alternatively be given as named parameters. For example:
```
postgresql:///mydb?host=localhost&port=5433
```
All named parameters must match key words listed in [Section 32.1.2](#LIBPQ-PARAMKEYWORDS), except that for compatibility with JDBC connection URIs, instances of `ssl=true` are translated into `sslmode=require`.
The connection URI needs to be encoded with [percent-encoding](https://datatracker.ietf.org/doc/html/rfc3986#section-2.1) if it includes symbols with special meaning in any of its parts. Here is an example where the equal sign (`=`) is replaced with `%3D` and the space character with `%20`:
```
postgresql://user@localhost:5433/mydb?options=-c%20synchronous_commit%3Doff
```
The host part may be either a host name or an IP address. To specify an IPv6 address, enclose it in square brackets:
```
postgresql://[2001:db8::1234]/database
```
The host part is interpreted as described for the parameter [host](#LIBPQ-CONNECT-HOST). In particular, a Unix-domain socket connection is chosen if the host part is either empty or looks like an absolute path name, otherwise a TCP/IP connection is initiated. Note, however, that the slash is a reserved character in the hierarchical part of the URI. So, to specify a non-standard Unix-domain socket directory, either omit the host part of the URI and specify the host as a named parameter, or percent-encode the path in the host part of the URI:
```
postgresql:///dbname?host=/var/lib/postgresql
postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
```
It is possible to specify multiple host components, each with an optional port component, in a single URI. A URI of the form `postgresql://host1:port1,host2:port2,host3:port3/` is equivalent to a connection string of the form `host=host1,host2,host3 port=port1,port2,port3`. As further described below, each host will be tried in turn until a connection is successfully established.
### 32.1.1.3. Specifying Multiple Hosts [#](#LIBPQ-MULTIPLE-HOSTS)
It is possible to specify multiple hosts to connect to, so that they are tried in the given order. In the Keyword/Value format, the `host`, `hostaddr`, and `port` options accept comma-separated lists of values. The same number of elements must be given in each option that is specified, such that e.g., the first `hostaddr` corresponds to the first host name, the second `hostaddr` corresponds to the second host name, and so forth. As an exception, if only one `port` is specified, it applies to all the hosts.
In the connection URI format, you can list multiple `host:port` pairs separated by commas in the `host` component of the URI.
In either format, a single host name can translate to multiple network addresses. A common example of this is a host that has both an IPv4 and an IPv6 address.
When multiple hosts are specified, or when a single host name is translated to multiple addresses, all the hosts and addresses will be tried in order, until one succeeds. If none of the hosts can be reached, the connection fails. If a connection is established successfully, but authentication fails, the remaining hosts in the list are not tried.
If a password file is used, you can have different passwords for different hosts. All the other connection options are the same for every host in the list; it is not possible to e.g., specify different usernames for different hosts.
## 32.1.2. Parameter Key Words [#](#LIBPQ-PARAMKEYWORDS)
The currently recognized parameter key words are:
`host` [#](#LIBPQ-CONNECT-HOST) : Name of host to connect to. If a host name looks like an absolute path name, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored. (On Unix, an absolute path name begins with a slash. On Windows, paths starting with drive letters are also recognized.) If the host name starts with `@`, it is taken as a Unix-domain socket in the abstract namespace (currently supported on Linux and Windows). The default behavior when `host` is not specified, or is empty, is to connect to a Unix-domain socket in `/tmp` (or whatever socket directory was specified when PostgreSQL was built). On Windows, the default is to connect to `localhost`.
```
A comma-separated list of host names is also accepted, in which case
each host name in the list is tried in order; an empty item in the
list selects the default behavior as explained above. See
[Section 32.1.1.3](libpq-connect.md#LIBPQ-MULTIPLE-HOSTS) for details.
```
`hostaddr` [#](#LIBPQ-CONNECT-HOSTADDR) : Numeric IP address of host to connect to. This should be in the standard IPv4 address format, e.g., `172.28.40.9`. If your machine supports IPv6, you can also use those addresses. TCP/IP communication is always used when a nonempty string is specified for this parameter. If this parameter is not specified, the value of `host` will be looked up to find the corresponding IP address — or, if `host` specifies an IP address, that value will be used directly.
