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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 produc- tion. These variables are intended for internal use only, subject 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 ex- ists. CURL_DBG_SOCK_RBLOCK The percentage of recv() calls that should be answered with an EA- GAIN 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 limited to a maximum of 400 bytes. CURL_DBG_SOCK_WBLOCK The percentage of send() calls that should be answered with an EA- GAIN 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 EAGAIN 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 envi- ronment 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 protocol 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 environment variable to [IP:port] makes libcurl use that DNS server instead 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 exists. CURL_FORCETIME A time of 0 is used for AWS signatures and NTLM if this variable ex- ists. CURL_ENTROPY A fixed faked value to use instead of a proper random number so that functions in libcurl that are otherwise getting random outputs can be tested for what they generate. CURL_SIGPIPE_DEBUG When present, curl does not set SIGPIPE to ignore. This allows veri- fication 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 variables 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_LO- CAL_PORT), and in that case the value is not a timestamp. 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 encoding and decoding. CURL_WS_CHUNK_EAGAIN Used to simulate blocking sends after this chunk size for WebSocket 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 initiated by the curl command line tool. The value of the environment 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 han- dles that are run via easy_perform. The value of the environment 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 millisec- onds. CURL_DBG_RESOLV_FAIL_DELAY With a threaded resolver, delay each lookup by the given millisec- onds 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 an- swer 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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