第一种方式 启动浏览器添加命令行

格式 --user-agent="自定义的UA"

1、C:\Users\Administrator\AppData\Local\Google\Chrome\Application\chrome.exe --user-agent="Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/129.0.0.0 Safari/537.36 jdtest"

2、启动浏览器f12查看效果

可以看到 net render 和主进程以及f12开发者工具中 都已经追加成功自定义ua。

原因是 翻看源码【本例子122核】可以看到每次取UA优先从命令行，然后是默认UA

// A string used to override the default user agent with a custom one.
const char kUserAgent[] = "user-agent";

components\embedder_support\user_agent_utils.cc

absl::optional<std::string> GetUserAgentFromCommandLine() {
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (command_line->HasSwitch(kUserAgent)) {
    std::string ua = command_line->GetSwitchValueASCII(kUserAgent);
    if (net::HttpUtil::IsValidHeaderValue(ua)) {
      return ua;
    }
    LOG(WARNING) << "Ignored invalid value for flag --" << kUserAgent;
  }
  return absl::nullopt;
}


std::string GetUserAgent(
    UserAgentReductionEnterprisePolicyState user_agent_reduction) {
  //从命令行获取UA
  absl::optional<std::string> custom_ua = GetUserAgentFromCommandLine();
  if (custom_ua.has_value()) {
    return custom_ua.value();
  }
  
  //获取默认UA
  return GetUserAgentInternal(user_agent_reduction);
}

第二种方法如下：

1、在components\embedder_support\user_agent_utils.cc和content\common\user_agent.cc中直接修改源码：

1、components\embedder_support\user_agent_utils.cc

// Internal function to handle return the full or "reduced" user agent string,
// depending on the UserAgentReduction enterprise policy.
std::string GetUserAgentInternal(
    UserAgentReductionEnterprisePolicyState user_agent_reduction) {
  std::string product = GetProductAndVersion(user_agent_reduction);
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(kHeadless)) {
    product.insert(0, "Headless");
  }

#if BUILDFLAG(IS_ANDROID)
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kUseMobileUserAgent))
    product += " Mobile";
#endif

  // In User-Agent reduction phase 5, only apply the <unifiedPlatform> to
  // desktop UA strings.
  // In User-Agent reduction phase 6, only apply the <unifiedPlatform> to
  // android UA strings.
  return ShouldSendUserAgentUnifiedPlatform(user_agent_reduction)
             ? content::BuildUnifiedPlatformUserAgentFromProduct(product)
             : content::BuildUserAgentFromProduct(product);
}

absl::optional<std::string> GetUserAgentFromCommandLine() {
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (command_line->HasSwitch(kUserAgent)) {
    std::string ua = command_line->GetSwitchValueASCII(kUserAgent);
    if (net::HttpUtil::IsValidHeaderValue(ua)) {
      return ua;
    }
    LOG(WARNING) << "Ignored invalid value for flag --" << kUserAgent;
  }
  return absl::nullopt;
}

//主进程 net进程 和render进程都会调用此函数，稍后看调试堆栈
std::string GetUserAgent(
    UserAgentReductionEnterprisePolicyState user_agent_reduction) {
  absl::optional<std::string> custom_ua = GetUserAgentFromCommandLine();
  if (custom_ua.has_value()) {
    return custom_ua.value();
  }

  return GetUserAgentInternal(user_agent_reduction);
}

2、content::BuildUserAgentFromProduct函数实现在
content\common\user_agent.cc
std::string BuildUserAgentFromProduct(const std::string& product) {
  std::string os_info;
  base::StringAppendF(&os_info, "%s%s", GetUserAgentPlatform().c_str(),
                      BuildOSCpuInfo(IncludeAndroidBuildNumber::Exclude,
                                     IncludeAndroidModel::Include)
                          .c_str());
  return BuildUserAgentFromOSAndProduct(os_info, product);
}
//此处代码是否有种似曾相识感觉,其他的自行看源码吧。
std::string BuildUserAgentFromOSAndProduct(const std::string& os_info,
                                           const std::string& product) {
  // Derived from Safari's UA string.
  // This is done to expose our product name in a manner that is maximally
  // compatible with Safari, we hope!!
  std::string user_agent;
  base::StringAppendF(&user_agent,
                      "Mozilla/5.0 (%s) AppleWebKit/537.36 (KHTML, like Gecko) "
                      "%s Safari/537.36",
                      os_info.c_str(), product.c_str());
  return user_agent;
}

2、看下调用GetUserAgent的堆栈

 1）、地址栏输入www.baidu.com render进程调用GetUserAgent初始化UA

2)、我将UA追加 jdtest 编译看下效果：

至此：两种方法定义UA已经介绍完毕，至于动态定义UA读者可自行实现。