```
Using `hostaddr` allows the
application to avoid a host name look-up, which might be important
in applications with time constraints. However, a host name is
required for GSSAPI or SSPI authentication
methods, as well as for `verify-full` SSL
certificate verification. The following rules are used:
* If `host` is specified
without `hostaddr`, a host name lookup occurs.
(When using `PQconnectPoll`, the lookup occurs
when `PQconnectPoll` first considers this host
name, and it may cause `PQconnectPoll` to block
for a significant amount of time.)
* If `hostaddr` is specified without `host`,
the value for `hostaddr` gives the server network address.
The connection attempt will fail if the authentication
method requires a host name.
* If both `host` and `hostaddr` are specified,
the value for `hostaddr` gives the server network address.
The value for `host` is ignored unless the
authentication method requires it, in which case it will be
used as the host name.
Note that authentication is likely to fail if `host`
is not the name of the server at network address `hostaddr`.
Also, when both `host` and `hostaddr`
are specified, `host`
is used to identify the connection in a password file (see
[Section 32.16](libpq-pgpass.md)).
A comma-separated list of `hostaddr` values is also
accepted, in which case each host in the list is tried in order.
An empty item in the list causes the corresponding host name to be
used, or the default host name if that is empty as well. See
[Section 32.1.1.3](libpq-connect.md#LIBPQ-MULTIPLE-HOSTS) for details.
Without either a host name or host address,
libpq will connect using a local
Unix-domain socket; or on Windows, it will attempt to connect to
`localhost`.
```
`port` [#](#LIBPQ-CONNECT-PORT) : Port number to connect to at the server host, or socket file name extension for Unix-domain connections. If multiple hosts were given in the `host` or `hostaddr` parameters, this parameter may specify a comma-separated list of ports of the same length as the host list, or it may specify a single port number to be used for all hosts. An empty string, or an empty item in a comma-separated list, specifies the default port number established when PostgreSQL was built.
`dbname` [#](#LIBPQ-CONNECT-DBNAME) : The database name. Defaults to be the same as the user name. In certain contexts, the value is checked for extended formats; see [Section 32.1.1](#LIBPQ-CONNSTRING) for more details on those.
`user` [#](#LIBPQ-CONNECT-USER) : PostgreSQL user name to connect as. Defaults to be the same as the operating system name of the user running the application.
`password` [#](#LIBPQ-CONNECT-PASSWORD) : Password to be used if the server demands password authentication.
`passfile` [#](#LIBPQ-CONNECT-PASSFILE) : Specifies the name of the file used to store passwords (see [Section 32.16](/client-interfaces/libpq/libpq-pgpass.md)). Defaults to `~/.pgpass`, or `%APPDATA%\postgresql\pgpass.conf` on Microsoft Windows. (No error is reported if this file does not exist.)
`require_auth` [#](#LIBPQ-CONNECT-REQUIRE-AUTH) : Specifies the authentication method that the client requires from the server. If the server does not use the required method to authenticate the client, or if the authentication handshake is not fully completed by the server, the connection will fail. A comma-separated list of methods may also be provided, of which the server must use exactly one in order for the connection to succeed. By default, any authentication method is accepted, and the server is free to skip authentication altogether.
```
Methods may be negated with the addition of a `!`
prefix, in which case the server must *not* attempt
the listed method; any other method is accepted, and the server is free
not to authenticate the client at all. If a comma-separated list is
provided, the server may not attempt *any* of the
listed negated methods. Negated and non-negated forms may not be
combined in the same setting.
As a final special case, the `none` method requires the
server not to use an authentication challenge. (It may also be negated,
to require some form of authentication.)
The following methods may be specified:
`password`
: The server must request plaintext password authentication.
`md5`
: The server must request MD5 hashed password authentication.
### Warning
Support for MD5-encrypted passwords is deprecated and will be
removed in a future release of
PostgreSQL. Refer to
[Section 20.5](../../server-administration/client-authentication/auth-password.md) for details about migrating to
another password type.
`gss`
: The server must either request a Kerberos handshake via
GSSAPI or establish a
GSS-encrypted channel (see also
[gssencmode](libpq-connect.md#LIBPQ-CONNECT-GSSENCMODE)).
`sspi`
: The server must request Windows SSPI
authentication.
`scram-sha-256`
: The server must successfully complete a SCRAM-SHA-256 authentication
exchange with the client.
`oauth`
: The server must request an OAuth bearer token from the client.
`none`
: The server must not prompt the client for an authentication
exchange. (This does not prohibit client certificate authentication
via TLS, nor GSS authentication via its encrypted transport.)
