Squid Proxy Blocking curl POST Requests With Expect: 100-continue

When sending POST requests through a Squid proxy with curl, you may encounter a proxy error that blocks the request, even though the same request works fine with GET. The issue typically appears as:

ERROR
The requested URL could not be retrieved

The root cause is the Expect: 100-continue header that curl automatically adds to POST requests with a body. Many Squid proxy configurations don’t support this header, causing the request to be rejected before it even reaches your destination server.

Why GET works but POST doesn’t

When you switch from POST to GET:

# This fails (POST with body)
curl http://example.com/send --data-urlencode "data=hello"

# This works (GET with query string)
curl -G http://example.com/send --data-urlencode "data=hello"

The difference is that GET requests don’t include the Expect: 100-continue header. curl adds this header to POST requests with a body to ask the server “should I send this large request?” before transmitting it. Squid doesn’t handle this negotiation properly and blocks the request.

Solution: Remove the Expect header

The simplest fix is to explicitly remove the Expect header from your curl command:

curl http://example.com/send -H "Expect:" --data-urlencode "data=hello"

The empty Expect: header tells curl to remove that header entirely from the request.

Using curl options instead of -H

For better clarity, you can also use curl’s built-in option:

curl http://example.com/send --data-urlencode "data=hello" -H "Expect:"

Or if you want to handle multiple headers, keep them grouped:

curl http://example.com/send \
  --data-urlencode "data=hello" \
  -H "Expect:" \
  -H "User-Agent: MyApp/1.0"

In libcurl C code

If you’re using libcurl in your application:

CURL *curl = curl_easy_init();
struct curl_slist *headers = NULL;

// Remove the Expect header
headers = curl_slist_append(headers, "Expect:");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

// Set your POST data
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "data=hello");
curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/send");

// Perform the request
curl_perform(curl);

// Clean up
curl_slist_free_all(headers);
curl_easy_cleanup(curl);

In Python with requests

If you’re using the Python requests library, you can set headers similarly:

import requests

headers = {"Expect": ""}
response = requests.post(
    "http://example.com/send",
    data={"data": "hello"},
    headers=headers,
    proxies={"http": "http://proxy-server:3128"}
)

Or with urllib:

import urllib.request

req = urllib.request.Request(
    "http://example.com/send",
    data=b"data=hello",
    headers={"Expect": ""}
)

proxy_handler = urllib.request.ProxyHandler(
    {"http": "http://proxy-server:3128"}
)
opener = urllib.request.build_opener(proxy_handler)
response = opener.open(req)

Additional Squid proxy configuration considerations

If you control the Squid proxy, you can also configure it to handle Expect headers properly. Add this to your squid.conf:

# Allow 100-continue for proper handling
ignore_expect_100 off

However, for most users on corporate networks, modifying proxy settings isn’t an option, so removing the header from your client is the practical solution.

When to use this workaround

This fix is necessary when:

• You’re behind a corporate or ISP Squid proxy
• POST requests are being blocked but GET requests work
• You see 407 Proxy Authentication Required or similar errors with POST
• You don’t have control over the proxy configuration

The Expect: header removal has no negative side effects for most modern servers and is safe to apply universally to requests going through proxies.

2026 Comprehensive Guide: Best Practices

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Quick Reference Summary

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