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libcurl-env-dbg(3) Library Functions Manual libcurl-env-dbg(3) NAME libcurl-env-dbg - environment variables libcurl DEBUGBUILD understands DESCRIPTION This is a set of variables only recognized and used if libcurl was built "debug enabled", which should never be true for a library used in production. These variables are intended for internal use only, sub- ject to change and have many effects on the behavior of libcurl. Refer to the source code to determine how exactly they are being used. CURL_ALTSVC_HTTP Bypass the AltSvc HTTPS protocol restriction if this variable exists. CURL_DBG_SOCK_RBLOCK The percentage of recv() calls that should be answered with an EAGAIN at random. For TCP/UNIX sockets. CURL_DBG_SOCK_RMAX The maximum data that shall be received from the network in one recv() call. For TCP/UNIX sockets. This is applied to every recv. Example: CURL_DBG_SOCK_RMAX=400 means recv buffer size is lim- ited to a maximum of 400 bytes. CURL_DBG_SOCK_WBLOCK The percentage of send() calls that should be answered with an EAGAIN at random. For TCP/UNIX sockets. CURL_DBG_SOCK_WPARTIAL The percentage of data that shall be written to the network. For TCP/UNIX sockets. This is applied to every send. Example: CURL_DBG_SOCK_WPARTIAL=80 means a send with 1000 bytes would only send 800. CURL_DBG_QUIC_WBLOCK The percentage of send() calls that should be answered with EA- GAIN at random. QUIC only. CURL_DBG_SOCK_FAIL_IPV6 Fail opening of sockets for the IPv6 address family. CURL_DEBUG Trace logging behavior as an alternative to calling curl_global_trace(3). Example: CURL_DEBUG=http/2 means trace details about HTTP/2 han- dling. In the curl command line tool, built with --enable-debug, this environment variable adds to arguments like --verbose, -vvv. At least a single -v is needed to make the run emit trace output, but when it does, the contents of CURL_DEBUG are added and can override existing options. Example: CURL_DEBUG=tcp,-http/2 curl -vv url means trace proto- col details, triggered by -vv, add tracing of TCP in addition and remove tracing of HTTP/2. CURL_DEBUG_SIZE Fake the size returned by CURLINFO_HEADER_SIZE and CURLINFO_RE- QUEST_SIZE. CURL_DNS_SERVER When built with c-ares for name resolving, setting this environ- ment variable to [IP:port] makes libcurl use that DNS server in- stead of the system default. This is used by the curl test suite. CURL_FTP_PWD_STOP When set, the first transfer - when using ftp: - returns before sending the PWD command and stop any further progress. This is used to test an edge case CURL_GETHOSTNAME Fake the local machine's unqualified hostname for NTLM and SMTP. CURL_HSTS_HTTP Bypass the HSTS HTTPS protocol restriction if this variable ex- ists. CURL_FORCETIME A time of 0 is used for AWS signatures and NTLM if this variable exists. CURL_ENTROPY A fixed faked value to use instead of a proper random number so that functions in libcurl that are otherwise getting random out- puts can be tested for what they generate. CURL_SIGPIPE_DEBUG When present, curl does not set SIGPIPE to ignore. This allows verification that libcurl does not cause SIGPIPE to be raised. CURL_SMALLREQSEND An alternative size of HTTP data to be sent at a time only if smaller than the current. CURL_SMALLSENDS An alternative size of socket data to be sent at a time only if smaller than the current. CURL_TIME Fake Unix timestamp to use for AltSvc, HSTS and CURLINFO vari- ables that are time related. This variable can also be used to fake the data returned by some CURLINFO variables that are not time-related (such as CURLINFO_LOCAL_PORT), and in that case the value is not a time- stamp. CURL_TRACE LDAP tracing is enabled if this variable exists and its value is 1 or greater. OpenLDAP tracing is separate. Refer to CURL_OPENLDAP_TRACE. CURL_OPENLDAP_TRACE OpenLDAP tracing is enabled if this variable exists and its value is 1 or greater. There is a number of debug levels, refer to openldap.c comments. CURL_WS_CHUNK_SIZE Used to influence the buffer chunk size used for WebSocket en- coding and decoding. CURL_WS_CHUNK_EAGAIN Used to simulate blocking sends after this chunk size for Web- Socket connections. CURL_WS_FORCE_ZERO_MASK Used to set the bitmask of all sent WebSocket frames to zero. The value of the environment variable does not matter. CURL_FORBID_REUSE Used to set the CURLOPT_FORBID_REUSE flag on each transfer ini- tiated by the curl command line tool. The value of the environ- ment variable does not matter. CURL_GRACEFUL_SHUTDOWN Make a blocking, graceful shutdown of all remaining connections when a multi handle is destroyed. This implicitly triggers for easy handles that are run via easy_perform. The value of the en- vironment variable gives the shutdown timeout in milliseconds. CURL_DBG_RESOLV_MAX_THREADS Overrides the maximum number of threads for resolver. CURL_DBG_RESOLV_DELAY Makes ever threaded resolve experience an initial delay in mil- liseconds. CURL_DBG_RESOLV_FAIL_DELAY With a threaded resolver, delay each lookup by the given mil- liseconds and give a negative answer. CURL_DBG_RESOLV_FAIL_IPV6 Make libcurl fail a resolve for IPv6 only. CURL_QUICK_EXIT Make curl use the quick exit option, even when built in debug mode. CURL_DBG_AWAIT_HTTPSRR When happy eyeballing for https: wait for the HTTPS-RR resolve answer to arrive before starting any connect attempt. SEE ALSO libcurl-env(3) libcurl 2026-06-11 libcurl-env-dbg(3)
NAME | DESCRIPTION | SEE ALSO
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