```
`channel_binding` [#](#LIBPQ-CONNECT-CHANNEL-BINDING) : This option controls the client's use of channel binding. A setting of `require` means that the connection must employ channel binding, `prefer` means that the client will choose channel binding if available, and `disable` prevents the use of channel binding. The default is `prefer` if PostgreSQL is compiled with SSL support; otherwise the default is `disable`.
```
Channel binding is a method for the server to authenticate itself to
the client. It is only supported over SSL connections
with PostgreSQL 11 or later servers using
the `SCRAM` authentication method.
```
`connect_timeout` [#](#LIBPQ-CONNECT-CONNECT-TIMEOUT) : Maximum time to wait while connecting, in seconds (write as a decimal integer, e.g., `10`). Zero, negative, or not specified means wait indefinitely. This timeout applies separately to each host name or IP address. For example, if you specify two hosts and `connect_timeout` is 5, each host will time out if no connection is made within 5 seconds, so the total time spent waiting for a connection might be up to 10 seconds.
`client_encoding` [#](#LIBPQ-CONNECT-CLIENT-ENCODING) : This sets the `client_encoding` configuration parameter for this connection. In addition to the values accepted by the corresponding server option, you can use `auto` to determine the right encoding from the current locale in the client (`LC_CTYPE` environment variable on Unix systems).
`options` [#](#LIBPQ-CONNECT-OPTIONS) : Specifies command-line options to send to the server at connection start. For example, setting this to `-c geqo=off` or `--geqo=off` sets the session's value of the `geqo` parameter to `off`. Spaces within this string are considered to separate command-line arguments, unless escaped with a backslash (`\`); write `\\` to represent a literal backslash. For a detailed discussion of the available options, consult [Chapter 19](/server-administration/runtime-config.md).
`application_name` [#](#LIBPQ-CONNECT-APPLICATION-NAME) : Specifies a value for the [application\_name](/server-administration/runtime-config/runtime-config-logging.md#GUC-APPLICATION-NAME) configuration parameter.
`fallback_application_name` [#](#LIBPQ-CONNECT-FALLBACK-APPLICATION-NAME) : Specifies a fallback value for the [application\_name](/server-administration/runtime-config/runtime-config-logging.md#GUC-APPLICATION-NAME) configuration parameter. This value will be used if no value has been given for `application_name` via a connection parameter or the `PGAPPNAME` environment variable. Specifying a fallback name is useful in generic utility programs that wish to set a default application name but allow it to be overridden by the user.
`keepalives` [#](#LIBPQ-KEEPALIVES) : Controls whether client-side TCP keepalives are used. The default value is 1, meaning on, but you can change this to 0, meaning off, if keepalives are not wanted. This parameter is ignored for connections made via a Unix-domain socket.
`keepalives_idle` [#](#LIBPQ-KEEPALIVES-IDLE) : Controls the number of seconds of inactivity after which TCP should send a keepalive message to the server. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where `TCP_KEEPIDLE` or an equivalent socket option is available, and on Windows; on other systems, it has no effect.
`keepalives_interval` [#](#LIBPQ-KEEPALIVES-INTERVAL) : Controls the number of seconds after which a TCP keepalive message that is not acknowledged by the server should be retransmitted. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where `TCP_KEEPINTVL` or an equivalent socket option is available, and on Windows; on other systems, it has no effect.
`keepalives_count` [#](#LIBPQ-KEEPALIVES-COUNT) : Controls the number of TCP keepalives that can be lost before the client's connection to the server is considered dead. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where `TCP_KEEPCNT` or an equivalent socket option is available; on other systems, it has no effect.
`tcp_user_timeout` [#](#LIBPQ-TCP-USER-TIMEOUT) : Controls the number of milliseconds that transmitted data may remain unacknowledged before a connection is forcibly closed. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket. It is only supported on systems where `TCP_USER_TIMEOUT` is available; on other systems, it has no effect.
`replication` [#](#LIBPQ-CONNECT-REPLICATION) : This option determines whether the connection should use the replication protocol instead of the normal protocol. This is what PostgreSQL replication connections as well as tools such as pg\_basebackup use internally, but it can also be used by third-party applications. For a description of the replication protocol, consult [Section 54.4](/internals/protocol/protocol-replication.md).
```
The following values, which are case-insensitive, are supported:
`true`, `on`, `yes`, `1`
: The connection goes into physical replication mode.
`database`
: The connection goes into logical replication mode, connecting to
the database specified in the `dbname` parameter.
`false`, `off`, `no`, `0`
: The connection is a regular one, which is the default behavior.
In physical or logical replication mode, only the simple query protocol
can be used.
```
`gssencmode` [#](#LIBPQ-CONNECT-GSSENCMODE) : This option determines whether or with what priority a secure GSS TCP/IP connection will be negotiated with the server. There are three modes:
```
`disable`
: only try a non-GSSAPI-encrypted connection
`prefer` (default)
: if there are GSSAPI credentials present (i.e.,
in a credentials cache), first try
a GSSAPI-encrypted connection; if that fails or
there are no credentials, try a
non-GSSAPI-encrypted connection. This is the
default when PostgreSQL has been
compiled with GSSAPI support.
`require`
: only try a GSSAPI-encrypted connection
`gssencmode` is ignored for Unix domain socket
communication. If PostgreSQL is compiled
without GSSAPI support, using the `require` option
will cause an error, while `prefer` will be accepted
but libpq will not actually attempt
a GSSAPI-encrypted
connection.
```
`sslmode` [#](#LIBPQ-CONNECT-SSLMODE) : This option determines whether or with what priority a secure SSL TCP/IP connection will be negotiated with the server. There are six modes:
```
`disable`
: only try a non-SSL connection
`allow`
: first try a non-SSL connection; if that
fails, try an SSL connection
`prefer` (default)
: first try an SSL connection; if that fails,
try a non-SSL connection
`require`
: only try an SSL connection. If a root CA
file is present, verify the certificate in the same way as
if `verify-ca` was specified
`verify-ca`
: only try an SSL connection, and verify that
the server certificate is issued by a trusted
certificate authority (CA)
`verify-full`
: only try an SSL connection, verify that the
server certificate is issued by a
trusted CA and that the requested server host name
matches that in the certificate
See [Section 32.19](libpq-ssl.md) for a detailed description of how
these options work.
`sslmode` is ignored for Unix domain socket
communication.
If PostgreSQL is compiled without SSL support,
using options `require`, `verify-ca`, or
`verify-full` will cause an error, while
options `allow` and `prefer` will be
accepted but libpq will not actually attempt
an SSL
connection.
Note that if GSSAPI encryption is possible,
that will be used in preference to SSL
encryption, regardless of the value of `sslmode`.
To force use of SSL encryption in an
environment that has working GSSAPI
infrastructure (such as a Kerberos server), also set
`gssencmode` to `disable`.
```
`requiressl` [#](#LIBPQ-CONNECT-REQUIRESSL) : This option is deprecated in favor of the `sslmode` setting.
```
If set to 1, an SSL connection to the server
is required (this is equivalent to `sslmode`
`require`). libpq will then refuse
to connect if the server does not accept an
SSL connection. If set to 0 (default),
libpq will negotiate the connection type with
the server (equivalent to `sslmode`
`prefer`). This option is only available if
PostgreSQL is compiled with SSL support.
```
`sslnegotiation` [#](#LIBPQ-CONNECT-SSLNEGOTIATION) : This option controls how SSL encryption is negotiated with the server, if SSL is used. In the default `postgres` mode, the client first asks the server if SSL is supported. In `direct` mode, the client starts the standard SSL handshake directly after establishing the TCP/IP connection. Traditional PostgreSQL protocol negotiation is the most flexible with different server configurations. If the server is known to support direct SSL connections then the latter requires one fewer round trip reducing connection latency and also allows the use of protocol agnostic SSL network tools. The direct SSL option was introduced in PostgreSQL version 17.
```
`postgres`
: perform PostgreSQL protocol
negotiation. This is the default if the option is not provided.
`direct`
: start SSL handshake directly after establishing the TCP/IP
connection. This is only allowed with
`sslmode=require` or higher, because the weaker
settings could lead to unintended fallback to plaintext
authentication when the server does not support direct SSL
handshake.
```
`sslcompression` [#](#LIBPQ-CONNECT-SSLCOMPRESSION) : If set to 1, data sent over SSL connections will be compressed. If set to 0, compression will be disabled. The default is 0. This parameter is ignored if a connection without SSL is made.
```
SSL compression is nowadays considered insecure and its use is no
longer recommended. OpenSSL 1.1.0 disabled
compression by default, and many operating system distributions
disabled it in prior versions as well, so setting this parameter to on
will not have any effect if the server does not accept compression.
PostgreSQL 14 disabled compression
completely in the backend.
If security is not a primary concern, compression can improve
throughput if the network is the bottleneck. Disabling compression
can improve response time and throughput if CPU performance is the
limiting factor.
```
`sslcert` [#](#LIBPQ-CONNECT-SSLCERT) : This parameter specifies the file name of the client SSL certificate, replacing the default `~/.postgresql/postgresql.crt`. This parameter is ignored if an SSL connection is not made.
`sslkey` [#](#LIBPQ-CONNECT-SSLKEY) : This parameter specifies the location for the secret key used for the client certificate. It can either specify a file name that will be used instead of the default `~/.postgresql/postgresql.key`, or it can specify a key obtained from an external “engine” (engines are OpenSSL loadable modules). An external engine specification should consist of a colon-separated engine name and an engine-specific key identifier. This parameter is ignored if an SSL connection is not made.
`sslkeylogfile` [#](#LIBPQ-CONNECT-SSLKEYLOGFILE) : This parameter specifies the location where libpq will log keys used in this SSL context. This is useful for debugging PostgreSQL protocol interactions or client connections using network inspection tools like Wireshark. This parameter is ignored if an SSL connection is not made, or if LibreSSL is used (LibreSSL does not support key logging). Keys are logged using the NSS format.
```
### Warning
Key logging will expose potentially sensitive information in the
keylog file. Keylog files should be handled with the same care as
[sslkey](libpq-connect.md#LIBPQ-CONNECT-SSLKEY) files.
```
`sslpassword` [#](#LIBPQ-CONNECT-SSLPASSWORD) : This parameter specifies the password for the secret key specified in `sslkey`, allowing client certificate private keys to be stored in encrypted form on disk even when interactive passphrase input is not practical.
```
Specifying this parameter with any non-empty value suppresses the
`Enter PEM pass phrase:`
prompt that OpenSSL will emit by default
when an encrypted client certificate key is provided to
libpq.
If the key is not encrypted this parameter is ignored. The parameter
has no effect on keys specified by OpenSSL
engines unless the engine uses the OpenSSL
password callback mechanism for prompts.
There is no environment variable equivalent to this option, and no
facility for looking it up in `.pgpass`. It can be
used in a service file connection definition. Users with
more sophisticated uses should consider using OpenSSL engines and
tools like PKCS#11 or USB crypto offload devices.
```
`sslcertmode` [#](#LIBPQ-CONNECT-SSLCERTMODE) : This option determines whether a client certificate may be sent to the server, and whether the server is required to request one. There are three modes:
```
`disable`
: A client certificate is never sent, even if one is available
(default location or provided via
[sslcert](libpq-connect.md#LIBPQ-CONNECT-SSLCERT)).
`allow` (default)
: A certificate may be sent, if the server requests one and the
client has one to send.
`require`
: The server *must* request a certificate. The
connection will fail if the client does not send a certificate and
the server successfully authenticates the client anyway.
### Note
`sslcertmode=require` doesn't add any additional
security, since there is no guarantee that the server is validating
the certificate correctly; PostgreSQL servers generally request TLS
certificates from clients whether they validate them or not. The
option may be useful when troubleshooting more complicated TLS
setups.
```
`sslrootcert` [#](#LIBPQ-CONNECT-SSLROOTCERT) : This parameter specifies the name of a file containing SSL certificate authority (CA) certificate(s). If the file exists, the server's certificate will be verified to be signed by one of these authorities. The default is `~/.postgresql/root.crt`.
```
The special value `system` may be specified instead, in
which case the trusted CA roots from the SSL implementation will be loaded. The exact
locations of these root certificates differ by SSL implementation and
platform. For OpenSSL in particular, the
locations may be further modified by the `SSL_CERT_DIR`
and `SSL_CERT_FILE` environment variables.
### Note
When using `sslrootcert=system`, the default
`sslmode` is changed to `verify-full`,
and any weaker setting will result in an error. In most cases it is
trivial for anyone to obtain a certificate trusted by the system for a
hostname they control, rendering `verify-ca` and all
weaker modes useless.
The magic `system` value will take precedence over a
local certificate file with the same name. If for some reason you find
yourself in this situation, use an alternative path like
`sslrootcert=./system` instead.
```
`sslcrl` [#](#LIBPQ-CONNECT-SSLCRL) : This parameter specifies the file name of the SSL server certificate revocation list (CRL). Certificates listed in this file, if it exists, will be rejected while attempting to authenticate the server's certificate. If neither [sslcrl](#LIBPQ-CONNECT-SSLCRL) nor [sslcrldir](#LIBPQ-CONNECT-SSLCRLDIR) is set, this setting is taken as `~/.postgresql/root.crl`.
`sslcrldir` [#](#LIBPQ-CONNECT-SSLCRLDIR) : This parameter specifies the directory name of the SSL server certificate revocation list (CRL). Certificates listed in the files in this directory, if it exists, will be rejected while attempting to authenticate the server's certificate.
```
The directory needs to be prepared with the
OpenSSL command
`openssl rehash` or `c_rehash`. See
its documentation for details.
Both `sslcrl` and `sslcrldir` can be
specified together.
```
`sslsni` [#](#LIBPQ-CONNECT-SSLSNI) : If set to 1 (default), libpq sets the TLS extension “Server Name Indication” (SNI) on SSL-enabled connections. By setting this parameter to 0, this is turned off.
```
The Server Name Indication can be used by SSL-aware proxies to route
connections without having to decrypt the SSL stream. (Note that
unless the proxy is aware of the PostgreSQL protocol handshake this
would require setting `sslnegotiation`
to `direct`.)
However, SNI makes the destination host name appear
in cleartext in the network traffic, so it might be undesirable in
some cases.
```
`requirepeer` [#](#LIBPQ-CONNECT-REQUIREPEER) : This parameter specifies the operating-system user name of the server, for example `requirepeer=postgres`. When making a Unix-domain socket connection, if this parameter is set, the client checks at the beginning of the connection that the server process is running under the specified user name; if it is not, the connection is aborted with an error. This parameter can be used to provide server authentication similar to that available with SSL certificates on TCP/IP connections. (Note that if the Unix-domain socket is in `/tmp` or another publicly writable location, any user could start a server listening there. Use this parameter to ensure that you are connected to a server run by a trusted user.) This option is only supported on platforms for which the `peer` authentication method is implemented; see [Section 20.9](/server-administration/client-authentication/auth-peer.md).
`ssl_min_protocol_version` [#](#LIBPQ-CONNECT-SSL-MIN-PROTOCOL-VERSION) : This parameter specifies the minimum SSL/TLS protocol version to allow for the connection. Valid values are `TLSv1`, `TLSv1.1`, `TLSv1.2` and `TLSv1.3`. The supported protocols depend on the version of OpenSSL used, older versions not supporting the most modern protocol versions. If not specified, the default is `TLSv1.2`, which satisfies industry best practices as of this writing.
`ssl_max_protocol_version` [#](#LIBPQ-CONNECT-SSL-MAX-PROTOCOL-VERSION) : This parameter specifies the maximum SSL/TLS protocol version to allow for the connection. Valid values are `TLSv1`, `TLSv1.1`, `TLSv1.2` and `TLSv1.3`. The supported protocols depend on the version of OpenSSL used, older versions not supporting the most modern protocol versions. If not set, this parameter is ignored and the connection will use the maximum bound defined by the backend, if set. Setting the maximum protocol version is mainly useful for testing or if some component has issues working with a newer protocol.
`min_protocol_version` [#](#LIBPQ-CONNECT-MIN-PROTOCOL-VERSION) : Specifies the minimum protocol version to allow for the connection. The default is to allow any version of the PostgreSQL protocol supported by libpq, which currently means `3.0`. If the server does not support at least this protocol version the connection will be closed.
```
The current supported values are
`3.0`, `3.2`,
and `latest`. The `latest` value is
equivalent to the latest protocol version supported by the libpq
version being used, which is currently `3.2`.
```
`max_protocol_version` [#](#LIBPQ-CONNECT-MAX-PROTOCOL-VERSION) : Specifies the protocol version to request from the server. The default is to use version `3.0` of the PostgreSQL protocol, unless the connection string specifies a feature that relies on a higher protocol version, in which case the latest version supported by libpq is used. If the server does not support the protocol version requested by the client, the connection is automatically downgraded to a lower minor protocol version that the server supports. After the connection attempt has completed you can use [`PQfullProtocolVersion`](/client-interfaces/libpq/libpq-status.md#LIBPQ-PQFULLPROTOCOLVERSION) to find out which exact protocol version was negotiated.
```
The current supported values are
`3.0`, `3.2`,
and `latest`. The `latest` value is
equivalent to the latest protocol version supported by the libpq
version being used, which is currently `3.2`.
```
`krbsrvname` [#](#LIBPQ-CONNECT-KRBSRVNAME) : Kerberos service name to use when authenticating with GSSAPI. This must match the service name specified in the server configuration for Kerberos authentication to succeed. (See also [Section 20.6](/server-administration/client-authentication/gssapi-auth.md).) The default value is normally `postgres`, but that can be changed when building PostgreSQL via the `--with-krb-srvnam` option of configure. In most environments, this parameter never needs to be changed. Some Kerberos implementations might require a different service name, such as Microsoft Active Directory which requires the service name to be in upper case (`POSTGRES`).
`gsslib` [#](#LIBPQ-CONNECT-GSSLIB) : GSS library to use for GSSAPI authentication. Currently this is disregarded except on Windows builds that include both GSSAPI and SSPI support. In that case, set this to `gssapi` to cause libpq to use the GSSAPI library for authentication instead of the default SSPI.
`gssdelegation` [#](#LIBPQ-CONNECT-GSSDELEGATION) : Forward (delegate) GSS credentials to the server. The default is `0` which means credentials will not be forwarded to the server. Set this to `1` to have credentials forwarded when possible.
`scram_client_key` [#](#LIBPQ-CONNECT-SCRAM-CLIENT-KEY) : The base64-encoded SCRAM client key. This can be used by foreign-data wrappers or similar middleware to enable pass-through SCRAM authentication. See [Section F.38.1.10](/appendixes/contrib/postgres-fdw.md#POSTGRES-FDW-OPTIONS-CONNECTION-MANAGEMENT) for one such implementation. It is not meant to be specified directly by users or client applications.
`scram_server_key` [#](#LIBPQ-CONNECT-SCRAM-SERVER-KEY) : The base64-encoded SCRAM server key. This can be used by foreign-data wrappers or similar middleware to enable pass-through SCRAM authentication. See [Section F.38.1.10](/appendixes/contrib/postgres-fdw.md#POSTGRES-FDW-OPTIONS-CONNECTION-MANAGEMENT) for one such implementation. It is not meant to be specified directly by users or client applications.
`service` [#](#LIBPQ-CONNECT-SERVICE) : Service name to use for additional parameters. It specifies a service name in `pg_service.conf` that holds additional connection parameters. This allows applications to specify only a service name so connection parameters can be centrally maintained. See [Section 32.17](/client-interfaces/libpq/libpq-pgservice.md).
`target_session_attrs` [#](#LIBPQ-CONNECT-TARGET-SESSION-ATTRS) : This option determines whether the session must have certain properties to be acceptable. It's typically used in combination with multiple host names to select the first acceptable alternative among several hosts. There are six modes:
```
`any` (default)
: any successful connection is acceptable
`read-write`
: session must accept read-write transactions by default (that
is, the server must not be in hot standby mode and
the `default_transaction_read_only` parameter
must be `off`)
`read-only`
: session must not accept read-write transactions by default (the
converse)
`primary`
: server must not be in hot standby mode
`standby`
: server must be in hot standby mode
`prefer-standby`
: first try to find a standby server, but if none of the listed
hosts is a standby server, try again in `any`
mode
```
`load_balance_hosts` [#](#LIBPQ-CONNECT-LOAD-BALANCE-HOSTS) : Controls the order in which the client tries to connect to the available hosts and addresses. Once a connection attempt is successful no other hosts and addresses will be tried. This parameter is typically used in combination with multiple host names or a DNS record that returns multiple IPs. This parameter can be used in combination with [target\_session\_attrs](#LIBPQ-CONNECT-TARGET-SESSION-ATTRS) to, for example, load balance over standby servers only. Once successfully connected, subsequent queries on the returned connection will all be sent to the same server. There are currently two modes:
```
`disable` (default)
: No load balancing across hosts is performed. Hosts are tried in
the order in which they are provided and addresses are tried in
the order they are received from DNS or a hosts file.
`random`
: Hosts and addresses are tried in random order. This value is mostly
useful when opening multiple connections at the same time, possibly
from different machines. This way connections can be load balanced
across multiple PostgreSQL servers.
While random load balancing, due to its random nature, will almost
never result in a completely uniform distribution, it statistically
gets quite close. One important aspect here is that this algorithm
uses two levels of random choices: First the hosts
will be resolved in random order. Then secondly, before resolving
the next host, all resolved addresses for the current host will be
tried in random order. This behaviour can skew the amount of
connections each node gets greatly in certain cases, for instance
when some hosts resolve to more addresses than others. But such a
skew can also be used on purpose, e.g. to increase the number of
connections a larger server gets by providing its hostname multiple
times in the host string.
When using this value it's recommended to also configure a reasonable
value for [connect_timeout](libpq-connect.md#LIBPQ-CONNECT-CONNECT-TIMEOUT). Because then,
if one of the nodes that are used for load balancing is not responding,
a new node will be tried.
```
`oauth_issuer` [#](#LIBPQ-CONNECT-OAUTH-ISSUER) : The HTTPS URL of a trusted issuer to contact if the server requests an OAuth token for the connection. This parameter is required for all OAuth connections; it should exactly match the `issuer` setting in [the server's HBA configuration](/server-administration/client-authentication/auth-oauth.md).
```
As part of the standard authentication handshake, libpq
will ask the server for a *discovery document:* a URL
providing a set of OAuth configuration parameters. The server must
provide a URL that is directly constructed from the components of the
`oauth_issuer`, and this value must exactly match the
issuer identifier that is declared in the discovery document itself, or
the connection will fail. This is required to prevent a class of
["mix-up attacks"](https://mailarchive.ietf.org/arch/msg/oauth/JIVxFBGsJBVtm7ljwJhPUm3Fr-w/) on OAuth clients.
You may also explicitly set `oauth_issuer` to the
`/.well-known/` URI used for OAuth discovery. In this
case, if the server asks for a different URL, the connection will fail,
but a [custom OAuth flow](libpq-oauth.md#LIBPQ-OAUTH-AUTHDATA-HOOKS)
may be able to speed up the standard handshake by using previously
cached tokens. (In this case, it is recommended that
[oauth_scope](libpq-connect.md#LIBPQ-CONNECT-OAUTH-SCOPE) be set as well, since the
client will not have a chance to ask the server for a correct scope
setting, and the default scopes for a token may not be sufficient to
connect.) libpq currently supports the
following well-known endpoints:
* `/.well-known/openid-configuration`
* `/.well-known/oauth-authorization-server`
### Warning
Issuers are highly privileged during the OAuth connection handshake. As
a rule of thumb, if you would not trust the operator of a URL to handle
access to your servers, or to impersonate you directly, that URL should
not be trusted as an `oauth_issuer`.
```
`oauth_client_id` [#](#LIBPQ-CONNECT-OAUTH-CLIENT-ID) : An OAuth 2.0 client identifier, as issued by the authorization server. If the PostgreSQL server [requests an OAuth token](/server-administration/client-authentication/auth-oauth.md) for the connection (and if no [custom OAuth hook](/client-interfaces/libpq/libpq-oauth.md#LIBPQ-OAUTH-AUTHDATA-HOOKS) is installed to provide one), then this parameter must be set; otherwise, the connection will fail.
`oauth_client_secret` [#](#LIBPQ-CONNECT-OAUTH-CLIENT-SECRET) : The client password, if any, to use when contacting the OAuth authorization server. Whether this parameter is required or not is determined by the OAuth provider; "public" clients generally do not use a secret, whereas "confidential" clients generally do.
`oauth_scope` [#](#LIBPQ-CONNECT-OAUTH-SCOPE) : The scope of the access request sent to the authorization server, specified as a (possibly empty) space-separated list of OAuth scope identifiers. This parameter is optional and intended for advanced usage.
```
Usually the client will obtain appropriate scope settings from the
PostgreSQL server. If this parameter is used,
the server's requested scope list will be ignored. This can prevent a
less-trusted server from requesting inappropriate access scopes from the
end user. However, if the client's scope setting does not contain the
server's required scopes, the server is likely to reject the issued
token, and the connection will fail.
The meaning of an empty scope list is provider-dependent. An OAuth
authorization server may choose to issue a token with "default scope",
whatever that happens to be, or it may reject the token request
entirely.
```
***
原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/libpq-connect.html)（英文原文，待翻譯）
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